UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON D.C., 20460

	

MEMORANDUM					            	        June 15, 2009

Subject:	Registration Review –Preliminary Problem Formulation for
Ecological Risk and Environmental Fate, Endangered species and Drinking
Water Assessments for Tebuthiuron (PC Code 105501; DP Barcode D363982)

To:		Wilhelmena Livingston, Chemical Review Manager

Special Review Branch

Special Review and Reregistration Division (SRRD)

From:		Fred Jenkins, Biologist 

James Lin, Environmental Engineer

Environmental Risk Branch 1

Environmental Fate and Effects Division

Office of Pesticide Programs

Through:	Nancy Andrews, Chief

		Environmental Risk Branch 1

		Environmental Fate and Effects Division

		Office of Pesticide Programs

The Environmental Fate and Effects Division (EFED) has completed the
preliminary problem formulation (attached) for the ecological risk,
environmental fate, endangered species, and drinking water assessments
to be conducted as part of the Registration Review of the herbicide,
Tebuthiuron (DP Barcode D363982).  The problem formulation draws
information from both open literature and studies submitted by the
technical registrants in response to data requirements.  This document
is intended to provide an overview of what is currently known regarding
the environmental fate and ecological effects associated with
tebuthiuron and its degradates and outlines uncertainties regarding
attributes of the parent compound and its transformation products.  It
describes the preliminary ecological risk hypothesis and the processes
that will be used during the completion of drinking water and ecological
risk assessments in support of registration.

  SEQ CHAPTER \h \r 1 

				

Problem Formulation for the

Environmental Fate, Ecological Risk,

Endangered Species,

and Drinking Water Assessments

in Support of the Registration Review of 

Tebuthiuron

1-(5-tert-butyl-1,3,4-thiadiazol-2-yl)-1,3-dimethylurea

 

Tebuthiuron (CAS 34014-18-1 )

Prepared by:

Fred Jenkins, Biologist

James Lin, Environmental Engineer	U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch I

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Reviewed by:

Faruque Khan, Senior Scientist

Brian Anderson, RAPL 

Nancy Andrews, Branch Chief

	Table of Contents

  TOC \o "1-3" \h \z \u    HYPERLINK \l "_Toc232820829"  I.  Purpose	 
PAGEREF _Toc232820829 \h  4  

  HYPERLINK \l "_Toc232820830"  II.  Problem Formulation	  PAGEREF
_Toc232820830 \h  4  

  HYPERLINK \l "_Toc232820831"  A.  Nature of Regulatory Action	 
PAGEREF _Toc232820831 \h  4  

  HYPERLINK \l "_Toc232820832"  B.  Available Information from Previous
Risk Assessments	  PAGEREF _Toc232820832 \h  5  

  HYPERLINK \l "_Toc232820833"  III.  Stressor Source and Distribution	 
PAGEREF _Toc232820833 \h  5  

  HYPERLINK \l "_Toc232820834"  A.	Mechanism of Action	  PAGEREF
_Toc232820834 \h  5  

  HYPERLINK \l "_Toc232820835"  B.  Overview of Pesticide Usage	 
PAGEREF _Toc232820835 \h  6  

  HYPERLINK \l "_Toc232820836"  C.  Environmental Fate and Transport	 
PAGEREF _Toc232820836 \h  8  

  HYPERLINK \l "_Toc232820837"  IV.  Receptors	  PAGEREF _Toc232820837
\h  10  

  HYPERLINK \l "_Toc232820838"  A.  Effects to Aquatic Organisms	 
PAGEREF _Toc232820838 \h  11  

  HYPERLINK \l "_Toc232820839"  B.  Effects to Terrestrial Organisms	 
PAGEREF _Toc232820839 \h  12  

  HYPERLINK \l "_Toc232820840"  B.	Degradate toxicity	  PAGEREF
_Toc232820840 \h  14  

  HYPERLINK \l "_Toc232820841"  E.  Ecological Incidents	  PAGEREF
_Toc232820841 \h  15  

  HYPERLINK \l "_Toc232820842"  E.	Ecosystems Potentially at Risk	 
PAGEREF _Toc232820842 \h  16  

  HYPERLINK \l "_Toc232820843"  V.  Assessment Endpoints	  PAGEREF
_Toc232820843 \h  16  

  HYPERLINK \l "_Toc232820844"  VI. Conceptual Model	  PAGEREF
_Toc232820844 \h  16  

  HYPERLINK \l "_Toc232820845"  A.  Risk Hypothesis	  PAGEREF
_Toc232820845 \h  17  

  HYPERLINK \l "_Toc232820846"  B.  Conceptual Diagram	  PAGEREF
_Toc232820846 \h  17  

  HYPERLINK \l "_Toc232820847"  VII. Analysis Plan	  PAGEREF
_Toc232820847 \h  19  

  HYPERLINK \l "_Toc232820848"  A.  Stressors of Concern	  PAGEREF
_Toc232820848 \h  20  

  HYPERLINK \l "_Toc232820849"  B.  Measures of Exposure	  PAGEREF
_Toc232820849 \h  20  

  HYPERLINK \l "_Toc232820850"  C.  Measures of Effect	  PAGEREF
_Toc232820850 \h  22  

  HYPERLINK \l "_Toc232820851"  D.  Integration of Exposure and Effects	
 PAGEREF _Toc232820851 \h  23  

  HYPERLINK \l "_Toc232820852"  1.  Deterministic and Probabilistic
Assessment Methods	  PAGEREF _Toc232820852 \h  23  

  HYPERLINK \l "_Toc232820853"  E.  Endangered Species Assessments	 
PAGEREF _Toc232820853 \h  24  

  HYPERLINK \l "_Toc232820854"  F.  Preliminary Identification of Data
Gaps	  PAGEREF _Toc232820854 \h  24  

  HYPERLINK \l "_Toc232820855"  1.  Fate	  PAGEREF _Toc232820855 \h  24 


  HYPERLINK \l "_Toc232820856"  2. Effects	  PAGEREF _Toc232820856 \h 
26  

  HYPERLINK \l "_Toc232820857"  VIII.  References	  PAGEREF
_Toc232820857 \h  29  

  HYPERLINK \l "_Toc232820858"  IX.  Appendixes	  PAGEREF _Toc232820858
\h  31  

 I.  Purpose

The purpose of this problem formulation is to provide a plan for
evaluating the environmental fate and ecological effects of the
registered uses of tebuthiuron and to identify information gaps needed
to complete an ecological risk assessment.  Tebuthiuron is a systemic
relatively nonselective herbicide which is absorbed into the plant roots
and is then translocated into the plant. Once absorbed and translocated
into the plant, it acts by inhibiting photosynthesis.  It is used to
control broadleaf and woody weeds, grasses and brush on feed crop sites.
 Primary use sites include rangeland and near railroads and other
industrial facilities. 

This document will provide a plan for analyzing data relevant to
tebuthiuron and for conducting environmental fate, ecological risk, and
endangered species assessments for its registered uses.  Additionally,
this problem formulation is intended to identify data gaps,
uncertainties, and potential assumptions used to address those
uncertainties relative to characterizing the ecological risks associated
with the registered uses of tebuthiuron.

II.  Problem Formulation

	A.  Nature of Regulatory Action

The Food Quality Protection Act of 1996 mandated the EPA to implement a
new program for assessing the risks of pesticides, i.e., registration
review (  HYPERLINK "http://www.epa.gov/oppsrrd1/registration_review/" 
http://www.epa.gov/oppsrrd1/registration_review/ ). All pesticides
distributed or sold in the United States generally must be registered by
EPA.  The decision to register a pesticide is based on the consideration
of scientific data and other factors showing that it will not cause
unreasonable risks to human health, workers, or the environment when
used as directed on product labeling. 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 ensure 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 to
determine whether tebuthiuron continues to meet the FIFRA standard for
registration. This problem formulation for the environmental fate and
ecological risk assessment chapter in support of the registration review
is intended for the initial docket opening, which starts the public
phase of the review process. 

B.  Available Information from Previous Risk Assessments

The following tebuthiuron risk assessment related documents performed by
EFED are available in the docket, and will serve as the basis for this
problem formulation:

1.	Tier II Aquatic Exposure Assessment for Selected Tebuthiuron Uses on
Rangeland, Pastureland, and Other Non-Crop Lands in the Pacific
Northwest: Endangered Species (ES) Consultation Package.  Dated: July
12, 2004.  DP Barcode D304225.

2.  	Tier II Aquatic Exposure Assessment for Selected Tebuthiuron Uses
(Carrots, Alfalfa, Cotton, Grapes, Safflower, Tomatoes) in the
California: Endangered Species (ES) Consultation Package Dated: July 12,
2004 (provided to the Field and External Affairs Division (FEAD) to
support an endangered species consultation package for salmon in
California).

3.	Addendum to Drinking Water Assessment for the Tolerance Reassessment
(TRED) for Tebuthiuron.  Dated: February 20, 2002

4.	Tolerance Reassessment and Risk Management Decisions (TRED) for
Tebuthiuron, Dated: November 28, 2001.  .

Detailed water monitoring data for surface water and ground water were
discussed.  The modeling results with index reservoir were used for
surface water drinking water purpose. 

5.	Reregistration Eligibility Decision (RED) for Tebuthiuron.  Dated:
June 15, 1994

In regards to EFED’s risk assessment purposes, the most significant
findings in these documents are that tebuthiuron: 1) may pose a
significant risk to terrestrial, semi-aquatic and aquatic plants
including non-listed and listed species, and 2) has a high potential to
leach into ground water.

III.  Stressor Source and Distribution

Mechanism of Action

Tebuthiuron is a systemic relatively nonselective herbicide which is
absorbed into the plant roots and is then translocated into the plant. 
Once absorbed by the roots and translocated into the plant, it acts by
inhibiting photosynthesis.  

B.  Overview of Pesticide Usage

According to the United States Geological Survey’s (USGS) Pesticide
National Synthesis Project, the annual tebuthiuron use amount for 2002
is about 119,000 pounds  (see Figure 1;   HYPERLINK
"http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m1
963" 
http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m19
63 ).  Most of the uses are estimated in the States of Oklahoma and
Pennsylvania (Figure 1).  However, non-agricultural uses are not
included in these estimates.  More recent usage data are available from
California Department of Pesticide regulation (CA DPR).  Table 1
outlines the three most recent years available of tebuthiuron uses in
California.  Based on this use information, there is a trend of
increased usage of tebuthiuron in the State of California. 

Figure 1.   Distribution of agricultural applications of Tebuthiuron in
2002.   

Table 1.  California DPR Usage Data of Tebuthiuron 



Commodity	2004 pounds applied	2005 pounds applied	2006 pounds applied

Landscape Maintenance	2,407.47	2,529.10	2,510.49

Rights Of Way	5,755.78	7,599.04	7,997.16

Structural Pest Control	30.40	19.20	15.52

Uncultivated Non-Ag	No usage data available	3.20	9.60

Chemical Total	8.195.15	10,150.54	10,533.37

Note:  Date - January8, 2009.   

CA DPR data -   HYPERLINK "http://www.cdpr.ca.gov/docs/pur/purmain.htm" 
http://www.cdpr.ca.gov/docs/pur/purmain.htm .

The targeted uses for this registration review are listed in Table 2. 
Many of the current labels do not contain sufficient information to
limit maximum annual number of applications or maximum annual
application rates.  For uses without specifications on maximum annual
application numbers of rates, the applications will be estimated using
conservative assumptions.

Table 2.  Uses to be Considered for Registration Review (provide by
BEAD)



Use Site	Form Code1	Max App Rate / App	Unit	Max App Rate / crop cycle or
Yr

PASTURES	P/T	4	lb Acre	4 lb/yr 

RANGELAND	P/T	4	lb Acre	4 lb/yr 

RANGELAND	WP	6	lb Acre	Not specified (NS)

COMMERCIAL, INSTITUTIONAL, INDUSTRIAL PREMISES, EQUIPMENT (OUTDOOR)	G
0.0919	lb 1K sq.ft	.12 lb/yr 

DRAINAGE SYSTEMS	G	6	Lb Acre	NS

DRAINAGE SYSTEMS	P/T	6	Lb Acre	NS

INDUSTRIAL AREAS (OUTDOOR)	G	6	Lb Acre	NS

INDUSTRIAL AREAS (OUTDOOR)	P/T	4	Lb Acre	4 lb/yr 

NONAGRICULTURAL RIGHTS-OF-WAY, FENCEROWS, HEDGEROWS	G	6	Lb Acre	NS

NONAGRICULTURAL RIGHTS-OF-WAY, FENCEROWS, HEDGEROWS	P/T	4	Lb Acre	4
lb/yr 

NONAGRICULTURAL RIGHTS-OF-WAY, FENCEROWS, HEDGEROWS	WP	6	Lb Acre	NS

NONAGRICULTURAL UNCULTIVATED AREAS/SOILS	G	6	Lb Acre	NS

NONAGRICULTURAL UNCULTIVATED AREAS/SOILS	G	0.0919	lb 1K sq.ft	.12 lb/yr 

NONAGRICULTURAL UNCULTIVATED AREAS/SOILS	P/T	6	Lb Acre	NS

NONAGRICULTURAL UNCULTIVATED AREAS/SOILS	WP	6	Lb Acre	NS

PASTURES	WP	6	Lb Acre	NS

PAVED AREAS (PRIVATE ROADS/SIDEWALKS)	G	4	Lb Acre	NS

PAVED AREAS (PRIVATE ROADS/SIDEWALKS)	P/T	4	Lb Acre	4 lb/yr 

1Form Code:   P/T = Pelleted/Tableted; G = Granular; WP = Wettable
Powder 

NS=Not Specified on label



 

C.  Environmental Fate and Transport

Registrant-submitted data defining the physical, chemical, fate, and
transport characteristics associated with tebuthiuron are summarized in
Table 3.  The information will be used for performing quantitative
aquatic and terrestrial assessments.

   Table 3.  General chemical and environmental fate properties of
Tebuthiuron. 

Chemical/Fate Parameter	Value(s)	Source (MRID)

Molecular weight (MW) (g/mol)	214.3	Product Chemistry

Vapor pressure (VP) (torr) 	7.5 × 10-7	Product Chemistry

Water solubility (mg/L at 20(C)	0.8	Product Chemistry

Henry’s Law Constant (atm-m3/mol; at 25(C)	7.58 × 10-11	Calculated1

Hydrolysis half-lives (25(C) (days)	Stable (pH 5)

Stable (pH 7)

Stable (pH 9)	1994 RED

Water photolysis half-life	Stable	41365101

Soil photolysis half-life	Stable	41695501

Aerobic soil metabolism half-life (days)	2832	41328001

Aerobic Aquatic metabolism half-life (days)	683	41372501

Anaerobic Aquatic metabolism half-life (days)	Stable	41913101

Adsorption coefficient  (KD) (L/kg)	0.11 (lowest)	40768401

Octanol-water partition coefficient (KOW)	Log KOW = 1.79	Product
Chemistry

Fish bioconcentration	1.98 × (edible)

3.40 × (nonedible)

2.63 × (whole fish)	40819501

1 Calculated according to the formula :  KH = 
(VP*MW)÷(760*solubility).



Tebuthiuron is persistent, mobile and can leach to ground water, as
indicated by a small-scale retrospective ground water study (MRID
42390901).  It is resistant to biological and chemical degradation, and
its principle route of dissipation in the environment appears to be
mobility.  Transport to ground water through leaching and to surface
water through run-off and off-target spray drift are likely as a result
of tebuthiuron's environmental persistence and low adsorption to soil. 
Tebuthiuron has been detected in ground water in Texas and California. 
Because tebuthiuron has the potential to leach through the ground and
because of the detections in Texas and California, the Agency is
concerned about the potential for ground water contamination from
registered uses of tebuthiuron.  Considering its low Henry’s Law
constant, (7.58 × 10-11 atm-m3/mol; at 25(C), the potential of vapor
and long range transports are insignificant.  The environmental fate
study summaries are presented as Appendix B.

Based on available environmental fate laboratory studies, several minor
degradates (< 10%) were identified (Appendix C).  Among these
degradates, compound 104
(N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea) has the
highest detection, and was identified by Health Effects Division (HED)
as the only environmental fate degradate of tebuthiuron of toxicological
concern.  A cumulative residue approach to model total tebuthiuron
residues was applied for drinking water assessment purposes.  

The MARC Committee review of the compound 104 concluded that this
compound is persistent and mobile in the environment.  Additionally, the
degradate 104 was detected in a retrospective ground water monitoring
study and was a major degradate in a terrestrial field dissipation study
accounting for up to 23% of the mass applied.  The degradate 104 was
also found in an aerobic soil metabolism and soil photolysis studies
comprising close to 7% of the mass applied.  In addition, due to the
structural similarity of degradate 104 to tebuthiuron (104

lacks an N-methyl group) and the lack of toxicity information on
degradate 104, the MARC

committee assumed that it has similar toxicity to the parent.

In regards to the bioaccumulation potential of tebuthiuron, a 28-day
flow-though study in which bluegill sunfish were exposed to a nominal
tebuthiuron concentration of 5.0 ppm demonstrated bioconcentration
factors of 1.98, 3.40, and 2.63 for edible tissue, nonedible tissue, and
whole fish, respectively.  Residues in the tissues consisted primarily
of tebuthiuron and two metabolites
(N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methyl-N’-hydroxyme
thyl-urea [compound 109] and compound 103(OH), an hydroxylated form of
parent tebuthiuron).  Accumulated residues depurated rapidly from fish
tissue with depuration half-lives of 0.33 and 0.51 days reported for
edible and nonedible tissue, respectively (MRID 40819501).

IV.  Receptors

Consistent with the process described in the Overview Document (USEPA,
2004), the risk assessment for tebuthiuron will rely 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 to potential effects on
a variety of species (receptors) included under these taxonomic
groupings.

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 tebuthiuron on
aquatic and terrestrial receptors. This includes toxicity data on the
technical grade active ingredient, when degradates, and available,
formulated products (e.g. “Six-Pack” studies). The open literature
studies are identified using EPA’s ECOTOX database (USEPA 2007d). 
ECOTOX integrates three previously independent databases - AQUIRE,
PHYTOTOX, and TERRETOX - into a system that includes toxicity data
derived predominately from the peer-reviewed literature, for aquatic and
terrestrial organisms.  Other information that is used to evaluate
toxicity and potential ecological risks include use of the acute probit
dose-response relationship to establish the probability of an individual
effect and reviews of the Ecological Incident Information System (EIIS).
 A summary of the available ecotoxicity information and the incident
information for tebuthiuron are provided below.  

The evaluation of these sources of data can also provide insight into
the direct and indirect effects of tebuthiuron 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.

A.  Effects to Aquatic Organisms

Table 4 provides a summary of toxicity data for surrogate aquatic
species that the EPA plans to use to characterize potential acute and
chronic ecological effects of tebuthiuron.  This table provides only the
results of studies that indicate the greatest toxicity for each aquatic
taxonomic group for which toxicity data are available.  A complete
listing of all registrant submitted aquatic ecotoxicity data for
tebuthiuron known by the Agency is available in Appendix A. 

Table 4.  Toxicity results that the Agency plans to use to assess
ecological effects of Tebuthiuron to aquatic species and the associated
acute toxicity classification.



Taxonomic Group	Toxicity Type	Surrogate Species	Acute Toxicity

--

Chronic Toxicity	MRID, Study Id. No.	Study Classification	Acute Toxicity
Classification

  SEQ CHAPTER \h \r 1 Freshwater fish	Acute	Bluegill sunfish

(Lepomis macrochirus)

	96-hr LC50 = 106 ppm

	MRID 40098001	Acceptable	Practically nontoxic

	Chronic	No data available	--	--	--	--

Amphibian	Acute	No data available	--	--	--	--

  SEQ CHAPTER \h \r 1 Freshwater invertebrates	Acute	  SEQ CHAPTER \h \r
1 Water flea (Daphnia magna)	48-hr EC50 = 297 ppm	MRID 00041694

	Acceptable	Practically nontoxic



	Chronic	No data available	--	--	--	--

  SEQ CHAPTER \h \r 1 Estuarine/marine fish	Acute	No data available	--
--	--	--

Estuarine/marine invertebrates	Acute	Pink shrimp (Penaeus duorarum)
96-hr   SEQ CHAPTER \h \r 1 EC50 = 62 ppm

  SEQ CHAPTER \h \r 1 	MRID 

00041684	

Acceptable	Slightly toxic

Aquatic plants

(nonvascular)	Acute	Green algae

(Selenastrum capricornutum)	14-day EC50 = 0.05 ppm

NOEC = 0.013 µg/L	Study Id. No. NAOTEB07	Supplemental	--

Aquatic plants

(vascular)	Acute	Large duckweed

(Lemna gibba)	14-day EC50 = 0.135 ppm

NOEC = < 0.66 ppm 	MRID 41080404	Acceptable	--



Toxicity to fish and aquatic invertebrates

On an acute exposure basis, technical grade tebuthiuron is classified as
practically nontoxic to slightly toxic to fish and aquatic
invertebrates.  There are no fish chronic toxicity data available for
the Agency’s ecological risk assessment purposes. 

Toxicity to aquatic plants

The submitted aquatic plant toxicity data which demonstrated the most
sensitive aquatic plant toxicity endpoints indicate that tebuthiuron may
be toxic to some nonvascular and vascular aquatic plants at
concentrations of 0.135 ppm and 0.05 ppm respectively.

B.  Effects to Terrestrial Organisms

Table 5 provides a summary of toxicity data for surrogate terrestrial
species that the EPA plans to use to characterize potential acute and
chronic ecological effects of tebuthiuron.  This table provides only the
results of studies that indicate the greatest toxicity for each
terrestrial taxonomic group for which toxicity data are available.  The
remainder of the registrant submitted terrestrial ecotoxicology data for
tebuthiuron known by the Agency is provided in Appendix A.  

Table 5.  Toxicity results that the Agency plans to use to assess
potential ecological effects of Tebuthiuron to terrestrial species and
the associated acute toxicity classification

Taxonomic Group	Toxicity Type	Surrogate Species	Toxicity	

MRID	Study Classification	Acute Toxicity Classification

Birds1	Acute Oral

	  SEQ CHAPTER \h \r 1 Mallard duck (Anas platyrhynchos)	LD50  > 2500
mg/kg

	MRID 00041680	Supplemental	Practically nontoxic

	Subacute Dietary	Mallard duck

(Anas platyrhynchos)	LC50  > 5093 mg/kg-diet	MRID 40601002

	Acceptable	Practically nontoxic

	Chronic (Reproductive toxicity)	Mallard duck	NOAEL > 100 ppm	MRID
00104243 	Acceptable	--

  SEQ CHAPTER \h \r 1 Mammals	Acute	  SEQ CHAPTER \h \r 1 Laboratory rat

(Rattus norvegicus)	LD50  = 387.5 mg/kg	MRID 40583901, 00226375
Acceptable	Moderately toxic

	Chronic (2-Generation reproduction study)	  SEQ CHAPTER \h \r 1
Laboratory rat

(Rattus norvegicus)	NOAEL = 100 mg/kg/day

LOAEL = 200 mg/kg/day	MRID 00090108	Acceptable	--

  SEQ CHAPTER \h \r 1 Insects	Acute Contact	  SEQ CHAPTER \h \r 1 Honey
bee 

(Apis mellifera L.)	> 100 µg/bee	MRID 40840401	Acceptable	Practically
nontoxic

	Residues on foliage	No data available	--	--	--	--

  SEQ CHAPTER \h \r 1 .Plants	Vegetative Vigor	No data available	--	--
--	--

	Seedling Emergence	No data available	--	--	--	--

1 Birds represent surrogates for terrestrial-phase amphibians and
reptiles



Toxicity to Birds

Based on the avian toxicity data that produced the most sensitive
endpoints, tebuthiuron is classified as practically nontoxic to birds on
an acute oral and acute dietary basis.  Two avian reproductive studies,
one on the bobwhite quail and one on the mallard duck show no effect on
reproduction at dietary levels up to 100 ppm (MRIDs 001044243 and
00093690; Table 4).  

The most recent Tebuthiuron RED (data 1994) concluded the following as
stated regarding the chronic risk of tebuthiuron to birds: 

“When tebuthiuron is applied directly to vegetation at the maximum use
rate for wettable powders (6 lbs ai/A), the expected residue levels on
various avian food items (42 – 1440 ppm) would exceed 100 ppm.  Little
is known about the persistence of tebuthiuron on plant surfaces. 
However, significant chronic exposure to birds is not expected due to
the following information.  About two-thirds of the total tebuthiuron
use is on rangeland and pastureland.  The maximum application rate for
this use pattern is 4 lbs ai/A/year.  Actual plant residue data for
forage grasses exist for a rangeland use pattern.  Residue monitoring at
test sites covering a wide range of climatic, edaphic, and geographical
conditions all showed residues in grasses to be below 20 ppm. 
Tebuthiuron is always applied to the ground as a pelleted/tableted
formulation and is never applied directly to vegetation.  Therefore, the
residue monitoring values represent tebuthiuron that is taken up from
the soil and transported to the plant tissues.  These values are
probably realistic indicators of long-term exposure to avian species. 
As these residue values (20 ppm) do not exceed the NOEL of 100 ppm in
the avian reproduction studies, chronic risk to birds is not
expected.”

Currently however, EFED concludes that although tebuthiuron is only
applied to bare ground and not to foliage, the application of this
chemical to bare ground may still pose pose a chronic risk to birds. 
This is because tebuthiuron persistently remains in the environment
after it is applied especially on soil (as indicated by a stable soil
photolysis half-life and an aerobic soil metabolism half-life of 2832
days).  Additionally, many birds species may forage for insects on bare
ground in open fields.  If tebuthiuron were applied directly to bare
ground at the maximum use rate for wettable powders (6 lbs ai/A), the
maximum expected residue levels on small insects would be 810 ppm.  This
level would exceed 100 ppm which is currently the avian reproductive
toxicity NOEAL and the maximum dose tested in the avian repro. study. 
Thus, birds species which forage for small insects in the field may be
at risk of chronic exposure to concentrations of tebuthiuron that are
much higher than the maximum dose tested in the available avian
reproductive toxicity studies.

An additional avian reproductive toxicity study is needed in order to
assess the risk of chronic exposure to birds foraging on bare ground. 
This study should test the risk of chronic exposure at concentrations
that range from > 100 ppm to at least 810 ppm.

Toxicity to Mammals

The most sensitive available mammalian acute toxicity data indicate that
tebuthiuron is moderately toxic to mammals on an acute oral basis (LD50
= 387.5 mg/kg; MRID 40583901; Table 4).   The available 2-two generation
reproduction study testing rats demonstrated a reproductive toxicity
NOAEL and LOAEL of 100 mg/kg/day and 200 mg/kg/day, respectively.

	

Toxicity to Beneficial Insects

The acute contact honey toxicity data indicate that tebuthiuron is
practically nontoxic to beneficial insects (MRID 40840401; Table 4). 

	Toxicity to Terrestrial Plants

The 40 Code of Federal Regulations (40 CFR) requires tier II terrestrial
plant toxicity data for all herbicides.  The required tier II plant
toxicity studies are a tier II seedling emergence study and a vegetative
vigor study.   Currently, no tier II seedling emergence or vegetative
vigor studies testing tebuthiuron have been submitted to the Agency.  A
seedling germination toxicity study was submitted to the Agency by the
registrant (MRID 41066901 and 41066902).  However, based on EFED’s
policy this study does not fulfill the guideline requirement for tier II
terrestrial plant toxicity data (EFED Memo “Policy on Data
Requirements for Nontarget plant testing”; Dated July 1, 1999).  

Additionally, the MARC committee presumed that compound 104, the only
tebuthiuron degradate of toxicological concern, has the equivalent
toxicity of its’ parent compound because of its structural similarity
to the parent.   Thus, EFED presumes that compound 104 may also have
that the same herbicidal toxic mode of action because of its structural
similarity to its’ parent compound.  Tier II terrestrial plant
toxicity data testing compound 104 is needed to verify this presumption.


An ECOTOX review of open literature terrestrial plant studies
demonstrated several different terrestrial plant toxicity studies
(Appendix D; Attached Excel file entitled “Appendix D
TebuthiuronECOTOX”).  Review of these studies to determine which can
be used for risk assessment purposes is currently pending.

 

Degradate toxicity

The MARC committee review of the compound 104 concluded that this was
the only degradate of toxicological concern.  The MARC committee made
this conclusion because of the structural similarity of degradate 104 to
tebuthiuron (compared to its parent chemical compound 104 only lacks an
N-methyl group) and because of the lack of toxicity information on
degradate 104.  Thus, as previously mentioned, EFED presumes that
compound 104 may also have that same herbicidal toxic mode of action
because of its structural similarity to its’ parent compound.  Tier II
terrestrial plant toxicity data testing compound 104 is needed to verify
this presumption. 

	D. Open literature ecotoxicity data/ECOTOX

The ECOTOX database has identified several ecotoxicity studies for
tebuthiuron.  These studies include tests for terrestrial and aquatic
plants, birds, mammals, and aquatic invertebrates.  The review of these
studies is currently pending.  The review of these studies will
determine whether they may be used for the Agency’s ecological risk
assessment.   

E.  Ecological Incidents 

The Ecological Incident Information System (EIIS) was used to evaluate
ecological incidents associated with use of Tebuthiuron.  Incidents in
this database are only ones which have been: 1) investigated, 2) linked
to one or more pesticide active ingredient, and 3) reported to the
Office of Pesticide Programs.  The Agency believes that these incidents
represent only a fraction of the total number of incidents that have
occurred.  Incidents in this system are categorized by certainty, which
indicates the Agency’s judgment on the probability that Tebuthiuron
was the cause of the observed effects.  Ecological incidents in the EIIS
database are summarized in Table 6.

Table 6.  Summary of ecological incidents associated with Tebuthiuron
use, by certainty.



Incident Type	

Incident ID	Use Type	Legality	Certainty





	All (excluding unlikely)	Unlikely	Possible	Probable	Highly Probable

Terrestrial plant damage	

1003503-001	Power line 	Misuse (accidental)



	X

	1015921-002	Agricultural area	Undetermined

	X



	1-13603-002	Rangeland	Registered use



X



100512-001	Right-of-way, road	Misuse (accidental)



X

	

As demonstrated in table 6, there are four reported incidences for
tebuthiuron. Two incidents were attributed to misuse of tebuthiuron, one
to a registered use, and another was undetermined.  All of the incidents
involved reported damage to terrestrial plants.

 Ecosystems Potentially at Risk

The ecosystems potentially 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 might 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.  

V.  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 (EPA 1998).  For
Tebuthiuron, the ecological entities include the following:  birds,
reptiles, terrestrial-phase amphibians, mammals, freshwater fish,
freshwater aquatic-phase amphibians and invertebrates, estuarine/marine
fish and invertebrates, terrestrial plants, insects, aquatic plants, and
algae. The attributes for each of these entities include growth,
reproduction, and survival.  

VI. Conceptual Model tc \l2 "D.        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 Tebuthiuron provides a written description and
visual representation of the predicted relationships between
Tebuthiuron, 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).

A.  Risk Hypothesis tc \l3 "1.         Risk Hypotheses 

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

Tebuthiuron, when used in accordance with current labels may result in
adverse effects upon the survival, growth, and reproduction of
non-target terrestrial and aquatic organisms.  These nontarget organisms
include Federally-listed threatened and endangered species.

B.  Conceptual Diagram tc \l3 "2.         Diagram 

The environmental fate properties of tebuthiuron along with monitoring
data identifying its presence in surface waters and ground water
indicate that runoff, spray drift and leaching represent potential
transport mechanisms of tebuthiuron to aquatic and terrestrial
organisms.  Considering it’s low Herny’s Law constant, (7.58 ×
10-11 atm-m3/mol; at 25(C), the potential of vapor and long range
transports are insignificant.   These transport mechanisms and resulting
movement of tebuthiuron into aquatic habitats (water) and terrestrial
habitats (soil and foliage) depicted in Figures 1 and 2, respectively. 
These figures also depict direct and indirect exposure pathways for a
broad range of biological receptors of concern (nontarget animals) and
the potential attribute changes, i.e., effects such as reduced survival,
growth and reproduction that may occur in the receptors due to
tebuthiuron exposure.  Because tebuthiuron is not very lipophilic (log
kow = 1.97) and is metabolized relatively rapidly in organisms, exposure
to aquatic organisms through the diet is predicted to be small compared
to uptake through the gills and integument, and thus contribute little
to ecological risk in aquatic ecosystems.

 

Figure   SEQ Figure \* ARABIC  1 . Conceptual model for Tebuthiuron
effects on aquatic organisms.  Dotted lines indicate exposure pathways
that are expected to have a relatively small contribution to ecological
risk.

 

Figure   SEQ Figure \* ARABIC  2 .  Conceptual model for Tebuthiuron
effects on terrestrial organisms.

VII. Analysis Plan  tc \l2 "E.        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 tebuthiuron are characterized and integrated
to assess the risks.  This is accomplished using a risk quotient (ratio
of exposure concentration to effects concentration) approach for spray
applications and an LD50/square foot method for granular applications. 
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 tebuthiuron is estimated using the probit dose-response slope
and either the level of concern (discussed below) or 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 for Tebuthiuron.

A.  Stressors of Concern

The stressor of concern for this assessment is tebuthiuron.  Exposures
in aquatic and terrestrial habitats will be estimated for tebuthiuron. 

As discussed in Appendix C, based on available environmental fate
laboratory studies, several minor degradates (< 10%) were identified. 
Among these degradates, compound 104
(N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea) has the
highest detection (6.9%), and was identified by Health Effects Division
(HED) as the only environmental fate degradate of tebuthiuron of
toxicological concern. As previously mentioned a cumulative residue
approach to model total tebuthiuron residues will be applied to the
drinking water assessment.  Also previously explained, for ecological
assessment, only the parent, tebuthiuron, will be considered.  This is
because, EFED presumes that compound 104 will not be a degradate of
concern on an ecotoxicity basis because only 6.8%, 6.9%, 2.9% and 1.5%
of this degradate was available in the environmental respectively based
on soil photolysis (14 days after application), aerobic soil metabolism
(270 days after application), anaerobic soil metabolism (270 days after
application) and aerobic aquatic metabolism (2.9 days after
application).

Evaluation of pesticide mixtures is beyond the scope of this assessment
because of the myriad factors that cannot be quantified based on the
available data.  Those factors include identification of other possible
co-contaminants and their concentrations, differences in the pattern and
duration of exposure among contaminants, and the differential effects of
other physical/chemical characteristics of the receiving waters (e.g.
organic matter present in sediment and suspended water). Evaluation of
factors that could influence additivity/synergism is beyond the scope of
this assessment and is beyond the capabilities of the available data to
allow for an evaluation.  However, it is acknowledged that not
considering mixtures could over- or under-estimate risks depending on
the type of interaction and factors discussed above.  The assessment
will however, analyze the toxicity of formulated products if formulated
product data is available (including formulations involving more than
one active ingredient) and will determine  whether formulated products
are more toxic than the technical grade active ingredient data used for
assessing both direct and indirect risks.

B.  Measures of Exposure  tc "1.  Measures of Exposure " \l 3 

In order to estimate risks of tebuthiuron exposures in aquatic and
terrestrial environments, all exposure modeling and resulting risk
conclusions will be based on maximum application rates and methods cited
in Table 2 and will be estimated for each use of tebuthiuron.  Measures
of exposure are based on aquatic and terrestrial models that predict
estimated environmental concentrations (EECs) of tebuthiuron.  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 environmental
fate data from registrant submissions and the literature; PRZM/EXAMS
model input values will be consistent with the most recent version of
the input parameter guidance (Version 2; EFED 2000).

PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are
simulation models coupled with the input shell pe5.pl (Aug 2007).  The
models generate daily exposures and calculated 1-in-10 year EECs of
tebuthiuron that may occur in surface water bodies adjacent to
application sites receiving tebuthiuron 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 in the water body and estimates
resulting concentrations.  The standard scenarios used for ecological
pesticide assessments assume 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.  PRZM/EXAMS is used to estimate
screening-level exposure of aquatic organisms to tebuthiuron.  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 chronic exposure to aquatic invertebrates.

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 1.4.1, 10/09/2008).    SEQ CHAPTER \h \r 1 This model
incorporates the Kenega nomograph, as modified by Fletcher et al.
(1994), which is based on a large set of field residue data.  The upper
limit values from the nomograph represent high-end 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.
 

Two spray drift models, AGDisp and AgDRIFT are used to assess exposures
of terrestrial plants to Tebuthiuron deposited in terrestrial habitats
by spray drift.  AgDrift (version 2.01; dated 5/24/2001) is used to
simulate ground, aerial, and spray blast applications.  To estimate
potential spray drift deposition at distances that exceed 1000 feet,
AGDisp (version 8.13; dated 12/14/2004) (Teske and Curbishley 2003) is
used, which simulates aerial and ground applications using a Gaussian
far-field extension. 

At this time, the Agency does not have an approved model for estimating
atmospheric transport of pesticides and resulting exposure to organisms
in areas receiving pesticide deposition from the atmosphere. Methods to
describe the contributions of atmospheric transport and deposition of
tebuthiuron to exposures to non-target organisms will be explored and
incorporated into this risk assessment as part of registration review of
Tebuthiuron.

C.  Measures of Effect

Ecological effect data are used as measures of direct and indirect
effects to biological receptors. Data were obtained from
registrant-submitted studies or from literature studies identified by
ECOTOX. The ECOTOXicology database (ECOTOX) was searched in order to
provide more ecological effects data 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 (USEPA 2007d).

Information on the potential effects of Tebuthiuron on non-target
animals is also collected from the Ecological Incident Information
System (EIIS; USEPA 2007c). 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 have included behavioral changes (e.g., lethargy, changes in
coloration and effects olfaction).  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 the assessment of
potential risks to birds is protective of 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 screening-level
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.

In the absence of data for either acute or chronic effects, the
conservative assumption will be to presume that potential risks from
exposure to Tebuthiuron exceed levels of concern.  

D.  Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological
effects characterization to determine the potential ecological risk from
labeled uses of Tebuthiuron 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 Tebuthiuron 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 (LOCs) (USEPA 2004). 
These criteria are used to indicate when tebuthiuron’s uses, as
directed on the label, have the potential to cause adverse direct or
indirect effects to non-target organisms.  

1.  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 describe potential risks
using probabilistic tools that the Agency has developed.  These tools
have been reviewed by FIFRA Scientific Advisory Panels and have been
deemed as appropriate means of refining assessments where deterministic
approaches have identified risks that exceed concern levels.

Endocrine Disruption Potential

EPA is required under the FFDCA, as amended by FQPA, to develop a
screening program to determine whether certain substances (including all
pesticide active and other ingredients) “may have an effect in humans
that is similar to an effect produced by a naturally occurring estrogen,
or other such endocrine effects as the Administrator may designate.” 
Following the recommendations of its Endocrine Disruptor Screening and
Testing Advisory Committee (EDSTAC), EPA determined that there were
scientific bases for including, as part of the program, androgen and
thyroid hormone systems, in addition to the estrogen hormone system. 
EPA also adopted EDSTAC’s recommendation that the Program include
evaluations of potential effects in wildlife.  When the appropriate
screening and/or testing protocols being considered under the Agency’s
Endocrine Disrupter Screening Program (EDSP) have been developed and
vetted, “tebuthiuron” may be subjected to additional screening
and/or testing to better characterize effects related to endocrine
disruption.  For further information on the status of the Endocrine
Disrupter Screening Program please visit our website: 
http://www.epa.gov/endo/.

E.  Endangered Species Assessments

Consistent with the Agency’s responsibility under the Endangered
Species Act (ESA), EPA will evaluate potential risks to Federally-listed
threatened and/or endangered (listed) species from registered uses of
Tebuthiuron.  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 registrations of Tebuthiuron
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 Tebuthiuron application
occurs), plus all areas where offsite transport (i.e., spray drift,
runoff, long-range atmospheric transport, 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 is not spatially limited and is assumed to be the entire
United States.

F.  Preliminary Identification of Data Gaps  tc \l3 "1.        
Preliminary Identification of Data Gaps and Methods 

1.  Fate

Although many submissions have been made to provide data on the
environmental fate of tebuthiuron and its degradates, one data gap is
identified and discussed below.. 

a)  Explanation Anticipated Data Needs for Fate Studies

Study Title:  Aquatic field Dissipation 

Guideline Number:  835.6200; 164-2

Test Substance:  Tebuthiuron

Rationale for Requiring the Data

According to Reregistration Eligibility Decision (RED) for Tebuthiuron
(1994), the registrant dropped their support for the only aquatic use
site (ditchbanks); therefore, the environmental fate and residue
chemistry data required in the Registration Standard for aquatic use
sites are no longer required for tebuthiuron.  However, current labels
suggest that there is a use for drainages.  Therefore in this
registration review, the aquatic field dissipation study is required.

Tebuthiuron is generally persistent, however EFED lacks important
information to evaluate the behavior of tebuthiuron under field aquatic
conditions which might increase its persistence or alter degradates
produced.  To adequately determine the environmental fate of tebuthiuron
and its degradates, aquatic field dissipation data are needed.



Practical Utility of the Data

How will the data be used?

Data from field study can provide more realistic estimates of the
persistence and transport of an active ingredient and its degradates. 
The results can reduce potential overestimation of exposure and risk and
can confirm assumptions of low levels of toxic degradated.

No aquatic field dissipation studies that track the transformation of
tebuthiuron in field aquatic environments are currently available. 
These data will be used to characterize tebuthiuron’s fate behavior in
the aquatic environment.  For mitigation purpose, these data can be used
to propose scenario-specific effective risk mitigation..

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

Dissipation in the field is not the same as degradation of the pesticide
in individual laboratory studies.  Dissipation in the field results from
interaction of multiple processes at work in the entire aquatic system. 
Without these required data, the fate of tebuthiuron cannot be fully
characterized and exposure to potentially toxic degradates cannot be
quantified.  The persistence of tebuthiuron and the degradates which may
be found in aquatic environments and to which aquatic organisms will be
exposed is uncertain.  In the absence of acceptable data, conservative
assumptions will be made.  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
tebuthiuron which are unnecessarily severe.



2. Effects

Although many submissions have been made to provide data on the
ecological toxicity of tebuthiuron, several studies have been identified
as data gaps.  These studies are discussed below. 

b)  Explanation Anticipated Data Needs for Eco Studies

Several effects studies that are currently not available that would be
valuable to the ecological risk assessment of tebuthiuron.  These are
summarized below.  

Test Substance:  Tebuthiuron

Study Titles with Respective Guideline Numbers:  

1) Study Title: Marine/estuarine fish acute toxicity; Guideline no: 72-3
(a)/850.1075, 

2) Study Title: Bivalve acute toxicity on shell deposition and embryo
larvae; Guideline no: 72-3(b)/(850.1025), (850.1055) 

3) Study Title: Crustacean acute toxicity; Guideline no:
72-3(c)/(850.1035)(850.1045)

Rationale for Requiring the Data

Rationale: Acceptable acute marine/estuarine fish acute toxicity data
are required under the 40 CFR Part 158.  Additionally, tebuthiuron use
patterns entails a multitude of use areas including pasture and
rangeland and a variety of non-food crop sites such as airports/landing
fields, outdoor industrial areas, non-agricultural rights-of-way,
fencerows, hedgerows, uncultivated areas/soils, and under paved roads
and sidewalks in areas where no future landscaping is planned.   Any of
these areas could potentially be in close proximity to a
marine/estuarine habitat that could be exposed to tebuthiuron.  This is
especially possible since tebuthiuron has a high potential to move
through soil from the original site of application.  

Practical Utility of the Data

How will the data be used?

These data are necessary to evaluate the extent of potential acute risk
of tebuthiuron to marine and/or estuarine species and will be used to
calculate acute RQs for these organisms. 

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

These data will be used to evaluate potential risks to aquatic
organisms, including threatened and endangered species.  If these data
reveal potential risks to marine and/or estuarine organisms, the Agency
will be able to determine how to mitigate these risks.  



Test Substance:  Tebuthiuron

Study Titles with Respective Guideline Numbers:  

1) Study Title: Freshwater Fish early-life stage toxicity; Guideline no:
72-4 (a)/850.1400 

2) Study Title: Freshwater Aquatic  invertebrate life cycle; Guideline
no: 72-4(b)/

850.1300



Rationale for Requiring the Data

Rationale: Acceptable chronic acute toxicity data testing the early life
history stages of freshwater fish and invertebrate are required under
the 40 CFR Part 158.  Additionally, based on available fate data
tebuthiuron is expected to move off of site of its application and
persistent in the environment.  Tebuthiuron  has a high potential reach
aquatic habitats and persist in these habitats.  Thus, aquatic organisms
in these habitats are at risk of chronic exposure to tebuthiuron.  

Practical Utility of the Data

How will the data be used?

These data are necessary to evaluate the extent of potential chronic
risk of Tebuthiuron to the early life stages of freshwater fish and
invertebrates. 

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

If these data reveal significant risk to the early life stages of
freshwater fish and invertebrates, the Agency will be able to determine
how to mitigate these risks.  



Test Substance:  Tebuthiuron

Study Titles with Respective Guideline Numbers:  

1) Study Title: Tier II Seedling Emergence toxicity study; Guideline no:
123-1a/850.4100 

2) Study Title: Tier II Vegetative Vigor; Guideline no: 123-1b/850.4150



Rationale for Requiring the Data

Rationale: Acceptable tier II terrestrial toxicity data for tebuthiuron
are required under the 40 CFR Part  158.  This is particularly because
tebuthiuron’s use as an herbicide classifies it as a known
phytotoxicant.  

Practical Utility of the Data

How will the data be used?

These data are necessary to evaluate the extent of potential risks of
tebuthiuron to non-target terrestrial plants and to animals that depend
on plants for survival or reproduction.  As an herbicide, neither an
action area nor a distance from a treatment site to which potential
risks extend can be defined without acceptable terrestrial plant
toxicity studies.  

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

Without acceptable terrestrial plant toxicity data, evaluating potential
risks to terrestrial plants (direct effects) or terrestrial or aquatic
animals (indirect effects) cannot be evaluated.  Therefore,
effectiveness of potential mitigations cannot be evaluated without these
data.    



Test Substance:  Tebuthiuron

Study Titles with Respective Guideline Numbers:  

Study Title: Avian acute oral toxicity study testing a passerine
species; Guideline no.: 850.2100/71-1

Rationale for Requiring the Data

Rationale: An acceptable avian acute oral toxicity data for tebuthiuron
is required under the 40 CFR Part  158.  Acceptable avian acute oral
toxicity studies testing an upland game species and a waterfowl species
have been submitted to the Agency.  These studies demonstrated that
tebuthiuron is practically nontoxic to upland game species and waterfowl
species.  An acute oral study testing a passerine species is needed to
ensure that tebuthiuron does not pose an acute risk to passerine species
according to the 40 CFR Part 158 .  

Practical Utility of the Data

How will the data be used?

These data are necessary to evaluate the extent of potential acute risks
of tebuthiuron to passerine bird species.  

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

If the data reveals that tebuthiuron poses an acute risk to passerine
species, then the Agency will be able to determine how to mitigate these
risks.   



Test Substance:  Tebuthiuron

Study Titles with Respective Guideline Numbers:  

Study Title: Avian reproductive toxicity study (testing exposure
concentrations ranging from > 100 ppm to at least up to 810 ppm);
Guideline no.: 850.2200/71-4

Rationale for Requiring the Data

Rationale: Although tebuthiuron is only applied to bare ground and not
to foliage, the application of this chemical to bare ground may still
pose a chronic risk to birds.  This is because tebuthiuron persistently
remains in the environment after it is applied especially on soil (as
indicated by a stable soil photolysis half-life and an aerobic soil
metabolism half-life of 2832 days).  Additionally, many bird species may
forage for insects on bare ground in open fields.  If tebuthiuron were
applied directly to bare ground at the maximum use rate for wettable
powders (6 lbs ai/A), the maximum expected residue levels on small
insects would be 810 ppm which would exceed 100 ppm (the avian
reproductive toxicity NOEAL and the maximum dose tested in the avian
repro. study).  Thus, bird species which forage for small insects in the
field may be at risk of chronic exposure to concentrations of
tebuthiuron that are much higher than the maximum dose tested in the
available avian reproductive toxicity studies.

	

An additional avian reproductive toxicity study is needed in order to
assess the risk of chronic exposure to birds foraging on bare ground. 
This study should test the risk of chronic exposure at concentrations
that range from > 100 ppm to at least 810 ppm.

.  

Practical Utility of the Data

How will the data be used?

These data are necessary to evaluate the extent of potential chronic
risk to avian species.

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

If the data reveals that tebuthiuron poses a chronic risk to avian
species, then the Agency will be able to determine how to mitigate these
risks.   



VIII.  References

Fletcher, J.S, Nellessen, J.E. & Pfleeger, T.G. (1994) Literature review
and evaluation of the

EPA food-chain (Kenaga) nomagram, an instrument for estimating pesticide
residues on 

plants. Environ. Toxicol. Chem., 13, 1383-1391.

  SEQ CHAPTER \h \r 1 Hoerger, F. and E. E. Kenaga, 1972. Pesticide
Residues on Plants: Correlation of 

Representative Data as a Basis for Estimation of their Magnitude in the
Environment. In F. 

Coulston and F. Korte, eds., Environmental Quality and Safety:
Chemistry, Toxicology, and 

Technology, Georg Thieme Publ., Stuttgart, West Germany, pp. 9-28.

Teske ME, SL Bird, DM Esterly, SL Ray, and SG Perry. 2001. A User’s
Guide for AgDRIFT 

2.01: A Tiered Approach for the Assessment of Spray Drift of Pesticides,
Regulatory Version. 

Continuum Dynamics Report No 01-02.

Teske M.E., and T.B. Curbishley. 2003. “AGDISP Version 8.07 User
Manual.” Technical 

Note No. 02-06. Continuum Dynamics, Inc., Ewing, NJ.

U.S. Environmental Protection Agency (USEPA). 1992. Pesticides in Ground
Water Database 

- A Compilation of Monitoring Studies:1971- 1991. Office of Prevention,
Pesticides, and 

Toxic Substances, EPA 734-12-92-001.

U. S. Environmental Protection Agency (USEPA). 1994.  Reregistration
Eligibility Decision 

for Tebuthiuron.  http://epa.gov/oppsrrd1/REDs/old_reds/tebuthiuron.pdf

U.S. Environmental Protection Agency (USEPA). 1998. Guidelines for
Ecological Risk 

Assessment. Office of Research and Development, Washington DC.
EPA/630/R-95/002F. pp 

114.

U. S. Environmental Protection Agency (USEPA). 2002. Guidance for
Selecting Input 

Parameters in Modeling the Environmental Fate and Transport of
Pesticides, Version II. US 

Environmental Protection Agency, Washington DC. Online at: 

http://www.epa.gov/oppefed1/models water/input_guidance2_28_02.htm.

 

U.S. Environmental Protection Agency (USEPA). 2004. Overview of the
Ecological Risk 

Assessment Process in the Office of Pesticide Programs, U.S.
Environmental Protection 

Agency.  Endangered and Threatened Species Effects Determinations. 
Office of Prevention, 

Pesticides and Toxic Substances, Office of Pesticide Programs,
Washington, D.C.  January 

23, 2004.   HYPERLINK
"http://www.epa.gov/espp/consultation/ecorisk-overview.pdf" 
http://www.epa.gov/espp/consultation/ecorisk-overview.pdf  

U.S. Environmental Protection Agency (USEPA). 2005. Generic Format and
Guidance for the

Level I Screening Ecological Risk Assessments Conducted in the
Environmental Fate and 

Effects Division.  Office of Pesticide Programs, Washington, D.C. 
January 24, 2005. 

  HYPERLINK
"http://www.epa.gov/oppefed1/ecorisk_ders/index.htm#framework" 
http://www.epa.gov/oppefed1/ecorisk_ders/index.htm#framework  

U.S. Environmental Protection Agency (USEPA). 2002. Memo: Tebuthiuron.
(List A, Case No.0054) The Outcome of the HED Metabolism Assessment
Review Committee Meeting Held on 01/22/02. Chemical 105501. TXR# 0050409

U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries
Service (NMFS). 

1998. Endangered Species Consultation Handbook: Procedures for
Conducting Consultation 

and Conference Activities Under Section 7 of the Endangered Species Act.
Final Draft. March 

1998. 

IX.  Appendixes

APPENDIX A: Toxicity Data Tables of Registrant Submitted Data

Table A1. Fish acute toxicity data for Tebuthiuron

Species	%A.I.	LC50 mg/L (confidence interval)	MRID	Classification

Freshwater Species

Rainbow trout

(Oncorhynchus mykiss)

	98	143 (118-224)	00020661	Acceptable

Bluegill sunfish

(Lepomis macrochirus)

	98	106 (87-120)	00020661	Acceptable

Goldfish

(Carassius auratus)

	98	 > 160 (N.R.)	00020661	

Supplemental



Fathead minnow

(Pimephales promelas)

	98	> 180 (N.R.)	00041685	

Supplemental



Fathead minnow

(Pimephales promelas)

	80	> 180 (N.R.)	00041685	

Supplemental



Fathead minnow

(Pimephales promelas)

	20	> 180 (N.R.)	00041685	Supplemental



Estuarine/Marine Species

No data submitted	--	--	--	

--

--

*Note: The study was deemed supplemental because only seven fish were
tested rather than the recommended 30 fish per group.

Toxicity A2 Fish chronic toxicity data 

Species	%A.I.	LOEL mg/L	NOEL mg/L	Effect	MRID	Classification

Freshwater Species

No data submitted 	--	--	--	--	--	--



Table A3 Aquatic invertebrate acute toxicity data for Tebuthiuron

Species	%A.I.	EC50 mg/L (confidence interval) 	MRID	Classification

Freshwater Species

Water flea 

(Daphnia magna)

	99.2	297	00041694	Acceptable

Estuarine/Marine Species







Pink shrimp 

(Penaeus duorarum)

	98	62	00041684	Acceptable

Fiddler crab 

(Uca pugilator)

	98	100	00041684	Supplemental



Table A4 Aquatic invertebrate chronic toxicity data for tebuthiuron

Species	%A.I.	LOEL mg/L	NOEL mg/L	Effect	MRID	Classification

Freshwater Species

 No data submitted	--	--	--	--	--	--





Table A5. Aquatic plant toxicity data

Species	%A.I.	EC50 mg/L	MRID	Classification

Blue green algae (Anabaena flos-aquae)	99.08	4.06	41080401	

Acceptable

Marine diatom 

(Skeletonema costatum)

	99.08	0.06	41080402	

Acceptable

Freshwater diatom

(Navicula pelliculosa)

	99.08	0.193	41080403	

Acceptable

Duckweed 

(Lemna gibba)

	99.08	0.135	41080404	

Acceptable



Table A6.  Toxicity of Tebuthiuron to Terrestrial Plants (Tier 1;
Vegetative Vigor and Seedling Emergence Studies)

Species	%A.I.	Toxicity Value	Identification Number	Study Classification

No data available	--	--	--	--



Table A7. Avian Single Acute Oral Dose Toxicity Data for Tebuthiuron

Species	%A.I.	LD50 mg/kg	MRID	Classification



Mallard duck 

(Anas platyrhynchos)	98	> 2000	00041692	Acceptable



Mallard duck 

(Anas platyrhynchos)

	98	> 500	00020661	Supplemental

Bobwhite quail 

(Colinus virginianus)	98	> 500	00020661	Supplemental



Table A8. Avian Acute Dietary Dose Toxicity Data for Tebuthiuron

Species	%A.I.	LC50 mg/kg-diet	MRID	Classification



Mallard duck 

(Anas platyrhynchos)

	98	> 2500	

00041680	Acceptable



Table A9.  Avian Reproductive Toxicity for Tebuthiuron

Species	%A.I.	NOEC mg/kg-diet	MRID	Classification

Mallard duck

(Anas platyrhynchos)	98	100	

00093690	Acceptable

Bobwhite quail (Colinus virginianus)	98	100	001044243	Acceptable



Table A9. Mammal Acute and Chronic Toxicity Data

Study Type	Organism	Endpoint	MRID	Classification

Acute oral	  SEQ CHAPTER \h \r 1 Laboratory rat

(Rattus norvegicus)	LD50 = 387.5 mg/kg	40583901, 	Acceptable

Chronic (2-Generation reproduction study)	  SEQ CHAPTER \h \r 1
Laboratory rat

(Rattus norvegicus)	NOAEL = 100 mg/kg/day

LOAEL = 200 mg/kg/day	MRID 00090108	Acceptable



Table A10.  Non-Target Insect Toxicity Data

Species	Study Type	Results	MRID	Classification

Honey bee	Honeybee acute contact toxicity	LD50 > 100	40840401	Acceptable



APPENDIX B

Environmental Fate Studies Summary

Based on acceptable environmental fate laboratory data reviewed,
tebuthiurion is persistent and mobile.  The principal route of
dissipation appears to be transport to ground and surface water. 
Tebuthiuron is stable to hydrolysis and photodegradation in water (t½ =
much greater than 30 days).  It has a soil photolysis half-life of 39.7
days.  Tebuthiuron is metabolized very slowly in soil under aerobic (t½
= 35.4 months) and anaerobic conditions (t½ = much greater than 60
days).  In aerobic and anaerobic aquatic metabolism studies,
tebuthiuron’s respective half-lives were much greater than 1 month and
greater than 1 year.  Tebuthiuron is mobile with Kads for sand, sandy
loam, loam, and clay loam soils were 0.11, 0.62, 0.82, and 1.82,
respectively.  The compound’s Koc was reported as 4.  In addition,
aged leaching data indicate that one metabolite,
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (compound
104) has similar mobility to parent tebuthiurion.  

Terrestrial field dissipation studies were done in California, Nebraska,
and Florida.  The field half-lives were estimated at 1- 2 years.  In CA
and NE, tebuthiuron moved into the 6-12” soil depth, with small
quantities detected at 12-18” and 18-24”.  In a FL soil (92% sand),
tebuthiurion leached to a depth of > 72”.  Tebuthiuron has slight
potential to accumulate in fish with bioconcentration factors of 1.98,
3.40, and 2.63 for edible tissue, nonedible tissue, and whole fish,
respectively.  Accumulated residues depurated rapidly.

Degradation products identified in laboratory studies were
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (compound
104), N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-methylurea
(compound 105), 5-(1,1-dimethylethyl)-2 methylamino-1,3,4-thiadiazol
(compound 107), 5-(1,1-dimethylethyl)-2 amino-1,3,4-thiadiazol (compound
108), and
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-(hydroxymethyl)-N-m
ethylurea (compound 109).  After 9 months of aerobic soil incubation,
compound 104 accounted for 6.9% of the applied radioactivity.  

In summary, tebuthiuron is resistant to biological and chemical
degradation under environmental conditions.  Its principal route of
dissipation in the environment appears to be mobility; transport to
ground water (through leaching) and surface water (solubilize in runoff)
are likely to occur after the application of tebuthiuron.  Laboratory
data indicate that photodegradation on soil may occur slowly but is not
likely to be a route of dissipation in the environment.  Transport to
ground water (through leaching) and surface water (following runoff) are
likely as a result of tebuthiuron’s persistence and low adsorption to
soil.  According to the Pesticides in Ground Water Database (1992),
tebuthiuron has been detected in ground water in Texas (two wells) and
California (one well).  A small-scale retrospective ground water study
indicated that tebuthiuron is persistence and mobile and can leach
ground water.  The results of the study indicated that tebuthiuron was
persistent and mobile enough to leach at least 15 feet to the water
table, then still be present above minimum detection levels more than 4
years after the application.

Hydrolysis (MRID 00020779)

14C tebuthiuron (radiochemical purity ~ 99.5%), at 10 and 100 ppm, did
not degrade during 64 days of incubation in sterile aqueous solutions at
pH 3, 6, and 9 in the dark at 

25 oC.

Photodegradatoin in water (MRID 41328001)

Tebuthiuron did not photodegrade in sterile aqueous buffered (pH 5)
solutions that were continuously irradiated for 33 days with a xenon
light source at approximately 25 oC.  Tebuthiuron was the only compound
identified in the irradiated and dark control solutions at all sampling
intervals.  At 33 days posttreatment, tebuthiuron comprised 96.3 –
97.0% and 99.7 – 100% of the recovered radioactivity in the irradiated
and dark control solutions, respectively.  

Aerobic soil metabolism (MRID 41328001)

In a 9-month study, thiadiazole-labeled 14C tebuthiuron, at a
concentration of 6 ppm in sandy loam soil incubated in darkness at 24 oC
and 75% field moisture capacity, degraded with a half-life (calculated
by the registrant) of 35.4 months.  The degradates identified by
two-dimensional TLC were
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (compound
104), N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-methylurea
(compound 105), 5-(1,1-dimethylethyl)-2 methylamino-1,3,4-thiadiazol
(compound 107), and 5-(1,1-dimethylethyl)-2 amino-1,3,4-thiadiazol
(compound 108).  The concentration of compound 104, which accounted for
6.9% of the applied radioactivity after 9 months of incubation, appeared
to be increasing at the end of the experiment.  

Anaerobic soil metabolism (MRID41328002)

Following 30 days of aerobic incubation at 24 ± 1 oC and 75% of 0.33
bar moisture capacity and 60 days under flooded conditions in a sandy
loam soil, thiadiazole-labeled 14C tebuthiuron (nominal concentration 6
ppm) exhibited very little metabolism.  After 60 days of anaerobic
incubation, the concentration of parent tebuthiuron had decreased 4.7%
from the concentration at initiation of flooding.  Degradates identified
were N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea
(compound 104),
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-methylurea
(compound 105), and
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-(hydroxymethyl)-N-m
ethylurea (compound 109).

Anaerobic aquatic metabolism (MRID 41913101)

Tebuthiuron degraded with a half-life (calculated by the registrant) of
> 1 year in an anaerobic system containing pond water and sediment
incubated for 365 days in darkness at 25.5 ± 0.8 oC.  During the study
there was very little degradation of tebuthiuron, with 93.7% of the
applied radiocarbon remaining as parent material at day 365.  Degradates
were reported to comprise approximately 1.4% of the applied
radioactivity at the termination of the study.

Aerobic aquatic metabolism (MRID 41372501)

Tebuthiuron did not degrade appreciably in pond water and sediment that
was incubated in darkness at 24 ± 1 oC fro 4 weeks under aerobic
conditions. After 4 weeks of incubation, 95.2% of the applied
radioactivity was present in parent tebuthiuron.  Degradates identified
by two-dimensional TLC were
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea (compound
104), N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-methylurea
(compound 105), 5-(1,1-dimethylethyl)-2 methylamino-1,3,4-thiadiazol
(compound 107), 5-(1,1-dimethylethyl)-2 amino-1,3,4-thiadiazol (compound
108), and
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-(hydroxymethyl)-N-m
ethylurea (compound 109).

Leaching, Adsorption/Desorption (MRID 40768401)

Tebuthiuron is mobile in all 4 soils and has a Kads for sand, sandy
loam, loam, and clay loam of 0.11, 0.62, 0.82, and 1.82, respectively. 
The corresponding Koc values are 38, 76, 75, and 152, respectively.

Bioaccumulation in fish (MRID 40819501)

In a 28-day flow-through study in which bluegill sunfish were exposed to
a nominal tebuthiuron concentration of 5.0 ppm, bioconcentration factors
of 1.98, 3.40, and 2.63 were reported for edible tissue, nonedible
tissue, and whole fish, respectively.  Residues in the tissues consisted
primarily of tebuthiuron and two metabolites
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N’-(hydroxymethyl)-N-m
ethylurea (compound 109) and compound103(OH), an hydroxylated form of
parent tebuthiuron).  Accumulated residues depurated rapidly from fish
tissue with depuration half-lives of 0.33 and 0.51 days reported for
edible and nonedible tissue, respectively.  Based on the reported
octanol/water coefficient (log Kow = 1,79), there is slight potential
for tebuthiuron residues to accumulate in fish.

Terrestrial Field Dissipation (MRID 43318101)

Tebuthiuron was broadcast applied once as a spray at nominal application
rates of 6.6 lb ai/A on bareground plots of Lakeland fine sand
(Florida), Watsonville sandy loam (California), and Sharpsburg silty
clay loam (Nebraska).  EFED calculated soil dissipation half lives for
Florida of 385 days (r2 = 0.66) for the total parent (summed across 0 to
72 inches) and 123 days (r2 = 0.92) for the surface layer (0 to 6
inches), for California of 770 days (r2 = 0.89) for the total parent
(summed across 0 to 72 inches) and 462 days (r2 = 0.97) for the surface
layer (0 to 6 inches), and  for Nebraska of 575 days (r2 = 0.89) for the
total parent (summed across 0 to 72 inches) and 433 days (r2 = 0.91) for
the surface layer (0 to 6 inches).  The registrant calculated half lives
(total parent profile) of 383 days for Florida, 806 days for California,
and 575 days for Nebraska.  

Tebuthiuron was detected in the deepest soil sample (60 to 72 inches) at
the Florida site indicating that migration to ground water is a concern.
 Tebuthiuron was not detected below 24 to 30 inches at the California
site or below 30 to 36 inches at the Nebraska site, although these
depths indicate that tebuthiuron is leaching.  The metabolite (Compound
109383) was detected at all three sites but was not observed to have
migrated below 6 to 12 inches in Florida, 12 to 18 inches in California,
and 0 to 6 inches in Nebraska. The registrant did not calculate the half
life for the metabolite.  EFED estimated the half life for the
metabolite from each study by performing linear regression on the
decline pattern (as determined by peak concentration of metabolite). 
EFED calculated half lives for the total profile for Florida,
California, and Nebraska, respectively were 81 days (r2 = 0.92), 495
days (r2 = 0.96), and 385 days (r2 = 0.57).  

Finally, it should be noted that evaporation exceeded water input at the
California and Nebraska sites and therefore the vertical movement
(leaching potential) of tebuthiuron and its degradate may be
underestimated at these two sites.

The reported data indicate that tebuthiuron is mobile and persistent. 
Based on the data provided, leaching to ground water is likely to be a
significant route of dissipation.

Small Scale Retrospective Ground Water Monitoring Study (MRID 42390901) 

A small-scale retrospective ground water monitoring study was performed
on a ranch near Sarita, Texas, that has last been treated with
tebuthiuron on March 24, 1986.  The results of the study indicated that
tebuthiuron was mobile enough to leach at least 15 feet to the water
table, then still be present above minimum detection levels more than 4
years after the application.  Soil sampling at and near the study site
showed convincingly that tebuthiuron can persist at relatively high
concentrations in soil and soil water if restrictive layers blocks
leaching to the ground water.



APPENDIX C

DEGRADATE NAMES and STRUCTURES

The molecular structures of the tebuthiuron degradates detected in each
fate study were listed below.  Among these degradates, compound 104 was
identified by Health Effects Division (HED) as the only environmental
fate degradate of tebuthiuron of toxicological concern.

Parent / Degradate Name and Structure	Percent of Applied 	

MRID #	

Study Type 

	Maximum	Day





104:  N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-methylurea

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	6.8	14	41050201	soil photolysis

	6.9	270	41328001	aerobic soil metabolism

	2.9	60	41328002	anaerobic soil metabolism

	1.5	21	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	22.9	408	43318101	terrestrial field dissipation

	2.2 (edible)	21	40819501	bioaccumulation in fish

105:  N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N=-methylurea

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	3.5	19	41050201	soil photolysis

	0.4	90	41328001	aerobic soil metabolism

	0.2	60	41328002	anaerobic soil metabolism

	0.4	28	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	ND	N/A	43318101	terrestrial field dissipation

	4.7 (edible)	21	40819501	bioaccumulation in fish



106:  N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl] urea

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	2.7	5	41050201	soil photolysis

	ND	N/A	41328001	aerobic soil metabolism

	ND	N/A	41328002	anaerobic soil metabolism

	ND	N/A	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	ND	N/A	43318101	terrestrial field dissipation

	ND	N/A	40819501	bioaccumulation in fish

107:  5-(1,1-Dimethylethyl)-2-methylamino-1,3,4-thiadiazole

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	ND	N/A	41050201	soil photolysis

	1.1	270	41328001	aerobic soil metabolism

	ND	N/A	41328002	anaerobic soil metabolism

	0.3	21	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	ND	N/A	43318101	terrestrial field dissipation

	ND	N/A	40819501	bioaccumulation in fish



108:  2-dimethylethyl-5-amino-1,3,4-thiadiazole

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	ND	N/A	41050201	soil photolysis

	0.6	270	41328001	aerobic soil metabolism

	ND	N/A	41328002	anaerobic soil metabolism

	0.1	7	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	ND	N/A	43318101	terrestrial field dissipation

	ND	N/A	40819501	bioaccumulation in fish



109: 
N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N'-hydroxymethyl-N-methy
lurea

	ND	N/A	N/A	hydrolysis

	ND	N/A	41305101	aqueous photolysis

	ND	N/A	41050201	soil photolysis

	ND	N/A	41328001	aerobic soil metabolism

	0.2	60	41328002	anaerobic soil metabolism

	0.3	7	41372501	aerobic aquatic metabolism

	ND	N/A	41913101	anaerobic aquatic metabolism

	ND	N/A	40768401	batch equilibrium

	N/A	N/A	N/A	laboratory volatility

	N/A	N/A	N/A	field volatility

	ND	N/A	43318101	terrestrial field dissipation

	40.1 (edible)	21	40819501	bioaccumulation in fish



APPENDIX D ECOTOX Data (See: Attached Excel file entitled “Appendix D
TebuthiuronECOTOX”)

Page   PAGE  1  of   NUMPAGES  45 

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

Tebuthiuron applied

Attribute

Change

Receptors

Source

Stressor

Wet/dry deposition

Soil

Ground water

Uptake/gills 

or integument

Aquatic Plants

Non-vascular

Vascular

Uptake/cell, 

roots, leaves

Riparian plant terrestrial exposure pathways see Figure 2

Stressor

Source

Receptors

Attribute

Change

tebuthiuron applied..

Direct

application

Spray drift

Birds / Terrestrial-phase amphibians / reptiles / mammals

Juvenile

Adult

Terrestrial 

insects

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Food chain

Reduction in prey

Habitat integrity

Reduction in primary productivity

Reduced cover

Community change

Terrestrial/riparian plants

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

Runoff

Mammals

Exposure

Media

Soil

Ingestion

Ingestion

Ingestion

Ingestion

Dermal uptake/Ingestion

Long range atmospheric transport

Root uptake

Wet/dry deposition

Birds / Amphibians

Ingestion

