UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

	PC Code:  074801

	DP Barcode:  357536

	

MEMORANDUM	December 10, 2008

Subject:	Registration Review:  Preliminary Problem Formulation for
Ecological Risk, Endangered Species, and Drinking Water Exposure
Assessments for Tribufos

To:		Molly Clayton, Chemical Review Manager

Kevin Costello, Team Leader

Reregistration Branch 3

Special Review and Reregistration Division

Office of Pesticide Programs

From:		Melissa Panger, Ph.D., Biologist

		Greg Orrick, Environmental Scientist

		Environmental Risk Branch 4

		

Megan Thynge, Environmental Protection Specialist

		Environmental Information and Support Branch

Environmental Fate and Effects Division

		Office of Pesticide Programs

Through:	Elizabeth Behl, Chief

		Environmental Risk Branch 4

		Environmental Fate and Effects Division

		Office of Pesticide Programs

  SEQ CHAPTER \h \r 1 

				

Problem Formulation for the Ecological Risk, Endangered Species, and
Drinking Water Exposure Assessments in Support of the Registration
Review of Tribufos

S,S,S-tributyl phosphorotrithioate

CAS Registry Number: 78-48-8

PC Code: 074801

Prepared by:

Melissa Panger, Ph.D., Biologist

Greg Orrick, Environmental Scientist

Megan Thynge, Environmental Protection Specialist	U. S. Environmental
Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch IV

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460



Reviewed by:

Thomas Steeger, Ph.D., Senior Biologist

R. David Jones, Ph.D., Senior Agronomist

Elizabeth Behl, Branch Chief

	

December 10, 2008

Table of Contents

  TOC \o "1-3" \h \z \u    HYPERLINK \l "_Toc216592285"  1.  Purpose	 
PAGEREF _Toc216592285 \h  4  

  HYPERLINK \l "_Toc216592286"  2.  Problem Formulation	  PAGEREF
_Toc216592286 \h  4  

  HYPERLINK \l "_Toc216592287"  2.1.  Nature of Regulatory Action	 
PAGEREF _Toc216592287 \h  4  

  HYPERLINK \l "_Toc216592288"  2.2.  Previous Risk Assessments	 
PAGEREF _Toc216592288 \h  4  

  HYPERLINK \l "_Toc216592289"  3.  Stressor Source and Distribution	 
PAGEREF _Toc216592289 \h  5  

  HYPERLINK \l "_Toc216592290"  3.1.  Mechanism of Action	  PAGEREF
_Toc216592290 \h  5  

  HYPERLINK \l "_Toc216592291"  3.2.  Overview of Pesticide Use and
Usage	  PAGEREF _Toc216592291 \h  5  

  HYPERLINK \l "_Toc216592292"  3.3.  Environmental Fate and Transport	 
PAGEREF _Toc216592292 \h  8  

  HYPERLINK \l "_Toc216592293"  3.3.1.  Degradation	  PAGEREF
_Toc216592293 \h  9  

  HYPERLINK \l "_Toc216592294"  3.3.2.  Transport	  PAGEREF
_Toc216592294 \h  9  

  HYPERLINK \l "_Toc216592295"  3.3.3.  Field Dissipation	  PAGEREF
_Toc216592295 \h  10  

  HYPERLINK \l "_Toc216592296"  3.3.4.  Bioaccumulation	  PAGEREF
_Toc216592296 \h  10  

       HYPERLINK \l "_Toc216592297"  3.4.  Water Quality	  PAGEREF
_Toc216592297 \h  11  

  HYPERLINK \l "_Toc216592298"  4.  Receptors	  PAGEREF _Toc216592298 \h
 11  

  HYPERLINK \l "_Toc216592299"  4.1.  Effects to Aquatic Organisms	 
PAGEREF _Toc216592299 \h  12  

  HYPERLINK \l "_Toc216592300"  4.2.  Effects to Terrestrial Organisms	 
PAGEREF _Toc216592300 \h  14  

  HYPERLINK \l "_Toc216592301"  4.3.  Incident Database Review	  PAGEREF
_Toc216592301 \h  16  

  HYPERLINK \l "_Toc216592302"  4.4.  Ecosystems Potentially at Risk	 
PAGEREF _Toc216592302 \h  16  

  HYPERLINK \l "_Toc216592303"  5.  Assessment Endpoints	  PAGEREF
_Toc216592303 \h  16  

  HYPERLINK \l "_Toc216592304"  6.  Conceptual Model	  PAGEREF
_Toc216592304 \h  17  

  HYPERLINK \l "_Toc216592305"  6.1.  Risk Hypothesis	  PAGEREF
_Toc216592305 \h  17  

  HYPERLINK \l "_Toc216592306"  6.2.  Conceptual Diagram	  PAGEREF
_Toc216592306 \h  18  

  HYPERLINK \l "_Toc216592307"  7.  Analysis Plan	  PAGEREF
_Toc216592307 \h  20  

  HYPERLINK \l "_Toc216592308"  7.1.  Stressors of Concern	  PAGEREF
_Toc216592308 \h  21  

  HYPERLINK \l "_Toc216592309"  7.2.  Measures of Exposure	  PAGEREF
_Toc216592309 \h  21  

  HYPERLINK \l "_Toc216592310"  7.3.  Measures of Effect	  PAGEREF
_Toc216592310 \h  22  

  HYPERLINK \l "_Toc216592311"  7.4.  Integration of Exposure and
Effects	  PAGEREF _Toc216592311 \h  23  

  HYPERLINK \l "_Toc216592312"  7.5.  Deterministic and Probabilistic
Assessment Methods	  PAGEREF _Toc216592312 \h  24  

  HYPERLINK \l "_Toc216592313"  7.6.  Endangered Species Assessments	 
PAGEREF _Toc216592313 \h  24  

  HYPERLINK \l "_Toc216592314"  7.7.  Drinking Water Assessment	 
PAGEREF _Toc216592314 \h  24  

  HYPERLINK \l "_Toc216592315"  7.8.  Preliminary Identification of Data
Gaps	  PAGEREF _Toc216592315 \h  25  

  HYPERLINK \l "_Toc216592316"  7.8.1.  Fate	  PAGEREF _Toc216592316 \h 
25  

  HYPERLINK \l "_Toc216592317"  7.8.2.  Effects	  PAGEREF _Toc216592317
\h  29  

  HYPERLINK \l "_Toc216592318"  8.  References	  PAGEREF _Toc216592318
\h  32  

  HYPERLINK \l "_Toc216592319"  APPENDIX A:  Preliminary EECs for
Aquatic Habitats and RQs for Aquatic Organisms.	  PAGEREF _Toc216592319
\h  37  

  HYPERLINK \l "_Toc216592320"  APPENDIX B:  Preliminary EECs and
Associated RQs for Terrestrial Animals.	  PAGEREF _Toc216592320 \h  41  

  HYPERLINK \l "_Toc216592321"  APPENDIX C:  Preliminary EECs and
Associated RQs for Terrestrial and Riparian Plants.	  PAGEREF
_Toc216592321 \h  45  

  HYPERLINK \l "_Toc216592322"  APPENDIX D:  Data Call-In Tables.	 
PAGEREF _Toc216592322 \h  47  

 1.  Purpose

The purpose of this problem formulation is to provide an understanding
of what is known about the environmental fate and ecological effects of
tribufos, considering its currently registered uses.  Tribufos is an
organophosphate pesticide used only as a defoliant on cotton.  This
document will provide a plan for analyzing data relevant to tribufos and
for conducting ecological risk, endangered species, and drinking water
exposure assessments for its registered use on cotton.  Additionally,
this problem formulation is intended to identify data gaps,
uncertainties, and potential assumptions used to address those
uncertainties relative to characterizing the ecological risk associated
with the registered uses of tribufos.  

2.  Problem Formulation

2.1.  Nature of Regulatory Action

Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA),
all pesticides distributed or sold in the United States generally must
be registered by the Environmental Protection Agency (EPA).  In
determining whether a pesticide can be registered in the U.S., the
Agency evaluates its safety to non-target species based on a wide range
of environmental and health effects studies.  In 1996, FIFRA was amended
by the Food Quality Protection Act, and the Agency was mandated to
implement a new program for the periodic review of pesticides, i.e.,
registration review (  HYPERLINK
"http://www.epa.gov/oppsrrd1/registration_review/" 
http://www.epa.gov/oppsrrd1/registration_review/ ).  The registration
review program is intended to ensure that, as the ability to assess risk
evolves and as policies and practices change, all registered pesticides
continue to meet the statutory standard of no unreasonable adverse
effects to human health and the environment.  Changes in science, public
policy, and pesticide use practices will occur over time.  Through the
new registration review program, the Agency periodically reevaluates
pesticides to make sure that as change occurs, products in the
marketplace can be used safely. 

As part of the implementation of the new registration review program
pursuant to Section 3(g) of the Federal Insecticide, Fungicide and
Rodenticide Act (FIFRA), the Agency is beginning its evaluation of
tribufos to determine whether it continues to meet the FIFRA standard
for registration.  This problem formulation for the environmental fate,
ecological risk, endangered species, and drinking water assessment
chapter in support of the registration review will be posted in the
initial docket which will open the public phase of the review process.

2.2.  Previous Risk Assessments

A national-level ecological risk assessment was completed as part of the
interim reregistration eligibility decision (IRED) for tribufos in 2000
(USEPA 2000).  The reregistration eligibility decision (RED) was
finalized in 2006 after the cumulative human health risk assessment of
the organophosphate (OP) class of pesticides was completed (USEPA 2006).
 The ecological risk assessment included in the tribufos IRED was based
on laboratory studies of the environmental fate and ecotoxicological
effects of tribufos that were submitted by the registrant in support of
reregistration or collected from the publicly available literature.  The
ecological risk assessment was also based on incident reports of adverse
effects on non-target organisms associated with the use of tribufos. 
The IRED identified acute and chronic risks to birds, mammals, and
aquatic invertebrates (both freshwater and estuarine/marine) from the
registered use of tribufos on cotton.  Acute risks to estuarine/marine
fish, but not freshwater fish, were also identified.  Neither risks to
fish from chronic exposure to tribufos nor risks to plants could be
fully evaluated due to a lack of data.  After the IRED was signed, the
following studies were submitted to and reviewed by the Agency:

Terrestrial field dissipation study (on MS soil) (MRID 458637-01)

Washoff study on cotton leaves (MRID 451744-01)

Life-cycle toxicity study with saltwater mysid (MRID 458637-02)

Early life stage toxicity study with the fathead minnow (MRID 45863703)

Tier 2 seedling emergence and vegetative vigor phytotoxicity study with
DEF 6 (MRID 458637-04)

Vascular aquatic plant toxicity study (MRID 458637-05)

Acute toxicity study with the fathead minnow (MRID 458637-06)

Early life stage toxicity study with the sheepshead minnow (MRID
458637-07)

Therefore, the above studies were not considered during the IRED
process.  However, these studies were considered by the Agency in an
endangered species risk assessment for tribufos and the endangered
California red-legged frog (Rana aurora draytonii) (CRLF) that was
submitted to the U.S. Fish and Wildlife Service as part of formal
consultation on October 20, 2008.  EPA concluded in its endangered
species risk assessment that use of tribufos on cotton in California was
likely to adversely affect the California red-legged frog. 
Additionally, the above studies will be considered for the registration
review of tribufos.     

3.  Stressor Source and Distribution

3.1.  Mechanism of Action

Tribufos, S,S,S-tributyl phosphorotrithioate, is an organophosphate
chemical that, when applied to cotton plants and absorbed by its leaves,
injures the cells in the layer immediately beneath the leaf cuticle (via
esterase inhibition), causing stress to the plant, which stimulates the
production of ethylene (USEPA 2000).  The increased ethylene
concentrations result in the premature abscission of whole green leaves.
 Due to tribufos’ mechanism of action, effects to terrestrial plants
are primarily expected in those plants that form abscission zones (e.g.,
some woody, deciduous plants).  Therefore, the plant species recommended
in the Agency’s guidelines for Tier I and Tier II seedling emergence
and vegetative vigor studies (i.e., annuals) are not expected to be as
sensitive to tribufos as plants that form abscission zones.  

3.2.  Overview of Pesticide Use and Usage

Tribufos was first registered in the U.S. in 1961.  The only registered
use for tribufos in the U.S. is for use on cotton (as a defoliant) to
remove leaves prior to machine harvesting.  There are currently four
tribufos products registered in the U.S.: one product is a technical
grade for use in the manufacture of cotton defoliants and three are
end-use products (see Table 3.1).  There are no residential or other
agricultural uses of the compound.   

TABLE 3.1.  Summary of Tribufos Products Registered in the U.S.

Product Name (EPA Reg. No.)	Registrant	Percent Active Ingredient	Use

DEF® Technical

(264-720)	Bayer Crop Science	98	For use in the manufacture of cotton
defoliants

DEF® 6 Emulsifiable Defoliant

(264-730)	Bayer Crop Science	70.5	Cotton defoliant

Folex® 6 EC

(5481-504)	Amvac Chemical Corp.	70.5	Cotton defoliant

DFT® 6 EC Cotton Defoliant

(34704-867)	Loveland Products, Inc.	70.5	Cotton defoliant



Tribufos can be used nationally in areas where cotton is grown
(primarily the southeastern and southwestern U.S.).  The application
rates, application methods, and label restrictions are virtually the
same for all three of the currently registered tribufos end-use
products, however, the application rates vary geographically (see Table
3.2).  The maximum single and seasonal application rate (the same for
all three end-use products) is 1.9 lb a.i./acre in CA and AZ (see Table
3.2).  The maximum single and seasonal application rate in other states
(i.e., all states except CA and AZ) is 1.1 lb a.i./acre.  Tribufos is
most effective when it is applied to cotton leaves with thin leaf
cuticles (the waxy layer surrounding the leaf) and is applied at times
when light intensity and humidity are high (which result in more open
stomata).  Thick leaf cuticles, which occur in drier climates, and low
humidity result in lower uptake through the leaves.  That is why higher
rates of tribufos are required in CA and AZ.

In all states, two applications are allowed, not to exceed the seasonal
maximum application rate.  The application interval is not specified on
the current labels.  Applications are limited to foliar applications via
ground and aerial equipment; applications via irrigation systems
(chemigation) are not allowed.  According to the labels, higher
application rates and/or diesel oil rather than water as the spray
carrier is recommended in adverse conditions (e.g., low temperature,
especially temperatures below 60oF at night; low humidity) or when
plants are stressed.  

TABLE 3.2.  Summary of Tribufos Application Methods and Rates (for Use
as a Cotton Defoliant).

Product 	App. Method(s)	App. Rate (lbs a.i./A/season)	Max. No. of
App.s/Season	App. Interval	Label Restrictions

DEF® 6 Emulsifiable Defoliant 	Ground spray, 

aerial spray	All states except CA and AZ: 1.1

CA and AZ: 1.9 

	2	Not specified	Do not apply directly to water, or to areas where
surface water is present or to intertidal areas below the mean high
water mark.  Do not apply where runoff is likely to occur.  Do not apply
when weather conditions favor drift from areas treated.

Do not apply through any type of irrigation equipment.  Use higher rates
and/or diesel oil rather than water as the spray carrier in adverse
conditions (e.g., low temperature, especially temperatures below 60oF at
night; low humidity) or when plants are stressed.

Folex® 6 EC (70.5% a.i.)





	DFT® 6 EC Cotton Defoliant (70.5% a.i.)





	

According to the United States Geological Survey’s (USGS) national
pesticide usage data (based on information from 1999 to 2004), an
average of 2,407,397 lb of tribufos per year are applied nationally to
cotton in the U.S. (see Figure 3.1).  Tribufos usage is limited to
cotton-growing regions of the U.S. (primarily southwestern and
southeastern states).  Potential application sites of tribufos are
expected to include these states.  According to the October 2000 EPA
fact sheet on tribufos, there were 4.5 million pounds of active
ingredient applied annually to between 4-5 million acres or about 35% of
planted cotton acreage in the U.S.  Typical rates of application varied
between 0.5 to 1.9 lbs a.i./A (USEPA 2000a).

FIGURE 3.1.  Estimated Annual Tribufos Usage in the U.S. (from  
HYPERLINK
"http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m8
009" 
http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m80
09 ) [The pesticide use maps available from this site show the average
annual pesticide use intensity expressed as average weight (in pounds)
of a pesticide applied to each square mile of agricultural land in a
county.  The area of each map is based on state-level estimates of
pesticide use rates for individual crops that were compiled by the
CropLife Foundation, Crop Protection Research Institute during based on
information collected during 1999 through 2004 and on 2002 Census of
Agriculture county crop acreage.  The maps do not represent a specific
year, but rather show typical use patterns over the five year period
1999 through 2004.].

3.3.  Environmental Fate and Transport

Registrant-submitted data defining the physicochemical and environmental
fate characteristics of tribufos are summarized in Table 3.3.  These
data indicate that tribufos is immobile in soil and relatively
persistent and that the compound may be semi-volatile.  Therefore, there
is a potential for tribufos to accumulate in soil with repeated
applications.  Tribufos is expected to sorb to soil and not to leach to
ground water or move to surface water bodies through dissolved runoff. 
Surface water contamination may be expected to occur predominantly via
spray drift and/or from the transport of residues sorbed to sediment
that is suspended in runoff (i.e., erosion).  In either aquatic or
terrestrial environments, tribufos may bioaccumulate in organisms to
which it is exposed.

TABLE 3.3.  General Chemical Properties and Environmental Fate
Parameters of Tribufos.

Parameter	Value	Reference

Selected Physical/Chemical Parameters

Molecular mass	314.5 g/mol	MRID 41618803

Vapor pressure (20°C)	1.7 x 10-6 torr	MRID 41618803

Water solubility  (20°C)	2.3 mg/L	MRID 41618803

Henry’s law constant (20°C)	3.1 x 10-7 atm-m3/mol	(calculated)

Octanol-air partition coefficient (KOA)	2.6 x 1010	EpiSuite v3.20

Octanol-water partition coefficient (KOW) (20°C)	3.31 x 105	MRID
41618803

Fish Bioconcentration Factor (35 d)	300X   (edible)

1300X (inedible)

730X   (whole fish)	71-88% depurated after 14 days.	MRID 41618811

MRID 43080401

Persistence

Hydrolysis half-life (25°C; pH 5-9)	No evidence of degradation	MRID
41618814

Aqueous photolysis half-life	No evidence of degradation	MRID 41719401

Soil photolysis half-life	No evidence of degradation	MRID 41618816

Aerobic soil metabolism half-life	745 d (sandy loam)	MRID 42007204

Anaerobic soil metabolism half-life	389 d (sandy loam)	MRID 42007205

Anaerobic aquatic metabolism half-life	ca. 100 d (flooded silty clay
sediment)

(observed DT50 of 4-6 months)	MRID 43325504

Mobility

Freundlich adsorption coefficient (KF) 	60.6 L/kg (sandy loam)

66.8 L/kg (sand)

74.3 L/kg (silt loam)

106 L/kg (clay loam)	MRID 41618817

Field Dissipation

Terrestrial field dissipation half-life	13.9 d (not detected below 15
cm)	MRID 45863701



3.3.1.  Degradation

Tribufos does not appear to degrade by the abiotic processes hydrolysis
or photolysis (MRID 41618814, 41618816, 41719401).  The compound
degraded very slowly (745-day half-life) in sandy loam soil incubated
aerobically in the dark at 25°C for 360 days (MRID 42007204). 
Therefore, tribufos is more persistent than is typical for most
organophosphate chemicals.  The only major degradate observed in the
aerobic soil metabolism study was 1-butane sulfonic acid (1-BSA),
identified at about 10% of the applied radioactivity 9 months after
treatment.

Under anaerobic conditions, a half-life of 389 days was calculated for
tribufos in sandy loam soil (MRID 42007205) and a half-life of
approximately 100 days was calculated for tribufos in flooded silty clay
sediment (MRID 43325504).  The primary degradate, 1-BSA, was a minor
degradate in anaerobic soil.  However, it was a major degradate in the
anaerobic aquatic system, at up to 29.5% to 30.6% of the applied
radioactivity at 6 to 9 months after treatment.  The anaerobic aquatic
metabolism half-life is uncertain because tribufos degraded with a
biphasic pattern, degrading slowly during the first 2 months and more
rapidly afterward.  The observed DT50 was 4 to 6 months; however, the
linearly regressed half-life for the duration of the study was near 2
months.  No explanation was provided for the atypical degradation
pattern.  A single, nonlinearly regressed half-life of 100 days was
selected to approximate the uncertain degradation rate. 

The primary degradate of tribufos, 1-butane sulfonic acid (1-BSA), does
not retain the organophosphate substructure of the parent compound and
is not expected to be approximately as or more acutely toxic than
tribufos.  Furthermore, 1-BSA is not listed is an “apparent
exceeder” for chronic effects and cancer (USEPA 1996).  Therefore,
unless data are received that raise toxicity concerns for 1-BSA,
tribufos alone is the residue of concern for the ecological risk,
endangered species, and drinking water exposure assessments.

3.3.2.  Transport 

Tribufos is moderately soluble, with an aqueous solubility of 2.3 mg/L
at 20°C (MRID 41618803).  Tribufos is expected to be relatively
immobile in soil, based on Freundlich soil adsorption values that range
from 61 to 106 L/kg (MRID 41618817).  Because the coefficient of
variation (COV) for Freundlich soil adsorption (KF) values (0.26) is
less than that for values normalized for organic carbon (KFOC) (0.36),
tribufos does not conform to the organic carbon partitioning model
(USEPA 2006a).  However, appreciable surface water contamination may
occur via spray drift and/or from the transport of residues sorbed to
sediment that is suspended in runoff (i.e., erosion).  Based on the
tendency of the compound to bind to soil, ground water contamination as
a result of tribufos use is not expected.  

The potential for volatilization from soil and water is expected to be
low to moderate.  Tribufos’ moderately low vapor pressure (1.7 x 10-6
torr at 20ºC; MRID 41618803) and Henry’s law constant (calculated to
be 3.1 x 10-7 atm-m3/mol at 20°C) indicate that the compound may be
semi-volatile.  A submitted foliar washoff study that is currently in
review indicates that 15 days after foliar application, 7.6% of the
applied tribufos volatilizes.  However, volatilization was not observed
in the remaining submitted environmental fate studies.

The potential for moderate volatilization has been reviewed by the
California Department of Pesticide Regulation (CDPR).  A scientific
review panel for the CDPR reviewed the open literature on ambient air
concentrations of tribufos as of August 1998 (Scientific Review Panel
1999).  The panel highlighted two monitoring studies in which air
concentrations in California Central Valley towns near cotton fields
ranged from nondetectable (i.e., less than 1.1 ng/m3) to 548 ng/m3. 
Furthermore, CDPR has found that tribufos is readily photodegraded to
the volatile transformation product, n-butyl mercaptan, and possibly to
n-butyl disulfide (CDPR 1999).  However, these photodegradates were not
observed in environmental fate studies submitted to the Agency.  CDPR
listed DEF (i.e., tribufos) as a Toxic Air Contaminant (TAC) due to a
slight carcinogenic risk from tribufos in ambient air.

3.3.3.  Field Dissipation

In a supplemental terrestrial field dissipation study conducted in
Mississippi (MRID 45863701), the half-life of tribufos in the top 15 cm
of sandy loam soil was 13.9 days.  The dissipation pattern was biphasic
in this study, with an initial rapid decline through approximately 28
days post-treatment, followed by slower dissipation for the duration of
the study period.  The major route of dissipation of tribufos under
terrestrial field conditions was not reported.  The patterns of
formation and decline of transformation products were not addressed and
the transformation products were not identified in the study; therefore,
it could not be determined whether the study duration was adequate to
capture such information.  In addition, only one site representative of
the intended tribufos use site (cotton) was studied.  The dissipation
half-life from this study is not consistent with the laboratory fate
data and suggests that there are dissipation pathways that have not been
addressed in laboratory fate studies.

		3.3.4.  Bioaccumulation

Two bioaccumulation studies on bluegill sunfish (Lepomis macrochirus)
were submitted for tribufos; one for the in-life portion study of
bioaccumulation (MRID 41618811) and one to supplement that study with
metabolite identification (MRID 43080401).  In the in-life study,
bluegill sunfish were continuously exposed to a mean concentration of
6.2 µg/L of tribufos (dissolved in acetone) for 35 days.  After 14 days
of depuration, 71-88% of residues were eliminated from fish tissues. 
Bioconcentration factors (BCF) were determined to be 300X for edible
tissue, 1300X for inedible tissue, and 730X for whole fish tissues.  The
metabolite identification study found that tribufos was the only
compound detected in the edible tissue and water samples.  In inedible
tissue, tribufos parent was the major extractable residue at 33% of
radioactivity.  In addition to parent, 47 other metabolites were
extracted from the inedible tissue; however only three were identified
at between 3% - 4% of the measured radioactivity.  The BCF values
indicate that tribufos has the potential to bioaccumulate.

Consistent with these BCF values, an octanol-water partition coefficient
(KOW) of 3.31 x 105 (log KOW of 5.520) was determined for tribufos,
which means that tribufos is oliophilic (MRID 41618803).  Also, EpiSuite
(v3.20; Feb., 2007) estimates for tribufos an octanol-air partition
coefficient (KOA) of 2.6 x 1010 (log KOA of 10.41).  Kelly and Gobas
(2003) and Armitage and Gobas (2007) concluded that chemicals with a log
KOW >2 and a log KOA >5 have the potential to biomagnify in terrestrial
habitats.  Therefore, tribufos has the potential to biomagnify in
terrestrial habitats.

	3.4.  Water Quality

Tribufos has not been identified as a cause of impairment for any water
bodies listed as impaired under section 303(d) of the Clean Water Act,
based on information provided at   HYPERLINK
"http://iaspub.epa.gov/tmdl/waters_list.impairments?p_impid=3" 
http://iaspub.epa.gov/tmdl/waters_list.impairments?p_impid=3 .  The
Agency invites submission of water quality data for this pesticide.  To
the extent possible, data should conform to the quality standards in
Appendix A of the OPP Standard Operating Procedure: Inclusion of
Impaired Water Body and Other Water Quality Data in OPP’s Registration
Review Risk Assessment and Management Process (see:   HYPERLINK
"http://www.epa.gov/oppfead1/cb/ppdc/2006/november06/session1-sop.pdf" 
http://www.epa.gov/oppfead1/cb/ppdc/2006/november06/session1-sop.pdf ),
in order to ensure they can be used quantitatively or qualitatively in
pesticide risk assessments.

4.  Receptors

Consistent with the process described in the Overview Document (USEPA
2004), the risk assessment for tribufos relies on a surrogate species
approach.  T  SEQ CHAPTER \h \r 1 oxicological data generated from
surrogate test species, which are intended to be representative of broad
taxonomic groups, are used to extrapolate the potential effects on a
variety of species (receptors) included under these taxonomic groupings.
 Based on previous assessments (e.g., USEPA 1996), the primary degradate
of tribufos, 1-BSA, is not of toxicological concern.  Therefore, unless
data are received to suggest otherwise, the focus of the assessment is
expected to be the parent tribufos.  However, the Agency will review all
available current degradate ecotoxicity data to identify any additional
stressors of concern for this assessment.

Acute and chronic toxicity data from studies submitted by pesticide
registrants along with the available open literature are used to
evaluate the potential direct and indirect effects of tribufos to
aquatic and terrestrial receptors.  This includes toxicity on the
technical grade active ingredient, degradates, and when available,
formulated products (e.g., “Six-Pack” studies).  The open literature
studies are identified through EPA’s ECOTOXicology (ECOTOX) database
(USEPA 2007d), which employs a literature search engine for locating
chemical toxicity data for aquatic life, terrestrial plants, and
wildlife.  The evaluation of both sources of data may also provide
insight into the direct and indirect effects of tribufos on biotic
communities from loss of species that are sensitive to the chemical and
from changes in structure and functional characteristics of the affected
communities.  Open literature data were obtained from ECOTOX information
obtained on August 2, 2007.    

A summary of the aquatic and terrestrial toxicity data used to derive a
preliminary evaluation of risk for tribufos is provided in Sections 4.1
and 4.2, respectively.  In addition, a summary of ecological incidents
associated with tribufos and a description of ecosystems potentially at
risk are provided in Sections 4.3 and 4.4, respectively.

4.1.  Effects to Aquatic Organisms

A summary of the most sensitive aquatic toxicity data from
registrant-submitted studies is provided in Table 4.1.  All of the
endpoints for freshwater fish, freshwater invertebrates, and aquatic
plants used for RQ calculations in this assessment come from submitted
studies; there were no acceptable studies in the open literature that
resulted in more sensitive endpoints for these taxa.  Tribufos is
classified as highly toxic to freshwater and estuarine/marine fish and
aquatic plants and very highly toxic to invertebrates (freshwater and
estuarine/marine) on an acute exposure basis.

For freshwater fish, the chronic endpoint was derived using the
acute-to-chronic ratio (ACR) for the bluegill sunfish.  Toxicity data
for sheepshead minnow (Cyprinodon variegatus), an estuarine/marine
species, were used to derive the ACR because they were the only fish
species that had adequate toxicity data available for both acute and
chronic exposures to tribufos.  Additionally, the consistency of the
acute toxicity values for saltwater and freshwater species suggests that
saltwater species are not considerably more or less sensitive to
tribufos than freshwater species.  The sheepshead minnow studies
resulted in an LC50 of 767 µg a.i./L (MRID: 418963-02) and a NOAEC of
11 µg a.i./L (MRID 458637-07) based on clinical signs of intoxication,
reduced post-hatch survival, and reduced growth.  The calculated NOAEC
value for bluegill sunfish is 3.5 µg a.i./L based on the sheepshead
minnow-derived ACR (ACR = 69.7).  The chronic endpoint for freshwater
invertebrates [Daphnia magna NOAEC = 1.56 µg a.i./L (MRID 439782-01)]
is based on reduced number of young/adult/day and reduced adult length
in daphnids.

As noted above, the chronic endpoint for estuarine/marine fish [NOAEC =
11 µg a.i./L (MRID 458637-07)] is based on clinical signs of
intoxication, reduced post-hatch survival, and reduced growth in
sheepshead minnows.  For estuarine/marine invertebrates, there were no
effects seen at the highest concentration tested (NOAEC=0.26 µg a.i./L)
in a chronic study using mysids (MRID 458637-02).

TABLE 4.1.  Summary of the Most Sensitive Endpoints from Submitted
Aquatic Toxicity Studies for Tribufos.  

Species	Taxa Represented	Toxicity Value 	MRID # 	Classification	Comment

Bluegill sunfish

(Lepomis macrochirus)	Freshwater fish and aquatic-phase amphibians	96-hr
LC50 = 245 µg a.i./L (slope = 6.91, 95% CI; 3.11, 10.71)	400980-01

	Supplemental 	some deviations from guideline requirements



NOAEC = 3.5 µg a.i./L	N/A	Based on acceptable study	The NOAEC is based
on acute to chronic ratio using sheepshead minnow endpoints
[0.767/0.011= 69.7 (ACR)]

Daphnid

(Daphnia magna)	Freshwater invertebrates	48-hr EC50 = 6.8 µg a.i./L
400980-01	Supplemental  	raw data were not available for verification



NOAEC = 1.56 µg a.i./L

LOAEC = 3.23 µg a.i./L	439782-01	Acceptable	the NOAEC is based on
reduced number of young/adult/day and reduced adult length

Sheepshead minnow (Cyprinodon variegatus)	Estuarine/marine fish	96-hr
LC50 = 767 µg a.i./L	418963-02	Acceptable



	NOAEC = 11 µg a.i./L

LOAEC = 19 µg a.i./L	458637-07

	Acceptable	the NOAEC is based on clinical signs of intoxication,
post-hatch survival, and growth

Eastern oyster (Crassostrea virginica)	Estuarine/marine invertebrates
EC50 = 39.6 µg a.i./L	420832-01	Acceptable	flow through shell
deposition

Mysid (Mysidopsis bahia)	Estuarine/marine invertebrates	96-hr LC50 =
11.53 µg/L	418963-01	Acceptable



	NOAEC = 0.26 µg a.i./L (highest concentration tested, no endpoints
affected)	458637-02	Supplemental	Analytical variation in the measured
test concentrations exceeded 20% among replicate results; but no effects
seen at any concentration

Algae

(Selenastrum capricornutum)	Non-vascular aquatic plants	NOAEC = 58.5 µg
a.i./L 

EC50 = 148 µg a.i./L	416188-13	Acceptable	Tier II, 7-day test

Duckweed

(Lemna gibba)	Vascular aquatic plants	NOEAC = <17.2 µg a.i./L

EC05 = 140 µg a.i./L

EC50 = 1,100 µg a.i./L	458637-05	Supplemental	analytical verification
indicated that the test material was not stable; no definitive NOAEC
value; the NOAEC is based on reduced frond count.



4.2.  Effects to Terrestrial Organisms

Table 4.2 summarizes the most sensitive terrestrial toxicity endpoints
for tribufos, based on an evaluation of both the submitted studies and
the open literature.  All of the endpoints for birds, mammals,
terrestrial invertebrates, and terrestrial plants that will be used for
RQ calculations come from submitted studies; there were no acceptable
studies in the open literature that resulted in more sensitive endpoints
for these taxa.

Tribufos is classified as slightly to moderately toxic to birds on an
subacute dietary and acute oral exposure basis, respectively.  Tribufos
is considered moderately toxic to mammals and practically non-toxic to
terrestrial invertebrates on an acute exposure basis.

For chronic exposure, the NOAEC for birds is 150 mg/kg-diet based on
decreased egg shell thickness in bobwhite quail (Colinus virginianus)
(MRID 407571-01).  For mammals, the chronic endpoint is a NOAEL of 32
mg/kg-diet (1.7 mg/kg/day) based on a significant increase in the number
of litters with stillborn pups and pup death (including cannibalism)
through lactation; a decrease in F1 and F2 pup body weights; and a
significant increase in the F1 gestation period in laboratory rats
(Rattus norvegicus) (MRID 420402-01).

The submitted Tier II seedling emergence study (MRID 45867-04) indicated
that onion was the most sensitive monocot species tested (NOAEC 0.45 lb
a.i./A; EC25 >2.0 lb a.i./A) based on reduced plant height.  Soybean was
the most sensitive dicot species tested, also based on reduced plant
height, with a NOAEC of 0.22 lb a.i./A and an EC25 of > 2.0 lb a.i./A. 
All other species (monocots and dicots) had NOAEC and EC25 values at 2.0
lb a.i./A and > 2.0 lb a.i./A (the highest concentration tested) with
the exception of turnip, a dicot species with a NOAEC of 0.94 lb a.i./A.

The Tier II vegetative vigor study (MRID 45867-04) indicated that corn
(monocot) and buckwheat (dicot) were the most sensitive species based on
reduced dry weight.  The NOAEC and EC25 values for corn were 0.48 lb
a.i./A and 1.8 lb a.i./A, respectively.  The NOAEC and EC25 values for
buckwheat were 0.48 lb a.i./A and 1.3 lb a.i./A, respectively.  All
other species tested showed that dry weight was the most sensitive
endpoint for all species, though ryegrass (monocot) and sunflower
(dicot) did not exhibit effects at any test concentration.  

Because tribufos affects the abscission zone in plants, plants that do
not form abscission zones (such as those used in the vegetative vigor
and seedling emergence studies discussed above) are not expected to be
as sensitive to tribufos as plants that form abscission zones. 
Therefore, the plant species recommended in the Agency’s guidelines
for Tier I and Tier II seedling emergence and vegetative vigor studies
(i.e., annuals) are not expected to be as sensitive to tribufos as
plants that form abscission zones.

TABLE 4.2.  Summary of the Most Sensitive Endpoints from Submitted
Terrestrial Toxicity Studies for Tribufos.  

Species	Taxa Represented	Toxicity Value 	MRID # 	Classification	Comment

Bobwhite quail

(Colinus virginianus)	Birds, reptiles, and terrestrial-phase amphibians
LD50 = 151 mg/kg-bw (95% C.I. = 128 – 178)	00049258

00120771	Acceptable

	Bobwhite quail

LC50 = 1519 mg/kg-diet	416188-04	Acceptable	NOAEC = 556 mg/kg-diet,
based on reduced body weight gain

Bobwhite quail

NOAEC = 150 mg/kg-diet	407571-01	Acceptable 	NOAEC based on decreased
egg shell thickness at 280 mg/kg-diet; also reduced egg production and
hatchling survival at 410 mg/kg-diet

Laboratory rat

(Rattus norvegicus)	Mammals	LD50 = 192 mg/kg-bw	419549-03	Acceptable

	Laboratory rat

NOAEL = 32 mg/kg-diet (1.7 mg/kg/day)

LOAEL = 260 mg/kg-diet (15 mg/kg/day)	420402- 01 (based on 2-generation
reproduction study )	Acceptable	NOAEL based on  increase in number of
litters with stillborn pups and pup death through lactation; decrease in
F1 and F2 pup body weights; increase in F1 gestation period.  .

Honey bee

(Apis mellifera)	Terrestrial invertebrates	LD50 = >24.17 µg a.i./bee 
00001999 

(Atkins and Anderson 1967)	Supplemental	3% mortality at 24.17 µg
a.i./bee.  Value obtained from contact study.



Seedling Emergence

Monocots	Terrestrial plants	EC25 = >2.0 lb a.i./acre

	458637-04	Acceptable	Based on onion plant height



Seedling Emergence

Dicots

EC25 = >2 lb a.i./acre

	458637-04	Acceptable	Based on soybean plant height



Vegetative Vigor

Monocots

EC25 = 1.8 lb a.i./acre

	458637-04	Acceptable	Based on corn dry weight



Vegetative Vigor

Dicots

EC25 = 1.3 lb a.i./acre

	458637-04	Acceptable	Based on buckwheat dry weight

4.3.  Incident Database Review

A review of the Ecological Incident Information System (EIIS) database
for ecological incidents involving tribufos was completed on June 18,
2008.  There is one incident in the EIIS database associated with
tribufos (I016036-024).  This incident involved damage to 53 acres of
lettuce in Riverside, California, in 2004.  The damage was described as
‘uniform throughout’ the field, although the specific type of damage
to the lettuce was not specified in the incident report.  Damage was
attributed to spray drift of tribufos and diuron that had been legally
applied by air to a cotton field adjacent to the damaged lettuce field. 
The damaged lettuce tested positive for tribufos (50 ppb).  No other
residue information was provided in the report.

Due to limitations with data in the EIIS, a low number or lack of
reported incidents in the database cannot be used as evidence that
additional incidents have not occurred.  Incident reports for non-target
plants and animals typically provide information on mortality events
only.  Reports for other adverse effects, such as reduced growth or
impaired reproduction, are rarely received.  EPA’s changes in the
registrant reporting requirements of incidents may also account for the
reduced number of reported incidents.  Registrants are now only required
to submit detailed information on ‘major’ incidents.  Minor
incidents are generally reported aggregately and are not included in
EIIS.  In addition, there have been changes in state monitoring efforts
due to lack of resources.  However, the available incident report
indicates that there is a potential for field-observable effects to
non-target species from the use of tribufos.  

4.4.  Ecosystems Potentially at Risk

The ecosystems at risk are often extensive in scope; therefore, it may
not be possible to identify specific ecosystems during the development
of a nation-wide ecological risk assessment.  However, in general terms,
terrestrial ecosystems potentially at risk could include the treated
field and immediately adjacent areas that may receive drift or runoff. 
Areas adjacent to the treated field could include cultivated fields,
fencerows and hedgerows, meadows, fallow fields or grasslands,
woodlands, riparian habitats and other uncultivated areas.  

Aquatic ecosystems potentially at risk include water bodies adjacent to,
or down stream from, the treated field and could include impounded
bodies such as ponds, lakes and reservoirs, or flowing waterways such as
streams or rivers.  For uses in coastal areas, aquatic habitat also
includes marine ecosystems, including estuaries.  

5.  Assessment Endpoints

Assessment endpoints represent the actual environmental value that is to
be protected, defined by an ecological entity (species, community, or
other entity) and its attribute or characteristics (USEPA 1998).  For
tribufos, the ecological entities may include the following:  birds,
mammals, terrestrial-phase amphibians, reptiles, freshwater fish and
invertebrates, aquatic-phase amphibians, estuarine/marine fish and
invertebrates, terrestrial plants, insects, and aquatic plants and
algae.  The attributes for each of these entities may include growth,
reproduction, and survival.

6.  Conceptual Model

For a pesticide to pose an ecological risk, it must reach ecological
receptors in biologically significant concentrations.  An exposure
pathway is the means by which a pesticide moves in the environment from
a source to an ecological receptor.  For an ecological pathway to be
complete, it must have a source, a release mechanism, an environmental
transport medium, a point of exposure for ecological receptors, and a
feasible route of exposure.

The conceptual model for tribufos provides a written description and
visual representation of the predicted relationships between tribufos,
potential routes of exposure, and the predicted effects for the
assessment endpoint.  A conceptual model consists of two major
components: risk hypothesis and a conceptual diagram (USEPA 1998).

Based on the submitted environmental fate data, tribufos is expected to
persist in soil and to potentially move to surface water through erosion
and spray drift.  Given its relatively high BCF, KOW, and KOA values, it
is expected to bioaccumulate in aquatic and terrestrial food chains. 
Preliminary estimated environmental concentrations (EEC) based on these
fate data were compared to effects data in order to calculate
preliminary risk quotients (RQ) for aquatic organisms (APPENDIX A) and
terrestrial organisms (APPENDICES B and C).  Based on maximum
application rates, estimated acute RQ values are sufficient to exceed
levels of concern (LOC) for Federally-listed threatened/endangered
(listed) and non-listed aquatic invertebrates (freshwater and
estuarine/marine), mammals, and birds.  Risks are also expected for
listed fish (freshwater and estuarine/marine), terrestrial
invertebrates, and listed vascular aquatic plants due to acute exposures
to tribufos.  Estimated chronic RQ values are sufficient to exceed
levels of concern (LOCs) for listed and non-listed fish (freshwater and
estuarine/marine), aquatic invertebrates (freshwater and
estuarine/marine), mammals, and birds.  Risks are assumed as well for
non-listed and listed terrestrial plant species that have leaves that
form abscission zones.  Because of the potential risk from direct
effects to the listed and non-listed taxa described above, listed
species in all taxa may potentially be affected indirectly due to
alterations in their habitat and prey items (e.g., food sources,
shelter, and areas to reproduce).  These preliminary conclusions are
used to derive the risk hypothesis and conceptual diagram discussed
below. 

6.1.  Risk Hypothesis

A risk hypothesis describes the predicted relationship among the
stressor, exposure, and assessment endpoint response along with the
rationale for their selection.  For tribufos, the following ecological
risk hypothesis is being employed for this ecological risk assessment:

Based on the application methods, mode of action, fate and transport,
and the sensitivity of non-target aquatic and terrestrial species,
tribufos has the potential to reduce survival, reproduction, and/or
growth in non-target terrestrial and aquatic organisms when used in
accordance with the current labels.  Although tribufos is only
registered for use on cotton, its stability to abiotic routes of
degradation and limited biotic degradation will likely result in residue
accumulations in the field.  Treated foliage is expected to drop to the
ground and contribute to residues from preceding applications.  Although
the chemical readily sorbs to soil, it will likely move to surface
waters through a combination of both spray drift and erosion, where it
will be primarily associated with benthic sediments.  Given the high KOW
of the compound, it is likely that successively higher trophic levels
will bioaccumulate the compound.  As such, dietary intake may represent
a significant route of exposure.  Since tribufos ranges from highly to
very highly toxic to aquatic organisms and is capable of causing chronic
effects as well, there is a likelihood of adverse effects (risk) to
aquatic organisms.  Since tribufos ranges from slightly to moderately
toxic to birds and mammals and is capable of causing chronic effects as
well, there is a likelihood of adverse effects (risk) to terrestrial
vertebrates.  These non-target organisms include Federally-listed
threatened and endangered species as well as non-listed species.  

	6.2.  Conceptual Diagram

The environmental fate properties of tribufos indicate that runoff,
spray drift, direct spray, and long-range atmospheric transport
represent potential transport mechanisms of tribufos to aquatic and
terrestrial habitats where non-target organisms may be exposed.  These
transport mechanisms (i.e., sources) are depicted in the conceptual
models below (Figures 6.1 and 6.2) along with the receptors of concern
and the potential attribute changes in the receptors due to exposures of
tribufos.  

 

FIGURE 6.1.  Conceptual Model for Tribufos Effects on Aquatic Organisms.
 Dotted Lines Indicate Exposure Pathways that Have a Low Likelihood of
Contributing to Ecological Risk.

 

FIGURE 6.2.  Conceptual Model for Tribufos Effects on Terrestrial
Organisms.  

7.  Analysis Plan

In order to address the risk hypothesis, the potential for adverse
effects on the environment is estimated.  The use, environmental fate,
and ecological effects of tribufos are characterized and integrated to
assess the risks.  This is accomplished using a risk quotient (ratio of
exposure concentration to effects concentration) approach.  Although
risk is often defined as the likelihood and magnitude of adverse
ecological effects, the risk quotient-based approach does not provide a
quantitative estimate of likelihood and/or magnitude of an adverse
effect.  However, as outlined in the Overview Document (USEPA 2004), the
likelihood of effects to individual organisms from particular uses of
tribufos is estimated using the probit dose-response slope and either
the level of concern (discussed below) or the actual calculated risk
quotient value.  

This analysis plan will be revisited and may be revised depending upon
the information submitted by the public in response to the opening of
the Registration Review docket.

	7.1.  Stressors of Concern

As discussed above, tribufos degrades to a limited extent into 1-butane
sulfonic acid.  Based on previous assessments (e.g., USEPA 1996),
1-butane sulfonic acid is not of toxicological concern.  Therefore, the
focus of this assessment is expected to be the parent tribufos. 
However, the Agency will review all available current degradate
ecotoxicity data to identify any additional stressors of concern for
this assessment.   If any degradate is found to be of toxicological
concern, additional environmental fate and effects data may be requested
on that degradate(s) in order to fully assess the potential risk
associated with the use of tribufos.

In its ecological risk assessments, the Agency does not routinely
include an evaluation of mixtures of active ingredients, either those
mixtures of multiple active ingredients in product formulations or those
in the applicator’s tank.  In the case of the product formulations of
active ingredients (that is, a registered product containing more than
one active ingredient), each active ingredient is subject to an
individual risk assessment for regulatory decision regarding the active
ingredient on a particular use site.  If effects data are available for
a formulated product containing more than one active ingredient, the
data may be used qualitatively or quantitatively in accordance with the
Agency’s Overview Document and the Services’ Evaluation Memorandum
(USEPA 2004; USFWS/NMFS 2004).  

Available toxicity data for environmental mixtures of tribufos with
other pesticides will be presented as part of the ecological risk
assessment.  It is expected that the toxic effect of tribufos, in
combination with other pesticides used in the environment, is likely to
be a function of many factors including but not necessarily limited to:
(1) the exposed species, (2) the co-contaminants in the mixture, (3) the
ratio of tribufos and co-contaminant concentrations, (4) differences in
the pattern and duration of exposure among contaminants, and (5) the
differential effects of other physical/chemical characteristics of the
receiving waters (e.g. organic matter present in sediment and suspended
water).  Quantitatively predicting the combined effects of all these
variables on mixture toxicity to any given taxa with confidence is
beyond the capabilities of the available data and methodologies. 
However, a qualitative discussion of implications of the available
pesticide mixture effects data on the confidence of risk assessment
conclusions will be addressed as part of the uncertainty analysis.  At
this time, however, there are currently no registered multi-active
ingredient products that contain tribufos in the U.S., and no data on
mixtures including tribufos are available 

	7.2.  Measures of Exposure

In order to estimate risks of tribufos exposures in aquatic and
terrestrial environments, all exposure modeling and resulting risk
conclusions will be made based on maximum application rates for cotton
discussed in Section 5.  Measures of exposure are based on aquatic and
terrestrial models that predict estimated environmental concentrations
of tribufos using maximum labeled application rates and application
methods that have the greatest potential for off-site transport of the
chemical.  The models used to predict aquatic EECs are the Pesticide
Root Zone Model coupled with the EXposure Analysis Model System
(PRZM/EXAMS).  The model used to predict terrestrial EECs on food items
is T-REX.  The model used to derive EECs relevant to terrestrial and
wetland plants is TerrPlant.  These models are parameterized using
relevant reviewed registrant-submitted environmental fate data.

PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are screening
simulation models coupled with the graphical user interface, PE (v5.0,
November 2006) to generate daily exposures and 1-in-10 year estimated
environmental concentrations (EEC) of tribufos that may occur in surface
water bodies adjacent to application sites receiving tribufos through
runoff and spray drift.  PRZM simulates pesticide application, movement
and transformation on an agricultural field and the resultant pesticide
loadings to a receiving water body via runoff, erosion, and spray drift.
 EXAMS simulates the fate of the pesticide and resulting concentrations
in the water body.  The standard watershed geometry used for ecological
pesticide assessments assumes application to a 10-hectare agricultural
field that drains into an adjacent 1-hectare water body that is 2 meters
deep (20,000 m3 volume) with no outlet.  The composite model PRZM/EXAMS
is used to estimate screening-level exposure of aquatic organisms to
tribufos.  The measure of exposure for aquatic species is the 1-in-10
year return peak or rolling mean concentration.  The 1-in-10 year peak
is used for estimating acute exposures of direct effects to aquatic
organisms.  The 1-in-10-year 60-day mean is used for assessing chronic
exposure to fish and aquatic-phase amphibians.  The 1-in-10-year 21-day
mean is used for assessing aquatic invertebrate chronic exposure.

Exposure estimates for terrestrial animals assumed to be in the target
area or in an area exposed to spray drift are derived using the T-REX
model (version 1.3.1, 12/07/2006).    SEQ CHAPTER \h \r 1 This model
incorporates the Kenaga nomograph, as modified by Fletcher et al.
(1994), which is based on a large set of actual field residue data. The
upper limit values from the nomograph represent the 95th percentile of
residue values from actual field measurements (Hoerger and Kenega,
1972).  The Fletcher et al. (1994) modifications to the Kenega nomograph
are based on measured field residues from 249 published research papers,
including information on 118 species of plants, 121 pesticides, and 17
chemical classes.  EECs for terrestrial plants inhabiting dry and
wetland areas are derived using TerrPlant (version 1.2.2, 12/26/2006). 
This model uses estimates of pesticides in runoff and in spray drift to
calculate EECs.  EECs are based upon solubility, application rate and
minimum incorporation depth.  

The AgDRIFT spray drift model (v2.01; May 2001) is used to assess
exposures of organisms to tribufos deposited on terrestrial habitats by
spray drift.

The ecological risk assessment will include an assessment of the
bioaccumulation of this chemical in aquatic and terrestrial habitats,
which will likely include bioaccumulation modeling and an investigation
into the compound’s atmospheric transport potential.

	7.3.  Measures of Effect

 

Ecological effects data are used as measures of direct and indirect
effects to biological receptors.  Data are obtained from
registrant-submitted studies or from literature studies identified by
ECOTOX.  The ECOTOX database (USEPA 2007d) provides more ecological
effects data in an attempt to bridge existing data gaps.  ECOTOX is a
source for locating single chemical toxicity data and potential chemical
mixture toxicity data for aquatic life, terrestrial plants, and
wildlife.  ECOTOX was created and is maintained by the USEPA, Office of
Research and Development, and the National Health and Environmental
Effects Research Laboratory's Mid-Continent Ecology Division.

Information on the potential effects of tribufos on non-target animals
is also collected from the Ecological Incident Information System (EIIS;
USEPA 2007b).  The EIIS is a database containing adverse effect
(typically mortality) reports on non-target organisms where such effects
have been associated with the use of pesticides.   

Where available, sublethal effects observed in both registrant-submitted
and open literature studies will be evaluated qualitatively.  Such
effects may include behavioral changes (e.g., lethargy and changes in
coloration).  Quantitative assessments of risks, though, are limited to
those endpoints that can be directly linked to the Agency’s assessment
endpoints of impaired survival, growth and reproduction.

  SEQ CHAPTER \h \r 1 The assessment of risk for direct effects to
non-target organisms makes the assumption that toxicity of tribufos to
birds is similar to terrestrial-phase amphibians and reptiles.  The same
assumption is made for fish and aquatic-phase amphibians. 

The acute measures of effect used for animals in this assessment are the
LD50, LC50 and EC50.  LD stands for "Lethal Dose", and LD50 is the
amount of a material, given all at once, that is estimated to cause the
death of 50% of the test organisms.  LC stands for “Lethal
Concentration” and LC50 is the concentration of a chemical that is
estimated to kill 50% of the test organisms.  EC stands for “Effective
Concentration” and the EC50 is the concentration of a chemical that is
estimated to produce a specific effect in 50% of the test organisms. 
Endpoints for chronic measures of exposure for listed and non-listed
animals are the NOAEL/NOAEC and NOEC.  NOAEL stands for “No
Observed-Adverse-Effect-Level” and refers to the highest tested dose
of a substance that has been reported to have no harmful (adverse)
effects on test organisms.  The NOAEC (i.e.,
“No-Observed-Adverse-Effect-Concentration”) is the highest test
concentration at which none of the observed effects were statistically
different from the control.  The NOEC is the
No-Observed-Effects-Concentration.  For non-listed plants, only acute
exposures are assessed (i.e., EC25 for terrestrial plants and EC50 for
aquatic plants); for listed plants either the NOAEC or EC05 is used.  

	7.4.  Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological
effects characterization to determine the potential ecological risk from
the use of tribufos on cotton and the likelihood of direct and indirect
effects to non-target organisms in aquatic and terrestrial habitats. 
The exposure and toxicity effects data are integrated in order to
evaluate the risks of adverse ecological effects on non-target species. 
For the assessment of tribufos risks, the risk quotient (RQ) method is
used to compare exposure and measured toxicity values.  EECs are divided
by acute and chronic toxicity values.  The resulting RQs are then
compared to the Agency’s Levels of Concern (LOC) (USEPA 2004).  These
criteria are used to indicate when tribufos’ use, as directed on the
labels, has the potential to cause adverse direct or indirect effects to
non-target organisms.  In addition, incident data from the EIIS will be
considered as part of the risk characterization.  Furthermore, the
persistence, bioaccumulation, and toxicity (PBT) characteristics of
tribufos will be investigated.

	7.5.  Deterministic and Probabilistic Assessment Methods

The quantitative assessment of risk will primarily depend on the
deterministic point-estimate based approach described in the risk
assessment.  An effort will be made to further qualitatively describe
risk using probabilistic tools that the Agency has developed.  These
tools have been reviewed by FIFRA Scientific Advisory Panels ( 
HYPERLINK "http://www.epa.gov/scipoly/sap/index.htm" 
http://www.epa.gov/scipoly/sap/index.htm ) and have been deemed as
appropriate means of refining assessments where deterministic approaches
have identified risks.

	7.6.  Endangered Species Assessments

Consistent with the Agency’s responsibility under the Endangered
Species Act (ESA), the Agency will evaluate risks to Federally-listed
threatened and/or endangered (listed) species from registered uses of
tribufos.  This assessment will be conducted in accordance with the
Overview Document (USEPA 2004), provisions of the ESA, and the
Services’ Endangered Species Consultation Handbook (USFWS/NMFS, 1998).

The assessment of effects associated with the registration of tribufos
is based on an action area.  The action area is considered to be the
area directly or indirectly affected by the federal action, as indicated
by the exceedance of Agency Levels of Concern (LOCs) used to evaluate
direct or indirect effects.  The Agency’s approach to defining the
action area under the provisions of the Overview Document (USEPA 2004)
considers the results of the risk assessment process to establish
boundaries for that action area with the understanding that exposures
below the Agency’s defined LOCs constitute a no-effect threshold.  For
the purposes of this assessment, attention will be focused on the
footprint of the action (i.e., the area where tribufos application
occurs), plus all areas where offsite transport (i.e., spray drift,
runoff, etc.) may result in potential exposure that exceeds the
Agency’s LOCs.  Specific measures of ecological effect that define the
action area for listed species include any direct and indirect effects
and/or potential modification of its critical habitat, including
reduction in survival, growth, and reproduction as well as the full
suite of sublethal effects available in the effects literature. 
Therefore, the action area extends to a point where environmental
exposures are below any measured lethal or sublethal effect threshold
for any biological entity at the whole organism, organ, tissue, and
cellular level of organization.  In situations where it is not possible
to determine the threshold for an observed effect, the action area will
initially be assumed to include all counties where cotton is grown in
the United States until a more refined assessment is completed.

7.7.  Drinking Water Assessment

A drinking water assessment will be conducted to support future human
health risk assessments of tribufos.  The drinking water assessment will
incorporate model estimates of tribufos in surface and ground waters. 
Concentrations of tribufos in surface waters will be estimated using
PRZM/EXAMS (see description in Section 7.2).  Ground water estimates of
tribufos concentrations will be estimated using the Screening
Concentration in Ground Water (SCI-GROW) model (v.2.3, July 2003).  The
drinking water assessment will also include a summary of available
surface and ground water monitoring data.

Previous drinking water assessments include two memoranda for the
tribufos IRED.  The first memorandum (DP barcode not cited; Dec. 17,
1997) states that there is probably no drinking water concern.  The
second memorandum (DP barcode 268764; Sep. 6, 2000) includes Tier I
exposure estimates in surface water (5.8 ppb peak; 1.8 ppb annual; 1.1
ppb 30-year exposure) and in ground water (0.007 ppb).  Exposure
estimates in the drinking water assessment for registration review will
use current models and methodologies and are expected to be up to 2-fold
higher than previous estimates.

7.8.  Preliminary Identification of Data Gaps

		7.8.1.  Fate

The studies submitted to fulfill environmental fate data requirements
for tribufos are sufficient for exposure assessment, with two
exceptions.  Submission of an aerobic aquatic metabolism study is
recommended in order to describe the fate of tribufos in surface water
bodies.  Submission of an octanol-air partition coefficient (KOA) study
is also recommended in order to describe the fate of tribufos in the
atmosphere in support of bioaccumulation modeling.  Although not
necessary for exposure assessment, submission of analytical methods in
water and in sediment and associated independent laboratory validations
is also recommended in order to support enforcement and monitoring
purposes at the federal, state, and local levels.  The analytical method
in sediment is recommended because tribufos is expected to partition
predominately to sediment in surface water bodies.  If data are
submitted indicating that a previously submitted analytical method in
soil is functional for residues in sediment, then a separate analytical
method in sediment is not necessary.  Draft Data Call-In (DCI) tables
for these data are provided in APPENDIX D.

Table 7.1 identifies studies by MRID that offer data for each guideline
requirement, as well as study classifications, classification
justifications, and whether or not further data are needed in order to
support risk assessment (i.e., whether there is a data gap).  It is
notable that studies submitted under the guidelines for hydrolysis and
terrestrial field dissipation are not acceptable.  However, submission
of additional data under these guidelines is not recommended because
additional data are not expected to appreciably change the
characterization of the environmental fate of tribufos or to alter risk
conclusions.

The hydrolysis study is classified as ancillary because the kinetics and
degradate identification portions of the study were run at
concentrations (5 and 20 mg/L, respectively) that are higher than the
solubility limit (2.3 mg/L), and 11% to 30% of applied radioactivity
adsorbed to the glass vials.  However, a new study is not likely to
provide additional useful information, as tribufos was stable to
hydrolysis in the dark controls used in the submitted aqueous photolysis
study.

The submitted terrestrial field dissipation studies were classified
either unacceptable or supplemental.  The supplemental study was only
conducted for 120 days on one site.  For the study, degradates were not
identified nor were their kinetics addressed.  Also, the biphasic
pattern of decline of the parent compound was not explained by available
laboratory study data on degradation.  While a new study may provide
insight regarding the persistence of tribufos under field conditions, it
is not likely to provide information that alters risk conclusions.TABLE
7.1.  Environmental Fate Data Requirement Table for Tribufos.

OPPTS Guideline	Data Requirement	Submitted Studies (MRID)
Classification1	Classification Justification	Data Gap?

835.2120	Hydrolysis	41618814	Ancillary	Study concentrations exceeded the
water solubility limit of tribufos.  Residues adsorbed to the glassware.
Currently, no.

835.2240	Aqueous photolysis	41618815	(Unacceptable)	Sampling intervals
were too infrequent and the provided data were too variable to
accurately assess dissipation.  Individual degradates were not
identified or quantified.  The study was conducted outdoors during
winter, and the reported temperatures of the test solutions varied by
>20°C.  The sterility of the test solutions was not confirmed. 
Material balances steadily decreased during the study, and the study
authors stated that the volatile traps were probably not adequate.	No.



41719401	Acceptable	Study is consistent with recommended guidelines.

	835.2410	Soil photolysis	41618816	Acceptable	Study is consistent with
recommended guidelines.	No.

835.4100	Aerobic soil metabolism	42007204	Acceptable	Study is consistent
with recommended guidelines.  (Data on three additional soils would be
useful, but are not necessary.)	No.  

835.4200	Anaerobic soil metabolism	42007205	Acceptable	Study consistent
with recommended guidelines.	No.

835.4300	Aerobic aquatic metabolism	(none)	--	(No study was provided.)
Yes.

835.4400	Anaerobic aquatic metabolism	42007206	(Unacceptable)	Redox
potential, dissolved oxygen, pH, and temperature were not monitored. 
The pattern of formation and decline of the major degradate was not
established.  The test compound was applied at twice its reported
aqueous solubility, and there were indications of adsorption to the
glassware.  Partitioning of residues between the water and sediment
phases could not be adequately assessed due to inappropriate procedures.
 All major degradates may not have been identified.	No.



43325504	Acceptable	Study is consistent with recommended guidelines.

	835.1230

835.1240	Adsorption/ desorption and leaching	49255	(Unacceptable)
Complete experimental details were not reported.  The test soils were
over-sieved.  Detection limits were not reported.	No.



41618817	Acceptable	Study is consistent with recommended guidelines.



	42350004	Acceptable	Study is consistent with recommended guidelines.

	835.6100	Terrestrial field dissipation	49255	Unacceptable	The maximum
labeled application rate was not reported.  The application rate was not
verified using application monitoring devices.  The stability of
tribufos in samples was not demonstrated under typical laboratory
storage conditions.  This was a runoff study in which soil samples were
not collected for analysis of tribufos or its degradates; therefore, the
dissipation kinetics of residues in soil and the depth of leaching could
not be determined.	Currently, no.



104930	Unacceptable	Sampling intervals were inadequate to accurately
define the half-life of tribufos under field conditions at both test
sites.  The pattern of formation and decline of degradates was not
determined.  The depth of leaching was not determined.  The maximum
labeled application rate was not reported.  The application rate was not
verified using application monitoring devices.  Analytical methods were
not reported.  A site use history was not provided for either of the
test sites.



	42350005

41686103

42350007	Unacceptable	The data were too variable to accurately assess
the dissipation of tribufos.  The route of dissipation was not
identified.



	43325501

43325503	Unacceptable	Only 29% of the applied was accounted for
immediately after treatment.  The route of dissipation was not
identified.  



	45863701	Supplemental	The study was only conducted for 120 days. 
Degradation kinetics of the degradates were not addressed and degradates
were not identified.  Only one site was studied.



Freezer storage stability	42350008	(In review)	None.

	850.1730	Fish bioconcentration	41618811	Acceptable	None.	No.



43080401	Acceptable	None.

	850.7100	Analytical method – soil	42350006	Acceptable	None (review
cites MRID 43325502).	No.



43325502	(Incomplete submission)	An independent laboratory validation of
the method in soil was not submitted.



Analytical method - water	(none)	--	(No method was provided.)	Yes.

	Analytical method - sediment	(none)	--	(No method was provided.)	Yes,
if soil method isn’t applicable.

Non-guideline	Foliar washoff	45174401	(Unacceptable)	A complete mass
balance was not determined.  Only 39% of the applied was recovered after
15 days.  Loss was attributed to volatilization; however, a preliminary
study determined that 7.6% of the applied volatilized over 15 days.	No.

Classifications in parentheses are provisional.

7.8.2.  Effects

Although many submissions have been made to provide data on the effects
of tribufos to aquatic and terrestrial organisms, several data gaps
still exist (Tables 7.2-7.4).  Data gaps include the following: avian
acute oral toxicity, early-life stage freshwater fish toxicity,
terrestrial plant, and sediment toxicity studies. These data gaps are
discussed below. 

TABLE 7.2.  Available ecological effects data for terrestrial animals
exposed to tribufos and remaining data gaps.

Guideline	Description	MRID/

Accession	Classification	Data Gap?	Comments

850.2100	Avian oral toxicity	00049258*	Supplemental	Yes*	*Avian acute
oral toxicity data are not available for passerines, which are required
under the new 40 CFR Part 158.  There is evidence from other OPs (e.g.,
dimethoate) that passerines are significantly more sensitive to OPs when
compared to upland and game bird species tested.  Therefore, this is
identified as a data gap.





00120771	Acceptable



850.2200	Avian dietary toxicity 	41618805	Acceptable	No



	41618804	Acceptable



850.2300	Avian reproduction	40757101	Acceptable	No

	850.3020	Honeybee acute contact toxicity	00001999	Supplemental	No

	

TABLE 7.3.  Available ecological effects data for aquatic animals
exposed to tribufos and remaining data gaps.

Guideline	Description	MRID/ Accession	Classification	Data Gap?	comments

850.1075	Freshwater fish – 

Acute toxicity 	41618806	Acceptable	No	*Although a high level of
analytical variation was measured in the test concentrations, a
saltwater invertebrate life cycle test (850.1350) is not requested
because no effects were detected at any concentration.

**A freshwater fish early life cycle test (850.1400) is requested for
tribufos because the submitted study was not adequate for RQ
calculation, based on a high level of analytical variation.





41618807	Acceptable





41618808	Supplemental





45863706	Acceptable





40098001	Supplemental**



850.1075	Saltwater fish – 

Acute toxicity 	41896302	Acceptable	No

	850.1010	Freshwater invertebrates –

Acute toxicity	41689902	Acceptable	No



	R2023118	Supplemental





40098001	Supplemental**



850.1025

850.1035	Saltwater invertebrates –

Acute toxicity 	42083201	Acceptable	No



	41896301	Acceptable



850.1300	Freshwater  invertebrate –

 life cycle test	43978201	Acceptable	No

	850.1350	Saltwater invertebrates – 

life cycle test	45863702	Supplemental	No*

	850.1400	Freshwater fish – 

early life stage test	45863703	Supplemental	Yes**

	850.1400	Saltwater fish – 

early life stage test	45863707	Acceptable	No

	

TABLE 7.4.  Available ecological effects data for plants exposed to
tribufos and remaining data gaps.

Guideline	Description	MRID	Classification	Data Gap?	comments

850.4100	Terrestrial Plant toxicity: Tier I seedling emergence	None	Not
applicable	No	

*Although acceptable Tier II terrestrial plant studies are available for
tribufos, the plant species studies are not expected to be sensitive to
tribufos as other plants, due to its mechanism of action.



850.4100	Terrestrial Plant toxicity: Tier 2 seedling emergence	45863704
Acceptable	No*

	850.4150	Terrestrial Plant toxicity: Tier I vegetative vigor	None	Not
applicable	No

	850.4150	Terrestrial Plant toxicity: Tier 2 vegetative vigor	45863704
Acceptable	No*

	850.4400	Aquatic Plant Growth: algae	45863705	Supplemental	No

	850.4400	Aquatic Plant Growth: vascular plants	41618813	Acceptable	No

	

Avian Acute Oral Toxicity 

Acceptable acute avian oral toxicity data were submitted for exposures
of mallard duck and bobwhite quail to tribufos; however, data are not
available for passerines, which are required under the new 40 CFR Part
158 (CFR 40 2007) data requirements for conventional pesticides (72 FR
60934; USEPA 2007c).  The new Part 158 data requirements specify that
acute avian oral toxicity data be submitted for either a mallard duck or
bobwhite quail and a passerine species.  Based on the results of
previous ecological risk assessments for tribufos and derivation of
preliminary RQs in this problem formulation (APPENDIX B), risks are
expected for some non-listed and listed birds due to acute exposures to
tribufos.  Additionally, there is evidence to indicate that passerines
are more sensitive to at least some OPs when compared to data from
upland and game species.  For example, dimethoate (an OP) is an order of
magnitude more toxic to passerines than to upland/game species (USEPA
2008).  Therefore, an avian oral toxicity test (OPPTS Guideline
850.2100) is required for passerine birds, as specified in 40 CFR Part
158 (CFR40 2007).  EFED recommends that SRRD request submission of a
passerine study protocol for review by the Agency prior to initiation of
this study.  If oral acute toxicity data are not submitted for
passerines, EFED will assume acute risk for passerine species.

Chronic Toxicity Studies with Freshwater Fish

Chronic toxicity data are not available for freshwater fish.  Only one
early life stage freshwater fish study on the fathead minnow is
available for tribufos and it is classified as supplemental and not
adequate for RQ calculations because a high level of analytical
variation (>20%) was observed in the mean results.  In the absence of
these data, an acute-to-chronic ratio (ACR) would normally be derived,
and the chronic toxicity to freshwater fish would be estimated based on
the ACR for estuarine/marine fish.  This was calculated for a previous
assessment based on toxicity data for sheepshead minnow, an
estuarine/marine species.  Based on the results of previous ecological
risk assessments for tribufos and derivation of preliminary aquatic RQs
in support of this problem formulation (APPENDIX A), risks are expected
for non-listed and listed fish due to acute and chronic exposures to
tribufos in aquatic habitats.  In the absence of chronic toxicity data
for freshwater fish, EFED will use the ACR calculated from
estuarine/marine fish toxicity data.

Terrestrial Plant Studies 

Although acceptable Tier II terrestrial plant studies are available for
tribufos, the plant species studied (i.e., annual dicots and monocots)
are not expected to be as sensitive to tribufos as other plants (e.g.,
woody deciduous trees and shrubs) due to its mechanism of action.  The
available toxicity studies were conducted on plant species that do not
form abscission zones, and data from these studies may underestimate the
potential toxicity of tribufos to non-target plants that do form
abscission zones.  Therefore, based on the mode of action of tribufos as
a known defoliant and the lack of acceptable toxicity data for
terrestrial plants with abscission zones, it will be assumed that
tribufos has the potential to adversely affect both listed and
non-listed woody, deciduous, non-target plants.  EFED recommends that
SRRD request submission of a terrestrial plant studies on species
expected to be sensitive to tribufos.  Additionally, EFED recommends
that SRRD request submission of a study protocol for review by the
Agency prior to initiation of this study.  In the absence of data
specific for plants that form abscission zones, EFED will assume risk to
terrestrial plants.  

Sediment Toxicology Studies 

There are no available data for the toxicity of tribufos to benthic
freshwater or marine invertebrates.  Because tribufos is likely to
partition to the sediment and no sediment toxicity data are available
for review, toxicity to benthic invertebrates may be underestimated in
this assessment by using daphnid data to determine toxicity to
freshwater invertebrates.  EFED recommends that SRRD request submission
of sediment toxicology tests (OPPTS Guidelines 850.1735 and 850.1740). 
In the absence of such data, EFED will assume risk to benthic freshwater
and marine invertebrates.

8.  References

Armitage, J.M. and F.A.P.C. Gobas.  2007.  A terrestrial food-chain
bioaccumulation model for POPs.  Environmental Science and Technology,
41 (11): 4019-4025.

California Department of Pesticide Regulation (CDPR).  1999.  Evaluation
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California Environmental Protection Agency, California Department of
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"http://www.cdpr.ca.gov/docs/emon/pubs/tac/finaleval/def.htm" 
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Kelly, B.C. and F.A.P.C. Gobas.  2003.  An Arctic terrestrial food-chain
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Mayer, F.; Ellersieck, M.  1986.  Manual of Acute Toxicity: 
Interpretation and Data Base 410 Chemicals and 66 Species of Fresh-Water
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Scientific Review Panel.  1999.  Findings of the Scientific Review Panel
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U.S. Environmental Protection Agency (USEPA).  1996.  Tribufos (DEF):
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USEPA.  1998.  Guidelines for Ecological Risk Assessment.  Risk
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USEPA.  2000.   Interim Registration Eligibility Decision (IRED):
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USEPA.  2000a.  Tribufos Facts.  U.S. Environmental Protection Agency,
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USEPA.  2002.  Guidance for Selecting Input Parameters in Modeling the
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USEPA.  2004.  Overview of the Ecological Risk Assessment Process in the
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USEPA. 2006. Organophosphorus Cumulative Risk Assessment: 2006 Update. 
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USEPA.  2006a.  Standardized Soil Mobility Classification Guidance. 
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USEPA. 2006b.  Reregistration Eligibility Decision (RED) Tribufos.  U.S.
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USEPA. 2007c.  40 CFR Part 158.  Pesticides; Data Requirements for
Conventional Chemicals: Final Rule. 72 FR 60934.  October 26, 2007.

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HYPERLINK "http://cfpub.epa.gov/ecotox/"  http://cfpub.epa.gov/ecotox/ .

USEPA.  2008.  Risks of dimethoate use to the federally-listed
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U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries
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Procedures for Conducting Consultation and Conference Activities Under
Section 7 of the Endangered Species Act.  Final Draft.  March 1998.

 

SUBMITTED FATE STUDIES:

MRID 41618803.  Talbott, T.  1990.  Product Chemistry of DEF Technical:
Lab Project Number: 90606: 94500: 94682.  Unpublished study prepared by
Mobay Corp.  29 p.

MRID 41618811.  Cohle, P.  1990.  Uptake, Depuration and
Bioconcentration of (Carbon 14)-DEF by Bluegill (Lepomis macrochirus):
Lab Project Number: 38466: 100236.  Unpublished study prepared by
Analytical Bio- Chemistry Laboratories, Inc.  48 p.

MRID 41618814.  Schocken, M. and I. Philippson.  1987.  Stability of DEF
in Sterile Aqueous Buffer Solutions: Lab Project Number: 94918. 
Unpublished study prepared by Mobay Corp.  23 p.

MRID 41618816.  Jackson, S., A. Kesterson, L. Lawrence.  1988.  Soil
Surface Photolysis of [14C]DEF in Natural Sunlight: Lab Project Num-
ber: 1153: 206: 95673.  Unpublished study prepared by Pharmacology and
Toxicology Research Laboratory.  67 p.

MRID 41618817.  Daly, D.  1987.  Soil Adsorption/Desorption with
[14C]DEF: Lab Project Number: 36356: 95600.  Unpublished study prepared
by Analytical Bio-Chemistry Laboratories, Inc.  50 p.

MRID 41719401.  Carpenter, M.  1990.  Determination of the Aqueous
Photolysis Rate of DEF: Lab Project Number: 38876.  Unpublished study
prepared by Analytical Bio-Chemistry Laboratories, Inc.  41 p.

MRID 42007204.  Stevenson, T., W. Leimkuehler, A. Mathew.  1991.  The
Metabolism of Tribufos in Soil Under Aerobic Conditions: Lab Project
Number : DE042101: 100338.  Unpublished study prepared by Mobay Corp., R
& D Dept.  33 p.

MRID 42007205.  Stevenson, T., W. Leimkuehler.  1990.  The Metabolism of
Tribufos in Soil Under Anaerobic Conditions: Lab Project Number: 100333.
 Unpublished study prepared by Mobay Corp., R & D Dept.  31 p.

MRID 43080401.  Leimkuehler, W., K. Moor.  1993.  Identification and
Characterization of Radioactive Residues of [14C] Tribufos in Bluegill
Sunfish (Lepomis Macrochirus): Lab Project Number: DE040301. 
Unpublished study prepared by Miles Inc.  84 p.

MRID 43325504.  Schmidt, J.  1994.  Anaerobic Aquatic Metabolism of
14C-Tribufos: Lab Project Number: DE042402: 106767.  Unpublished study
prepared by ABC Lab., Inc.  61 p.

MRID 45863701.  Wood, S.  2002.  Terrestrial Field Dissipation of
Tribufos on Mississippi Soil, 2000: Lab Project Number: DE022102:
110590.  Unpublished study prepared by Stoneville R & D, Inc., A & L
Great Lakes Laboratories, and Bayer Corporation.  84 p.

SUBMITTED EFFECTS STUDIES:

40098001

	Mayer, F.; Ellersieck, M. (1986) Manual of Acute Toxicity: Inter-
pretation and Data Base for 410 Chemicals and 66 Species of Freshwater
Animals. US Fish & Wildlife Service, Resource Pub- lication 160. 579 p.

00001999	Atkins, L., Jr.; Anderson, L.D. (1967) Toxicity of Pesticides
and Other Agricultural Chemicals to Honey Bees: Laboratory Studies.
(Unpublished study received Jan 30, 1969 under 9G0802; prepared by Univ.
of California--Riverside, Dept. of Entomology, sub- mitted by Hercules,
Inc., Agricultural Chemicals, Wilmington, Del.; CDL:093111-D)

49258	Lamb, D.W.; Jones, R.E. (1972) Acute Oral Toxicity of ^(R)I DEF
Technical to Bobwhite Quail and Mallard Ducks: Report No. 35016.
(Unpublished study received Mar 22, 1976 under 3125-71; submit- ted by
Mobay Chemical Corp., Kansas City, Mo.; CDL:224114-P) 

120771	Lamb, D.; Jones, R. (1972) Acute Oral Toxicity of DEF Technical
to Bobwhite Quail and Mallard Ducks: Report No. 35016. (Unpub- lished
study received May 7, 1973 under 3125-71; submitted by Mobay Chemical
Corp., Kansas City, MO; CDL:007020-O) 

40757101	Beavers, J.; Marselas, G.; Jaber, M. (1988) DEF: A
One-generation Reproduction Study with the Bobwhite (Colinus
virginianus): Wildlife International Ltd. Project No. 149-127.
Unpublished study prepared by Wildlife International Ltd. 101 p. 

41618804	Grau, R. (1990) DEF Technical Grade: 5-day Dietary LC50 to
Bobwhite Quail: Lab Project Number: BV-007: E2950351-5: 100223. Unpub-
lished study prepared by Bayer Ag. 36 p. 

41618805	Grau, R. (1990) DEF Technical Grade: 5-day Dietary LC50 to
Mallard Duck: Lab Project Number: VE-002: E2970426-0: 100224. Unpub-
lished study prepared by Bayer Ag. 43 p. 

41618806	Grau, R. (1990) DEF Technical Grade: Acute Toxicity to Bluegill
Sunfish in a Flow-through Test: Lab Project Number: FB-003: E2860137:
100222. Unpublished study prepared by Bayer Ag. 36 p. 

41618807	Grau, R. (1990) DEF 6: Acute Toxicity to Bluegill Sunfish in a
Flow-through Test: Lab Project Number: FB-006: E02860355-9: 100219.
Unpublished study prepared by Bayer Ag. 40 p. 

41618808	Grau, R. (1990) DEF Technical Grade: Acute Toxicity to Rainbow
Trout in a Flow-through Test: Lab Project Number: FF-286: E2810136-1:
100221. Unpublished study prepared by Bayer Ag. 36 p. 

41618809	Grau, R. (1990) DEF 6: Acute Toxicity to Rainbow Trout in a
Flow- through Test: Lab Project Number: FF-228: E2510429: 100220. Un-
published study prepared by Bayer Ag. 39 p. 

41618810	Heimbach, F. (1990) Influence of DEF Technical on the
Reproduction Rate of Water Fleas: Lab Project Number: HBF/RDM29:
E3210393-1: 100235. Unpublished study prepared by Bayer Ag. 56 p. 

41618811	Cohle, P. (1990) Uptake, Depuration and Bioconcentration of
(Carbon 14)-DEF by Bluegill (Lepomis macrochirus): Lab Project Number:
38466: 100236. Unpublished study prepared by Analytical Bio- Chemistry
Laboratories, Inc. 48 p. 

41618813	Hughes, J. (1990) The Toxicity of DEF Technical to Selanastrum
capricornutum (Tier 2 Growth and Reproduction of Aquatic Plant- s): Lab
Project Number: BO59-02-2: 100230. Unpublished study prepared by Malcolm
Pirnie, Inc. 36 p. 

41689901	Heimbach, F. (1989) Acute Toxicity of DEF (Technical) to
Waterfleas (Daphnia magna): Lab Project Number: 99630. Unpublished study
prepared by Bayer AG. 32 p. 

41689902	Heimbach, F. (1990) Acute Toxicity of DEF 6 to Waterfleas
(Daphnia magna): Lab Project Number: HBF/DM96: E 320 0403-2: 100208. Un-
published study prepared by Bayer AG. 33 p. 

41896301	Ward, G. (1991) DEF: Acute Toxicity to The Mysid, Mysidopsis
bahia, Under Flow-Through Test Conditions: Lab Project Number: J900802-
4B: 101245. Unpublished study prepared by Mobay Corp. 45 p. 

41896302	Gagliano, G. (1991) Acute Toxicity of DEF Technical to the
Sheep- Shead Minnow (Cyprinodon variegatus) Under Flow-Through Condi-
tions: Lab Project Number: DE831201: 101240. Unpublished study prepared
by Mobay Corp. 31 p. 

41954903	Sheets, L. (1991) Acute Oral Toxicity Study with Technical
Grade Tribufos (DEF) in Rats: Lab Project Number: 90-012-ES: 100697.
Unpublished study prepared by Mobay Corp. 20 p. 

42007201	Sheets, L.; Phillips, S. (1991) 21-Day Dermal Toxicity Study
with Technical Grade Tribufos (DEF) in Rabbits: Lab Project Number:
90-125-FP. Unpublished study prepared by Mobay Corp., Toxico- logy Dept.
445 p. 

42007203	Christenson, W. (1991) Chronic Feeding Toxicity Study of
Technical Grade Tribufos (DEF) with Dogs: Lab Project Number: 88-274-AB.
Unpublished study prepared by Mobay Corp., Toxicology Dept. 585 p. 

42040201	Eigenberg, D.; Elcock, L. (1991) A Two-Generation Reproduction
Study in Rats Using Tribufos (DEF): Lab Project Number: 88-671- AK.
Unpublished study prepared by Mobay Corp. 1025 p. 

42083201	Wheat, J.; Ward, G. (1991) DEF Technical: Acute Effect on New
Shell Growth of, The Eastern Oyster, Crassostrea virginica: Lab Pro-
ject Number: 101959: J9008024C. Unpublished study prepared by Toxikon
Environmental Sciences. 53 p. 

43978201	Bowers, L. (1996) Chronic Toxicity of (carbon 14)-DEF to the
Waterflea (Daphnia magna) Under Static Renewal Conditions: Lab Project
Number: DE840701: 107327. Unpublished study prepared by Bayer Corp. 64
p. 

45863702	Blankinship, A.; Kendall, T.; Krueger, H. (2002) Tribufos
Technical: A Flow-Through Life-Cycle Toxicity Test with the Saltwater
Mysid (Mysidopsis bahia): Lab Project Number: 149A-133A: DE843101:
200363. Unpublished study prepared by Wildlife International, Ltd. 77 p.
{OPPTS 850.1350} 

45863703	Kern, M.; Lam, C. (2002) Early Life Stage Toxicity of Tribufos
to the Fathead Minnow (Pimephales promelas): Lab Project Number:
DE841201: 200308. Unpublished study prepared by Bayer Corporation. 79 p.


45863704	Bowers, L. (2002) Tier 2 Seedling Emergence and Vegetative
Vigor Nontarget Phytotoxicity Study Using DEF 6: Lab Project Number:
DE451601: DE451602: 200245. Unpublished study prepared by Bayer
Corporation. 105 p. 

45863705	Kern, M.; Lam, C. (2002) Toxicity of Tribufos Technical to
Duckweed (Lemna gibba G3): Lab Project Number: DE883701: 200301.
Unpublished study prepared by Bayer Corporation. 45 p. 

45863706	Kern, M.; Lam, C. (2002) Acute Toxicity of Tribufos Technical
to the Fathead Minnow (Pimpephales promelas) Under Flow-Through
Conditions: Lab Project Number: DE831202: 200144. Unpublished study
prepared by Bayer Corporation. 34 p. 

45863707	Palmer, S.; Kendall, T.; Krueger, H. (2002) Tribufos Technical:
An Early Life-Stage Toxicity Test with the Sheepshead Minnow (Cyprinodon
variegatus): Lab Project Number: 149A-134: DE842801: 200336. Unpublished
study prepared by Wildlife International, Ltd. 82 p. {OPPTS 850.1400} 



 APPENDIX A:  Preliminary EECs for Aquatic Habitats and RQs for Aquatic
Organisms.

PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are screening
simulation models coupled with the graphical user interface, PE (v5.0,
November 2006) to generate daily exposures and 1-in-10 year estimated
environmental concentrations (EEC) of tribufos that may occur in surface
water bodies adjacent to application sites receiving tribufos through
runoff and spray drift.  PRZM simulates pesticide application, movement
and transformation on an agricultural field and the resultant pesticide
loadings to a receiving water body via runoff, erosion, and spray drift.
 EXAMS simulates the fate of the pesticide and resulting concentrations
in the water body.  The standard scenario used for ecological pesticide
assessments assumes application to a 10-hectare agricultural field that
drains into an adjacent 1-hectare water body that is 2 meters deep
(20,000 m3 volume) with no outlet.  The composite model PRZM/EXAMS is
used to estimate screening-level exposure of aquatic organisms to
tribufos.  The measure of exposure for aquatic species is the 1-in-10
year return peak or rolling mean concentration.  The 1-in-10 year peak
is used for estimating acute exposures of direct effects to aquatic
organisms.  The 1-in-10-year 60-day mean is used for assessing chronic
exposure to fish and aquatic-phase amphibians.  The 1-in-10-year 21-day
mean is used for assessing aquatic invertebrate chronic exposure.  Input
parameters for PRZM/EXAMS and the resulting EECs are in Tables A.1 and
A.2, respectively.  Table A.3 lists Agency levels of concern with which
risk quotients (RQ) are compared.  Preliminary RQs for freshwater and
marine/estuarine animals are listed in Tables A.4 and A.5, respectively.
 Preliminary RQs for aquatic plants are listed in Table A.6.  Acute and
chronic risk levels of concern are exceeded for all assessed aquatic
animals using each modeled PRZM/EXAMS scenario.  Levels of concern are
exceeded, as well, for listed aquatic vascular plants using each modeled
PRZM/EXAMS scenario.

Table A.1.  PRZM/EXAMS Input Parameters.

Input Parameter	Value	Source	Comments

Chemical specific

Molecular Weight (g/mol)	314.5	MRID 41618803	Product chemistry data

Solubility in Water  (mg/L)	23	MRID 41618803	Represents 10x the measured
water solubility value1

Henry’s Law Constant

(atm-m3/mol)	3.1 x 10-7	MRID 41618803	Calculated value from vapor
pressure and water solubility

Soil-Water Partition Coefficient  (Kd) (L/kg)	60.6	MRID 41618817	Lowest
value for a non-sand soil

Aerobic Soil Metabolism Half-life (days)	2235	MRID 42007204	Represents 3
times the single available half-life (745 d)1

Aerobic Aquatic Metabolism Half-life (days)	4470	MRID 42007204
Represents 2 times the aerobic soil metabolism half-life input1

Anaerobic Aquatic Metabolism Half-life (days)	300	MRID 43325504
Represents 3 times the single available half-life (100 d)1

Hydrolysis Half-life (days)	0	MRID 41618814	Represents stability to
hydrolysis

Aqueous Photolysis Half-life (days) 	0	MRID 41719401	Represents
stability to photolysis

Use specific

Scenarios	CA cotton STD 

MS cotton STD

NC cotton STD

STX cotton NMC

TX cotton OP	Not applicable	Available scenarios for cotton

Application rate (kg ai/ha)	1.26 (MS, NC, TX)

2.102 (CA)	Label directions	Maximum labeled use patterns

Applications per year	1	Label directions	Maximum labeled use patterns

Application date	Nov. 11	CDPR 2008	Within the application window
reported for California usage in 2006 (CDPR 2008)

Application efficiency/

Spray drift fraction	0.95/0.05	Input parameter guidance	Standard values
for aerial application1

Chemical application method (CAM)	2	Label directions	Foliar applications

IPSCND Input	1	Not applicable	Foliar residue is applied to the field
after harvest

1 According to input parameter guidance for PRZM/EXAMS (USEPA 2002).



Table A.2.  Preliminary aquatic 1-in-10 year EECs for aerial
applications of tribufos to cotton.

PRZM scenario	Peak EEC

(µg/L)	21-day EEC

(µg/L)	60-day EEC

(µg/L)

1 aerial application per year at 1.1 lb a.i./A (1.26 kg a.i./ha)

MS cotton STD	40.7	38.4	33.8

NC cotton STD	31.0	29.8	28.4

STX cotton NMC	25.3	23.0	21.6

TX cotton OP	21.0	19.1	17.2

CA cotton STD	15.0	14.2	13.3



Table A.3.  Agency Levels of Concern (LOCs).

Risk Presumption	Taxa	LOC

Acute Risk	Birds, mammals, aquatic animals	0.5

	Plants	1

Acute Restricted Use	Birds, mammals	0.2

	Aquatic animals	0.1

Acute Endangered Species	Birds, mammals	0.1

	Aquatic animals	0.05

	Plants	1

Chronic Risk	Birds, mammals, aquatic animals	1



Table A.4. Preliminary RQs for freshwater fish and invertebrates.

Scenario	Acute Fish1	Chronic Fish 2	Acute Invert 3	Chronic Invert 4

1 aerial application per year at 1.1 lb a.i./A (1.9 lb a.i./A in CA)

MS cotton STD	0.17	9.7	6.0	25

NC cotton STD	0.13	8.1	4.6	19

STX cotton NMC	0.10	6.2	3.7	15

TX cotton OP	0.09	4.9	3.1	12

CA cotton STD	0.06	3.8	2.2	9.1

1 Based on LC50 = 245 ug/L for bluegill sunfish.	 	 

2 Based on NOAEC = 3.5 ug/L for bluegill sunfish.	 	 

3 Based on EC50 = 6.8 ug/L for waterflea.	 	 

4 Based on NOAEC = 1.56 ug/L for waterflea.	 	 

Bolded RQs values > acute endangered species LOC of 0.05 or > chronic
endangered species LOC of 1.



Table A.5. Preliminary RQs for estuarine/marine fish and invertebrates.

Scenario	Acute Fish1	Chronic Fish 2	Acute Invert 3	Chronic Invert 4

1 aerial application per year at 1.1 lb a.i./A (1.9 lb a.i./A in CA)

MS cotton STD	0.05	3.1	3.5	148

NC cotton STD	0.04	2.6	2.7	115

STX cotton NMC	0.03	2.0	2.2	88

TX cotton OP	0.03	1.6	1.8	74

CA cotton STD	0.02	1.2	1.3	55

1 Based on LC50 = 767 ug/L for sheepshead minnow.	 	 

2 Based on NOAEC = 11 ug/L for sheepshead minnow.	 	 

3 Based on EC50 = 11.53 ug/L for mysid shrimp.	 	 

4 Based on NOAEC = 0.26 ug/L for mysid shrimp.	 	 

Bolded RQs values > acute endangered species LOC of 0.05 or > chronic
endangered species LOC of 1.



Table A.6. Preliminary RQs for Aquatic Plants (Vascular and
Non-Vascular).

Scenario	Non-listed Non-vascular Aquatic Plant1	Listed Non-vascular
Aquatic Plant2	Non-listed Vascular Aquatic Plant 3	Listed Vascular
Aquatic Plant 4

1 aerial application per year at 1.1 lb a.i./A (1.9 lb a.i./A in CA)

MS cotton STD	0.27	0.69	0.04	>2.4

NC cotton STD	0.21	0.53	0.03	>1.8

STX cotton NMC	0.17	0.43	0.02	>1.5

TX cotton OP	0.14	0.36	0.02	>1.2

CA cotton STD	0.10	0.26	0.01	>0.87

1 Based on EC50 = 148 ug a.i./L for algae.	 	 

2 Based on NOAEC = 58.5 ug a.i./L for algae.	 	 

3 Based on EC50 = 1,100 ug/L for duckweed.	 	 

4 Based on NOAEC = <17.2 ug/L for duckweed.	 	 

Bolded RQs values > LOC of 1.

APPENDIX B:  Preliminary EECs and Associated RQs for Terrestrial
Animals.

The T-REX model (version 1.3.1) is used to calculate dietary and
dose-based EECs of tribufos for mammals, birds, and listed terrestrial
invertebrates.  Exposure to terrestrial animals is based on the maximum
application rates according to the registered tribufos labels for use on
cotton.  Input values for T-REX are located in Table B.1.  Upper-bound
Kenega nomogram values are utilized to derive EECs for tribufos
exposures to terrestrial mammals and birds based on dietary- and
dose-based exposures (Tables B.2 and B.3).  A 1-year time period is
simulated.  Consideration is given to different types of feeding
strategies for mammals and birds, including herbivores, insectivores and
granivores.  For dose-based exposures, three weight classes of mammals
(15, 35 and 1000 g) and birds (20, 100, and 1000 g) are considered. 
T-REX is also used to calculate EECs for terrestrial invertebrates
exposed to tribufos.  Dietary-based EECs calculated by T-REX for small
and large insects (units of a.i./g) are used to bound an estimate of
exposure to bees.  Available acute contact toxicity data for bees
exposed to tribufos (in units of µg a.i./bee), are converted to µg
a.i./g (of bee) by multiplying by 1 bee/0.128 g.  The EECs are later
compared to the adjusted acute contact toxicity data for bees in order
to derive RQs.  Toxicity values used to define effects to terrestrial
animals from acute and chronic exposures are described in the effects
characterization section of this document.  

TABLE B.1.  Input Parameters for Deriving Terrestrial EECs for Tribufos
Using T-REX.

      Parameter Description 	Value

Application Rate (lbs a.i./A)	1.875a	1.125b

Foliar Half-life (days)	35c	35c

Number of Applications	1	1

a Based on the maximum single application rate allowed in CA and AZ.

b Based on the maximum single application rate allowed in all states
except CA and AZ.

C This is a default value.



TABLE B.2.  Upper Bound, T-REX Calculated EECs of Tribufos Residues on
Food Items (1.9 lb a.i./acre).

Food Type	Dietary-Based 

(ppm)

(mammals, birds, and terrestrial invertebrates)	Dose-Based 

(mg/kg-bw)

(mammals)	Dose-Based 

(mg/kg-bw)

(birds)



Small 

(15 g)	Medium 

(35 g)	Large 

(1000 g)	Small 

(20 g)	Medium 

(100 g)	Large 

(1000 g)

Short Grass 	450	429	297	69	513	292	131

Tall Grass	206	197	136	32	235	134	60

Broadleaf plants/sm insects	253	241	167	39	288	164	74

Fruits/pods/lg insects	28	27	19	4	32	18	8

Seeds (granivore)	28	27	19	4	32	18	8





TABLE B.3.  Upper Bound, T-REX Calculated EECs of Tribufos Residues on
Food Items (1.1 lb a.i./acre).

Food Type	Dietary-Based 

(ppm)

(mammals, birds, and terrestrial invertebrates)	Dose-Based 

(mg/kg-bw)

(mammals)	Dose-Based 

(mg/kg-bw)

(birds)



Small 

(15 g)	Medium 

(35 g)	Large 

(1000 g)	Small 

(20 g)	Medium 

(100 g)	Large 

(1000 g)

Short Grass 	270	257	178	41	308	175	79

Tall Grass	124	118	82	19	141	80	36

Broadleaf plants/sm insects	152	145	100	23	173	99	44

Fruits/pods/lg insects	17	16	11	3	19	11	5

Seeds (granivore)	17	16	11	3	19	11	5



Acute dose-based RQs for mammals are derived using the reported LD50 192
mg/kg-bw.  The acute listed species LOC of 0.1 is exceeded for mammals
(all size classes) that feed on short and tall grass, broadleaf plants,
and small insects based on acute, dose-based exposures of mammals to
tribufos at both the 1.1 and 1.9 lb a.i./acre application rates (Table
B.4).  The acute restricted use LOC of 0.2 is exceeded for all small and
medium mammals that feed on short and tall grass, broadleaf plants, and
small insects, at both application rates.  Additionally, large mammals
that feed on short grass (at the 1.1 and 1.9 lb a.i./acre application
rates) and large mammals that feed on tall grass, broadleaf plants, and
small insects (at the 1.9 lb a.i./acre application rate) also exceed the
acute restricted use LOC.  Chronic dietary-based RQs for mammals are
derived using the reported NOAEC of 32 mg/kg-diet.  Chronic dose-based
RQs are calculated using the NOAEL of 1.6 mg/kg-bw/day.  For chronic
dietary- and dose-based exposures, the chronic LOC is exceeded for
almost all mammal size and dietary categories for both the 1.1 and 1.9
lb a.i./acre application rates (Table B.5).  The RQs that do not exceed
the chronic LOC are the dietary-based RQs for mammals (all size classes)
that eat fruits/pods/large insects and seeds and dose-based RQs for
medium and large mammals that eat seeds at the 1.1 lb a.i./acre
application rate (all other chronic RQs exceed the chronic LOC of 1).

TABLE B.4.  Acute RQs for Mammals of Different Size and Feeding Classes
(Tribufos Application Rates = 1.1 and 1.9 lb a.i./acre). 

FOOD TYPE	DOSE-BASED RQs

	Small (15 g)	Medium (35 g)	Large (1000 g)

Application Rate (lb a.i./acre)	1.1	1.9	1.1	1.9	1.1	1.9

Short Grass 	0.61*	1.02*	0.52*	0.87*	0.28*	0.47*

Tall Grass	0.28*	0.47*	0.24*	0.40*	0.13*	0.21*

Broadleaf plants/sm insects	0.34*	0.57*	0.29*	0.49*	0.16*	0.26*

Fruits/pods/lg insects	0.04	0.06	0.03	0.05	0.02	0.03

Seeds (granivore)	0.01	0.01	0.01	0.01	0.00	0.01

Bolded RQs values > acute LOC of 0.5.

* RQ value > acute endangered species LOC of 0.1.





TABLE B.5.  Chronic RQs for Mammals of Different Size and Feeding
Classes (Tribufos Application Rates = 1.1 and 1.9 lb a.i./acre). 

FOOD TYPE	DIETARY-BASED RQs	DOSE-BASED RQs

	All Size Classes	Small (15 g)	Medium (35 g)	Large (1000 g)

Application Rate (lb a.i./acre)	1.1	1.9	1.1	1.9	1.1	1.9	1.1	1.9

Short Grass 	8	14	73	122	63	104	34	56

Tall Grass	4	6	34	56	29	48	15	26

Broadleaf plants/sm insects	5	8	41	69	35	59	19	31

Fruits/pods/lg insects	0.53	0.88	5	8	4	7	2	3

Seeds (granivore)	0.53	0.88	1	2	0.87	1	0.47	0.78

Bolded values > chronic LOC of 1.0 (for listed and non-listed species).



Acute dose-based RQ values are calculated using the value available for
the bobwhite quail (LD50 = 151 mg a.i./kg-bw).  The acute endangered
species LOC of 0.1 is exceeded for almost all birds for all size classes
and feeding types for both the 1.1 and 1.9 lb a.i./acre application
rates.  The only exceptions are for the 100g and 1,000g birds that eat
fruits, pods, seeds, and large insects at the 1.1 lb a.i./acre
application rate and the 1,000g birds that eat fruits, pods, seeds, and
large insects at the 1.9 lb a.i./acre application rate (these RQs do not
exceed the acute endangered species LOC) (Table B.6).  The acute
restricted use LOC of 0.2 is exceeded for birds of all size classes that
feed on short and tall grass, broadleaf plants, and small insects at
both application rates, with the exception of 1,000g birds feeding on
tall grass at the 1.1 lb a.i./acre application rate.  The acute
restricted use LOC is also exceeded for 20g birds feeding on fruits,
pods, seeds, and large insects at the 1.9 lb a.i./acre application rate.
 Additionally, the acute risk LOC is exceeded for 20g and 100g birds
that eat short grass, tall grass, and broadleaf plants and small insects
for both application rates.  For 1,000g birds, the only RQs that exceed
the acute risk LOC are for birds that eat short grass (at both the 1.1
and 1.9 lb a.i./acre application rates).

TABLE B.6.  Dose-Based RQ Values for Acute Exposures to Birds (Tribufos
Application Rates = 1.1 and 1.9 lb a.i./acre).

FOOD TYPE	20 g	100 g	1000 g

Application Rate (lb a.i./acre)	1.1	1.9	1.1	1.9	1.1	1.9

Short Grass 	3*	5*	1*	2*	0.4*	0.67*

Tall Grass	1*	2*	0.58*	0.97*	0.18*	0.31*

Broadleaf plants/sm insects	2*	3*	0.71*	1*	0.23*	0.38*

Fruits/pods/seeds/lg insects	0.18*	0.29*	0.08	0.13*	0.03	0.04

Bolded RQs values > acute LOC of 0.5.

* RQ value > acute endangered species LOC of 0.1.



Acute dietary-based RQ values, which are calculated using the LC50 for
bobwhite quail (1519 mg/kg-diet), exceed the acute endangered species
LOC for birds that feed on short grass (both application rates), tall
grass (1.9 lb a.i./acre application rate), broadleaf plants, and small
insects (both application rates) (Table B.7).  None of the acute RQs
exceed the acute risk LOC for non-listed birds.  Chronic dietary-based
RQ values are calculated using the bobwhite quail NOAEC (150
mg/kg-diet).  The RQs for birds that eat short grass (both application
rates), tall grass (1.9 lb a.i./acre application rate), and broadleaf
plants/small insects (both application rates) exceed the Agency’s
chronic LOCs for listed and non-listed birds. 

TABLE B.7.  Acute and chronic, Dietary-Based RQ s for Birds by Food Type
(Tribufos Application Rates = 1.1 and 1.9 lb a.i./acre).

FOOD TYPE	Acute RQ 	Chronic RQ 

Application Rate (lb a.i./acre)	1.1	1.9	1.1	1.9

Short Grass 	0.18*	0.30*	2	3

Tall Grass	0.08	0.14*	0.83	1

Broadleaf plants/small insects	0.10*	0.17*	1	2

Fruits/pods/seeds/large insects	0.01	0.02	0.11	0.19

Bolded acute RQ value > acute risk LOC of 0.5 or chronic RQ value >
chronic risk LOC of 1.

* acute RQ values >  endangered species LOC of 0.1.



The toxicity value for terrestrial invertebrates is calculated by
multiplying the lowest available acute contact LD50 of >24.2 µg
a.i./bee by 1 bee/0.128g, which is based on the weight of an adult honey
bee.  EECs (µg a.i./g of bee) calculated by T-REX for small and large
insects (Tables B.2 and B.3) are divided by the calculated toxicity
value for terrestrial invertebrates, which is >189 ppm (i.e., µg
a.i./g).  Resulting acute RQs are <1.34 (253 ppm/189 pmm) and <0.80 (152
ppm/189 pmm) for small insects at the 1.9 and 1.1 lb a.i./acre
application rates, respectively.  For large insects, the resulting RQs
are <0.15 (28 ppm/189 ppm) and <0.09 (17 ppm/189 ppm) at the 1.9 and 1.1
lb a.i./acre application rates, respectively.  Since the toxicity data
for honeybees are based on a single concentration that resulted in 3%
mortality, a discreet LD50 for terrestrial insects could not be
determined.  Although it is unclear whether the non-definitive “less
than” RQ values exceed the LOC of 0.05, potential risks to listed
terrestrial invertebrates cannot be precluded.  

Additional information from the open literature (Greenberg et al., 2004;
ECOTOX Ref.: 92450) indicates mortality to individuals and population
effects in boll weevils exposed to tribufos at an application rate of
0.42 lb a.i./acre (using the DEF 6 formulation), which is 22% of the
maximum allowable labeled rate.  Therefore, based on the effects
demonstrated in boll weevil study and the indiscreet LD50 for honey
bees, tribufos has the potential to adversely affect listed terrestrial
invertebrates.

APPENDIX C:  Preliminary EECs and Associated RQs for Terrestrial and
Riparian Plants. 

TerrPlant (Version 1.1.2) is used to calculate EECs for non-target plant
species inhabiting dry and semi-aquatic areas.  Parameter values for
application rate, drift assumption, and incorporation depth are based
upon the use and related application method (Table C.1).  A runoff value
of 0.01 is utilized based on tribufos’ solubility, which is classified
by TerrPlant as <10 mg/L.  For aerial and ground application methods,
drift is assumed to be 5% and 1%, respectively.  EECs relevant to
terrestrial plants consider pesticide concentrations in drift and in
runoff.  These EECs for non-listed and listed plants are listed in
Tables C.2 and C.3, respectively.  

TABLE C.1.   TerrPlant Inputs and Resulting EECs for Plants Inhabiting
Dry and Semi-aquatic Areas Exposed to Tribufos via Runoff and Drift
(Tribufos Application Rates = 1.1 and 1.9 lb a.i./acre).

Use	Application rate

(lbs a.i./A)	Application method	Drift Value (%)	Spray drift EEC 

(lbs a.i./A)	Dry area EEC 

(lbs a.i./A)	Semi-aquatic area EEC

(lbs a.i./A)

Cotton

	1.9	Foliar - ground	1	0.019	0.038	0.206



Foliar - aerial	5	0.094	0.113	0.281

	1.1	Foliar - ground	1	0.011	0.023	0.124



Foliar - aerial	5	0.056	0.068	0.169



TABLE C.2.   RQs for Non-Listed Monocots and Dicots Inhabiting Dry and
Semi-Aquatic Areas Exposed to Tribufos via Runoff and Drift (Tribufos
Application Rates = 1.1 and 1.9 lb a.i./acre).

Use	Application rate

(lbs a.i./A)	Application method	Drift Value (%)	Plant Type	Spray drift
RQ	Dry area RQ	Semi-aquatic area RQ

Cotton	1.9	Foliar – ground	1	Monocot	<0.1	<0.1	0.10





Dicot	<0.1	<0.1	0.10



Foliar – aerial	5	Monocot	<0.1	<0.1	0.14





Dicot	<0.1	<0.1	0.14

	1.1	Foliar – ground	1	Monocot	<0.1	<0.1	<0.1





Dicot	<0.1	<0.1	<0.1



Foliar – aerial	5	Monocot	<0.1	<0.1	<0.1





Dicot	<0.1	<0.1	<0.1





TABLE C.3.   RQs for Listed Monocots and Dicots Inhabiting Dry and
Semi-Aquatic Areas Exposed to Tribufos via Runoff and Drift (Tribufos
Application Rates = 1.1 and 1.9 lb a.i./acre).

Use	Application rate

(lbs a.i./A)	Application method	Drift Value (%)	Plant Type	Spray drift
RQ	Dry area RQ	Semi-aquatic area RQ

Cotton	1.9	Foliar – ground	1	Monocot	<0.1	<0.1	0.10





Dicot	<0.1	0.17	0.94



Foliar – aerial	5	Monocot	0.20	<0.1	0.14





Dicot	0.20	0.51	1.28*

	1.1	Foliar – ground	1	Monocot	<0.1	<0.1	<0.1





Dicot	<0.1	0.10	0.56



Foliar – aerial	5	Monocot	0.12	<0.1	<0.1





Dicot	0.12	0.31	0.77

* RQs exceed the listed species LOC for terrestrial plants.



The only terrestrial plant RQ that exceeds an Agency LOC is the RQ for
listed semi-aquatic dicots.  Because tribufos affects the abscission
zone in plants, plants that do not form abscission zones (such as those
used in the vegetative vigor and seedling emergence studies discussed
above) are not expected to be as sensitive to tribufos as plants that
form abscission zones.  One study identified from the open literature
(Greenberg et al., 2004; ECOTOX Ref.: 92450) does describe effects
(defoliation) to cotton from a tribufos application at a rate of 0.42 lb
a.i./acre (using the DEF 6 formulation).  At this application rate
(which is 22% of the maximum allowable labeled rate), an average of
80.2% (± SD of 6.1) of the leaves per plant (N = 9) were dropped 7 days
post-application, whereas in the untreated control plot, 3.3% (± SD of
1.6) of the leaves per plant (N = 10) were dropped during the same time
period.  This study is considered supplemental and not adequate for RQ
calculation.

 

Because the available toxicity studies were conducted on plant species
that do not form abscission zones, data from these studies may
underestimate the potential toxicity of tribufos to non-target plants
that do form abscission zones (e.g., some woody, deciduous plants). 
Therefore, based on the mode of action of tribufos as a known defoliant
and the lack of acceptable toxicity data for terrestrial plants with
abscission zones, it will be assumed that tribufos has the potential to
adversely affect both listed and non-listed woody, deciduous, non-target
plants.   

APPENDIX D:  Data Call-In Tables.

The Environmental Fate and Effects Division (EFED) has completed a Data
Call-In (DCI) table for the tribufos environmental fate and effects data
gaps identified in registration review.  The attached DCI table, which
includes the guideline number and study title for required data, also
provides a rationale for requiring the data, an explanation of how the
data will be used, and a brief description of how the data could impact
the Agency’s future decision-making.

Guideline Number:  (Non-guideline)

Study Title: Partition Coefficient (n-Octanol/Air)

Rationale for Requiring the Data

The Agency has a limited understanding of how tribufos behaves in the
environment once applied.  Depending on how the compound partitions
between environmental organic surfaces and air, tribufos may or may not
pose potential exposure concern in terrestrial environments.  Because
tribufos partitioning between organic surfaces and air is not well
understood, the Agency is requiring data.

Practical Utility of the Data

How will the data be used?

n-Octanol-air partitioning data will facilitate a better understanding
of the bioaccumulation potential of tribufos residues in terrestrial
environments.  If data indicate that tribufos is highly oliophilic
relative to air, then the Agency could potentially determine that
tribufos readily bioaccumulates in terrestrial organisms.  If data
indicate that tribufos is not highly oliophilic relative to air, then
the Agency could conclude that tribufos does not readily bioaccumulate
in terrestrial organisms.

How could the data change the Agency’s decision or impact the
Agency’s future decision-making?  

In the absence of the requested data, the n-octanol-air partitioning of
tribufos will be estimated with the KOAWIN model, the results of which
are characterized by high uncertainty.  If the required data indicate
that tribufos does not readily bioaccumulate in terrestrial organisms,
then risk assessment conclusions and associated labeled use precautions
and/or restrictions could be made less restrictive.



Guideline Number:  835.4300

Study Title: Aerobic Aquatic Metabolism

Rationale for Requiring the Data

The Agency has a limited understanding of how tribufos behaves in the
environment once applied.  Depending on how quickly, to what extent, and
to which transformation products the compound degrades, tribufos may or
may not pose potential exposure concern in water.  Because tribufos
degradation in water is not well understood, the Agency is requiring
data.

Practical Utility of the Data

How will the data be used?

Aerobic aquatic metabolism data will facilitate a better understanding
of the fate of tribufos residues in water.  If data indicate that
degradation in aerobic conditions occurs rapidly in water and produces
nontoxic transformation products, then the Agency could potentially
determine that tribufos residues of concern do not persist in water
under aerobic conditions.  If data indicate that degradation is not
appreciable in water or that the transformation products are of similar
or higher toxicity than the parent compound, then the Agency could
conclude that tribufos residues of concern persist in water under
aerobic conditions.

How could the data change the Agency’s decision or impact the
Agency’s future decision-making?  

In the absence of the requested data, tribufos aerobic aquatic
metabolism half-lives will be conservatively assumed two-fold greater
than aerobic soil metabolism half-lives.  If the required data indicate
that tribufos in aerobic aqueous environments is rapidly biodegraded,
then risk assessment conclusions and associated labeled use precautions
and/or restrictions could be made less restrictive.



Guideline Number: 850.1700 

Study Title: Analytical Methods (and Associated Independent Laboratory
Validations) for Residues in Water and Sediment

Rationale for Requiring the Data

Independently validated analytical methods for residues in soil, water,
sediment, and plant material (environmental chemistry methods) submitted
by the registrant are used to evaluate analyses described in submitted
environmental fate and ecological effects studies.  Submitted analytical
methods are also used by various Federal, State, Tribal, and local
agencies to detect and monitor residues that are or are suspected to be
in environmental compartments due to outdoor uses and accidental
releases.  Independent laboratory validations for submitted analytical
methods are necessary to confirm that the methods are functional and to
confirm the levels of detection and quantitation reported in
registrant-prepared validations.

Use of tribufos may impact surface water quality due to runoff of rain
water and drift of residues.  This is especially true for poorly
draining soils, erodible soils, and soils with shallow ground water. 
Tribufos has a medium potential for reaching both surface water and
aquatic sediment via runoff for several months or more after
application.  Therefore, outside agencies may request analytical methods
with which to monitor for tribufos residues in water and in soil or
sediment.

Environmental chemistry methods for tribufos residues in soil were
submitted by the registrant (MRID 42350006 and 43325502).  However,
environmental chemistry methods were not submitted for tribufos residues
in water or sediment.  Therefore, environmental chemistry methods, and
associated independent laboratory validations, for tribufos residues of
concern in water and in sediment are required to support monitoring for
tribufos residues and to evaluate any newly submitted environmental fate
and ecological effects data.  If data are submitted indicating that a
previously submitted analytical method in soil is functional for
residues in sediment, then a separate analytical method in sediment is
not required.

Practical Utility of the Data

How will the data be used?

In the absence of independently validated environmental chemistry
methods, submitted environmental fate and ecological effects data may
not be reviewable and entities outside the Agency may lack
chemical-specific methods for analyses in environmental compartments. 
Independently validated environmental chemistry methods will be used to
evaluate any newly submitted environmental fate and ecological effects
data and will be made available for enforcement and monitoring purposes
at the federal, state, and local levels.





Guideline Number:  850.2100 

Study Title:  Avian Acute Oral Toxicity Test

Rationale for Requiring the Data

Acceptable acute avian oral toxicity data were submitted for exposures
of mallard duck and bobwhite quail to tribufos; however, data are not
available for a passerine species, which is now required under the 40
CFR Part 158 (CFR 40 2007) data requirements for conventional
pesticides. The new Part 158 data requirements specify that acute avian
oral toxicity data be submitted for either mallard duck or bobwhite
quail and a passerine species. The available acute oral toxicity data
for mallard ducks and bobwhite quails indicates that tribufos may be
moderately toxic to birds on an acute oral exposure basis. Additionally,
there is evidence from other OPs (e.g., dimethoate) that passerines are
significantly more sensitive to at least some OPs when compared to
upland and game bird species tested.  Therefore, an avian oral toxicity
test is required for passerine birds, as specified in 40 CFR Part 158. A
passerine study protocol must be submitted for review by the Agency
prior to initiation of this study.

Practical Utility of the Data

How will the data be used?

Acute avian oral toxicity data for passerine species will be used to
refine the screening-level assessment by determining whether there are
differences in avian species sensitivity to tribufos between passerines
and upland game and waterfowl species. Based on the currently submitted
acute avian data, tribufos is classified as moderately toxic to birds on
an acute oral basis and slightly toxic on a sub-acute dietary exposure
basis. If oral acute toxicity data are not submitted for passerines,
risk will be assumed for all passerine species.

How could the data impact the Agency’s future decision-making?

If future endangered species risk assessments are performed without
these data, the Agency would have to assume that tribufos “may
affect” listed birds directly (and listed species from other taxa
indirectly), and use of tribufos and its formulated products may need to
be restricted in areas where listed species could be exposed. The lack
of these data will limit the flexibility the Agency and registrants have
in coming into compliance with the Endangered Species Act and could
result in use restrictions for tribufos use that are unnecessarily
severe.





Guideline Number:  850.1400 

Study Title:  Freshwater Fish Early Life-cycle Toxicity Test

Rationale for Requiring the Data

Acceptable chronic freshwater fish toxicity data are not available for
tribufos. A study was submitted for chronic toxicity to the fathead
minnow; however, this study was not adequate for RQ calculations because
a high level of analytical variation was observed in the mean results.
The Part 158 data requirements (40 CFR Part 158) require the submission
of chronic freshwater fish toxicity data for all conventional pesticides
with outdoor uses. Therefore, a freshwater fish early life-cycle
toxicity test is required, as specified in 40 CFR Part 158.  

Practical Utility of the Data

How will the data be used?

Chronic freshwater fish toxicity data will be used to determine whether
reproductive effects are possible for fish exposed to tribufos. If
chronic toxicity data are not submitted for freshwater fish, an
acute-to-chronic ratio (ACR) will be calculated for tribufos based on
toxicity data obtained from estuarine/marine fish.

How could the data impact the Agency’s future decision-making?

If future endangered species risk assessments are performed without
these data, the Agency would have to use the ACR value for its
freshwater fish chronic toxicity value.  Previous endangered species
assessments have indicated that risks are expected for listed and
non-listed fish due to acute and chronic exposures to tribufos in
aquatic habitats, based on this ACR value. Therefore, use of tribufos
and its formulated products may need to be restricted in areas where
listed species could be exposed. The lack of these data will limit the
flexibility the Agency and registrants have in coming into compliance
with the Endangered Species Act and could result in use restrictions for
tribufos that are unnecessarily severe.



Guideline Number:  N/A 

Study Title:  Terrestrial Plant Toxicity Test

Rationale for Requiring the Data

Although acceptable Tier II terrestrial plant studies are available for
tribufos, the plant species studied (i.e., annual dicots and monocots)
are not expected to be as sensitive to tribufos as other plants (e.g.,
woody deciduous trees and shrubs) due to its mechanism of action.
Available toxicity studies were conducted on plants that do not form
abscission zones, and data from these studies may underestimate the
potential toxicity of tribufos to non-target plants that do form
abscission zones. Therefore, additional terrestrial plant toxicity
studies on plants expected to be sensitive to tribufos’ mode of action
are required. 

Practical Utility of the Data

How will the data be used?

Vegetative vigor and seedling emergence data for terrestrial plants that
form abscission zones will be used to determine the potential for
tribufos to affect non-target plant species in the terrestrial
environment. In the absence of data specific for these plants, risk to
terrestrial plants will be assumed.

How could the data impact the Agency’s future decision-making?

If future endangered species risk assessments are performed without
these data, the Agency would have to presume risk to non-target
terrestrial plants from use of tribufos. Previous endangered species
assessments have presumed risk to terrestrial plants due to lack of data
on those species anticipated to be sensitive to tribufos. Therefore, use
of tribufos and its formulated products may need to be restricted in
areas where listed species could be exposed. The lack of these data will
limit the flexibility the Agency and registrants have in coming into
compliance with the Endangered Species Act and could result in use
restrictions for tribufos that are unnecessarily severe.



Guideline Number:  850.1735 and 850.1740 

Study Title:  Sediment Toxicity Testing on Freshwater and Marine
Invertebrates

Rationale for Requiring the Data

There are no available data for the toxicity of tribufos to benthic
freshwater or marine invertebrates.  Because tribufos could potentially
partition to the sediment and no sediment toxicity data are available
for review, toxicity to benthic invertebrates is unknown. The Part 158
data requirements (40 CFR Part 158) require the submission of sediment
toxicity testing for chemicals such as tribufos that have a half-life
exceeding 10 days and a Kd value above 50. Therefore, sediment toxicity
testing on freshwater and marine invertebrates is required, as specified
in 40 CFR Part 158.  

Practical Utility of the Data

How will the data be used?

Sediment toxicity data for freshwater and marine invertebrates will be
used to determine whether the potential for tribufos to affect the
benthic invertebrate community via its potential to partition to
sediment. Using daphnid data to determine toxicity to freshwater
invertebrates may over- or under- estimate benthic invertebrate
toxicity. In the absence of sediment toxicity data, EFED will assume
risk to benthic invertebrates.

How could the data impact the Agency’s future decision-making?

If future endangered species risk assessments are performed without
these data, the Agency would have to assume that tribufos has the
potential to affect the benthic invertebrate community, which could
indicate indirect concerns to listed species in marine and aquatic
environments that may depend on benthic invertebrates as potential prey
items. Therefore, use of tribufos and its formulated products may need
to be restricted in areas where listed species could be affected. The
lack of these data will limit the flexibility the Agency and registrants
have in coming into compliance with the Endangered Species Act and could
result in use restrictions for tribufos that are unnecessarily severe.



 PAGE   

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Ingestion

Birds / Amphibians

Wet/dry deposition

Root uptake

Long-range atmospheric transport

Dermal uptake/Ingestion

Ingestion

Ingestion

Ingestion

Ingestion

Soil

Exposure

Media

Mammals

Runoff

Terrestrial/riparian plants

grasses/forbs, fruit, seeds (trees, shrubs)

Habitat integrity

Reduction in primary productivity

Reduced cover

Community change

Food chain

Reduction in prey

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Terrestrial 

insects

Birds / Terrestrial-phase amphibians / reptiles / mammals

Juvenile

Adult

Spray drift

Direct

application

Tribufos applied to use site (cotton)

Attribute

Change

Receptors

Source

Stressor

Riparian plant terrestrial exposure pathways see Figure 3

Uptake/cell, 

roots, leaves

Aquatic Plants

Non-vascular

Vascular

Uptake/gills 

or integument

Ground water

Soil

Wet/dry deposition

Long-range atmospheric transport

Ingestion

Ingestion

Uptake/gills 

or integument

Exposure

Media

Aquatic Animals

Invertebrates

Vertebrates

Runoff

Surface water/

Sediment

Habitat integrity

Reduction in primary productivity

Reduced cover

Community change

Food chain

Reduction in algae

Reduction in prey

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Fish/aquatic-phase amphibians

Eggs     

Larvae 

Juveniles / Adults

Spray drift

Tribufos applied to use site (cotton)

Attribute

Change

Receptors

Source

Stressor

