-2 of 41-

1) Aerobic soil metabolism (OCSPP guideline # 835.4100). Though there is acceptable 

aerobic soil metabolism data for one type of soil, the guideline stipulates that results from 

at least four relevant soils of different organic carbon content, pH, clay content, and 

microbial biomass must be submitted in order to characterize the variability in the aerobic 

biodegradation of Pryithiobac-Na in the environment. 

2) Aerobic aquatic metabolism (OCSPP guideline # 835.4300). There are no acceptable 

aerobic aquatic metabolism studies. This study is necessary in order to describe the fate 

of Pyrithiobac-Na in surface water bodies down-gradient from use sites (e.g., rice 

paddies). The guideline stipulates that at least two different soil:water systems be 

submitted that are similar to the potential use sites in the United States. There is currently 

one supplemental study that lacks information on the aerobicity of the soil:water system.

3) Anaerobic aquatic metabolism (OCSPP guideline # 835.4400). Submission of an 

acceptable anaerobic aquatic metabolism study is necessary to understand the degradation 

of Pyrithiobac-Na in oxygen-limiting environments.

4) Environmental chemistry methods in water being reviewed by EFED. However, 

definitive conclusions cannot be made until the final report is reviewed which will be 

done prior to completion of the draft risk assessment. Although these studies have been 

submitted to EPA they will be considered data gaps until the data have been reviewed 

and found to be acceptable.

5) An independent laboratory validation (ILV) report for Environmental chemistry method 

in water is needed.

The available effects data are considered incomplete for the purposes of risk assessment. The 

following data gaps exist:

1) Avian acute oral toxicity (OCSPP guideline # 850.2100). Avian acute oral toxicity data 

are not available for passerines, which are required under the new 40 CFR Part 158. 

2) Seedling emergence (OCSPP guideline # 850.4100). There is no acceptable tier II data 

for seedling emergence. The new 40 CFR Part 158 requires the use of typical end use 

product (TEP), while the submitted study used the technical grade active ingredient 

(TGAI). Without data on the effects of the TEP on seedling emergence, the data are not 

sufficient for risk assessment. Herbicidal active ingredients are usually formulated with 

adjuvants or surfactants that enhance the activity of the pesticide. Therefore, the 

sensitivity of terrestrial plants to Pyrithiobac-Na may be underestimated. 

3) Vegetative vigor (OCSPP guideline # 850.4150). There is no acceptable tier II data for 

vegetative vigor. The new 40 CFR Part 158 requires the use of typical end use product 

(TEP), while the submitted study used the technical grade active ingredient (TGAI). 

Without data on the effects of the TEP on vegetative vigor, the data are not sufficient for 

risk assessment. Herbicidal active ingredients are usually formulated with adjuvants or 

surfactants that enhance the activity of the pesticide. Therefore, the sensitivity of 

terrestrial plants to Pyrithiobac-Na may be underestimated. 

Additional detail on the current data gaps for environmental fate and effect is provided in Section 

8.0.
-3 of 41-

Problem Formulation for the Environmental Fate and 

Ecological Risk, Endangered Species, and Drinking 

Water Assessments in Support of the Registration 

Review of Pyrithiobac-Na

Sodium 2-chloro-6-[(4,6-dimethoxypryrimidin-2-yl0thio]benzoate

CAS Registry Number: 123343-16-8

PC Code: 078905

Prepared by:

Elizabeth Riley, Biologist

Ibrahim Abdel-Saheb, Ph.D., Environmental Scientist U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch VI

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460

Reviewed by:

Mark Corbin, Branch Chief

William P. Eckel, Ph.D., Senior Science Advisor

Mary Frankenberry, Ph.D., Statistician

Stephanie Syslo, RAPL
-4 of 41-

May 25, 2011

1. Purpose

The purpose of this problem formulation is to provide an understanding of what is known about 

the environmental fate and ecological effects of the registered uses of Pyrithiobac-Na (PC Code 

078905). Pyrithiobac-Na is a selective pre- and post-emergent herbicide. This document will 

provide a plan for analyzing data relevant to Pyrithiobac-Na and for conducting environmental 

fate and ecological risk, endangered species, and drinking water 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 

risk associated with the registered uses of Pyrithiobac-Na. 

2. Problem Formulation

2.1. Nature of Regulatory Action

Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), all pesticides distributed 

or sold in the United States generally must be registered by the Environmental Protection 

Agency (EPA). As part of determining whether a pesticide can be registered in the U.S., the 

Agency evaluates its safety to non-target species based on a wide range of environmental and 

health effects studies. In 1996, FIFRA was amended by the Food Quality Protection Act, and the 

Agency was mandated to implement a new program for the periodic review of pesticides, i.e., 

Registration Review (http://www.epa.gov/oppsrrd1/registration_review/). The Registration 

Review program is intended to ensure that, as the ability to assess risk evolves and as policies 

and practices change, all registered pesticides continue to meet the statutory standard of no 

unreasonable adverse effects to human health and the environment. Changes in science, public 

policy, and pesticide use practices occur over time. Through the new Registration Review 

program, the Agency periodically reevaluates pesticides to make sure that as change occurs, 

products in the marketplace can be used safely. 

As part of the implementation of the Registration Review program pursuant to Section 3(g) of 

the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), the Agency is beginning its 

evaluation of Pyrithiobac-Na to determine whether it continues to meet the FIFRA standard for 

registration. This problem formulation for the environmental fate, ecological risk, endangered 

species, and drinking water assessment chapter in support of the Registration Review will be 

posted in the initial docket, which will open the public phase of the review process.

2.2. Previous Risk Assessments

The Most recent ecological risk assessment was conducted in 1995 in support of PyrithiobacNa’s initial registration (New Chemical Review of Environmental Fate and Transport Data for 

the Herbicide Pyrithiobac-Na (DPX-PE350, STAPLE®) on Cotton, 1995, DP Barcodes 215858, 
-5 of 41-

215730, 208476, and 208481 and New Chemical Review of Environmental Effects Data for the 

Herbicide Pyrithiobac-Na (DPX-PE350, STAPLE®) on Cotton, 1995, DP Barcode 208477 and 

208482).

Review of the environmental fate data determined that there were some concerns regarding the 

use of Pyrithiobac-Na on cotton because the submitted field and laboratory studies suggest it is 

mobile and slightly to moderately persistent. Downward transport of Pyrithiobac-Na may occur 

in certain environments where it could potentially leach to ground water (i.e., coarse textured 

soils, shallow depth to ground water) with surplus rainfall or irrigation.

The Agency recommended the proposed label to include the following language:

“This chemical demonstrates the properties and characteristics associated with chemicals 

detected in groundwater. The use of this chemical in areas with coarse textured soils 

where the water table is shallow may result in groundwater contamination”

“Application should not be made to field areas which are less than 100 feet from 

permanent natural bodies of surface water or reservoirs”

Based on the submitted field and laboratory studies that were suggesting mobility and 

persistence of Pyrithiobac-Na in the environment, in 1995 EFED asked for a prospective 

groundwater monitoring study to be conducted as a registration requirement.

Results from the small-scale prospective groundwater monitoring study conducted in North 

Carolina indicated that Pyrithibac-Na may not be stable enough to leach to ground water at most 

sites, even in sandy soils. This study appears to have been conducted at a site with groundwater 

highly vulnerable to contamination. However, it is not clear that the amounts and timing of water 

input were such that the leaching would be significantly more likely than in an average year at 

the test site.

Review of the environmental effects data determined that there were no risks to terrestrial and 

aquatic animals and invertebrates. However, data on the effects of Pyrithiobac-Na on avian 

reproduction were not available, so this data was requested to address the uncertainties. An 

acceptable avian reproduction study was submitted to the Agency in response to this Data Call In 

(DCI). 

This review also determined that use of Pyrithiobac-Na does pose a significant risk to terrestrial 

and aquatic vascular plants, which is expected from an herbicide. The reviewer suggested that 

mitigation measures be put in place for use of Pyrithiobac-Na, including: geographic restrictions 

on location and soil type, application rate reduction, reduction in the number of annual 

applications, minimizing drift, and establishment of buffer zones. 

In addition to mitigation measures to reduce impact of Pyrithiobac-Na on non-target plant 

species, the following labeling language was suggested:
-6 of 41-

End use product: “Do not apply directly to water, or areas where surface water is present 

or to intertidal areas below the mean high water mark. Do not contaminate water when 

disposing of equipment washwater or rinsate”

Manufacturing use: “Do not discharge affluent containing this product into lakes, 

streams, ponds, estuaries, oceans, or other waters unless in accordance with requirements 

of a National Pollutant Discharge Elimination System (NPDES) permit and the 

permitting authority has been notified in writing prior to discharge. Do no discharge 

effluent containing this product to sewer systems without previously notifying the sewage 

treatment plant authority. For guidance, contact your local State Water Board or Regional 

Office of the EPA.”

3. Stressor Source and Distribution

3.1. Mechanism of Action

The mode of action for Pyrithiobac-Na is inhibition of a specific plant enzyme, acetolactate 

synthase (acetohydoxyacid synthase AHAS), resulting in interference with normal branched 

chain amino acid biosynthesis, cell division, and plant growth. There is some evidence1

3.2. Overview of Pesticide Use and Usage

to 

suggest plant reproduction may be affected by ALS inhibitors, such as sulfonylurea herbicides, at 

levels below effects on vegetative growth or visual injury (Fletcher et al., 1993). Pyrithiobac-Na 

can terminate both monocotyledonous and dicotyledonous plant species following application at 

relatively low rates (35-140 g/ha) and has demonstrated a high selectivity for cotton plants 

(Shimizu et al. 1994). 

Pyrithiobac-Na is a selective pre- and postemergent herbicide used to control a variety of 

broadleaf weeds in cotton fields. Pyrithiobac-Na is also registered for use on kenaf in three 

states under a section 24c special local needs registration. The registered end-use formulations of 

Pyrithiobac-Na include a water soluble powder and soluble-concentrate. Pyrithiobac-Na is 

applied by aerial or ground spray, broadcast, band, and spot treatment. Use practice limitations 

prohibit applying or allowing Pyrithiobac-Na to drift to areas where endangered or desired plant 

species exist and discharging into bodies of water.

There are currently six Section 3 registered products as well as three SLN registrations for 

Pyrithiobac-Na (Table 3.1). 

TABLE 3.1. Summary of Registered Products Containing Pyrithiobac-Na in the U.S.

Registration # Name Company Name Percent Pyrithiobac

352-575 DUPONT KIH-2031 

(DPX-PE350) 

TECHNICAL

E. I. DU PONT DE NEMOURS AND CO., INC 96.4

 1 Snipes et al. 1992, Bansal et al. 1999; Fletcher et al. 1993, 1995,1996; Bhatti et al. 1995; Al-Khatib et al. 1992; 

Boutin et al. 2000; Asghari and Evans 1992
-7 of 41-

Registration # Name Company Name Percent Pyrithiobac

352-576 DUPONT STAPLE 

HERBICIDE E. I. DU PONT DE NEMOURS AND CO., INC 85

352-613 DUPONT STAPLE 

LX HERBICIDE E. I. DU PONT DE NEMOURS AND CO., INC 33.6

66222-175 PYRIMAX 3.2 L 

HERBICIDE

MAKHTESHIM-AGAN OF NORTH 

AMERICA INC 32.88

83558-11

PYRITHIOBACSODIUM 

TECHNICAL

CELSIUS PROPERTY B.V., AMSTERDAM 

(NL) 96.5

85588-11 PYSONEX 

HERBICIDE AGSURF CORPORATION 33.6

NC030002 DUPONT STAPLE 

HERBICIDE

E. I. DU PONT DE NEMOURS AND CO., INC 85

OK970001 DUPONT STAPLE 

HERBICIDE

E. I. DU PONT DE NEMOURS AND CO., INC 85

TN050005 DUPONT STAPLE 

HERBICIDE

E. I. DU PONT DE NEMOURS AND CO., INC 85

A summary of currently registered Pyrithiobac-Na uses, along with their respective methods and 

rates of application is presented in Table 3.2. For two registered uses listed below (registration 

numbers 352-613 and 66222-175), there is a discrepancy recorded on the label between the 

number of applications that can be made in one year and the application interval. Based on the 

label’s seasonal single application rate and application interval, the number of applications 

should be equal to 2.4 and not 1/yr as indicated on label.

Table 3.2. Registered Use Patterns for Pyrithiobac-Na.

Use Pattern

(Reg. No)

Single App. 

Rate

(lbs ai/A)

Number of 

App.

Seasonal 

App. Rate 

(lbs ai/A)

App. 

Interval 

(days)

Application 

Method Comments

Cotton

(352-576) 0.095625 Not specified 0.1275 7

Aerial, ground

and band

spray

Pre- and postemergence

Cotton

(352-613) 0.05292 1/year 0.12852 7 Ground spray Pre-emergence 

Cotton

(352-613) 0.09576 Not specified 0.12852 7

Aerial, ground 

and band 

spray

Post-emergence

Cotton

(66222-175) 0.05259516 1/year 0.12773109 -- Ground Pre-emergence

Cotton

(66222-175) 0.09517219 Not specified 0.12773109 Not 

specified

Aerial, ground 

and band 

spray

Pre- and postemergence
-8 of 41-

3.3. Environmental Fate and Transport

As stated previously, Pyrithiobac-Na is an herbicide used for weed control on cotton. 

At this time, the following data requirements for Pyrithiobac-Na have been satisfied:

835.2120- Hydrolysis 

835.2240- Photodegradation in Water 

835.2410- Photodegradation on Soil 

835.4200- Anaerobic Soil Metabolism'

835.1230/1240- Mobility (Leaching/Sorption)

835.6100- Terrestrial Field Dissipation 

For details, see Table 8.1.

Pyrithiobac-Na

Empirical Formula: C13H11CIN2NaO4S

CAS Registry No.: 123342-93-8

PC Code: 078905

Pyrithiobac-Na is a pale yellow powder with a faint ester-like odor and a melting point of 233-

234oC. Pyrithiobac-Na solubility varies with pH and ranges from 117-728 mg/L water. 

Table 3.4 summarizes the submitted environmental fate data for Pyrithiobac-Na. Additional 

characterization of the submitted environmental fate data is located in Appendix A.

Kenaf

(NC030002)

(TN050005)

0.06375 Not specified 0.1275 Not 

specified Foliar Post-emergence
-9 of 41-

Parameter Value Reference (MRID #)

Physical/Chemical Parameters

Molecular mass (g/mol) 348.74 Footprint Pesticide Properties 

Database available @ 

http://sitem.herts.ac.uk/aeru/foo

tprint/en/

accessed on April 12, 2011.

Vapor pressure (mmHg @ 25°C) 4.80 X 10-06

Henry’s Law Constant (25oC) 9.44 X 10-16

Water solubility (g/L @ 20°C) Varies with pH from 117 to 

690 mg/L MRID# 42856907

Octanol-water partition coefficient (KOW) 4.02 (pH 5)

0.15 (pH 7)

0.06 (pH 9)

MRID# 42856907

Dissociation constant (pKa) (20oC) 2.34 ±0.21 MRID# 42856907

Mobility

Organic carbon partitioning coefficient (KOC, L·kg-1) 16 (sandy loam)

13.9 (silt loam)

12(silt loam)

27.1 (silty clay loam)

42909211

Small-scale Prospective Groundwater Monitoring 

Study

Final results are under 

review

43530102

Persistence in Water

Hydrolysis half-life (25°C, pH 5, 7, & 9) Stable 42856945

Aqueous photolysis half-life (25°C) 9-15 days 42856946

Aerobic aquatic metabolism half-life (20oC) No data available Based on the Terrestrial NonFood use pattern, this data 

requirement is applicable. 

Aerobic aquatic metabolism 

data are not available, which 

are required under the new 40 

CFR Part 158. Therefore, this 

is identified as a data gap.

Anaerobic aquatic metabolism half-life (20oC) No data available Based on the Terrestrial NonFood use pattern, this data 

requirement is applicable. 

Anaerobic aquatic metabolism 

data are not available, which 

are required under the new 40 

CFR Part 158. Therefore, this 

is identified as a data gap.

Persistence in Soil

Soil photolysis half-life (25°C) 30.7-55.0 days (silt loam) 42856947

Aerobic soil metabolism half-life (25°C) † 60 days 42856948

Anaerobic soil metabolism half-life (25 oC) 128 days 43455601
-10 of 41-

Parameter Value Reference (MRID #)

Field Dissipation

Terrestrial field dissipation half-life; maximum 

leaching depth (cm)

46 (CA); 60-75 cm

11 (TX); 15 cm

43455603

43317212

Aquatic Bioconcentration

Bioconcentration Factor (BCF) in fish Assumed negligible based on 

low Kow (≤ 4.0)

Dietrich, R. F. 1993. Waiver Request: 

Fish accumulation potential for JSIH203 1

(DPX-PE350). Du Pont Document No. 

AMR 2728-93. Submitted by E.I. du 

Pont de

Nemours and Company, Wilmington, 

DE

Waiver request accepted

 3.3.5. Bioaccumulation

There are no data on the bioconcentration potential of Pyrithiobac-Na in fish. The octanol-water 

partition coefficient is sufficiently low, a value of ≤ 4 at 20 o

C, that bioaccumulation in aquatic 

organisms is not expected. 

4. Receptors

Consistent with the process described in the Overview Document (USEPA 2004), the risk 

assessment for Pyrithiobac-Na relies on a surrogate species approach. Toxicological data 

generated from surrogate test species, which are intended to be representative of broad 

taxonomic groups, are used to extrapolate the potential effects on a variety of species (receptors) 

included under these taxonomic groupings. 

Acute and chronic toxicity data from studies submitted by pesticide registrant, along with the 

available open literature, will be used to evaluate the potential direct and indirect effects of 

Pyrithiobac-Na to aquatic and terrestrial receptors. This includes toxicity on the technical grade 

active ingredient, degradates, and when available, formulated products (e.g., “Six-Pack” studies). 

Open literature studies were identified through EPA’s publicly available ECOTOXicology 

(ECOTOX) database (April 2011), which employs a literature search engine for locating 

chemical toxicity data for aquatic and terrestrial plants and wildlife. The evaluation of both 

sources of data may also provide insight into the direct and indirect effects of Pyrithiobac-Na

usage 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 brief summary of the aquatic and terrestrial toxicity data available for the species most 

sensitive to Pyrithiobac-Na is provided in Sections 4.1 and 4.2, respectively. A more complete 

summary of the available data is presented in Appendix B. In addition, a summary of ecological 

incidents associated with Pyrithiobac-Na and a description of ecosystems potentially at risk are 

provided in Sections 4.3 and 4.4, respectively. 
-11 of 41-

Toxicity to fish and aquatic invertebrates is categorized using the system shown in Table 4.1

(US EPA, 2004). Toxicity to terrestrial fauna (birds and mammals) is categorized using the 

system shown in Table 4.2. Toxicity categories for plants have not been defined.

Table 4.1. Categories of Acute Toxicity for Aquatic Animals

LC50 (ppm) Toxicity Category

< 0.1 Very highly toxic

> 0.1 - 1 Highly toxic

> 1 - 10 Moderately toxic

> 10 - 100 Slightly toxic

> 100 Practically nontoxic

Table 4.2. Categories of Acute Toxicity for Terrestrial Animals

LD50 (mg/kg) LC50 (ppm) Toxicity Category

<10 <50 Very highly toxic

10-50 50-500 Highly toxic

51-500 501 - 1000 Moderately toxic

501-2000 1001 - 5000 Slightly toxic

>2000 >5000 Practically nontoxic

4.1. Effects to Aquatic Organisms

The aquatic toxicity data from registrant-submitted studies are provided in Table 4.3. There are 

several studies available in ECOTOX which will be reviewed for the risk assessment. Based on 

the available information, Pyrithiobac-Na is classified as practically nontoxic to fish and aquatic 

invertebrates on both an acute and chronic exposure basis. Both vascular and nonvascular aquatic 

plants are affected by Pyrithiobac-Na.
-12 of 41-

Table 4.3. Aquatic Toxicity Profile for Pyrithiobac-Na

Taxonomic Group Species

(Common Name)

Endpoint 

(mg ai/L)

Acute Classification/ 

Chronic Effect

Reference

(MRID) 

Freshwater Fish

Lepomis 

macrochirus 

(Bluegill Sunfish)

LC50>930 Practically nontoxic 42856914

Oncorynchus 

mykiss

(Rainbow Trout)

LC50>1,000 Practically nontoxic 42856915

Ictalurus punctatus

(Channel Catfish) LC50>970 Practically nontoxic 43317204

Pimephales 

promelas 

(Fathead Minnow)

LOEC>100

NOEC=100

MATC>100

None 43317207

Freshwater 

Invertebrates

Procambarus 

clarkia

(Crayfish)

EC50>910 Practically nontoxic 43317206

Daphnia magna

(Waterflea ) EC50>100 Practically nontoxic 42856916

Waterflea

LOEC=500

NOEC=260

MATC=360

Survival 43317208

Estuarine/marine Fish

Cyprinodon 

variegatus 

(Sheepshead 

Minnow)

LC50>145 Practically nontoxic 42856917

Estuarine/marine 

Invertebrates

Crassostrea 

virginica 

(Eastern Oyster)

LC50>130 Practically nontoxic 42856919

Mysidopsis bahia

(Mysid Shrimp) LC50>140 Practically nontoxic 42856918

Mysid Shrimp LOEC>240

NOEC=240 none 43530101

Vascular plants Lemna gibba

(duckweed)

EC50=0.92 μg 

ai/L

NOAEC=0.27 

μg ai/L

--b 42611701

Nonvascular plants

Navicula 

pelliculosa

EC50=10.3 

NOAEC=6.3 -- 42560301

Selenastrum 

capricornutum

EC50=95 μg ai/L

NOAEC=22 μg 

ai/L

-- 42909204

Skeletonema 

costatum

EC50=63

NOAEC=32 -- 43577501

Anabaena flosaquae

EC50=1178 μg 

ai/L

NOAEC=71 μg 

ai/L

-- 42909205

a

No data available

b Toxicity categories for plants have not been defined.
-13 of 41-

4.2. Effects to Terrestrial Organisms

Table 4.3 summarizes the terrestrial toxicity endpoints for Pyrithiobac-Na, based on an 

evaluation of the submitted data. Open literature studies have been identified through ECOTOX 

and will be reviewed for the risk assessment. Pyrithiobac-Na is classified as slightly toxic to 

birds on an acute oral exposure basis. Avian reproduction data shows that Pyrithiobac-Na has an 

effect on both the number of viable embryos (30% decrease) and hatchling weight (5% 

decrease). Pyrithiobac-Na is classified as practically nontoxic to small mammals on an acute oral 

exposure basis. Pyrithiobac-Na is classified as practically nontoxic to young adult honey bees 

(Apis mellifera) on an acute contact exposure basis. 

As expected with an herbicide, Pyrithiobac-Na has adverse effects on plants. The studies were 

conducted with technical Pyrithiobac-Na rather than typical end use product (TEP) as required 

under 40 CFR Part 158. Herbicidal active ingredients are usually formulated with adjuvants or 

surfactants that enhance the activity of the pesticide. Therefore, the sensitivity of terrestrial 

plants to formulated Pyrithiobac-Na may be underestimated. 

Table 4.3 Terrestrial Toxicity Profile for Pyrithiobac-Na

Taxonomic 

Group

Surrogate Species

(Common Name) Endpoint Acute Classification/ Chronic effect Reference (MRID)

Birds

Colinus 

virginianus

(Northern 

Bobwhite)

LD50=1599 mg 

ai/kg bw Slightly toxic 42856911

Northern Bobwhite

NOEC=60 mg 

ai/kg 

LOEC=300 mg 

ai/kg

Number of viable embryos 43811101

Anas 

platyrhynchos

(Mallard Duck)

LC50>5620 mg 

ai/kg diet Practically nontoxic 42856913

Mallard Duck

NOEC=300 mg 

ai/kg 

LOEC=1500 mg 

ai/kg

Reduced hatchling weight 43811102

Mammals Rattus norvegicus

(Laboratory Rat)

LD50=3200 mg 

ai/kg bw Practically non toxic 42856920

Terrestrial 

Invertebrates Apis mellifera

(Honey bee)

LD50>25 μg/bee

NOAEC=25 

μg/bee

Practically nontoxic 42856944

Seedling 

emergence

Sorghum bicolor

(Sorghum

EC25=2.63 x 10-4

lb ai/A

EC05a=7.5 x 10-6

lb ai/A

--b 42856943
-14 of 41-

Taxonomic 

Group

Surrogate Species

(Common Name) Endpoint Acute Classification/ Chronic effect Reference (MRID)

Brassica napus

(Rape)

EC25=2.63 x 10-4

lb ai/A

NOAEC=2.44 x 

10-4 lb ai/A

-- 42856943

Vegetative vigor

Sorghum bicolor

(Sorghum)

EC25=4.34 x 10-5

lb ai/A

EC05a=1.25 x 10-5

lb ai/A

-- 42856943

Brassica napus

(Rape)

EC25=8.75 x 10-5

lb ai/A

EC05a=2.5 x 10-5

lb ai/A

-- 42856943

a

EC05 used in lieu of a reported NOAEC b Toxicity categories for plants have not been defined.

Although avian subacute and mammalian acute endpoints are nondefinitive values, preliminary 

calculation of RQs (not shown) indicate that the highest dose tested (e.g. 5000 mg aeai/kg-bw) 

are sufficiently higher than the EECs that further data are not needed at this time.

4.3. Incident Database Review

A review of the Ecological Incident Information System (EIIS) database for incidents involving 

Pyrithiobac-Na was conducted on May 13, 2011. One plant incident is reported in the database. 

Similarly, a search of the Avian Incident Monitoring System (AIMS; 

http://www.abcbirds.org/abcprograms/policy/pesticides/aims/aims/index.cfm), a database 

administered by the American Bird Conservancy which contains publicly available data on 

reported avian incidents involving pesticides did not report any incidents for Pyrithiobac-Na as 

of April 6, 2011. Pesticide registrants report certain types of incidents to the Agency as 

aggregate counts of incidents occurring per product per quarter. One aggregate plant incident 

has been reported to the Agency associated with Pyrithiobac-Na in an aggregated incident report 

from June, 2006. Although only one incident associated with the use of Pyrithiobac-Na has been 

reported to the EPA, the lack of incident reports cannot be construed as the absence of incidents 

as there can be many reasons why the Agency is not notified of adverse effects resulting from the 

use of a pesticide. Unless additional information on this 2006 aggregated incident report 

becomes available, it will be assumed to be due to a registered use of Pyrithiobac-Na in the risk 

assessment. 
-15 of 41-

4.4. Ecosystems Potentially at Risk

The ecosystems at risk are often extensive in scope; therefore, it may not be possible to identify 

specific ecosystems during the development of a nation-wide ecological risk assessment. 

However, in general terms, terrestrial ecosystems potentially at risk could include the treated 

field and immediately adjacent areas that may receive drift or runoff. Areas adjacent to the 

treated field could include cultivated fields, fencerows and hedgerows, meadows, fallow fields or 

grasslands, woodlands, riparian habitats and other uncultivated areas. 

Aquatic ecosystems potentially at risk include water bodies adjacent to, or downstream from, the 

treated field and could include impounded bodies such as ponds, lakes and reservoirs, or flowing 

waterways such as streams or rivers. For uses in coastal areas, aquatic habitat also includes 

marine ecosystems, including estuaries. 

Pyrithiobac-Na is mainly used on cotton. According to the USDA National Agricultural 

Statistics Service (NASS 2009-2010), cotton was grown in the following states: AL, AZ, AR, 

CA, FL, GA, KS, LA, MO, MS, NC, NM, OK, SC, TN, TX, and VA 

See Figure 1. Pyrithiobac-Na is also used on kenaf in the following states only: NC, OK, and 

TN. 
-16 of 41-

Figure 1. The 2002 estimated annual agricultural use of pyrithiobac-Na according to the USGSNAWQA data.

(http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m2020)

5. Assessment Endpoints

Assessment endpoints are defined as “explicit expressions of the actual environmental value that 

is to be protected.”2 Selection of the assessment endpoints is based on valued entities (e.g., fish, 

birds), the ecosystems potentially at risk (e.g., waterbodies, riparian vegetation, and upland 

habitats), the migration pathways of Pyrithiobac-Na (e.g., runoff, drift, etc.), and the routes by 

which ecological receptors are exposed to Pyrithiobac-Na (e.g., direct contact, etc.).

Assessment endpoints for Pyrithiobac-Na include direct toxic effects on the survival, 

reproduction, and growth of the receptors, as well as indirect effects, such as reduction of the 

prey base or modification of habitat. Each assessment endpoint requires one or more “measures 

of ecological effect,” defined as changes in the attributes of an assessment endpoint or changes in 

a surrogate entity or attribute in response to exposure to a pesticide. Specific measures of 

ecological effect are generally evaluated based on acute and chronic toxicity information from 

registrant-submitted guideline tests that are performed on a limited number of organisms. 

Additional ecological effects data from the open literature will also be considered.

6. Conceptual Model

For a pesticide to pose an ecological risk, it must reach ecological receptors in biologically 

significant concentrations. An exposure pathway is the means by which a pesticide moves in the 

environment from a source to an ecological receptor. For an ecological pathway to be complete, 

it must have a source, a release mechanism, an environmental transport medium, a point of 

exposure for ecological receptors, and a feasible route of exposure.

The conceptual model for Pyrithiobac-Na provides a written description (risk hypothesis) and 

visual representation (conceptual diagram) of the predicted relationships between PyrithiobacNa, 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 1998b).

Based on the available data, Pyrithiobac-Na is highly to very highly soluble, stable in water 

without light, and unstable in water and on soil under light. .Pyrithiobac-Na is considered 

persistent in soil under aerobic conditions, and very stable in soil and sediment under anaerobic 

conditions. Pyrithiobac-Na is very mobile in soil, Pyrithiobac-Na is not volatile, and will not 

likely accumulate in aquatic food webs. With these properties, it appears that Pyrithiobac-Na is 

highly mobile and has a strong potential to leach into ground water or move offsite into surface 

water. However, and based on the small-scale prospective groundwater monitoring study 

 2 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001.
-17 of 41-

conducted in North Carolina is being reviewed by EFED. Preliminary results indicated that 

Pyrithibac-Na may not be stable enough to leach to ground water at most sites, even in sandy 

soils. However, definitive conclusions cannot be made until the final report is reviewed which 

will be done prior to completion of the draft risk assessment.

Given its relatively low Kow, Pyrithiobac is not expected to bioaccumulate in aquatic food chains.

6.1. Risk Hypothesis

A risk hypothesis describes the predicted relationship among the stressor, exposure, and 

assessment endpoint response along with the rationale for their selection. For Pyrithiobac-Na, 

the following ecological risk hypothesis will be employed for the national-level ecological risk 

assessment:

Given the uses of Pyrithiobac-Na and its environmental fate properties, there is a likelihood of 

exposure to non-target terrestrial and aquatic organisms. When used in accordance with the 

label, Pyrithiobac-Na may result in potential adverse effects upon the survival, growth, and 

reproduction of non-target terrestrial and aquatic organisms. 

6.2. Conceptual Diagram

The conceptual model used to depict the potential ecological risk associated with the use of 

Pyrithiobac-Na (the stressor) assumes that as a pesticide, Pyrithiobac-Na can adversely affect 

terrestrial and aquatic organisms (the receptors) if environmental concentrations exceed toxic 

thresholds as a result of application according to the label directions (Figure 1). Ecological 

receptors that may potentially be exposed to Pyrithiobac-Na include terrestrial and semi-aquatic 

wildlife (e.g., mammals, birds, terrestrial-phase amphibians and reptiles), terrestrial and semiaquatic plants, and terrestrial invertebrates (e.g., honey bees) (Figure 2). Additionally, given the 

mobility of Pyrithiobac-Na, aquatic organisms (i.e., freshwater and estuarine/marine fish and 

invertebrates, aquatic-phase amphibians, and aquatic plants) are potential receptors in adjacent 

water bodies through off-site transport of Pyrithiobac-Na from the application site through 

runoff, erosion, and spray drift.
-18 of 41-

Figure 1. Conceptual model for Pyrithiobac-Na exposure to aquatic organisms. Dashed lines 

indicate route of exposure considered of lower importance for Pyrithiobac-Na.
-19 of 41-

Figure 2. Conceptual model for Pyrithiobac-Na exposure to terrestrial organisms. Dashed lines 

indicate route of exposure considered of lower importance for Pyrithiobac-Na.
-20 of 41-

7. Analysis Plan

In order to address the risk hypothesis, the potential for adverse effects on the environment will 

be estimated. The use, environmental fate, and ecological effects of Pyrithiobac-Na will be 

characterized and integrated to assess the risks. This will be accomplished using a risk quotient 

(ratio of exposure concentration to effects concentration) approach. Although risk is often 

defined as the likelihood and magnitude of adverse ecological effects, the risk quotient-based 

approach does not provide a quantitative estimate of likelihood and/or magnitude of an adverse 

effect. However, as outlined in the Overview Document (USEPA 2004), the likelihood of 

effects to individual organisms from particular uses of Pyrithiobac-Na may be estimated using 

the probit dose-response slope and either the level of concern (discussed below) or the actual 

calculated risk quotient value. 

This analysis plan will be revisited and may be revised depending upon the information 

submitted by the public in response to the opening of the Registration Review docket for 

Pyrithiobac-Na.

7.1. Stressors of Concern

The stressor of concern in the assessment will be Pyrithiobac-Na. There are no known 

applications of specific mixtures of Pyrithiobac-Na with other chemicals. In its ecological risk 

assessments, the Agency does not routinely include an evaluation of mixtures of active 

ingredients, either those mixtures of multiple active ingredients in product formulations or those 

in the applicator’s tank. In the case of the product formulations of active ingredients (that is, a 

registered product containing more than one active ingredient), each active ingredient is subject 

to an individual risk assessment for regulatory decision regarding the active ingredient on a 

particular use site. If effects data are available for a formulated product containing more than 

one active ingredient, the data may be used qualitatively or quantitatively in accordance with the 

Agency’s Overview Document and the Services’ Evaluation Memorandum (USEPA, 2004; 

Williams and Hogarth, 2004).

7.2. Measures of Exposure

In order to estimate risks of Pyrithiobac-Na exposures in aquatic and terrestrial environments, 

exposure modeling will be based on maximum application rates and methods cited in Table 3.2.

Measures of exposure are based on aquatic and terrestrial models that predict estimated 

environmental concentrations (EECs) of Pyrithiobac-Na. The models used to predict aquatic 

EECs are the Pesticide Root Zone Model coupled with the Exposure Analysis Modeling System 

(PRZM/EXAMS). The model used to predict terrestrial EECs on food items is the Terrestrial 

Residue Exposure model (T-REX). The model used to derive EECs relevant to terrestrial and 

wetland plants is TerrPlant. The AgDrift model will be used to estimate the distance off-field that 

effects may occur. These models are parameterized using relevant reviewed environmental fate 

data from registrant submissions and the literature. Model input values will be consistent with 

the most recent version of the input parameter guidance (Version 2.1; USEPA 2009).
-21 of 41-

PRZM (v3.12.2; 5/15/05) and EXAMS (v2.98.04.06; 4/25/05) are simulation models coupled 

with the linkage program shell: PE5 (PRZM EXAMS Model Shell; v5.0; 11/15/06), which 

incorporates the standard scenarios developed by EFED. The models generate daily exposures 

and calculated 1-in-10 year EECs of pyrithiobac-Na and possibly (depending on the outcome of 

environmental fate studies) toxicologically significant degradatation products (e.g., 

sulfanilamide). PRZM simulates pesticide fate and transport as a result of leaching, direct spray 

drift, runoff and erosion from an agricultural field, and EXAMS estimates environmental fate 

and transport of pesticides in a surface water body for a 30-year period. The combined model is 

designed to estimate pesticide concentrations found in water (standard pond) at the edge of the 

treated field. As such, it provides high-end values of the pesticide concentrations that might be 

found in ecologically sensitive environments following pesticide application. The location of the 

field is specific to the crop being simulated using site-specific information on the soils, weather, 

cropping, and management factors associated with the scenario. The crop/location scenario is 

intended to represent a high-end exposure site on which the crop is normally grown. Based on 

historical rainfall patterns, the receiving water body receives multiple runoff events during the 

years simulated. Weather and agricultural practices are simulated for 30 years so that the 10-year 

exceedance probability at the site can be estimated. The simulation is generated using 30 years of 

meteorological data, encompassing the years from 1961 to 1990. Additional information on these 

models can be found at: http://www.epa.gov/oppefed1/models/water/index.htm. 

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 Pyrithiobac-Na. The measure of exposure for aquatic species is the 1-in10 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 organisms assumed to be in the target area or in an area 

exposed to off-site deposition are derived using the T-REX model (version 1.4.1, 10/09/2008). 

This model incorporates the Kenaga 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 

the upper bound of residue values from actual field measurements (Hoerger and Kenaga 1972). 

The Fletcher et al. (1994) modifications to the Kenaga 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. Based on these data, T-REX assumes a 35-day foliar 

dissipation half-life in the absence of other data.

In order to determine if Pyrithiobac- Na residues in drinking water is a potential pathway of 

concern for avian and mammalian species, the Screening Imbibition Program (SIP v.1.0, 

Released June 15, 2010) was used to calculate an upper bound estimate of exposure using 

Pyrithiobac-Na’s solubility (728 ppm), the most sensitive acute and chronicavian toxicity 

endpoints (LD50=1599 mg ai/kg-bw; NOAEL= 60 mg ai/kg-bw) and the most sensitive acute and 

chronic mammalian toxicity endpoints (LD50=3200 mg ai/kg-bw; NOAEL not available). 

Drinking water exposure alone was determined to be a potential pathway of concern for avian 
-22 of 41-

species on an acute and chronic basis. Drinking water exposure alone was determined to not be a 

potential pathway of concern for mammalian species on an acute basis but cannot be precluded 

on a chronic basis due to insufficient data. This pathway will be explored further with the 

development of SIP v.2.0 in the risk assessment for Pyrithiobac-Na. Detailed information about 

the SIP v.1.0, as well as the tool, can be found on the EPA’s website at 

http://www.epa.gov/pesticides/science/models_pg.htm#terrestrial.

EECs for terrestrial plants will be derived using TerrPlant (version 1.2.2, 12/26/2006). This 

model uses estimates of pesticides in runoff and in spray drift to calculate EECs. EECs are based 

upon solubility, application rate and minimum incorporation depth. 

The AgDRIFT (v2.01; May 2001) or AgDisp (version 8.15) spray drift model may be used to 

assess exposures of organisms to Pyrithiobac-Na deposited on terrestrial habitats by spray drift.

7.3. Measures of Effect

Ecological effects data are used as measures of direct and indirect effects to biological receptors. 

Effects data are obtained from registrant-submitted studies or from literature studies identified by 

the ECOTOX database. 

The acute measures of effect used for animals in this assessment are the LD50, LC50 and EC50. 

LD stands for "Lethal Dose", and LD50 is the amount of a material, given all at once, that is 

estimated to cause the death of 50% of the test organisms. LC stands for “Lethal Concentration” 

and LC50 is the concentration of a chemical that is estimated to kill 50% of the test organisms. 

EC stands for “Effective Concentration” and the EC50 is the concentration of a chemical that is 

estimated to produce a specific effect in 50% of the test organisms. Endpoints for chronic 

measures of exposure for listed and non-listed animals are the NOAEL/NOAEC and NOEC. 

NOAEL stands for “No Observed-Adverse-Effect-Level” and refers to the highest tested dose of 

a substance that has been reported to have no harmful (adverse) effects on test organisms. The 

NOAEC (i.e., “No-Observed-Adverse-Effect-Concentration”) is the highest test concentration at 

which none of the observed effects were statistically different from the control. 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. 

Where available, sublethal effects observed in both registrant-submitted and open literature 

studies will be evaluated qualitatively. Such effects may include behavioral changes (e.g., 

lethargy and changes in coloration). Quantitative assessments of risks, though, are limited to 

those endpoints that can be directly linked to the Agency’s assessment endpoints of impaired 

survival, growth and reproduction.

In the absence of taxon-specific data, the assessment of risk for direct effects to non-target 

organisms makes the assumption that toxicity of Pyrithiobac-Na to birds is similar to terrestrialphase amphibians and reptiles. The same assumption is made for fish and aquatic-phase 

amphibians. In the absence of data for either acute or chronic effects, the conservative 

assumption will be to presume that Pyrithiobac-Na is toxic. 
-23 of 41-

7.4. Integration of Exposure and Effects

Risk characterization is the integration of exposure and ecological effects characterization to 

determine the potential ecological risk from the registered uses of Pyrithiobac-Na 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 Pyrithiobac-Na 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 

Pyrithiobac-Na use, as directed on the labels, has the potential to cause adverse direct or indirect 

effects to non-target organisms. In addition, incident data from the EIIS will be considered as 

part of the risk characterization. 

7.5. Deterministic and Probabilistic Assessment Methods

The quantitative assessment of risk will primarily depend on the deterministic point-estimate 

(RQ) based approach described in the risk assessment. An effort will be made to further 

qualitatively describe risk using probabilistic tools that the Agency has developed. These tools 

have been reviewed by FIFRA Scientific Advisory Panels 

(http://www.epa.gov/scipoly/sap/index.htm) and have been deemed as appropriate means of 

refining assessments where deterministic approaches have identified risks. Newer tools may be 

available to assess the routes of exposure (e.g. TIM and SWAMP models) and will be applied as 

appropriate in Registration Review.

7.6. Endocrine Disruptor Screening Program

As required under FFDCA section 408(p), EPA has developed the Endocrine Disruptor 

Screening Program (EDSP) to determine whether certain substances (including pesticide active 

and other ingredients) may have an effect in humans or wildlife similar to an effect produced by 

a “naturally occurring estrogen, or other such endocrine effects as the Administrator may 

designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect.

Between October 2009 and February 2010, EPA issued test orders/data call-ins for the first 

group of 67 chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. 

This list of chemicals was selected based on the potential for human exposure through pathways 

such as food and water, residential activity, and certain post-application agricultural scenarios. 

This list should not be construed as a list of known or likely endocrine disruptors.
-24 of 41-

Pyrithiobac-Na is not among the group of 58 pesticide active ingredients on the initial list to be 

screened under the EDSP. Under FFDCA sec. 408(p) the Agency must screen all pesticide 

chemicals. Accordingly, EPA anticipates issuing future EDSP test orders/data call-ins for all 

pesticide active ingredients. 

For further information on the status of the EDSP, the policies and procedures, the list of 67 

chemicals, the test guidelines and the Tier 1 screening battery, please visit our website: 

http://www.epa.gov/endo/.

7.7. Endangered Species Assessment

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the 

Agency will evaluate risks to Federally-listed threatened and/or endangered (listed) species from 

registered uses of Pyrithiobac-Na. This assessment will be conducted in accordance with the 

Overview Document (USEPA 2004), provisions of the ESA, and the Services’ Endangered 

Species Consultation Handbook (USFWS/NMFS, 1998).

The assessment of effects associated with the registration of Pyrithiobac-Na 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 Pyrithiobac-Na

application occurs), plus all areas where offsite transport (i.e., spray drift, runoff, etc.) may result 

in potential exposure that exceeds the Agency’s LOCs. Specific measures of ecological effect 

that define the action area for listed species include any direct and indirect effects and/or 

potential modification of its critical habitat, including reduction in survival, growth, and 

reproduction as well as the full suite of sublethal effects available in the effects literature. 

Therefore, the action area extends to a point where environmental exposures are below any 

measured lethal or sublethal effect threshold for any biological entity at the whole organism, 

organ, tissue, and cellular level of organization. In situations where it is not possible to 

determine the threshold for an observed effect, the action area will initially be assumed to 

include all counties where Pyrithiobac-Na is used in the United States. 

7.8. Drinking Water Assessment

In order to bring the drinking water exposure assessment up to date with current data, models 

and simulation model guidance, a new drinking water assessment will be conducted to support 

future human health dietary risk assessments of Pyrithiobac-Na. The drinking water exposure 

assessment will be conducted for the maximum labeled use in support of the human health 

dietary risk assessment. The drinking water assessment will incorporate model estimates of 

Pyrithiobac-Na alone in surface water and ground water including any residues that are 
-25 of 41-

determined to be of toxicological concern. Concentrations of Pyrithiobac-Na in surface waters 

will be estimated using PRZM/EXAMS (see description above) but using the Index Reservoir 

scenario rather than the standard pond used with aquatic exposure assessment.

An Estimated Drinking Water Concentration (EDWC) of Pyrithiobac-Na in ground water will be 

estimated using EFED’s Tier I aquatic model SCIGROW (Screening Concentration in Ground 

Water, version 2.3; 8/8/2003). SCIGROW is a regression model used as a screening tool to 

estimate pesticide concentrations found in ground water used as drinking water. The output of 

SCIGROW represents the concentrations that might be expected in shallow unconfined aquifers 

under sandy soils, which is representative of the ground water most vulnerable to pesticide 

contamination likely to serve as a drinking water source. If significant amounts of PyrithiobacNa are found to deposit in the environment due to releases from the use on cotton, a drinking 

water assessment will be performed for this use using PRZM/EXAMS and SCIGROW. In this 

case, the deposited residues will be treated as if they were a soil application of a pesticide in 

PRZM.

The drinking water assessment will also include available surface and groundwater monitoring 

data with consideration of changes in use patterns that may have occurred. States are encouraged 

to submit monitoring data for review.

8. Preliminary Identification of Data Gaps

8.1. Environmental Fate

The studies submitted to fulfill environmental fate data requirements for Pyrithiobac-Na are not 

complete to conduct an exposure assessment. The current 40 CFR (2010) requires the following 

fate studies for terrestrial uses of pesticides:

1) Aerobic soil metabolism (OCSSP guideline # 835.4100). Though there is acceptable 

aerobic soil metabolism data for one type of soil, the guideline stipulates that results from 

at least four relevant soils of different organic carbon content, pH, clay content, and 

microbial biomass must be submitted in order to characterize the variability in the aerobic 

biodegradation of Pryithiobac-Na in the environment.

2) Aerobic aquatic metabolism (OCSSP guideline # 835.4300). There are no acceptable 

aerobic aquatic metabolism studies. This study is necessary in order to describe the fate 

of Pyrithiobac-Na in surface water bodies down-gradient from use sites (e.g., rice 

paddies). The guideline stipulates that at least two different soil:water systems be 

submitted that are similar to the potential use sites in the United States. There is currently 

one supplemental study that lacks information on the aerobicity of the soil:water system.

3) Anaerobic aquatic metabolism (OCSSP guideline # 835.4400). Submission of an 

acceptable anaerobic aquatic metabolism study is necessary to understand the degradation 

of Pyrithiobac-Na in oxygen-limiting environments.

4) Environmental chemistry methods in water being reviewed by EFED. However, 

definitive conclusions cannot be made until the final report is reviewed which will be 

done prior to completion of the draft risk assessment. Although these studies have been 
-26 of 41-

submitted to EPA they will be considered data gaps until the data have been reviewed 

and found to be acceptable.

5) An independent laboratory validation (ILV) report for Environmental chemistry method 

in water is needed.

Table 8.1. Summary of available environmental fate data.

Guideline Study Title MRID Classification Data Gap? Comments

835.2120 Hydrolysis 42856945 Acceptable No No significant issues affecting the acceptability of 

the study were noted.

835.2240 Photodegradation in 

Water 42856946 Acceptable No No significant issues affecting the acceptability of 

the study were noted.

835.2410 Photodegradation on 

Soil 42856947 Acceptable No No significant issues affecting the acceptability of 

the study were noted.

835.4100 Aerobic Soil 

Metabolism 42856948 Acceptable Yes

Per 40 CFR part 158, promulgated July 1, 2009, 

requires aerobic soil metabolism data for 

pesticide food and nonfood terrestrial uses. 

Currently the Agency has received one 

acceptable study on one soil types. 

Per OCSPP guideline 835.4100, at least three 

additional soils should be used to determine 

transformation rate.

835.4200 Anaerobic Soil 

Metabolism 43455601 Acceptable No No significant issues affecting the acceptability of 

the study were noted.

835.4300 Aerobic Aquatic 

Metabolism N/A N/A Yes

Based on the Terrestrial Non-Food use 

pattern, this data requirement is applicable. 

Aerobic aquatic metabolism data are not 

available, which are required under the new 

40 CFR Part 158. Therefore, this is 

identified as a data gap.

835.4400 Anaerobic Aquatic 

Metabolism N/A N/A Yes

Based on the Terrestrial Non-Food use 

pattern, this data requirement is applicable. 

Anaerobic aquatic metabolism data are not 

available, which are required under the new 

40 CFR Part 158. Therefore, this is 

identified as a data gap.

835.1240

Mobility - 

Adsorption / 

Desorption

42909209 Acceptable

No

No significant issues affecting the acceptability of 

the study were noted.

42909210 Acceptable No significant issues affecting the acceptability of 

the study were noted.

835.1230

4290921 

1 Acceptable No significant issues affecting the acceptability of 

the study were noted.
-27 of 41-

Guideline Study Title MRID Classification Data Gap? Comments

835.6100 Terrestrial Field 

Dissipation 

43455603

Acceptable

No

No significant issues affecting the acceptability of 

the study were noted.

43317212

Acceptable No significant issues affecting the acceptability of 

the study were noted.

43455602 Supplemental

Limited rainfall confounds mobility interpretation

43647401 Supplemental

850.1730 Fish Accumulation N/A

A waiver request 

from the 

registrant, was 

accepted 

No This study is not required because the KOW is below 

4.

835.7100

Prospective 

Groundwater 

Monitoring

43530102

Pyrithibac-Na 

may not be 

stable enough to 

leach to ground 

water even in 

sandy soils.

No

This study appears to have been conducted at a site 

with groundwater highly vulnerable to 

contamination. However, it is not clear that the 

amounts and timing of water input were such that the 

leaching would be significantly more likely than in 

an average year at the test site. 

8.2. Effects

The available ecological effects data for Pyrithiobac-Na consists of acute and chronic effects 

studies on animals and effects to plants. There are no chronic data on estuarine/marine fish. 

Additionally, an avian acute oral study with a passerine is now required under the current 40 

CFR Part 158 and is not yet been submitted for Pyrithiobac-Na. Available data on the toxicity of 

Pyrithiobac-Na to terrestrial plants are based on technical Pyrithiobac-Na rather than the required 

TEP. Therefore effects to non-target plants may be appreciably underestimated by the available 

studies. The available effects studies for Pyrithiobac-Na are presented in Tables 8.2-8.4. The 

following data gaps have been identified:

850.2100- Avian Oral Toxicity, Passerine 

850.4100- Terrestrial Plants: Seedling Emergence, Tier II

850.4150- Terrestrial Plants: Vegetative Vigor, Tier II
-28 of 41-

TABLE 8.2. Available ecological effects data for terrestrial animals exposed to Pyrithiobac-Na.

Guideline Description MRID/

Accession Classification Data Gap? Comments

850.2100 Avian oral toxicity

(for bobwhite quail) 42856911 Acceptable Yes* *Avian acute oral toxicity data are not available for 

passerines, which are required 

under the new 40 CFR Part 

158. Therefore, this is 

identified as a data gap.

850.2200

Avian dietary 

toxicity 

(for bobwhite quail 

and mallard)

42856912

42856913 Acceptable No

850.2300

Avian reproduction

(for bobwhite quail 

and mallard)

43811101

43811102 Acceptable No

850.3020 Honeybee acute 

contact toxicity 42856944 Acceptable No

TABLE 8.3. Available ecological effects data for aquatic animals exposed to Pyrithiobac-Na.

Guideline Description MRID/ 

Accession Classification Data Gap? Comments

850.1075 Freshwater fish –

Acute toxicity 

42856914 Acceptable

42856915 Acceptable No

43317204 Acceptable

850.1075 Saltwater fish –

Acute toxicity 42856917 Acceptable No

850.1010

Freshwater 

invertebrates –

Acute toxicity

42856916 Acceptable

No

43317206 Supplemental

850.1025

850.1035

Saltwater 

invertebrates –Acute 

toxicity 

42856918 Acceptable

No

42856919 Acceptable

850.1300

Freshwater 

invertebrate –

life cycle test

43317208 Acceptable No

850.1350

Saltwater 

invertebrates –

life cycle test

43530101 Acceptable No

850.1400 Freshwater fish –

early life stage test 43317207 Acceptable No

850.1400 Saltwater fish –

early life stage test None No

Per 40 CFR Part 158, this test 

is conditionally required; 

conditions for requiring this 

test are not met. 
-29 of 41-

TABLE 8.4. Available ecological effects data for plants exposed to Pyrithiobac-Na.

Guideline Description MRID Classification Data 

Gap? comments

850.4100

Terrestrial Plant 

toxicity: Tier I seedling 

emergence

none No

Per 40 CFR Part 158, not 

required for known 

phytotoxicants.

850.4225

Terrestrial Plant 

toxicity: Tier II 

seedling emergence

42856943 Supplemental Yes

Study used technical grade 

product while 40 CFR Part 

158 requires the formulated 

product. Herbicidal active 

ingredients are usually 

formulated with adjuvants or 

surfactants that enhance the 

activity of the pesticide. 

Therefore, the sensitivity of 

terrestrial plants to 

Pyrithiobac-Na may be 

underestimated. 

850.4150

Terrestrial Plant 

toxicity: Tier I 

vegetative vigor

none No

Per 40 CFR Part 158, not 

required for known 

phytotoxicants.

850.4250

Terrestrial Plant 

toxicity: Tier II 

vegetative vigor

42856943 Supplemental Yes

Study used technical grade 

product while 40 CFR Part 

158 requires the formulated 

product. Herbicidal active 

ingredients are usually 

formulated with adjuvants or 

surfactants that enhance the 

activity of the pesticide. 

Therefore, the sensitivity of 

terrestrial plants to 

Pyrithiobac-Na may be 

underestimated. 

850.5400 Aquatic Plant Growth: 

algae

42909204 Acceptable

No

42909205 Acceptable

42909206 Acceptable

42909207 Invalid

43577501 Acceptable

850.4400 Aquatic Plant Growth: 

vascular plants

42909208 Supplemental No

43577502 Acceptable

9. References

Open Literature Citations
-30 of 41-

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

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

plants. Environ. Tox. Chem. 13:1383-1391.

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.

Shimizu, T., I. Nakayama, T. Nakao, Y. Nezu, and H. Abe. 1994. Inhibition of Plant 

Acetolactate Synthase by Herbicides, Pyrimidinylsalicylic Acids. Journal of Pesticide 

Science. 19: 56-97. 

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.

USDA. 2010. Acreage. National Agricultural Statistics Service (NASS), Agricultural Statistics 

Board, U.S. Department of Agriculture. May 2011. 

http://usda.mannlib.cornell.edu/usda/current/Acre/Acre-06-30-2010.pdf

USEPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. U.S. Environmental Protection Agency, Office of Prevention, Pesticides and 

Toxic Substances, Office of Pesticide Programs, Washington DC. January 23, 2004.

USEPA. 1998. Guidelines for Ecological Risk Assessment. Risk Assessment Forum, Office of 

Research and Development, Washington, D.C. EPA/630/R-95/002F. April 1998. 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=30759

Williams S and W Hogarth. 2004. Letter to Susan B Hazen regarding the USFWS and NMFS 

(Services) evaluation of US EPA’s Overview of the Ecological Risk Assessment Process 

in the Office of Pesticide Programs, Environmental Protection Agency (January 2004).

Willis, G.H. and L.L. McDowell. 1987. Pesticide Persistence on Foliage in Reviews of 

Environmental Contamination and Toxicology. 100:23-73.

Submitted Fate Studies

MRID Citation Reference

42856945 Bramble, F.; Shalaby, L. (1993) Hydrolysis of (carbon 14)-KIH-2031 (DPXPE350) in Buffer Solutions of pH 5, 7, and 9: Lab Project Number: AMR 2093-
-31 of 41-

91. Unpublished study prepared by Du Pont Agricultural Products. 25 p. 

42856946 Shalaby, L.; Bramble, F. (1993) Photodegradation of (carbon 14)-KIH-2031 

(DPX-PE350) in Buffer Solutions of pH 5, 7, and 9: Lab Project Number: 

AMR 2094-91. Unpublished study prepared by Du Pont Agricultural Products. 

63 p. 

42856947 Dietrich, R.; Taylor, T. (1993) Photodegradation of Radiolabeled KIH-2031 

(DPX-PE350) on Soil Conducted in Simulated Sunlight: Lab Project Number: 

AMR 2310-92. Unpublished study prepared by Du Pont Agricultural Products. 

53 p. 

42856948 McFetridge, R.; Houben, K. (1993) Aerobic Soil Metabolism of (carbon 14)-

KIH-2031 (DPX-PE350): Lab Project Number: AMR 2076-91. Unpublished 

study prepared by Du Pont Agricultural Products. 59 p. 

43455601 McFetridge, R.; Houben, K. (1993) Anaerobic Soil Metabolism of (carbon 14) 

KIH-2031 (DPX-PE350): Lab Project Number: AMR/2077/ 91. Unpublished 

study prepared by E.I. du Pont de Nemours and Co. 54 p. 

42909209 Hawkins, D.; Kirkpatrick, D.; Aikens, P.; et al. (1993) Soil Column Leaching 

of (Carbon 14)-KIH-2031 (DPX-PE350): Lab Project Number: AMR 2359-92: 

HRC/DPT 277/921239. Unpublished study prepared by Huntingdon Research 

Centre, Ltd. 87 p. 

42909210 Hawkins, D.; Kirkpatrick, D.; Dean, G.; et al. (1993) Soil Mobility of (Carbon 

14)-KIH-2031 (DPX-PE350) and (Carbon 14)-IN-JW212 By Soil Thin-Layer 

Chromatography: Lab Project Number: AMR 2462-92: HRC/DPT 285/921353. 

Unpublished study prepared by Huntingdon Research Centre, Ltd. 45 p. 

42909211 Hoffman, D. (1993) Batch Equilibrium (Adsorption/Desorption) Studies of 

Radiolabeled KIH-2031 (DPX-PE350): Lab Project Number: AMR 2123-91. 

Unpublished study prepared by E. I. du Pont De Nemours and Co. 49 p. 

43317212 McFetridge, R. (1994) Field Soil Dissipation of DPX-PE350 Experimental 

Herbicide: Lab Project Number: AMR 2289-92: 92-P1289.REP. Unpublished 

study prepared by DuPont Agricultural Products, Global Technology Division 

and Enviro-Test Labs. 273 p. 

43317213 Houben, K. (1994) Freezer Storage Stability of KIH-2031 (DPX-PE350) and 

IN-JW212 in Soil: Lab Project Number: AMR 2392-92. Unpublished study 

prepared by DuPont Agricultural Products, Global Technology Division. 41 p. 

43455602 McFetridge, R.; Dietrich, R. (1994) Supplement No. 1 to: Field Soil 

Dissipation of DPX-PE350 Experimental Herbicide: DPX-PE350: Lab Project 

Number: AMR/2289/92. Unpublished study prepared by E.I. du Pont de 

Nemours and Co. 10 p. 

43455603 McFetridge, R.; Houben, K. (1994) Terrestrial Field Soil Dissipation of (carbon 

14) KIH-2031 (DPX-PE350) in Madera, California: Lab Project Number: 

92121: 2333/92. Unpublished study prepared by E.I. du Pont de Nemours and 

Co.; Pan- Agricultural Lab., Inc.; and Agvise Lab. 182 p. 

43577503 Houben, K. (1995) Freezer Storage Stability of KIH-2031 (DPX-PE350) and 

IN-JW212 in Soil: Revision No. 1: Lab Project Number: AMR 2392-92, 

REVISION NO. 1. Unpublished study prepared by E. I. du Pont de Nemours 

and Co. 51 p. 

43647401 McFetridge, R.; Houben, K. (1995) Field Soil Dissipation of (carbon 14) KIH2031 (DPX-PE350) Experimental Herbicide in Greenville, Mississippi: Lab 
-32 of 41-

Project Number: AMR 2290-92. Unpublished study prepared by DuPont 

Agricultural Products and Harris Environmental Technologies, Inc. 214 p. 

42856950 Gaddamidi, V.; Hashinger, B.; Bookhart, S. (1993) Confined Accumulation 

Study of (carbon 14)KIH-2031 (DPX-PE350) in the Rotational Crops: Lab 

Project Number: AMR 2020-91. Unpublished study prepared by Du Pont 

Agricultural Products. 119 p. 

43404602 Gaddamidi, V.; Hashinger, B. (1994) Supplement No. 1 To: Confined 

Accumulation Study of (carbon 14)KIH-2031 (DPX-PE350) in the Rotational 

Crops: Lab Project Number: AMR 2020-91. Unpublished study prepared by E. 

I. du Pont de Nemours and Co. 8 p. 

43812701 Hiscock, A. (1995) A Small-Scale Prospective Ground-Water Monitoring 

Study for Pyrithiobac Sodium (Stable, DPX-PE350, KIH-2031: Progress 

Report Number 3: Lab Project Number: AMR 2870-93: 423.06 2870-93. 

Unpublished study prepared by Blasland, Bouck & Lee, Inc.; Midwest Labs, 

Inc.; and DuPont Agricultural Products. 71 p. 

43963401 Hiscock, A. (1996) A Small-Scale Prospective Ground-Water Monitoring 

Study for Pyrithiobac Sodium (Staple, DPX-PE350, KIH-2031): Progress 

Report No. 4: Lab Project Number: AMR 2870-93-PR4: AMR 2870-93: 95220. 

Unpublished study prepared by Stone Environmental, Inc.; DuPont Agricultural 

Products and Blasland, Bouck & Lee, Inc. 100 p. 

44373804 Hiscock, A. (1997) A Small-Scale Prospective Ground-Water Monitoring 

Study for Pyrithiobac Sodium (Staple, DPX-PE350, KIH-2031): Final Report: 

Lab Project Number: AMR 2870-93-REPFI: 95220: AMR 2870-93. 

Unpublished study prepared by Stone Environmental, Inc. and Blasland, Bouck 

& Lee, Inc. 562 p. 

43588101 Hiscock, A. (1995) A Small-Scale Prospective Ground-Water Monitoring 

Study for Pyrithiobac-Sodium (Staple, DPX-PE350, KIH-2031): Site Selection, 

Characterization, and Instrument Report: Lab Project Number: AMR 2870-93: 

423.06. Unpublished study prepared by Blasland, Bouck & Lee, Inc.; Lancaster 

Labs; and DuPont Agricultural Products. 260 p. 

43588102 Hiscock, A. (1995) A Small-Scale Prospective Ground-Water Monitoring 

Study for Pyrithiobac-Sodium (Staple, DPX-PE350, KIH-2031): Progress 

Report Number 1: Lab Project Number: AMR 2870-93-PR1: 423.06: AMR 

2870-93. Unpublished study prepared by Blasland, Bouck & Lee, Inc.; 

Lancaster Labs; and DuPont Agricultural Products. 82 p. 

43647601 Goetz, M.; Tillotson, P.; Hiscock, A. (1995) A Small-Scale Prospective 

Ground-Water Monitoring Study for Pyrithiobac-Sodium (STAPLE, DPXPE350, KIH-2031): Progress Report No. 2: Lab Project Number: 

AMR/2870/93: 423/06: AMR/2870/93/PR2. Unpublished study prepared by 

Blasland, Bouck & Lee, Inc.; Midwest Labs, Inc. and Dupont Agricultural 

Products. 74 p. 

43530102 Mulderig, L.; Ledeker, K. (1995) Analytical Method for the Determination of 

Pyrithiobac Sodium in Water Using Liquid Chromatography with UV 

Absorption and Confirmation by Mass Spectrometric Detection: Lab Project 

Number: AMR 3050-94. Unpublished study prepared by DuPont Agricultural 

Products. 64 p. 

43317214 ECM Evaluation in Water

44373805 Sumpter, S.; Peterson, B.; Muldering, L. et al. (1997) Analytical Method for the 
-33 of 41-

Determination of Pyrithiobac Sodium in Soil Using Subcritical Water 

Extraction, Graphitized Carbon Clean-Up, and Column-Switching LC/UV 

Analysis with Confirmation by LC/MS: Revision No. 1: Lab Project Number: 

AMR 2745-93. Unpublished study prepared by Dupont Agricultural Products. 

84 p. 

Submitted Effects Studies

MRID Citation Reference

42856911 Campbell, S.; Smith, G. (1993) KIH-2031 (DPX-PE350): An Acute Oral 

Toxicity Study with the Northern Bobwhite: Lab Project Number: 112-288A: 

42-93. Unpublished study prepared by Wildlife International Ltd. 22 p. 

42856912 Campbell, S.; Lynn, S. (1992) KIH-2031 (DPX-PE350): A Dietary LC50 Study 

with the Northern Bobwhite: Lab Project Number: 112-286: 141-92. 

Unpublished study prepared by Wildlife International Ltd. 27 p. 

42856913 Campbell, S.; Lynn, S. (1992) KIH-2031 (DPX-PE350): A Dietary LC50 Study 

with the Mallard: Lab Project Number: 112-287: 140-92. Unpublished study 

prepared by Wildlife International Ltd. 27 p. 

43811101 Beavers, J.; Sipler, O.; Marselas, G.; et al. (1994) H-19,187 (KIH-2031, DPXPE350-10): A One-Generation Reproduction Study with the Northern 

Bobwhite (Colinus virginianus): Lab Project Number: 112-297: HLO 88-94. 

Unpublished study prepared by Wildlife International Ltd. 240 p. 

43811102 Beavers, J.; Sipler, O.; Marselas, G.; et al. (1994) H-19,187 (KIH-2031, DPXPE350-10): A One-Generation Reproduction Study with the Mallard (Anas 

platyrhynchos): Lab Project Number: 112-298: HLO 89-94. Unpublished study 

prepared by Wildlife International Ltd. 249 p. 

42856914 Baer, K. (1992) Static, Acute, 96-Hour LC50 of KIH-2031 (DPX-PE350-10) to 

Bluegill Sunfish, Lepomis macrochirus: Lab Project Number: 188-92. 

Unpublished study prepared by Du Pont's Haskell Laboratory for Toxicology 

and Industrial Medicine. 21 p. 

42856915 Baer, K. (1992) Static, Acute, 96-Hour LC50 of KIH-2031 (DPX-PE350-10) to 

Rainbow Trout, Oncorhynchus mykiss: Lab Project Number: 184-92: 4581-

946. Unpublished study prepared by Du Pont's Haskell Laboratory for 

Toxicology and Industrial Medicine. 21 p. 
-34 of 41-

43317204 Kreamer, G. (1994) Static, Acute, 96-Hour LC50 of KIH-2031 (DPX-PE350-9) 

to Channel Catfish, Ictalurus punctatus: Lab Project Number: 450-93: HLR 

450-93: 9602-001. Unpublished study prepared by DuPont Haskell Lab for 

Toxicology and Industrial Medicine. 32 p. 

42856916 Baer, K. (1992) Static, Acute, 48-Hour EC50 of KIH-2031 (DPX-PE350-10) to 

Daphnia magna: Lab Project Number: 172-92: 4581-946. Unpublished study 

prepared by Du Pont's Haskell Laboratory for Toxicology and Industrial 

Medicine. 22 p. 

43317205 Baer, K. (1994) Supplement No. 1 to Static, Acute, 48-Hour EC50 of KIH2031 (DPX-PE350-10) to Daphnia magna: Lab Project Number: HLR 172-92: 

172-92: 4581-946. Unpublished study prepared by DuPont Haskell Lab for 

Toxicology and Industrial Medicine. 6 p. 

43317206 Bowman, J.; Yang, Y. (1994) Acute Flow-Through Toxicity of KIH-2031 

(DPX-PE350-9) to Crayfish (Procambarus clarkii): Lab Project Number: HLO 

309-94: 40983: 309-94. Unpublished study prepared by ABC Labs., Inc. 155 p. 

42856917 Ward, T.; Boeri, R. (1992) Acute Flow-Through Toxicity of KIH-2031 (DPXPE350-10) to the Sheepshead Minnow, Cyprinodon variegatus: Lab Project 

Number: 202-92: 91203-DU: 4581-946. Unpublished study prepared by 

EnviroSystems Division, Resource Analysts, Inc. 32 p. 

42856918 Ward, T.; Boeri, R. (1992) Acute Flow-Through Toxicity of KIH-2031 (DPXPE350-10) to the Mysid, Mysidopsis bahia: Lab Project Number: 165-92: 

91202-DU: 4581-946. Unpublished study prepared by EnviroSystems Division, 

Resource Analysts, Inc. 32 p. 

42856919 Ward, T.; Boeri, R. (1992) Acute Flow-Through Mollusc Shell Deposition Test 

with KIH-2031 (DPX-PE350-10): Lab Project Number: 244-92: 91201-DU: 

4581-946. Unpublished study prepared by EnviroSystems Division, Resource 

Analysts, Inc. 31 p. 

43317207 Baer, K. (1994) Early Life-Stage Toxicity Test of KIH-2031 (DPX-PE350-9) 

with Fathead Minnows, Pimephales promelas: Lab Project Number: 9602-001: 

786-93: HLR 786-93. Unpublished study prepared by DuPont Haskell Lab for 

Toxicology and Industrial Medicine. 271 p. 

43317208 Kreamer, G. (1994) Chronic Toxicity of KIH-2031 (DPX-PE350-9) to Daphnia 

magna: Lab Project Number: 536-93: 9602-001: HLR 536-93. Unpublished 

study prepared by DuPont Haskell Lab for Toxicology and Industrial Medicine. 

397 p. 

43530101 Ward, T.; Kowalski, P.; Boeri, R. (1995) Life-cycle Toxicity of KIH-2031 

(DPX-PE350-9) to the Mysid, Mysidopsis bahia: Lab Project Number: 9981-

001: 519-DU: HLO 24-95. Unpublished study prepared by T. R. Wilbury Labs, 

Inc. 419 p. 

42856943 Carski, T. (1993) Influence of KIH-2031 (DPX-PE350) on Seed Germination, 

Seedling Emergence, and Vegetative Vigor of Several Terrestrial Plants: Lab 

Project Number: AMR 2303-92. Unpublished study prepared by Du Pont's 

Stine-Haskell Research Center. 239 p. 

43317211 McKelvey, R. (1994) Supplement No. 1 to: Influence of KIH 2031 (DPXPE350) on Seed Germination, Seedling Emergence, and Vegetative Vigor of 

Several Terrestrial Plants: Lab Project Number: AMR 2303-92. Unpublished 

study prepared by DuPont Agricultural Products, Stine-Haskell Research 

Center. 15 p. 
-35 of 41-

42856943 Carski, T. (1993) Influence of KIH-2031 (DPX-PE350) on Seed Germination, 

Seedling Emergence, and Vegetative Vigor of Several Terrestrial Plants: Lab 

Project Number: AMR 2303-92. Unpublished study prepared by Du Pont's 

Stine-Haskell Research Center. 239 p. 

43317211 McKelvey, R. (1994) Supplement No. 1 to: Influence of KIH 2031 (DPXPE350) on Seed Germination, Seedling Emergence, and Vegetative Vigor of 

Several Terrestrial Plants: Lab Project Number: AMR 2303-92. Unpublished 

study prepared by DuPont Agricultural Products, Stine-Haskell Research 

Center. 15 p. 

42909204 Hughes, J.; Alexander, M. (1993) KIH-2031 (DPX-PE350): Toxicity to 

Selenastrum capricornutum: Lab Project Number: B382-158-1: AMR 2449-92. 

Unpublished study prepared by Malcolm Pirnie, Inc. 29 p. 

42909205 Hughes, J.; Alexander, M. (1993) KIH-2031 (DPX-PE350): Toxicity to 

Anabaena flos-aquae: Lab Project Number: B382-158-2: AMR 2450-92. 

Unpublished study prepared by Malcolm Pirnie, Inc. 30 p. 

42909206 Hughes, J.; Alexander, M. (1993) KIH-2031 (DPX-PE350): Toxicity to 

Navicula pelliculosa: Lab Project Number: B382-158-3: AMR 2451-92. 

Unpublished study prepared by Malcolm Pirnie, Inc. 29 p. 

42909207 Hughes, J.; Alexander, M. (1993) KIH-2031 (DPX-PE350): Toxicity to 

Skeletonema costatum: Lab Project Number: B382-158-4: AMR 2452-92. 

Unpublished study prepared by Malcolm Pirnie, Inc. 30 p. 

42909208 Hughes, J.; Alexander, M. (1993) KIH-2031 (DPX-PE350): Toxicity to Lemna 

gibba G3: Lab Project Number: B382-158-5: AMR 2453-92. Unpublished 

study prepared by Malcolm Pirnie, Inc. 29 p. 

43577501 Thompson, S. (1995) DPX-PE350: Influence on Growth and Reproduction of 

Skeletonema costatum: Lab Project Number: AMR 3234-94: 112A-135. 

Unpublished study prepared by Wildlife International, Ltd. 45 p. 

43577502 Kannuck, R.; Sloman, T. (1995) DPX-PE350: Influence on Growth and 

Reproduction of Lemna gibba G3: Lab Project Number: AMR 3222-94: HA95-006. Unpublished study prepared by DuPont's Stine-Haskell Research 

Center. 45 p. 

42856944 Hoxter, K.; Bernard, W.; Smith, G. (1993) KIH-2031 (DPX-PE350): An Acute 

Contact Toxicity Study with the Honey Bee: Final Report: Lab Project 

Number: 112-289: 101-93: 9329-001. Unpublished study prepared by Wildlife 

International Ltd. 17 p. 

47007907 Brown, A. (2006) Metabolism of N-Acetylglyphosate (IN-MCX20) in Plants 

and Animals. Project Number: DUPONT/21333. Unpublished study prepared 

by E. I. du Pont de Nemours and Co, Inc. 16 p.
-36 of 41-

Appendix A Summary of available fate studies

1- Persistence in Water

An acceptable hydrolysis study (MRID# 42856945 ) shows that Pyrithiobac-Na is stable to 

degradation by abiotic hydrolysis. On the other hand, it degrades by aqueous photolysis with 

half-lives range between 9-15 days (MRID# 42856946). The degradates identified in the 

irradiated solutions were: ammonium 2-chloro-6-sul fobenzoate (mono- and di -salt of 

INKD303); 4,6-dimethoxy-2-pyrimidinol; urea; and methyl 1 -ureacarboxylate .

2- Persistence in soil

In acceptable study (MRID# 42856948, Pyrithiobac-Na degrades with half-life of 60 days in 

silt loam soil. The degradates identified i n the soil were: carbon dioxide; 2-chloro-6-[(4-

hydroxy-6-methoxypyrimidin-2-y1 ) thiobenzoic acid (IN-JW212); and 4,6-dimethoxy-2-

pyrimidinol.

Under anaerobic soil conditions, Pyrithiobac-Na degrades with half-life of 128 days in an

anaerobic (nitrogen atmosphere) silt loam soi1:water system (MRID# 43455601). The nonvolatile degradate identified was (2-chloro-6-[(4-hydroxy-6-methoxypyrimidin-2-

yl)thio]benzoic acid) with a maximum concentration of = 15 % of the applied radioactivity .

3- Mobility

Based on acceptable batch equi1ibrium experiments (MRID# 42909211), pyrithiobac-Na was 

very mobile in sandy loam, si1t loam, and silty clay loam. Freundlich Kads values were 0.06 

for the silt loam soil (1.1% OM) , 0.08 for the sandy loam soi1 (1.1%OM). 0.32 for the silt 

loam soi1 (5.1% OM), and 0.61 for the silty clay loam soil (5.0% OM).

In another acceptable studies (MRID# 42909209, and 42909210), pyrithiobac-Na

and metabolite (2-chl oro-6-C(4-hydroxy-6-methoxypyrimidin-2-yl )thio]benzoic acid appear 

to be mobile in sandy loam, silty clay loam and silt loam soils.

4- Dissipation :Terrestrial field 
-37 of 41-

Results of the four terrestrial field dissipation studies indicate Pyrithiobac-Na dissipates 

rapidly in Texas (acceptable; MRID # 43317212 ) and Mississippi field locations

(supplemental, MRID# 43455602, 43647401), and was moderately persistent in a 

California study using irrigation (acceptable, MRID# 43455603). Some downward 

movement of Pyrithiobac-Na was observed in the irrigated California study; however, 

this result would be anticipated because the study was conducted with weekly irrigation 

in a coarse-textured soil.

Appendix B Summary of available effects studies

I. Aquatic Organisms

Freshwater fish

An acceptable 96-hr acute toxicity study with Bluegill Sunfish (Lepomis macrochirus) is 

available for Pyrithiobac-Na (MRID 42856914). The fish were exposed to mean-measured 

concentrations of 0 (control), 400, 540, 590, 770, and 930 mg ai/L. No effects were reported at 

any concentration and the LC50 is reported to be >930 mg ai/L.

An acceptable study with Rainbow Trout (Oncorynchus mykiss; MRID 00098505) is available 

for Pyrithiobac-Na. Trout were exposed to mean-measured concentrations of 0 (control), 120, 

190, 340, 580, 1000 mg ai/L. No effects were reported at any concentration and the LC50 is 

reported to be >1000 mg ai/L.

An acceptable study with Channel Catfish (Ictalurus punctatus; MRID 43317204) is available 

for Pyrithiobac-Na. The fish were exposed to mean-measured concentrations of 0 (control), 120, 

210, 350, 580, 970 mg ai/L. No effects were reported at any concentration and the LC50 is 

reported to be > 970 mg ai/L.

As a result of these studies, Pyrithiobac-Na is classified as practically nontoxic to freshwater fish 

on an acute exposure basis.

Freshwater invertebrates

An acceptable 48-hr acute study with the freshwater invertebrate Daphnia magna is available for 

Pyrithiobac-Na (MRID 42856916). Twenty daphnids per level were exposed at mean measured 

concentrations of 0 (negative control), 430, 540, 670, 840, and 1100 mg ai/L. No effects were 

reported at any concentration and the EC50 is reported to be > 1100 mg ai/L. 
-38 of 41-

A supplemental 48-hr acute study with the freshwater invertebrate Procambrus clarkia (crayfish; 

MRID 43317206) is also available for Pyrithiobac-Na. Crayfish were exposed at measured 

concentrations of 0 (negative controls), 130, 230, 350, 520, and 910 mg ai/L. No effects were 

reported at any concentration and the EC50 is reported to be > 910 mg ai/L. 

An acceptable chronic toxicity study with the freshwater invertebrate Daphnia magna is 

available for Pyrithiobac-Na (MRID 43317208). Daphnids were exposed at mean measured 

concentrations of 0 (negative control), 20, 36, 68, 130, 260, 500, 1000 mg ai/L. The most 

sensitive endpoint was survival, with a NOAEC of 260 mg ai/L, a LOAEC of 500 mg ai/L, and a 

MATC of 360 mg ai/L. 

Based on these studies, Pyrithiobac-Na is classified as practically nontoxic to freshwater 

invertebrates on an acute and chronic exposure basis.

Estuarine/marine fish

An acceptable 96-hr acute toxicity study with Pyrithiobac-Na was conducted on the Sheepshead 

Minnow (Cyprinodon variegatus; MRID 42856917). Fish were exposed at a mean-measured 

concentration of 145 mg ai/L. No mortality was reported so the LC50 is reported to be >145 mg 

ai/L. 

There are no data assessing the chronic effects of Pyrithiobac-Na on estuarine/marine fish. 

Based on this study, Pyrithiobac-Na is classified as practically nontoxic to estuarine/marine fish 

on an acute exposure basis. 

Estuarine/marine invertebrates

An acceptable 96-hr shell deposition study was conducted on the Eastern Oyster (Crassostrea 

virginica; MRID 42856919) is available for Pyrithiobac-Na. Oysters were exposed to a meanmeasured concentration of 130 mg ai/L. No effects were reported so the EC50 is reported to be > 

130 mg ai/L. 

An acceptable acute flow-through study with Pyrithiobac-Na was conducted on the 

estuarine/marine mysid shrimp (Mysidopsis bahia; MRID 42856918). Shrimp were exposed at a 

mean-measured concentration of 140 mg ai/L. No mortality was reported so the LC50 is reported 

to be >140 mg ai/L. 

An acceptable life-cycle study with Pyrithiobac-Na was also conducted on the mysid shrimp 

(MRID 43530101). Shrimp were exposed at mean-measured concentrations of 0 (control), 13, 

28, 59, 120 and 240 mg ai/L. No effects were reported so the NOAEC is 240 mg ai/L and the 

LOAEC is > 240 mg ai/L. 

Based on these studies, Pyrithiobac-Na is classified as practically nontoxic to estuarine/marine 

invertebrates on an acute and chronic exposure basis.
-39 of 41-

Plants

Two guideline studies are available assessing the toxicity of Pyrithiobac-Na to the aquatic 

vascular plant duckweed (Lemna gibba). In the first study, duckweed was exposed to nominal 

concentrations of Pyrithiobac-Na of 0 (control) 0.063, 0.125, 0.25, 0.5, 1.0, 2.0, and 40 μg ai/L. 

The 14-day EC50, based on frond number, is 0.9 μg ai/L, the NOAEC is 0.5 μg ai/L, and the 

LOAEC is 1.0 μg ai/L (MRID 42909207; Supplemental).

In the second study, duckweed was exposed to mean-measured concentrations of Pyrithiobac-Na 

of 0 (control), 0.077, 0.13, 0.27, 1.2, and 5.5 μg ai/L. The 14-day EC50, based on frond number, 

is 0.92 μg ai/L and the NOAC is 0.27 μg ai/L (MRID 43577502; Acceptable).

Four guideline studies are available assessing the toxicity of Pyrithiobac-Na to aquatic 

nonvascular plants. The green algae Pseudokirchneriella subcapitata (formerly known as 

Selanastrum capricornutum) was exposed to mean-measured concentrations of Pyrithiobac-Na

of 0 (control), 13.55, 21.87, 56.12, 95.47, 181.74, 369.18, and 756.05 μg ai/L. The five-day

EC50, based on cell density, is 95 μg ai/L and the NOAEC is 22 μg ai/L (MRID 42909204; 

Acceptable).

The cyanobacteria Anabaena flos aqua (MRID 42909205; Acceptable) was exposed to initialmeasured concentrations of Pyrithiobac-Na of 0 (control), 71, 289, 514, 1190, 2320, and 4810 μg 

ai/L. The five-day EC50, based on cell density, 1178 μg ai/L and the NOAEC is 71 μg ai/L.

The diatom Skeletonema costatum (MRID 43577501; Acceptable) was exposed to meanmeasured concentrations of Pyrithiobac-Na of 0 (control), 7.8, 16, 32, 64, 130, and 250 mg ai/L. 

The five-day EC50, based on cell density, is 63 mg ai/L and the NOAEC is 32 mg ai/L.

The diatom Navicula pellicosa (MRID 42909206; Acceptable) was exposed to mean-measured 

concentrations of pyrithiobac-Na of 0 (control), 0.83, 1.66, 3.23, 6.31, 12.7, 27.5, and 49.5 mg 

ai/L. The five-day EC50, based on cell density, is 10.3 mg ai/L and the NOAEC is 6.3 mg ai/L.

II. Terrestrial Organisms

Avian 

An acceptable acute oral toxicity study of Pyrithiobac-Na was conducted on the Northern 

Bobwhite (Colinus virginianus; MRID 42856911). The birds were exposed to Pyrithiobac-Na at 

doses of 0 (control), 998, 1175, 1382, 1626, 1913, and 2250 mg ae/kg-bw. The LD50 for the 

Northern Bowhite, based on reduction in body weight, is 1599 mg ai/kg-bw and the NOAEC is 

998 mg ai/kg-bw. Pyrithiobac-Na is classified as slightly toxic to avian species on an acute oral 

exposure basis. There is no data available for a passerine species; therefore, sensitivity to avian 

species may be underestimated on an acute oral basis. 

The 5-day subacute dietary toxicity of Pyrithiobac-Na was studied in the northern bobwhite

(MRID 42856912; Acceptable). Birds were exposed to nominal concentrations of 0 (control), 

562, 1000, 1780, 3160, and 5620 mg ai/kg-bw. Signs of toxicity, based on reduced body weight, 
-40 of 41-

was recorded for birds exposed to 5620 mg ai/kg-bw for the first portion of the study. Based on 

these data, the LC50 is greater than 5620 mg ai/kg-bw and the NOAEC was 1780 mg ai/kg-bw. 

The 5-day subacute dietary toxicity of Pyrithiobac-Na was studied in mallard ducklings (MRID 

42856913; Acceptable). Birds were exposed to nominal concentrations of 0 (control), 562, 1000, 

1780, 3160, and 5620 mg ai/kg-bw. There were no mortalities or signs of toxicity reported. 

Based on these data, the LC50 is greater than 5620 mg ai/kg-bw.

An acceptable avian reproduction study was conducted on the Northern Bobwhite (MRID 

43811101). Birds were exposed to Pyrithiobac-Na at nominal concentrations of 0 (control) 60, 

300, and 1500 mg ai/kg-bw. Based on a significant decrease in the number of viable embryos 

and live 3-week embryos, the NOAEC is 60 mg ai/kg-bw and the LOAEC is 300 mg ai/kg-bw. 

An acceptable avian reproduction study was conducted on mallard ducklings (MRID 43811102). 

Birds were exposed to Pyrithiobac-Na at nominal concentrations of 0 (control) 60, 300, and 1500 

mg ai/kg-bw. Based on a reduction in hatchling weight, the NOAEC is 300 mg ai/kg-bw and the 

LOAEC is 1500 mg ai/kg-bw.

Mammals

Mammal toxicity data are drawn from the Health Effects Division (HED) chapter in support of 

the RED. The data are summarized below.

Acute toxicity testing with the TGAI (85% Pyrithiobac-Na) produced an acute oral LD50= 3200 

mg ae/kg-bw. Based on these data, Pyrithiobac-Na is classified as practically nontoxic to 

mammals on an acute exposure basis (MRID 42856920).

Chronic toxicity data is not reported. 

Terrestrial Invertebrates

An acceptable 48-hr acute contact study with young adult honeybees (Apis mellifera; MRID 

42856944) exposed honeybees to concentrations of Pyrithiobac-Na of 0 (control), 1.6, 3.1, 6.3, 

12.5, and 25 μg ai/bee. There were no treatment related effects on honeybees so the LD50 is 

greater than 25 μg ai/bee and the NOAEC is 25 μg ai/bee. This data indicates Pyrithiobac-Na is 

practically nontoxic to honeybees. 

Plants

The toxicity of Pyrithiobac-Na to terrestrial plant seedling emergence and vegetative vigor was 

assessed in separate studies reported under MRID 42856943 (Supplemental). The studies were 

conducted with technical Pyrithiobac-Na rather than typical end use product as required under 40 

CFR Part 158. Herbicidal active ingredients are usually formulated with adjuvants or surfactants 
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that enhance the activity of the pesticide. Therefore, the sensitivity of terrestrial plants to 

Pyrithiobac-Na may be underestimated. 

For seedling emergence, corn (Zea mays), cucumber (Cucumis sativus), soybean (Glycine max), 

sorghum (Sorghum bicolor), sugar beet (Beta vulgaris), pea (Pisum sativum), tomato (Solanum 

lycopersicum), onion (Allium cepa), wheat (Triticum aestivum), and rape (Brassica napa) were 

tested at measured concentrations of 0 (control), 0.00781, 0.0156, 0.0312, 0.0625, 0.125 oz ai/A. 

Biomass (dry weight), a preferred parameter, was not measured in the seedling emergence test. 

Of the parameters reported, shoot length was the most sensitive for all species. Rape was the 

most sensitive dicot with an EC25 of 0.0042 oz ai/A (2.63 x 10-4 lb ai/A) and a NOAEC of 0.0039 

oz ai/A (2.44 x 10-4 lb ai/A). Sorghum was the most sensitive monocot, with an EC25 of 0.0067 

oz ai/A (2.63 x 10-4 lb ai/A); since NOAEC was not provided the EC05, 0.0012 oz ai/A (7.5 x 10-

6 lb ai/A), will be used for the listed species assessment. 

For vegetative vigor corn (Zea mays), cucumber (Cucumis sativus), soybean (Glycine max), 

sorghum (Sorghum bicolor), sugar beet (Beta vulgaris), pea (Pisum sativum), tomato (Solanum 

lycopersicum), onion (Allium cepa), wheat (Triticum aestivum), and rape (Brassica napa) were 

tested at measured concentrations of 0 (control), 0.00781, 0.0156, 0.0312, 0.0625, 0.125 oz ai/A. 

Root weight was the most sensitive parameter reported. Rape was the most sensitive dicot with 

an EC25 of 0.0014 oz ai/A (8.75 x 10-5 lb ai/A); since a NOAEC was not provided the EC05, 

0.0004 oz ai/A (2.5 x 10-5 lb ai/A), will be used for the listed species assessment. Sorghum was 

the most sensitive monocot with an EC25 of 0.0007 oz ai/A (4.34 x 10-5 lb ai/A); since a NOAEC 

was not provided the EC05, 0.0002 oz ai/A (1.25 x 10-5 lb ai/A), will be used for the listed species 

assessment. 

There is some evidence3 to suggest plant reproduction may be affected by sulfonylurea 

herbicides at levels below effects on vegetative growth or visual injury (Fletcher et al., 1993). To 

the extent that terrestrial plant reproduction are more sensitive to Pyrithiobac-Na than growth 

endpoints, risks to terrestrial plants may be underestimated. 

 3 Snipes et al. 1992, Bansal et al. 1999; Fletcher et al. 1993, 1995,1996; Bhatti et al. 1995; Al-Khatib et al. 1992; 

Boutin et al. 2000; Asghari and Evans 1992