       U. S. ENVIRONMENTAL PROTECTION AGENCY

Washington, D.C. 20460

OFFICE OF             

PREVENTION, PESTICIDES

AND TOXIC SUBSTANCES 

PC Code: 053501	

DP BARCODE: D308030

MEMORANDUM							DATE:  December 13, 2006

SUBJECT: 	Response to Registrant’s Comments on Methyl Parathion

		

FROM:	Amer Al-Mudallal, Chemist

		Edward Odenkirchen, Ph.D., Senior Biologist

		Environmental Risk Branch 1

		Environmental Fate and Effects Division (7507P)

THRU:	Nancy Andrews, Branch Chief

Environmental Risk Branch 1

		Environmental Fate and Effects Division (7507P)

TO:		Laura Parsons, Risk Manager

John Pates, RM Reviewer			

		  SEQ CHAPTER \h \r 1 Special Review and Reregistration Division
(7508P)

		

Attached are EFED’s responses to the Registrant’s comments on Methyl
Parathion.  If you have any questions, Please contact Amer Al-Mudallal
at (703) 605-0566 or Edward Odenkirchen at (703) 305-6449.  Thank you.

III. Comments on EPA’s Ecological Risk Assessment 

Formulations (page 14 of 40)

EFED Response:  The registrant claims that EFED’s risk assessment
approach for conducting a combined ecological risk assessment for the EC
and encapsulated (MCAP) formulations of methyl parathion “distorts”
the risk evaluation for both formulations.  No further details were
provided to substantiate this claim.  EFED cannot fully address this
issue without a more complete discussion on toxicity and environmental
fate differences for the different formulations of methyl parathion. 

The ecological risk assessment considered all submitted data regardless
of formulation. This strategy is consistent with the screening-level
ecological risk assessment process to address toxicity and environmental
fate processes of the active ingredient. Formulation effects on
environmental fate properties are considered in volatility and field
dissipation studies.  Additionally, formulation effects on toxicity are
considered when there are sufficient data to ensure differential
toxicity among formulations.  However, the most sensitive species
endpoints are used for calculation of risk quotients. For methyl
parathion, the registrant did not provide a comparison of environmental
fate and toxicity data for the EC and MCAP formulations to warrant
characterization of formulation effects.          

Aquatic Organism Risk Assessment 

EPA’s Model Inputs

●  Soil absorption constant. (page 15 of 40)

EFED Response: The Koc of 5100 used is described in Hornsby, et
al.(1996) as “an ‘estimate,’ meaning either (a) an unusually wide
range of values have been reported and we had no reason to select any
one value as a ‘best’ value, or (b) no experimental value is
available but a reasonable estimation was possible” or a value was
“calculated from some more fundamental property”. The Koc of 487
ml/g used by EFED was chosen based on experimental data submitted by the
registrant.  A repeat the adsorption-desorption laboratory study,
because the soils of the original study were autoclaved, has not been
submitted for consideration.

The Koc of 487 ml/g used by EFED is conservative, but not unreasonable
given the data reported in outside literature.  Simple partition
coefficients for methyl parathion can range from 3 to 147 (median=50)
(Howard, 1991). Sanchez-Martin and Sanchez-Camazanoe, (1991) found that
the methyl parathion sorption on soil was correlated to the soil organic
carbon content.  They found the mean Koc for methyl parathion is 697
ml/g across 8 mineral soils.  Hornsby, et al. (1996) reports organic
carbon partitioning coefficients of 424 and 14000 ml/g in addition to
the 5100 discussed by the registrant.  

EFED discussed the adsorption coefficient of ethyl parathion further in
the revised EFED ethyl parathion RED chapter. EFED stands by the use of
the use of a Koc of 487 ml/g.

●  Aerobic soil half-life. (page 15 of 40)

EFED Response: The half-life for parent methyl parathion (3.75 days) was
derived from upgradable supplemental data; the study provided useful
information for methyl parathion, but the study was deemed as
supplemental because of  inadequate identification of degradation
products. Quantitatively, the aerobic soil metabolism half-life of
methyl parathion, assuming a normal distribution, can range anywhere
from an approximate mean half-life or central tendency of 3.75 days to
an upper bound 90th percentile upper confidence limit (UCL) of the mean
(11.25 days).  As per interim guidance on model input parameters, the
90th percentile UCL of the mean is used in PRZM-EXAMS modeling in
situations where the only one half-life is available. This correction
factor assumes that half-lives in different soils are normally
distributed. The correction factor is used to incorporate uncertainty in
the variability of the half-lives.    

Until such time that EFED adopts a probabilistic exposure assessment,
EFED will use the 90th percentile UCL of the mean in an attempt to be
reasonable and protective of non-target populations.

ions less than 100 μg/g.  

Finally, for foliar-applied insecticides such as methyl parathion, the
relationship between application interval and the aerobic soil
metabolism half-life may not be strong.

●  Foliar dissipation. (page 16 of 40) 

EFED Response: EFED did include foliar dissipation in the original EECs
to the extent possible in the PRZM-EXAMS modeling. The model includes
two input cards for foliar dissipation. The first, the “foliar
extraction” card, regulates what mass fraction of the pesticide will
wash off the leaves with a centimeter of rainfall. EFED sets this to 0.5
as a default. 

The second input card is decay rate on foliage. Foliar decay data was
not available for methyl parathion. Foliar dissipation includes not only
decay, but other processes such as washoff and volatilization.
Therefore, the use of foliar dissipation in this input slot would
account for decay, but double-count for foliar washoff. Therefore, it
would have been inappropriate to use the foliar dissipation value in the
model.

Bacterial degradation in water. (page 16 of 40)

EFED Response: The registrant is correct  in noting that  a standard
fitting procedure of first-order degradation kinetics (ln transformed
concentration vs time) does not   describe the initial rapid degradation
phase of methyl parathion in MRID  42069601.   EFED recalculated the
half-life estimate with both linear regression fitting procedures using
ln transformed concentration data v time  and non-linear regression
fitting of the 2 parameter, exponential decay model using
non-transformed concentration data v time.   Fitting procedures were
conducted using Sigmaplot 2000 Regression Wizard.    This analysis
yields the following results:

Co (y-intercept)	 			K(days-1)	    Half-life         	 R2

Linear Regression        2.9548 (19.19% of initial dose)(P=0.0049)  
0.1705 (P=0.0202)	   4.06		0.62	

Non-Linear Regression	89.24 % of initial dose (P<0.0001)	         0.5432
(P<0.0001) 	   1.3		0.99

This analysis indicates that methyl parathion degradation appears to
follow a hockey-stick type degradation pattern with rapid degradation
rate   followed by slower degradation rate.   The incorporation of  a
non-linear half-life in the PRZM/EXAMS modeling is not expected to alter
the risk assessment because current EFED Input Parameter Guidance
recommends using an estimate of the 90th percentile of the mean
half-life, which is  3 * 1.3 days or 3.9 days.  This value is not
different than the 4.1 days used in the original exposure assessment.

Appropriation of EPA’s model for estimating concentrations of
pesticides in water (page 17 of 40)

EFED Response:  The registrant claims the Pesticide Root Zone Model
(PRZM) and EXposure Analysis Modeling System (EXAMS) have not been
validated according to the Guidelines for Exposure Assessment
(FRC-4129-5).  Therefore, they believe the Agency is in violation of the
Data Quality Control Act.

 

The PRZM and EXAMS  models were developed by the Office of Research and
Development in the US Environmental Protection Agency.  Since the
initial versions of PRZM and EXAMS were developed, these models have
undergone continuous upgrading and evaluation to improve alogrithims and
capabilities for predicting pesticide concentrations in surface water 
and ground water.  Validation and evaluation of  the models has been
conducted by academics, regulated industry, and government agencies.   

Examples of open literature validation and calibration exercises for
PRZM :

Muller, T., R. E. Jones, P. B. Bush and P. A. Banks. 1991.  Comparison
of  PRZM and GLEAMS Computer Model Predictions with Field Data for
Alachlor, Metribuzin, and Norflurazon Leaching. Environmental Toxicity
and Chemistry 11:427-436.

Durburow, T. E., N. L. Barnes, S. Z. Cohen, G. L.  Horst, and A. E.
Smith. 2000.  Calibration and Validation of Runoff and Leaching Models
for Turf Pesticides, and Comparison with Monitoring Results in Fate and
Management of Turfgrass Chemicals editors J. Marshall Clark and Michael
P. Kenna.  ACS Symposium Series 743.  ACS, Washington D.C.

Parrish, R. S., C. N. Smith and F. K. Fong.  1992.  Tests of Pesticide
Root Zone Model and the Aggregate Model for Transport and Transformation
of Aldicarb, Metolachlor, and Bromide. J Environ. Qual. 21: 685-697     

Pennel, K. D.,  A. G. Hornsby, R.E. Jessup, and P. S. C. Rao. 1990
Evaluation of Five Simulation Models for Predicting Aldicarb and Bromide
Behavior Under Field Conditions. Water Resources Research.  26(11):
2679-2693.  

Ma, Q.L., Wauchope, R.D., Hook, J.E.; Johnson, A.W., Truman, C.C.,
Dowler, C.C., Gascho, G.J.; Davis, J.G., Sumner, H.R., Chandler, L.D.
1998. GLEAMS, Opus, and PRZM-2  model predicted versus measured runoff
from a coastal plain loamy sand. Transactions of the ASAE.1998, 41: 1,
77-88. 

Mueller, T.C. (1994): Comparison of PRZM computer model predictions with
field lysimeter data for dichlorprop and bentazon leaching. Journal of 

Environmental Science and  Health. Part A, Environmental Science and
Engineering. 1994, 29: 6, 1183-1195.  

Sadeghi, A.M.; Isensee, A.R.; Shirmohammadi, A. (1995): Atrazine
movement in soil: comparison of field observations and PRZM simulations.
Journal of Soil   Contamination.1995, 4: 2, 151-161.  

Industry efforts to validate the PRZM model:

FIFRA Environmental Modeling Validation Task Force.

PRZM/EXAMS evaluations to support risk assessments:

 

FIFRA Science Advisory Panel Meeting, December 10-11, 1997. Use of Water
Models in FQPA Drinking Water Assessments. Crystal City, VA

  

FIFRA Science Advisory Panel Meeting,  July 29-30, 1998.  Proposed
Methods for Basin-scale Estimation of Pesticide Concentrations in
Flowing Water and Reservoirs for Tolerance Reassessment. Crystal City,
VA.

The registrant indicates PRZM/EXAMS modeling is not accurate because
methyl parathion detection frequencies from USGS studies are lower than
detection frequencies predicted from modeling.  Additionally, they
believe the dissipation rate from PRZM simulations are different than
observed in the field.  Therefore, they believe the model is not
accurately predicting all dissipation pathways for methyl parathion.  

The Agency believes the PRZM/EXAMS and their use in FIFRA and FQPA risk
assessments have been extensively peer-reviewed.  The PRZM/EXAMS model
approach has been generally accepted as an appropriate refined screening
level model for FIFRA and FQPA risk assessments.  The comparison of
model predicted concentrations with ambient water monitoring data such
as USGS/NAWQA requires a complete understanding of the targeted nature
of the monitoring relative to pesticide use site and pesticide use
timing.  The Agency expects the PRZM/EXAMS models to over predict  peak
pesticide concentrations and detection frequencies when compared to
USGS/NAWQA  monitoring studies.  The Agency is willing to consider any
targeted field scale monitoring data for methyl parathion in  the risk
characterization for methyl parathion.   Differences in model predicted
and field dissipation rates may suggest the model is not accounting for
all dissipation pathways.  However, it may also indicate the variable
nature of field dissipation processes among fields.  

Incident data cited by EFED for aquatic organisms (page 18 of 40)

EFED Response:  Should a redraft of the ecological risk assessment be
warranted, the information presented would be considered in any
reevaluation of incident data.	

Terrestrial Organisms  Risk Assessment 

            1.  Limitations of the assessment (page 19 of 40)

EFED Response:  EFED acknowledges that the tools for risk assessment are
similar to the screening level assessment methods.  However, risk
management decisions involve multiple lines of evidence which are
incorporated in the document.  EFED can apply other tools (e.g.
probabilistic approaches) should risk managers decide this is necessary.

2. 	Use of data that do not adhere to the principals of Data Quality
Control Act. (page 20 of 40)

EFED Response: Re-evaluation of the Penwalt study could be performed
under the Agency’s current literature evaluation procedure.  Switching
to next highest dietary acute LC50 could be performed for RQ
calculations.  It would only make a difference to seeds RQ all other RQs
would still be above non-listed acute LOC.

Use of certain field studies (page 21 of 40)

EFED Response: Discussion of the relevance of these studies to current
labels should be included if a redrafting of the ecological risk
assessment is warranted.

Use of literature data concerning the sub-lethal effects of methyl
parathion on birds (page 21 of 40)

EFED Response: EFED discussed effects on avian maternal behavior and
provided both effects levels and indications of exposure (in bold) in
the following examples:

“Brewer et al. (1988; MRID 44371604) reported brood abandonment and
mortality among wood duck and teal hens in a field treated with 1.25 lb
ai/acre methyl parathion, but not in a control field. Two-thirds of the
nesting hens from the treated field had significantly depressed brain
cholinesterase levels.  Mortality among ducklings in the treated field
(84%) was greater than that in the control field (42%) by 22 days
post-spray.”	

“Kendall, et al. (1984; MRID 44413601) reported a 39% increase in
mortality among nesting starlings in a treated field.  Since this effect
did not correlate with ChE depression, the authors surmised that changes
in maternal behavior or depressed food abundance might have been to
blame. This same study reported nest abandonment by mallards and teals
adjacent to a field treated at 0.6 lbs ai/A.”

“Mineau (1991) reports that two-week old northern bobwhite quail did
not discriminate between untreated food and diets containing 45 or 90
ppm methyl parathion, and initially (0-24 hour post-dose) chose treated
over untreated food. This indicates that there will be little avoidance
of treated food sources.”

“Edwards (1968; MRID 00090488) noted that birds sprayed with 0.5 lb
ai/A of methyl parathion suffered a 20% weight loss shortly after the
spraying, but recovery was rapid.”

These data are presented to provide further context with regard to risks
that are based on the RQ calculations.

Conclusions about reproductive effects on birds (Page 21 of 40)

EFED Response:  While shell breaking strength was a sensitive endpoint
in the Bennett and Bennett (1990) study, egg production (eggs/hen/day)
was also a sensitive endpoint and was an observed effect in the northern
bobwhite quail reproduction study used in the risk assessment.  Egg
production data in Bennett and Bennett (1990) show a NOEC of 14 ppm and
a LOEC of 20 ppm.  These concentrations compare closely with the NOEC
and LOEC values used in the risk assessment for RQ calculation (see the
following table).



Avian Reproduction Endpoints from the Registration Eligibility Document
Ecological Risk Assessment





Species/ 

Study Duration 	

% ai	

NOEC

 (ppm)	

LOEC

(ppm)	

LOEC

Endpoints 	

MRID No.

Author/Year	

Study Classification



Northern bobwhite quail

(Colinus virginianus)

	

Tech	

6.27	

15.5	

Number of eggs laid; eggs set/hen; adult female bodyweight	

41179302

Beavers/1988	

Core



Mallard duck

(Anas platyrhynchos)	

Tech	

14.7	

>14.7	

No effects at highest conc.	

41179301

Beavers/1988	

Supplemental



Cheminova  is correct that Bennett and Bennett (1990) did indicate that
the dose-related reproduction effects observed were apparently due to
dose-related reductions in food consumption.  However, Cheminova has
failed to mention that the authors also indicated that pesticide-induced
anorexia is “frequently observed in laboratory studies with
organophosphates and confounds the ability to determine whether are due
to chemical toxicity, reduced food consumption, a change in metabolic
efficiency due to chemical exposure, or a combination of reasons.”

Cheminova is correct that egg production effects in Bennett and Bennett
(1990) were observed by 3 days of methyl-parathion exposure.  Cheminova
appears to be arguing that a more protracted exposure period than the
single day modeled for the RQ calculations is warranted.  This argument
was considered in the context of the expected residue decline following
the maximum EEC estimate.  A 2.4-day foliar dissipation half-life and a
first order decline function were used to estimate the number of days
following maximum EEC for which residues of methyl-parathion would
exceed the reproduction NOEC of 6.27 ppm.  The following table presents
these estimates.  

It is important to note that for all single application and multiple
application scenarios modeled, methyl-parathion residues exceed the NOEC
for well over 3 days.  This is the time for the onset of egg production
effects in Bennet and Bennett (1990)) for at least 3 of the avian  food
groups modeled.  Even if a egg production NOEC of 14 ppm from the
Bennett and Bennett (1990) study was substituted for the NOEC used to
calculate RQs, the number of days residues would exceed this toxicity
threshold would still be greater than 3 days for almost all food groups
in almost all exposure scenarios modeled.  One should also recall that
the foliar dissipation decline does not account for the potential for
the highly toxic oxon moiety of methyl parathion to be formed and so
exposures estimated in this fashion are not likely to be conservative
representation of total toxic exposure. 

Avian Reproduction Risk Quotients for Single and Multiple Applications
for Major Use Crops and the Number of Days Following Maximum EEC Where
Exposure Would Equal or Exceed Reproduction NOEC (6.27 ppm) Based on a
2.4 Day Foliar Half-Life



	

	

	

	

Single Application	

Multiple Application



Site1 (# Apps, 

App. Interval in days)	

App.Rate 

(lbs ai/A)	

Food Items	

Maximum EEC (ppm)	

Reproduction

RQ

(EEC/

NOEC)	

Exposure days above NOEC	

Reproduction

RQ

(EEC/

NOEC)	

Exposure days above NOEC



Rice, Grasses

(6,3)	

0.79	

Short 

grass	

190	

30.3	

11.8	

181.8	

18.0





Tall

grass	

87	

13.9	

9.1	

83.4	

15.3





Broadleaf

plants/Insects	

107	

17.1	

9.8	

102.6	

16.0





Seeds	

12	

1.9	

2.2	

11.4	

8.4



Sunflower

(3,5)	

1	

Short

grass	

240

	

38.3	

12.6	

114.9	

16.4





Tall

grass	

110

	

17.5	

9.9	

52.5	

13.7





Broadleaf

plants/Insects	

135	

21.5	

10.6	

64.5	

14.4





Seeds	

15

	

2.4	

3.0	

7.2	

6.8



Soybean,

 (6,3)

Corn (all)

 (6,2)

	

1	

Short

grass	

240	

38.3	

12.6	

229.8	

18.8





Tall

grass	

110	

17.5	

9.9	

105.0	

16.1





Broadleaf

plants/Insects	

135	

21.5	

10.6	

129.0	

16.8





Seeds	

15	

2.4	

3.0	

14.4	

9.2



Alfalfa

(4,42)	

1	

Short

grass	

240	

38.3	

12.6	

153.2	

17.4





Tall

grass	

110	

17.5	

9.9	

70.0	

14.7





Broadleaf

plants/Insects	

135	

21.5	

10.6	

86.0	

15.4





Seeds	

15	

2.4	

3.0	

9.6	

7.8



Cotton

(10,3)

	

3	

Short

grass	

720	

114.8	

16.4	

1,148.0	

24.4





Tall

grass	

330	

52.6	

13.7	

526.0	

21.7





Broadleaf

plants/Insects	

405	

64.6	

14.4	

645.9	

22.4





Seeds	

45	

7.2	

6.8	

71.8	

14.8



Cheminova indicates that the  Meyers et al. (1990) study involving a
single oral dose exposure to methyl parathion would be the most
appropriate study to compare with the single day exposure assumption. 
However, it appears that the Meyers et al.  (1990) approach with a
single oral dose would not capture the more prolonged exposure profile
suggested by the above analysis.

Appropriate data for estimating dietary residues (page 22-30 of 40)

EFED Response: 

●  The field data indicating a rapid decline of methyl parathion in
food items has been incorporated in into the terrestrial wildlife risk
assessment, where default foliar dissipation half-life assumption of 35
days has been replaced by a half-life of 2.4 days.

●  The nomogram is only dependent upon application rate.  The residue
modeling that incorporates the nomogram (e.g. FATE and ELFATE) also
considers application number, application interval, and foliar
dissipation.  The comment is correct that, should higher tier
assessments be conducted, additional information could be considered
with respect to other factors related to application, and the use of
such information would be dictated by its quality.

●  While the Agency agrees that understory plants and the insects
residing on them may be important food sources for some species, they do
not constitute the only food sources within an agroecosystem.  Seeds and
fruit present on plant stalks and insects in the upper canopy could also
be important food sources for avian wildlife that is adapted to a
surface gleaning feeding guild.  Indeed, the environment immediately
adjacent to a treated crop, receiving much the same application rate as
the crop itself, is likely to be important feeding habitat for
edge-residing wildlife.  In such cases the effects of crop canopy
interception would not be as extensive as directly on the field.  The
Agency agrees that, should a higher Tier assessment be performed with
methyl-parathion, feeding habits, canopy effects and
timing漠⁦灡汰捩瑡潩⁮潣汵⁤敢洠牯⁥畦汬⁹癥污慵整
Ɽ椠⁦慤慴漠⁮敲楳畤獥‬楷摬楬敦戠桥癡潩Ⱳ愠摮愠
牧湯浯捩瀠慲瑣捩獥愠敲愠慶汩扡敬മ

●  The Agency agrees that reducing daily consumption of
methyl-parathion contaminated dietary items would reduce daily exposure.
 However, given the high reproduction RQs from the existing assessment,
pesticide-contaminated dietary items would have to be assumed to
comprise less than 1% of the diet for there not to be a dietary risk. 
The commenter suggests that data are available showing avian and
mammalian avoidance of methly-parathion.  Without a reference the Agency
cannot follow up this assertion and indeed, assuming such behavior would
have to involve near total avoidance for it to reduce exposure levels
below reproduction risk levels of concern.

●  The Agency agrees that the likelihood that a bird will consistently
feed in areas with the highest residues decreases with the number of
days of exposure modeled.  However, the risk assessment has demonstrated
that exposure times for both lethal and reproduction effects with methyl
parathion are very short.  Of course the full distribution of potential
combinations of daily exposures over a variety of exposure windows could
be explored by probabilistic methods, should a refinement to the
existing risk assessment be warranted.

●  EFED has incorporated the referenced data into its refined risk
assessment approaches.  Should a reevaluation of the ecological risk
assessment be warranted, these data could be used in the context of
these refined exposure assessment tools.

Estimates of dietary exposure of methyl parathion to mammals (Page 30 of
40)

EFED Response: EFED used the standard dietary ingestion rate assumptions
in this risk assessment.  To determine the impact of the alternative
values presented in the comments, EFED looked at the existing mammal 
RQs for 15 and 35 g mammals and reduced them by a factor reflecting the
difference between standard intake assumptions and alternatives in the
comments.  Data for the prairie vole from the comment was excluded in
this analysis, as the study involved laboratory feeding of the animals
with dried oats and dry grass not fresh material as is modeled for the
risk assessment.  Cited data for the deer mouse were also excluded as
the intake numbers reflect laboratory consumption of dry rat chow not
natural food items  In no comparison of standard assumption to
alternative ingestion rates from acceptable data cited in the comments
is the difference more than a factor of 2 (e.g, 15 g insectivore 95% vs
49%, 15 g seed eater 21% versus 18%, 35 g 66% vs 30% ).  Such a factor
would not reduce the RQs below acute risk levels of concern for any 15
or 35g mammals except for 35g granivores on for three use scenarios
modeled: Grasses rice 0.79 lb/acre, field/sweet corn 1 lb/acre,
alfalfa/barley/oats/rye/wheat 1.25 g/acre.  It should be noted that no
multiple application scenarios were modeled, so it is possible that
multiple applications for the above use scenarios (to the extent that
the label allows) could still exceed the acute risk LOC.

●  NRDC Comment 1

 

. . . we find that the half-life for methyl parathion in aquatic media
is 175 days and the half-life for methylparathion in soil is anywhere
from 10 days to two months (PAN-UK 1998). These half-lives for both
aquatic and terrestrial media are not all that short and still result in
what we believe to be significant opportunities for exposure in humans,
fish and wildlife and the environment. After all, you have to keep in
mind that the acute toxicity of methyl parathion is so high, and uptake
is so

efficient (oral, dermal, and inhalation or any combination thereof) that
in many instances it only takes one exposure to kill an organism.

EFED Response:  For aquatic exposure modeling EFED used environmental
fate property values in accordance with established procedures.  The
acute effects risk assessment for aquatic organisms is based on an
estimated single day exposure.  Similarly, both acute lethality and
reproduction risk assessments for terrestrial vertebrates are based on a
peak single day exposure.

●  NRDC Comment 2

. . . the initial degradation (and metabolic) product is methyl
paraoxon, a compound that is considered at least three orders of
magnitude MORE TOXIC than the parent compound methyl parathion due to
its incredible cholinesterase-inhibiting abilities.

EFED Response:  Available dietary residue studies do not show
significant detections of the paraxon in crops treated in the field with
methyl parathion.  Similarly, while finished drinking water may show
formation of the oxon, raw surface waters do not.

●  NRDC Comment 3

In amphibians, Johnson and Prine (1976) found that exposure to only 25
μg/l methylparathion over 24 hrs caused juvenile western toads (Bufo
boreas) to lose their tolerance to temperature, a sublethal effect that
likely becomes lethal rather rapidly. Methyl parathion (Folidol M50
formulation) contained in simulated rice paddy overflow water was found
to cause mortality in giant toads (Bufo marinus) at levels as low as
280μg/l (Calumpanget al. 1997). In another study, frogs exposed to 1.0
mg/l methyl parathion suffered changes in the composition of their
connective tissue matrices, which  resulted in malformations of the
spinal column (scoliosis) and/or limbs (short and think long bones  with
the epiphyses grossly twisted; Alvarez et al. 1995). Further, a study by
Fleming et al. (1982) showed that parathion actually bioaccumulated in
cricket frogs (Acris crepitans), thereby posing a serious secondary
poisoning hazard to American kestrels (Falco sparverius). Studies by
Hall and Kolbe (1980) and Hall (1990) further document bioaccumulation
of parathion and other organophosphorus insecticides in amphibians,
which is cause for great concern, particularly for amphibian predators
that are likely to die from secondary poisoning. 

EFED Response:  Discussion of the relevance of these studies to
assessing risks to amphibians and their predators should be included if
a redrafting of the ecological risk assessment is warranted.  The extent
to whcic such data are quantitatively incorporated in any revised risk
assessment would be based on a review of the quality of each study.

●  NRDC Comment 4

h”

$

Ø	@

”

Ð

Ñ

Ý

ï

ÿ

Ò

Õ

h”

”

•

–

—

˜

™

š

›

œ

Ð

Ñ

î

ï

Ô

Õ

&

&

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

摧ᐻ£

 is commonly used in other countries (e.g.,Venezuela) as an avicide to
intentionally kill songbirds. The work of Basili and Temple (1999) has
documented huge mass mortality events involving dickcissels (Spiza
americana) that flock in agricultural areas. These international
mortality incidents cannot be ignored and must be included in any
ecological risk assessment of methyl parathion.

EFED Response:   EFED has documented the avian incident data available
at the time of the drafting of the risk assessment.  If other data are
pertinent to the application rates and methods for methyl parathion in
the United States, this information should be considered in any
warranted redrafting of the risk assessment.

●  NRDC Comment 5

In addition to lethal effects, there are a wide range of debilitating
sublethal effects seen following exposure to methyl parathion. Both
methyl parathion and its closely related chemical analog ethyl parathion
have been found to cause reproductive, physiological, biochemical, and
behavioral effects that ultimately lead to premature mortality in
exposed organisms.  Reproductive effects such as decreased productivity
(% eggs hatching/% nests successful) have been documented in songbirds
exposed to parathion. Physiological effects such as reduced daily food
consumption (anorexia) and decreased body temperature (hypothermia) have
been documented in songbirds and upland game birds exposed to parathion.
Behavioral effects may be the most insidious and damaging of the
sublethal effects. Decreased ability to avoid predators, decreased nest
attentiveness during incubation and maternal care following hatching,
and increased brood abandonment by females have been documented in
upland gamebirds, gulls, and waterfowl (King et al. 1984, White et al.
1979, 1983).

EFED Response:   The avian risk assessment has included discussions of
the potential of methyl parathion to adversely affect birds through
effects on egg production, maternal behavior, anorexia, and increases in
sensitivity as a result of exposure
to漠桴牥攠癮物湯敭瑮污猠牴獥潳獲‮䤠⁴獩椠灭牯慴
瑮琠⁯潮整琠慨⁴桴⁥楲歳愠獳獥浳湥⁴污潳焠慵瑮瑩
瑡癩汥⁹楤捳獵敳⁳潣据牥獮映牯氠瑥慨楬祴愠摮映慲
歮爠灥潲畤瑣潩⁮晥敦瑣⁳獡椠摮捩瑡摥戠⁹桴⁥兒挠
污畣慬楴湯⁳湡⁤潣灭牡獩湯⁳楷桴攠瑳扡楬桳摥䰠癥
汥⁳景䌠湯散湲മ

●  NRDC Comment 6

 

In mammals, exposure to the OP insecticide methyl parathion has been
found to result in loss of motor coordination in bats (Clark 1986),
reduced predator escape response (Galindo et al. 1985), altered hearing
ability in squirrel monkeys (Reischl et al. 1975), a decreased ability
to learn (Reiter et al. 1973), and suppression of the immune system
(Street and Sharma 1975). In rats, Zhu et al. (2001) found that repeated
dermal exposures of as little as 1 mg/kg/day methyl parathion resulted
in sustained inhibition of cholinesterase activity and impairment of
both motor function and memory. These subtle yet insidious sublethal
effects are never monitored in wildlife species, so they almost always
go undetected unless morbid animals are found by accident. It seems that
we often miss these types of sublethal effects in humans even when we
are looking hard for them, so it is clear that we are barely seeing the
tip of the iceberg when it comes to negative impacts of methyl parathion
on ourselves and our environment.

EFED Response:   The mammalian wildlife risk assessment identifies
concerns for acute lethal and reproduction risks for methyl parathion
for all uses assessed.  While other sublethal effects may be evident it
is unclear how this information would alter the existing conclusions of
the mammalian wildlife risk assessment.

●  NRDC Comment 7

f both EC and microencapsulated (ME) formulations. The LD50s for each
formulation (0.111 μg/bee and 0.214μg/bee, respectively) indicate very
high toxicity to bees. The ecosystem service that bees and other 
pollinators provide to humans is invaluable and many of our most
important food crops require  pollination by bees and other organisms
that continue to decline in the face of our continued heavy reliance on
pesticides.

EFED Response:  The ecological risk assessment includes a discussion of
the impacts of various formulations of methyl parathion on pollinators. 

