Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 1 - 

REGISTRATION REVIEW

ECOLOGICAL RISK ASSESSMENT

PROBLEM FORMULATION FOR:

FOSTHIAZATE

O-ETHYL-S-(1-METHYLPROPYL)(2-OXO-3-THIAZOLIDINYL)-

PHOSPHONOTHIOATE 

CAS# 98886-44-3; PC Code 129022

PREPARED BY: 

Keara Moore, Chemist 

Environmental Risk Branch 3 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

APPROVED BY: 

Dana Spatz, Branch Chief 

Environmental Risk Branch 3 

Environmental Fate and Effects Division 

Office of Pesticide Programs 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 2 - 

1. STRESSOR SOURCE AND DISTRIBUTION

Fosthiazate is used for control of a broad spectrum of nematode species in tomatoes and 

also suppresses insects including sweet potato whitefly, tomato pinworm, and beet 

armyworm. It is an organophosphate insecticide which exerts toxic action through 

inhibition of acetylcholinesterase, an enzyme that is necessary for normal nerve function. 

 

Current labels allow application of fosthiazate at a maximum annual rate of 1.5 lb active 

ingredient/A. It must be applied through drip irrigation systems placed under a plastic 

mulch. Registered formulations of fosthiazate include the technical product (Reg. No. 

71512-6) as well as three end use products (Fosthiazate 900EC, Reg. No. 71512-4; 

Nematachide 500EC, Reg. No. 71512-5; and Fosthiazate 75% EC, Reg. No. 71512-17). 

All labels are time-limited and are scheduled to be terminated March 30, 2009. 

 

2. INTEGRATION OF AVAILABLE INFORMATION

The risk assessments available in the docket, and which serve as the basis for this 

problem formulation, include the following: 

• New Chemical Section 3 assessment (December 9, 2002), including Amendment 

(February 27, 2003) and Amendment II (July 10, 2003) 

• Organophosphate Cumulative Risk Assessment – 2006 Update (August, 2006) 

• Review of Environmental Fate Assessment for Fosthiazate Groundwater/ 

Drinking Water Modeling Estimates (February 26, 2008) 

3. ECOLOGICAL EFFECTS

TOXICITY STUDIES

Acute toxicity 

The available acute toxicity data on the active ingredient indicate that fosthiazate is 

highly toxic to birds (LD50 = 16.6 mg/kg; LC50 = 85 ppm), moderately toxic to small 

mammals (LD50 = 51 mg/kg, female rat), practically non toxic to freshwater fish (LC50 = 

111 ppm–157 ppm), highly toxic to freshwater invertebrates (EC50 = 0.26 ppm), slightly 

toxic to estuarine/marine fish (LC50 = 40.6 ppm), and highly to very highly toxic to 

estuarine/marine invertebrates (EC50 = 0.43 ppm-14.1 ppm). 

Chronic toxicity; aquatic animals 

Chronic effects in the freshwater fish early life stage using the fathead minnow 

(Pimephales promelas) included reduced dry weight and length of young fish (NOAEC = 

2.32 ppm, LOAEC 4.48 ppm). The freshwater invertebrate life-cycle study determined 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 3 - 

an NOAEC of 0.061 ppm and an LOAEC of 0.128 ppm based on reduced number of 

offspring produced and parental mortality. No chronic toxicity data are available for 

estuarine/marine fish or invertebrates. 

Chronic toxicity; terrestrial animals 

Both the mallard duck and bobwhite quail avian reproduction studies had the same 

NOAEC (21.7 ppm) and LOAEC (42.6 ppm). Significant effects on male and female 

weight gain, decreased eggshell thickness and increased cracked eggs were observed in 

the bobwhite quail. A 2 generation rat study yielded a reproductive NOAEL of 10 ppm 

(or 0.91 and 0.91 mg/kg/day for males and females respectively) and a LOAEL of 30 

ppm (equivalent to 2.09 and 2.60 mg/kg/day males and females, respectively) based on 

decreased viability index, litter size and pup weight for the f1 generation. 

Toxicity to plants 

An extensive battery of Tier I and Tier II terrestrial plant toxicity tests were performed on 

technical fosthiazate in which dicots appear to be more sensitive than monocots. Based 

on a Tier II seedling emergence test on technical fosthiazate, the NOAEC and EC25 for 

mustard are 0.8 and 1.0 lb ai/A respectively. Although the Tier I test results showed 

peppers were more sensitive (27 %) than mustard (<25 %) no test was conducted with 

peppers. For Tier II vegetative vigor, the most sensitive terrestrial plant is the dicot 

buckwheat (NOAEC = 0.8 lbs ai/A; EC25 = 3.3 lbs ai/A). Sorghum was the only monocot 

tested (NOAEC = 3.0 lbs ai/A and the EC50 = > 6.0 lbs ai/A). 

No acute risk LOCs were exceeded for non-vascular plants based on a Tier I test with the 

green alga (Selenastrum capricornatum) which found no adverse effects at a 

concentration equal to 4.5 ppm. There were no data for vascular plants. 

Toxicity to terrestrial invertebrates 

The acute contact honey bee LD50 is 0.247 ug/bee (MRID 41347636). Since the LD50 

is less than 2 ug per bee fosthiazate TGAI is categorized as highly toxic to pollinators on 

an acute contact basis. (MRID 41347636). 

INCIDENT REPORTS

The Agency has no incident reports in the Ecological Incident Information System (EIIS) 

for adverse effects to fish, wildlife, or plants that were attributed to fosthiazate use. 

However, the lack of reported incidents may be due to the fact that there is currently no 

actual usage of fosthiazate (see Section 6), rather than to low risk. 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 4 - 

4. EXPOSURE CHARACTERISTICS

In the terrestrial environment, laboratory studies suggest that persistence of fosthiazate in 

soils will be on the scale of weeks to months, with rates depending on soil pH. In aerobic 

soils, measured half-lives were 14.0 to 57.7 days with significantly slower metabolism in 

acidic soils (t1/2 = 41.8 to 57.7 days) than in alkaline soils (t1/2 =14.0 to 20.7 days). 

Substantial degradation was seen in sterile controls as well, suggesting that some of the 

transformation occurs through abiotic processes. Fosthiazate has low sorption to soil and, 

based on Koc values ranging from 25 to 285.7 mL/goc, is classified as mobile to 

moderately mobile by the FAO classification scheme. 

The persistence and mobility of fosthiazate are conducive to runoff, but currently 

approved application methods are designed to minimize potential transport to surface 

water. Labels require application through drip irrigation under plastic mulch. Therefore, 

spray drift to surface water is not a concern, and the potential for runoff is reduced by the 

direct soil application and by the protection from precipitation. If fosthiazate does reach 

aquatic environments, it is stable to aquatic photolysis and has limited hydrolysis in 

neutral and acidic conditions, but it hydrolyzes rapidly in alkaline conditions. It is 

moderately persistent in water-sediment systems, with an average half-life in aerobic 

environments of 32 days and in anaerobic environments of 41 days. 

Terrestrial field dissipation (TFD) studies were conducted with 4 lbs a.i./A applied to 

bare ground and incorporated 4 to 5 inches with no plastic mulch. In three sites, 

measured half-lives were 9 to 17 days, and in a fourth site, the half-life was 28 days. All 

studies were conducted in acidic soils (pH <6.4), where the leaching potential of 

fosthiazate is expected to be the highest. In most cases, residues were detected to no 

more than 15-30 cm below the soil surface, but in three of eight tests, residues were 

found at low levels up to 46 cm below the soil surface. 

 

In the currently registered use, the plastic mulch cover would minimize volatilization 

and runoff, therefore increasing the amount of the chemical available for leaching. 

However, with the drip irrigation method, a small amount of water is slowly dripped into 

soils precisely where it is needed, thus lessening the amount of water flowing down 

through the soil past the root zone where it cannot be used by the crop. This should 

greatly reduce the potential for the chemical to reach ground water systems.

As summarized in Table 1, carbon dioxide and bound residues were the primary end 

products in most cases. Additionally, six major degradates were detected along with 

unidentified organic volatiles. Study lengths were not sufficient to determine the pattern 

of formation and decline for all degradates. The only degradates found in terrestrial field 

dissipation studies were MBSo/MBSx and BSA, at maximum amounts of 1 to 4% of the 

applied. 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 5 - 

Table 1. Major degradates of fosthiazate, as a percentage of the parent applied.

Study Type (Study Length)a

Hydrolysis Degradates b

pH 5 

(30 d) 

pH 7 

(30 d)

pH 9 

(14 d)

Aerobic 

Soil 

(150 d) 

Aerobic 

Aquatic

(140 d) 

Anaerobic 

Aquatic Soil 

(120 d) 

DETO ~13 % ~13 % up to 10% c minor 

DBTO 45% 

TZO 40% 32% 

(60 d) 

DTTO 44% up to 10% c minor 

MBSo/ 

MBSx d 11% 

(28 – 35 d) 

16.4% 

(60 d) 

BSA 20% 

Unid'd organic 

volatile ~30% 8% 

a

 Unless otherwise noted, maximum concentrations occurred at study termination. 

b

DETO: (RS)-S-sec-butyl hydrogen 2-oxo-1,3-thiazolidin-3-ylphosphorothioate; DTTO: (RS)-S-sec-butyl O-ethyl 

hydrogen phosphorothioate; MBSo: sec-butyl methyl sulfone; MBSx: sec-butyl methyl sulfoxide; BSA: 2-

butanesulfonic acid. 

c

 Aerobic aquatic study did not separate DETO/DTTO. 

d

 MBSo/MBSx not separated. 

5. CLEAN WATER ACT

 

Fosthiazate is not identified as a cause of impairment for any water bodies listed as 

impaired under section 303(d) of the Clean Water Act, based on information provided at 

http://oaspub.epa.gov/tmdl/waters_list.impairments?p_impid=3. 

The Agency invites submission of water quality data for this pesticide. To the extent 

possible, data should conform to the quality standards in Appendix A of the OPP 

Standard Operating Procedure: Inclusion of Impaired Water Body and Other Water 

Quality Data in OPP’s Registration Review Risk Assessment and Management Process 

(see: http://www.epa.gov/oppfead1/cb/ppdc/2006/november06/session1-sop.pdf), in 

order to ensure they can be used quantitatively or qualitatively in pesticide risk 

assessments. 

6. CHARACTERISTICS OF ECOSYSTEMS POTENTIALLY AT RISK

For fosthiazate and pesticides in general, the ecosystems at greatest risk are those in close 

proximity to the use areas. These would include agricultural fields (surrounding nonagricultural terrestrial habitats) and water bodies directly adjacent to treated fields that 

may receive chemical residues via drift, volatilization, and/or runoff. Within water 

bodies, the water column, sediments, and pore water are all compartments of concern. 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 6 - 

Fosthiazate is registered for use on tomatoes only. According to data from the United 

States Department of Agriculture’s National Agriculture Statistics Service (USDANASS) pesticide usage data from 2007 and 20081

, tomatoes are grown primarily in 

California (78% total national acreage) and Florida (9%). However, the Screening Level 

Usage Analysis (SLUA) done by the Biological and Economic Analysis Division 

(BEAD) found no use data for fosthiazate (Use Report Package Memo; October 28, 

2008). This is based on data from the Screening Level Usage Analysis (SLUA) database, 

which is a compilation of NASS (National Agricultural Statistics Survey) and proprietary 

data from 2001-2007, and the California Department of Pesticide Regulation (CDPR) 

database was checked as well. 

7. ASSESSMENT ENDPOINTS

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

value that is to be protected.” Defining an assessment endpoint involves two steps: 1) 

identifying the valued attributes of the environment that are considered to be at risk; and 

2) operationally defining the assessment endpoint in terms of an ecological entity (i.e., a 

community of fish and aquatic invertebrates) and its attributes (i.e., survival and 

reproduction). Therefore, selection of the assessment endpoints is based on valued 

entities (i.e., ecological receptors), the ecosystems potentially at risk, the migration 

pathways of pesticides, and the routes by which ecological receptors are exposed to 

pesticide-related contamination. The selection of clearly defined assessment endpoints is 

important because they provide direction and boundaries in the risk assessment for 

addressing risk management issues of concern. Changes to assessment endpoints are 

typically estimated from the available toxicity studies, which are used as the measures of 

effects to characterize potential ecological risks associated with exposure to a pesticide, 

such as fosthiazate. 

To estimate exposure concentrations, the ecological risk assessment considers application 

at the maximum application rate to fields that have vulnerable soils. The most sensitive 

toxicity endpoints are used from surrogate test species to estimate treatment-related direct 

effects on acute mortality and chronic reproductive, growth and survival assessment 

endpoints. Toxicity tests are intended to determine effects of pesticide exposure on birds, 

mammals, fish, terrestrial and aquatic invertebrates, and plants. These tests include shortterm acute, sub-acute, and reproduction studies and are typically arranged in a 

hierarchical or tiered system that progresses from basic laboratory tests to applied field 

studies. The toxicity studies are used to evaluate the potential of a pesticide to cause 

adverse effects, to determine whether further testing is required, and to determine the 

need for precautionary label statements to minimize the potential adverse effects to nontarget animals and plants. 

Fosthiazate is classified as highly toxic to birds and moderately toxic to mammals on an 

acute oral basis. It is also highly toxic to aquatic invertebrates and honeybees on an acute 

basis, but it is only slightly toxic to fish. Chronic toxicity data that are available for these 

 

1 http://www.nass.usda.gov/QuickStats/indexbysubject.jsp?Text1=&site=NASS_MAIN&select=Select+a+State&Pass_ 

name=&Pass_group=Crops+%26+Plants&Pass_subgroup=Vegetables. Accessed 2/26/2009.
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 7 - 

taxa indicate that chronic effects may be of concern for exposed animals that do not 

experience acute lethality. 

8. CONCEPTUAL MODEL

In order for a chemical 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 exposure 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 (Figure 1) depicts the potential pathways for ecological risk 

associated with fosthiazate use. The conceptual model provides an overview of the 

expected exposure routes for organisms within the fosthiazate action area. For terrestrial 

organisms, the major route of exposure considered is the dietary route; consumption of 

food items such as seeds or insects that have fosthiazate residues as a result of contact in 

the soil. Foliar residues are not expected because there are no spray applications. For 

aquatic animal species, the major routes of exposure are considered to be via the 

respiratory surface (gills) or the integument. Direct contact and/or root uptake is the 

major route of exposure for terrestrial and wetland (riparian) plants, while aquatic plants 

may be exposed via direct uptake and adsorption. Estimated exposure concentrations for 

all organisms are obtained through the use of several Agency exposure models. 

RISK HYPOTHESIS

Based on an examination of the physical/chemical properties of fosthiazate, the fate and 

disposition in the environment, and mode of application, a conceptual model was 

developed that represents the possible relationships between the stressor, ecological 

receptors, and the assessment endpoints. 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 8 - 

Stressor Fosthiazate, applied through drip irrigation under plastic mulch

Source/

Transport

Pathways

Direct

Deposition

Runoff/ 

Erosion

Leaching

(infiltration/ 

percolation)

Source/

Exposure

Media

Terrestrial Food 

Residues in soil

(seeds, insects)

Upland

Foliage/Soil

Riparian/

Wetland 

Foliage/Soil

Water 

Column/ 

Sediment

Groundwater

Exposure

Point

Ingestion Direct contact Direct contact Gill/

Integument Uptake

Receptors

Attribute

Changes

Terrestrial Vertebrates

Birds, Small Mammals,

Reptiles, Terrestrial 

Insects

Terrestrial

Upland

Plants

Individual Animals

Reduced survival

Reduced growth

Reduced reproduction

Aquatic Invertebrates

Aquatic Vertebrates

Amphibians

Aquatic Plants

Wetland/ 

Riparian

Plants

Individual Plants

Seedling emergence

Vegetative vigor

Individual 

Vertebrates and 

Invertebrates

Reduced

survival

Reduced

growth

Reduced

reproduction

Food Web Exposure 

(biconcentration)

Piscivourous

Vertebrates

Birds, 

Mammals

Individual 

Animals

Reduced

survival,

growth, and

reproduction

Plant 

Population

Reduced

population

growth

Figure 1. Conceptual model for potential effects of fosthiazate application to nontarget aquatic and terrestrial organisms. 

9. ANALYSIS PLAN OPTIONS

In Registration Review, pesticide ecological risk assessments will follow the Agency’s 

Guidelines for Ecological Risk Assessment, will be in compliance with the paper titled 

“Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs, U.S. Environmental Protection Agency” (“Overview Document”) (January 

2004), and will be done in accordance with Section 7 of the Endangered Species Act. 

ECOLOGICAL RISK ASSESSMENT

Previously completed screening level ecological risk assessments for application of 

fosthiazate at 1.5 lb a.i./A through drip irrigation under plastic mulch have found that this 

use poses minimal acute or chronic risk to endangered and non-endangered non-target 

plants and animals. Therefore, no additional analysis to refine conclusions will be 

necessary for registration review. 

Due to the application method, aquatic exposure will be low and risk to aquatic 

organisms is minimal. In the new chemical assessment, uses of fosthiazate at twice the 

currently labeled rate applied through broadcast spray did not exceed any aquatic levels 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 9 - 

of concern. With the drip irrigation method required by current labels, drift to surface 

water is not a factor, and because fosthiazate is applied directly to soil in low amounts of 

water and is protected from precipitation by the plastic mulch, runoff is unlikely. 

Therefore, previous assessments concluded that any aquatic exposure from the current 

use on tomatoes will be substantially lower than in those estimated for other uses and so 

there will still be no exceedances. 

In the terrestrial environment, there is some potential for dietary exposure to birds and 

mammals who probe beneath the surface for seeds and insects or which feed on emerging 

seedlings containing translocated residues. This exposure is unquantifiable and expected 

to be negligible, so there is minimal risk to terrestrial organisms as well. 

For all use patterns applied by spray methods, the previous assessment found that 

fosthiazate would pose acute risk to birds, mammals and pollinators such as honey bees 

as well as chronic risk to birds and mammals. It would pose acute and chronic risk to 

endangered birds and mammals as well as acute risk to endangered insects. Acute or 

chronic risk LOCs were not exceeded for non-endangered and endangered freshwater 

fish, marine/estuarine fish or invertebrates, or to aquatic plants. At the highest 

application rates, fosthiazate exceeded the LOC for acute risk to endangered freshwater 

aquatic invertebrates. Because these spray uses are no longer supported, additional 

analysis to refine these conclusions will not be necessary. 

Although the Agency anticipates that no quantitative ecological risk assessment will be 

needed, a qualitative endangered species assessment will be conducted for this fosthiazate 

use. 

DRINKING WATER ASSESSMENT

The most recent drinking water assessment considered exposure from the currently 

labeled use on tomatoes using drip irrigation under plastic mulch (Review of 

Environmental Fate Assessment for Fosthiazate Groundwater/ Drinking Water Modeling 

Estimates; February 26, 2008). Estimated drinking water concentrations in ground water 

were modeled using the most up to date version of SCI-GROW (v2.3, Jul. 29, 2003) and 

the most recent data. Therefore, no refinement is necessary unless requested by the 

Health Effects Division. 

That assessment found exposure to fosthiazate in drinking water from surface water 

sources to be unlikely due to the application method. In ground water, the estimated 

drinking water concentration was 1.8 ug/L. There is some uncertainty in this result 

because the model does not account for the drip irrigation application method or for the 

plastic mulch. Drip irrigation is likely to lead to lower exposure because the low amount 

of water used would limit leaching. However, additional uncertainty results from the pH 

dependence of fosthiazate fate behavior. The modeled results are based on sorption and 

persistence data that represent the average of all soils, while there may be more leaching 

and higher exposure in acidic soils where fosthiazate is more persistent. 
Registration Review 

Ecological Risk Assessment 

Problem Formulation for the Nematicide Fosthiazate 3/3/2009 

- 10 - 

ANTICIPATED DATA NEEDS

The Agency does not foresee requiring any additional ecological effects or environmental 

fate studies listed in 40 CFR Part 158 in order to complete registration review for the 

currently registered use on tomatoes using drip irrigation. Although the new chemical 

assessment identifies data gaps, those gaps are only relevant to uses that are no longer 

supported, and the assessment concludes that for the drip irrigation method, all fate and 

ecotoxicity requirements have been met. Because no additional analysis is needed to 

refine risk conclusions from the currently registered us, no additional data are needed. 

No search of open literature will be conducted.