Risks of Acephate Use to the Federally 

Listed California Red Legged Frog 

(Rana aurora draytonii) 

Pesticide Effects Determination 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

Washington, D.C. 20460 

July 19, 2007 

1
Primary Authors 

Michael Davy, Agronomist 

William P. Eckel, Ph.D., Agronomist 

Carolyn Hammer, Environmental Scientist 

Secondary Review 

Donna Randall, Senior Biologist 

Nelson Thurman, Senior Science Advisor 

Dana Spatz, Acting Branch Chief, 

Environmental Risk Assessment Branch 2 

2
Table of Contents 

1. Executive Summary.................................................................................................................6 

2. Problem Formulation ............................................................................................................10 

2.1 Purpose..........................................................................................................................10 

2.2 Scope..............................................................................................................................12 

2.2.1. Degradates...............................................................................................................12 

2.2.2. Mixtures....................................................................................................................13 

2.2.3. Other use sites not quantitatively assessed...........................................................15 

2.3 Previous Assessments...................................................................................................16 

2.3.1 Acephate Assessments ..............................................................................................16 

2.3.2 California Red-legged Frog Assessments...........................................................16 

2.4 Stressor Source and Distribution ...............................................................................17 

2.4.1 Environmental Fate Assessment.........................................................................17 

2.4.2 Environmental Transport Assessment...............................................................18 

2.4.3 Mechanism of Action...........................................................................................19 

2.4.4 Use Characterization ...............................................................................................20 

2.5 Assessed Species...........................................................................................................28 

2.5.1 Distribution...........................................................................................................28 

2.5.2 Reproduction........................................................................................................33 

2.5.3 Diet ........................................................................................................................33 

2.5.4 Habitat ..................................................................................................................34 

2.6 Designated Critical Habitat ........................................................................................35 

2.6.1. Special Rule Exemption for Routine Ranching Activities....................................37 

2.7 Action Area...................................................................................................................38 

2.8 Assessment Endpoints and Measures of Ecological Effect ......................................44 

2.8.1. Assessment Endpoints for the CRLF.................................................................44 

2.8.2. Assessment Endpoints for Designated Critical Habitat...................................46 

2.9 Conceptual Model........................................................................................................49 

2.9.1 Risk Hypotheses...................................................................................................49 

2.9.2 Diagram ................................................................................................................50 

2.10 Analysis Plan ................................................................................................................53 

2.10.1 Exposure Analysis...................................................................................................53 

2.10.2 Effects Analysis........................................................................................................54 

2.10.3 Action Area Analysis...............................................................................................55 

3. Exposure Assessment.............................................................................................................56 

3.1 Label Application Rates and Intervals .........................................................................56 

3.2 Aquatic Exposure Assessment.......................................................................................57 

3.2.1. Conceptual Model of Exposure...............................................................................57 

3.2.2 Existing Monitoring Data.........................................................................................57 

3.2.3 Modeling Approach ..................................................................................................57 

3.2.4. Aquatic EEC Results...............................................................................................60 

3.3. Terrestrial Exposure Assessment.................................................................................60 

3.3.1 Conceptual Model of Exposure................................................................................61 

3.3.2. Modeling Approach .................................................................................................61 

3.3.3. Model Inputs.............................................................................................................62 

3
3.3.4 Results........................................................................................................................62 

4. Effects Assessment.................................................................................................................64 

4.1 Evaluation of Aquatic and Terrestrial Ecotoxicity Studies........................................64 

4.1.1 Toxicity to Freshwater Aquatic Animals................................................................69 

4.1.2 Toxicity to Amphibians – Aquatic Phase................................................................73 

4.1.3 Toxicity to Freshwater Invertebrates.................................................................74 

4.1.4. Toxicity to Birds........................................................................................................75 

4.1.5. Toxicity to Mammals..............................................................................................78 

4.1.6 Toxicity to Insects .....................................................................................................80 

4.1.7. Summary of Effects Assessment.........................................................................81 

5. Risk Characterization............................................................................................................86 

5.1 Risk Estimation...............................................................................................................86 

5.1.1 Aquatic Direct and Indirect Effects........................................................................86 

5.1.2 Terrestrial Phase Direct Effects ..............................................................................89 

5.1.3. Individual Effects Calculation for Direct Acute Effect on CRLF......................91 

5.1.4. Indirect Effects, Terrestrial Phase .........................................................................93 

5.2 Risk Description...........................................................................................................94 

5.2.1 Direct Effects to the California Red Legged Frog ............................................95 

5.2.2. Indirect Effects to the CRLF ................................................................................100 

5.3 Action Area.....................................................................................................................100 

5.3.1. Aquatic Phase.........................................................................................................101 

5.3.2. Terrestrial Phase....................................................................................................102 

5.4 Listed Species Effect Determination for the California Red-Legged Frog .............105 

5.4.1. “May Affect” Determination ................................................................................105 

5.4.2 “Adverse Effect” Determination ...........................................................................105 

5.5 Risk Hypotheses Revisited ...........................................................................................108 

6. Uncertainties.........................................................................................................................110 

6.1.1 Maximum Use Scenario.........................................................................................110 

6.1.2 Usage Uncertainties ...............................................................................................110 

6.2. Exposure Assessment Uncertainties..........................................................................110 

6.2.1 PRZM Modeling Inputs and Predicted Aquatic Concentrations..................111 

6.2.2 Aquatic Exposure Estimates.................................................................................112 

6.3.3 Residue Levels Selection........................................................................................113 

6.3.4 Dietary Intake.........................................................................................................113 

6.4 Effects Assessment Uncertainties .............................................................................114 

6.4.1 Age Class and Sensitivity of Effects Thresholds.............................................114 

6.4.2 Extrapolation of Long-term Environmental Effects from Short-Term 

Laboratory Tests...............................................................................................................114 

6.4.3 Sublethal Effects.....................................................................................................114 

6.4.4 Location of Wildlife Species....................................................................................115 

6.5. Use of avian data as surrogate for amphibian data.................................................115 

6.6. Assumptions Associated with the Acute LOCs.......................................................115 

6.8. Action Area.................................................................................................................115 

7. References.............................................................................................................................117 

4
Appendices 

Appendix A Ecological Effects Data 

Appendix B Aquatic Exposure Modeling Runs 

Appendix C Incident Database Information 

Appendix D ECOTOX Database, Accepted 

Appendix D1 ECOTOX Database, Excluded 

Appendix D2 ECOTOX Database, Mixures 

Appendix E Toxicity Categories and LOCs 

Appendix F T-REX Model Outputs 

Appendix F1 T-HERPS Model Outputs 

Appendix G Acephate Label Information 

Appendix H Acephate Mixture Product Names 

Appendix I Fate Properties of Acephate 

Appendix J Usage Data 

Appendix K Geographic Information systems (GIS) maps 

Appendix L Terrestrial Invertebrate Exposure 

Attachments 

1. Life History of the California Red-Legged Frog 

2. Baseline Cumulative Effects 

5
1. Executive Summary 

Background 

Acephate is an organophosphate insecticide currently registered for use on a variety of 

field, fruit, and vegetable crops; in food handling establishments; rights-of-ways, 

fencerows, sewage disposal areas, drainage systems, and on ornamental plants both in 

greenhouses and outdoors (including lawns, turf, and cut flowers). Acephate use in 

California entails a variety of application techniques and varies greatly in amounts 

applied, number of applications, application intervals, and timing of applications. 

Acephate may move through the environment and be transported away from the site of 

application by run-off or spray drift. The toxic degradate of acephate, methamidophos, is 

also considered in this assessment. 

Exposure pathways to the CRLF or its critical habitat were considered non-existent for 

indoor uses that remain indoors which includes use in eating establishments, food 

processing facilities and other indoor applications. These uses are therefore considered to 

have “no effect” on the CRLF or its critical habitat. The following summary addresses 

effects to the CRLF and its critical habitat from the remaining labeled uses. 

There were insufficient monitoring data to support an aquatic evaluation based on 

concentrations found in water samples; specifically there were no targeted monitoring 

data on acephate for this region. Therefore, it was necessary to estimate exposure based 

on modeled results. 

The mode of action for acephate is similar to other organophosphate insecticides in that 

the chemical inhibits an enzyme, acetylcholinesterase. This action causes disruption to 

the central nervous system. 

Aquatic Phase 

Direct, acute effects to the aquatic phase CRLF are not expected as there are no acute 

listed LOC exceedences for freshwater fish, the surrogate test species for the aquatic 

phase CRLF. An acute-to-chronic ratio analysis with other organophosphate insecticides 

indicated no LOC exceedence for reproductive effects. Indirect effects to the aquatic 

phase of the frog, due to effects on critical habitat are not expected, since there were no 

LOC exceedences to aquatic plants, nor effects to water quality. Indirect effects to 

CRLF, based on food availability are not expected, because the effect on invertebrate 

food sources is discountable. Thus it was determined that acephate use is not likely to 

adversely affect the aquatic phase CRLF, or its critical habitat. 

Terrestrial Phase 

Acephate use is likely to adversely affect the terrestrial phase of the CRLF directly, as 

determined by acute and chronic LOC exceedences for birds, the surrogate test species 

for terrestrial phase CRLF. Avian reproductive effects indicate direct chronic fecundity 

6
effects to CRLF. Toxic effects on the CRLF prey base are likely to adversely affect the 

terrestrial phase CRLF as several taxa from the CRLF diet exceed the LOC. Birds, 

mammals, insects, and small amphibians are all part of the terrestrial CRLF diet. 

Because multiple components of the diet are expected to be affected, including mammals, 

birds and insects, an LAA determination was made for indirect effects. An LAA 

determination for terrestrial critical habitat was concluded based on adverse modification 

of terrestrial food resources. 

Based on LOC exceedences, the overlap of use sites with frog habitat and core areas, and 

other factors, the following table summarizes the effects determination for the CRLF 

from methamidophos use. 

Assessment Effects Basis for Determination Endpoint determination

Aquatic Phase 

(Eggs, larvae, tadpoles, juveniles, and adults) 

Direct Effects and Critical Habitat Effects 

1. Survival, growth, All Acute and Chronic RQ are below the listed LOC for and reproduction of No Effect surrogate species (rainbow trout) CRLF 

Indirect Effects 

2. Reduction or 

modification of 

aquatic prey base 

May Affect, Acute LOC is exceeded for aquatic invertebrates, 

Not Likely to however effect is considered discountable based on 

Adversely Affect low likelihood of individual effect. 

3. Reduction or 

modification of No Effect No LOC Exceedences for any plant species aquatic plant 

community 

4. Degradation of 

riparian vegetation 

No LOC Exceedences for any plant species. No 

No Effect adverse aquatic critical habitat modification is 

expected. 

Terrestrial Phase 

(Juveniles and Adults)

Direct Effects 

5. Survival, growth, 

and reproduction of 

CRLF 

Acute and Chronic LOC exceedences for birds, the 

surrogate species for direct effects to frogs. Initial May Affect, Area of Concern overlaps habitat. Use is widespread Likely to (nearly all counties). Use is documented in all months. Adversely Affect Probability of effect approaches 100% at calculated 

RQs. 

Indirect Effects and Critical Habitat Effects 

6. Reduction or 

modification of 

terrestrial prey base 

Acute and Chronic LOC exceedences for multiple 

components of CRLF prey base (mammals, birds, and 

May Affect, terrestrial invertebrates). LAA to terrestrial phase 

Likely to CRLF and its critical habitat based on acute RQs 

Adversely Affect exceeding 0.5 and chronic RQs over LOC for 

mammals, insects, birds. Adverse terrestrial critical 

habitat modification is expected. 

7
Assessment Effects Basis for Determination Endpoint determination

7. Degradation of No Effect No plant LOC exceedences. riparian vegetation 

Effects on Primary Constituent Elements of the Critical Habitat 

 Aquatic Breeding and Non-breeding Habitat 

Adverse effects on the aquatic critical habitat are not expected, as there are is No Effect 

via aquatic plants, and the effect on invertebrates is discountable. 

 Upland and Dispersal Habitat 

There may be effects on these habitats through reduction in prey base (invertebrates, and 

small mammals, birds, and amphibians). 

There may also be a reduction in shelter for the CRLF (small mammal burrows) due to 

the effects on mammals. 

Action Area 

Based on chronic effects to small birds consuming short grass food items, a terrestrial 

buffer zone of 2,913 feet is needed to delineate the Action Area. This is the distance 

from the edge of the use site needed to reduce exposure to below the Level of Concern 

for all taxa considered. 

The aquatic Action Area is based on effects to prey items (invertebrates) exposed to the 

degradate methamidophos. Based on the RQs, terrestrial effects are expected to dominate 

the Action Area. 

When evaluating the significance of this risk assessment’s direct/indirect and adverse 

habitat modification effects determinations, it is important to note that pesticide 

exposures and predicted risks to the species and its resources (i.e., food and habitat) are 

not expected to be uniform across the action area. In fact, given the assumptions of drift 

and downstream transport (i.e., attenuation with distance), pesticide exposure and 

associated risks to the species and its resources are expected to decrease with increasing 

distance away from the treated field or site of application. Evaluation of the implication 

of this non-uniform distribution of risk to the species would require information and 

assessment techniques that are not currently available. Examples of such information and 

methodology required for this type of analysis would include the following: 

• Enhanced information on the density and distribution of CRLF life stages 

within specific recovery units and/or designated critical habitat within the 

action area. This information would allow for quantitative extrapolation 

8
of the present risk assessment’s predictions of individual effects to the 

proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

• Quantitative information on prey base requirements for individual aquaticand terrestrial-phase frogs. While existing information provides a 

preliminary picture of the types of food sources utilized by the frog, it 

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

biologically relevant thresholds of effects on the prey base, and ultimately 

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

repeated exposure events and the inherent demographic characteristics of 

the prey population play into the extent to which prey resources may 

recover is not predictable. An enhanced understanding of long-term prey 

responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and 

together with the information described above, a more complete prediction 

of effects to individual frogs and potential adverse modification to critical 

habitat. 

9
2. Problem Formulation 

Problem formulation provides a strategic framework for the risk assessment. By 

identifying the important components of the problem, it focuses the assessment on the 

most relevant life history stages, habitat components, chemical properties, exposure 

routes, and endpoints. The structure of this risk assessment is based on guidance 

contained in U.S. EPA’s Guidance for Ecological Risk Assessment (U.S. EPA 1998), the 

Services’ Endangered Species Consultation Handbook (USFWS/NMFS 1998) and is 

consistent with procedures and methodology outlined in the Overview Document (U.S. 

EPA 2004) and reviewed by the U.S. Fish and Wildlife Service and National Marine 

Fisheries Service (USFWS/NMFS 2004).. 

2.1 Purpose 

The purpose of this endangered species assessment is to evaluate potential direct and 

indirect effects on individuals of the federally threatened California red-legged frog 

(Rana aurora draytonii) (CRLF) arising from FIFRA regulatory actions regarding use of 

acephate on labeled use sites. In addition, this assessment evaluates whether these 

actions can be expected to result in the destruction or adverse modification of the species’ 

critical habitat. Key biological information for the CRLF is included in Section 2.5, and 

designated critical habitat information for the species is provided in Section 2.6 of this 

assessment. This ecological risk assessment has been prepared as part of the Center for 

Biological Diversity (CBD) vs. EPA et al. (Case No. 02-1580-JSW(JL)) settlement 

entered in the Federal District Court for the Northern District of California on October 

20, 2006. 

In this endangered species assessment, direct and indirect effects to the CRLF and 

potential adverse modification to its critical habitat are evaluated in accordance with the 

methods (both screening level and species-specific refinements, when appropriate) 

described in the Agency’s Overview Document (U.S. EPA 2004). 

In accordance with the Overview Document, provisions of the ESA, and the Services’ 

Endangered Species Consultation Handbook, the assessment of effects associated with 

registrations of acephate are 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. It is acknowledged that the action area for a national-level FIFRA regulatory

decision associated with a use of acephate may potentially involve numerous areas 

throughout the United States and its Territories. However, for the purposes of this 

assessment, attention will be focused on relevant sections of the action area including 

those geographic areas associated with locations of the CRLF and its designated critical 

habitat within the state of California. 

 

As part of the “effects determination,” one of the following three conclusions will be 

reached regarding the potential for registration of acephate at the use sites described in 

10
this document to affect CRLF individuals and/or result in the destruction or adverse 

modification of designated CRLF critical habitat: 

• “No effect”; 

• “May affect, but not likely to adversely affect”; or 

• “May affect and likely to adversely affect”. 

Critical habitat identifies specific areas that have the physical and biological features, 

(known as primary constituent elements or PCEs) essential to the conservation of the 

listed species. The PCEs for CRLFs are aquatic and upland areas where suitable breeding 

and non-breeding aquatic habitat is located, interspersed with upland foraging and 

dispersal habitat (Section 2.6). 

If the results of initial screening-level assessment methods show no direct or indirect 

effects (no LOC exceedances) upon individual CRLFs or upon the PCEs of the species’ 

designated critical habitat, a “no effect” determination is made for the FIFRA regulatory 

action regarding acephate as it relates to this species and its designated critical habitat. If, 

however, direct or indirect effects to individual CRLFs are anticipated and/or effects may 

impact the PCEs of the CRLF’s designated critical habitat, a preliminary “may affect” 

determination is made for the FIFRA regulatory action regarding acephate. 

If a determination is made that use of acephate within the action area(s) associated with 

the CRLF “may affect” this species and/or its designated critical habitat, additional 

information is considered to refine the potential for exposure and for effects to the CRLF 

and other taxonomic groups upon which these species depend (e.g., aquatic and terrestrial 

vertebrates and invertebrates, aquatic plants, riparian vegetation, etc.). Additional 

information, including spatial analysis (to determine the geographic proximity of the 

CRLF habitat and175% acephate use sites) and further evaluation of the potential impact 

of acephate on the PCEs is also used to determine whether destruction or adverse 

modification to designated critical habitat may occur. Based on the refined information, 

the Agency uses the best available information to distinguish those actions that “may 

affect, but are not likely to adversely affect” from those actions that “may affect and are 

likely to adversely affect” the CRLF and/or the PCEs of its designated critical habitat. 

This information is presented as part of the Risk Characterization in Section 5 of this 

document. 

The Agency believes that the analysis of direct and indirect effects to listed species 

provides the basis for an analysis of potential effects on the designated critical habitat. 

Because acephate is expected to directly impact living organisms within the action area 

(defined in Section 2.7), critical habitat analysis for acephate is limited in a practical 

sense to those PCEs of critical habitat that are biological or that can be reasonably linked 

to biologically mediated processes (i.e., the biological resource requirements for the listed 

species associated with the critical habitat or important physical aspects of the habitat that 

may be reasonably influenced through biological processes). Activities that may destroy 

or adversely modify critical habitat are those that alter the PCEs and appreciably diminish 

the value of the habitat. Evaluation of actions related to use of acephate that may alter 

11
the PCEs of the CRLF’s critical habitat form the basis of the critical habitat impact 

analysis. Actions that may affect the CRLF’s designated critical habitat have been 

identified by the Services and are discussed further in Section 2.6. 

2.2 Scope 

Acephate (O, S-Dimethyl acetylphosphoramidothioate) is an organophosphate insecticide 

currently registered for use on a variety of field, fruit, and vegetable crops; in food 

handling establishments; on ornamental plants both in greenhouses and outdoors 

(including lawns, turf, and cut flowers); and in and around the home. Acephate was first 

registered in 1973 for ornamental uses and in 1974 for food uses (agricultural crops). 

Target pests include: Armyworms, aphids, beetles, bollworms, borers, budworms, 

cankerworms, crickets, cutworms, fire ants, fleas, grasshoppers, leafhoppers, loopers, 

mealybugs, mites, moths, roaches, spiders, thrips, wasps, weevils, and whiteflies. 

California’s Department of Pesticide Regulation maintains a database of all pesticide 

applications throughout the state and provides this information to the public. According 

to the Summary of Pesticide Use Reporting Data (2005), the reported pounds of acephate 

used have decreased by nearly 60%, from approximately 458,000 lbs in 1995 to 194,000 

lbs in 2005. The acreage to which acephate was applied has also decreased during that 

time period, from about 490,000 acres in 1995 to 198,000 acres in 2005. Since 

California’s Department of Pesticide Regulation (CDPR) Pesticide Use Reporting (PUR) 

data does not account for residential uses, the actual pounds used could be higher. 

The end result of the EPA pesticide registration process (the FIFRA regulatory action) is 

an approved product label. The label is a legal document that stipulates how and where a 

given pesticide may be used. Product labels (also known as end-use labels) describe the 

formulation type (e.g., liquid or granular), acceptable methods of application, approved 

use sites, and any restrictions on how applications may be conducted. Thus, the use or 

potential use of acephate in accordance with the approved product labels for California is 

“the action” being assessed. 

Although current registrations of acephate allow for use nationwide, this ecological risk 

assessment and effects determination addresses currently registered uses of acephate in 

portions of the action area that are reasonably assumed to be biologically relevant to the 

CRLF and its designated critical habitat. Further discussion of the action area for the 

CRLF and its critical habitat is provided in Section 2.7. 

 2.2.1. Degradates 

The major degradate of acephate in aerobic soil metabolism studies is methamidophos, 

(up to 23%) which is itself a registered insecticide. Methamidophos was also found in 

the aqueous photolysis study (maximum formation of 1.6% of the applied amount), the 

soil photolysis study (5.3 to 8.4% in both irradiated and control), the anaerobic aquatic 

metabolism study (5% at 7 days), and the aerobic aquatic metabolism study (<1.6%). 

12
Methamidophos is more toxic to vertebrates and invertebrates than is acephate, so its 

effects will be considered in the aquatic risk assessment. Because terrestrial LOCs are 

expected to be exceeded based on exposure to the parent acephate alone, methamidophos 

exposure will not be considered quantitatively for terrestrial wildlife. 

FIFRA methamidophos registered uses are also being assessed for the CRLF. Therefore, 

the effects characterization for methamidophos will be found in detail in the 

Methamidophos Assessment for CRLF and only summarized within this assessment as 

needed. 

Since there is information on the maximum formation of methamidophos in soil after 

application of acephate (23% at 32 days in a Fresno loam soil, less than 10% in two other 

soils), the exposure of aquatic organisms to methamidophos can be quantified in PRZMEXAMS modeling by assuming that the starting concentration of methamidophos in the 

soil is 23% of the application rate of acephate. The fate and transport properties of 

acephate and methamidophos are very similar, thus separate modeling runs are not 

necessary. 

 2.2.2. Mixtures 

Product Formulations Containing Multiple Active Ingredients 

The Agency does not routinely include, in its risk assessments, 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, they 

may be used qualitatively or quantitatively in accordance with the Agency’s Overview 

Document and the Services’ Evaluation Memorandum (U.S., EPA 2004; USFWS/NMFS 

2004). 

Formulated Product Data and Aquatic Exposure 

The limitation on the quantitative exposure modeling for formulations is based on the 

expectation that the varying physical-chemical properties of individual components of 

pesticide formulations will result in progressively different formulation constituents in 

environmental media over time. As the proportions of formulation components in 

environmental media differ from the proportions in the tested formulation, the 

assumption that environmental residues are toxicologically equivalent to tested 

formulations cannot be supported beyond the time period immediately following 

product application. This assumption is especially important in the case of runoff from 

treated areas to surface waters. In this case, varying fate and transport properties for 

each formulation component will result in a final proportion of the residues of these 

components in the receiving surface waters that is significantly different than the 

proportion of ingredients that are applied and that were tested in an aquatic organism 

13
toxicity study using the formulated product. Therefore available formulated product 

data for products directly applied to aquatic environments or that may drift into aquatic 

environments were considered, and only for effects resulting from acute exposure to the 

formulated product (see Overview Document section V.B.1.b.(2) and Services 

Evaluation memorandum). 

Formulated Product Data and Terrestrial Exposure 

In situations where available toxicity data indicate that a formulated product may be more 

toxic to terrestrial wildlife than indicated by active ingredient effects testing, it may be 

necessary to consider exposure to the formulation. Exposure modeling in these instances 

is limited to dietary exposure to residues for a time period immediately following 

pesticide product application. 

The limitation on the quantitative exposure modeling for formulations is based on the 

expectation that the varying physical-chemical properties of individual components of 

pesticide formulations will result in progressively different formulation constituents in 

environmental media over time. Because the proportions of formulation components in 

environmental media differ from the proportions in the tested formulation, the 

assumption that environmental residues are toxicologically equivalent to tested 

formulations cannot be supported beyond the time period immediately following 

product application. Therefore, available formulated product data for terrestrial 

applications were considered only for effects resulting from acute dietary exposure to 

the formulated product (see Overview Document section V.B.1.c.(2) and Services 

Evaluation memorandum). 

Acephate has registered products containing multiple active ingredients. These products, 

their product registration numbers, the active ingredient(s) in the product in addition to 

acephate, the percentage of each active ingredient in the product, and the available 

product formulation data are listed in Table 2.1 below. The ECOTOX search strategy 

for public scientific literature identifies studies addressing multiple active ingredients (see 

Overview Document and Services Evaluation memorandum). If a multiple active 

ingredient study was performed on any of the formulated pesticide products in Table 2.1, 

this assessment considers those studies. In addition to public literature, if the registrant 

has submitted data on any of the formulated products in Table 2.1, those data as well 

have been considered and noted. 

The below registered products containing multiple active ingredients are in pressurized 

containers used only for residential and greenhouse uses. Exposure pathways from 

indoor use to the CRLF are considered unlikely and therefore these uses have “no effect” 

on the CRLF. A quantitative risk assessment from mixtures to the CRLF was not done 

due to discountable nature of exposure of the mixtures. 

14
Table 2.1 Data on Acephate Mixtures with Other Active Ingredients 

Registrant 

Submitted 

Studies 

Public Scientific 

Literature 

Studies* 

Formulated 

EPA Reg# Product Formulation 

00023902476 

ORTHO SYSTEMIC 

ROSE & FLORAL 

SPRAY 

Acephate (0.25%) 

Resmethrin (0.1%) 

Triforine (0.1%) 

none 

00023902594 

ORTHENEX 

INSECT & 

DISEASE 

CONTROL 

FORMULA III 

Acephate (4%) 

Fenbutatinoxide 

(0.75%) 

Triforine (3.25%) 

none 

00023902595 

ISOTOX INSECT 

KILLER FORMULA 

IV 

Acephate (8%) 

Fenbutatinoxide (0.5%) 

none 

00049900441 WHITMIRE TC 136 

Acephate (1.5%) 

Fenpropathrin(1%) 

See Appendix D2 for 

listing of ECOTOX 

literature studies. 

none 

As summarized in Appendix H there are no product LD50 values, with associated 95% 

Confidence Intervals (CIs) available. 

As discussed in U.S. EPA (2000), a quantitative component-based evaluation of mixture 

toxicity requires data of appropriate quality for each component of a mixture. In this 

mixture evaluation, an LD50 with associated 95% CI is needed for the formulated 

product. The same quality of data is also required for each component of the mixture. 

Given that the formulated products for acephate do not have LD50 data available, it is not 

possible to undertake a quantitative or qualitative analysis for potential interactive effects. 

However, because the active ingredients are not expected to have similar mechanisms of 

action, metabolites, or toxicokinetic behavior, it is reasonable to conclude that an 

assumption of dose-addition would be inappropriate. Consequently, an assessment based 

on the toxicity of acephate is the only reasonable approach that employs the available 

data to address the potential acute risks of the formulated products in Appendix H. 

 2.2.3. Other use sites not quantitatively assessed 

Structural pest control applications, while a significant contribution to the total pounds of 

acephate reportedly applied each year, are not considered as likely exposure pathways to 

the CRLF or its critical habitat and are therefore considered to have “no effect” on the 

CRLF or its critical habitat. Indoor uses for eating establishments, food processing 

facilities and other indoor applications are not considered further in this assessment as the 

products are to be used indoors, in small amounts and disposed of according to the label 

instructions which will not result in exposure to the CRLF or its critical habitat. 

Therefore these indoor uses are also considered to have “no effect” on the CRLF or its 

critical habitat. 

15
2.3 Previous Assessments 

2.3.1 Acephate Assessments 

The Agency published an Interim Reregistration Eligibility Decision for Acephate in 

September 2001 which identified numerous human health and ecological risks associated 

with the labeled uses of acephate. Upon completion of the assessment, the Agency 

decided on a number of label amendments to address the worker, residential, and 

ecological concerns. Acephate and its degradate methamidophos are highly toxic to 

honey bees and beneficial predatory insects on an acute contact basis. Acute and chronic 

risks to birds and chronic risk to mammals were also of concern. The document is 

available on the web, at: http://www.epa.gov/oppsrrd1/REDs/acephate_ired.pdf. 

Numerous mitigation requirements (label amendments) resulted from the IRED 

assessment. However, at this time only those label amendment changes that are reflected 

by the current labels were assessed as the product user will follow the label rather than 

the IRED. 

Some changes include requiring labeling to protect honeybees and to reduce the potential 

for spray drift. Also, aerial applications to turf have been deleted as have residential 

indoor uses also been deleted. 

On March 31, 2004 EPA released an assessment of the potential effects of acephate to 26 

listed Environmentally Significant Units (ESUs) of Pacific salmon and steelhead. That 

assessment concluded that acephate would have no effect on the species under 

consideration. While acephate was noted to have significant toxicity to aquatic 

invertebrates, as does this assessment, the minimal usage, the size of the watersheds 

under consideration and the volume of the water bodies serving as habitat to these species 

taken together, resulted in the determination of no effect to the listed salmon and 

steelhead. 

2.3.2 California Red-legged Frog Assessments 

The Agency is currently developing a number of risk assessments for the CLRF, each 

addressing different pesticide active ingredients. A total of 66 chemicals will be 

assessed. Metolachlor is among the first group of ten chemicals to be completed. For 

information regarding the other chemicals in this group1

 please see the relevant 

document. 

 

1

 Other chemicals assessed in the first group include methamidaphos, methomyl, azinphos-methyl, 

acephate, imazpyr, aldicarb, metam sodium, diazinon and chloropicrin 

16
2.4 Stressor Source and Distribution 

Acephate technical is a colorless to white solid with a melting point of 81-91° C. 

Acephate is highly soluble in water (79 g/100 ml), acetone (151 g/100 ml), and ethanol 

(>100 g/100 ml), and is soluble in methanol (57.5 g/100 ml), ethyl acetate (35.0 g/100 

ml), benzene (16.0 g/100 ml), and hexane (<0.1g/100 ml) at 25° C. Acephate degrades to 

another, registered organophosphate insecticide, methamidophos.

Case number: 0042 

CAS registry number: 30560-19-1 

OPP chemical code: 103301 

Empirical formula: C4H10NO3PS 

Molecular weight: 183.16 g/mol 

Vapor Pressure: 1.7 x 10-6 mm Hg at 24°C 

Trade and other names: Orthene® 

Technical registrants: Valent U.S.A. Corporation; Micro-Flo Company LLC; United 

Phosphorous Ltd.; Drexel Chemical Corporation

2.4.1 Environmental Fate Assessment 

Aerobic soil metabolism is the main degradation process for acephate. Observed halflives are less than two days under the nominal or expected use conditions, producing the 

intermediate degradate methamidophos, which is also an insecticidally active compound. 

Methamidophos is itself rapidly metabolized by soil microorganisms to carbon dioxide 

and microbial biomass (half-lives of < 10 days). Acephate is stable against hydrolysis 

except at high pH (half-life at pH 9 of 18 days) and does not photodegrade. Acephate is 

not persistent in anaerobic clay sediment: creek water systems in the laboratory, with a 

half-life of 6.6 days. The major degradates under anaerobic conditions were carbon 

dioxide and methane, comprising > 60% of the applied acephate after 20 days of 

anaerobic incubation. No other anaerobic degradates were present at > 10% during the 

incubation. There are no acceptable data for the aerobic aquatic metabolism of acephate; 

supplemental information indicates that acephate degrades more rapidly in aquatic 

systems when sediment is present. Appendix I describes the fate properties of acephate 

in greater detail. 

Acephate is very soluble (80.1-83.5g/100 mL) and very mobile (Koc = 2.7) in the 

laboratory. Only one Koc value is available, because acephate was adsorbed in only one of 

the five soils (a clay loam) used in the batch equilibrium studies. When tested in the same 

soils, methamidophos was determined to be more mobile than acephate; again, only one 

Koc value is available (Koc = 0.9 in the clay loam soil). Because acephate is not persistent 

under aerobic conditions, very little acephate is expected to leach to groundwater. If any 

acephate does reach ground water, it would not be expected to persist, due to its short 

anaerobic half-life. Volatilization from soil or water is not expected to be a route of 

dissipation for either acephate or methamidophos. 

17
Field studies conducted in Mississippi (tobacco on silt loam soil), California (bell peppers 

on silt loam soil), Florida (cauliflower on sand soil) and Iowa (soybeans on loam soil) 

produced dissipation half-lives of 2 days or less with no detections of parent or the 

degradate methamidophos below a depth of 50 cm. Laboratory studies showed that 

bioaccumulation of acephate in bluegill sunfish was insignificant. A maximum 

bioaccumulation factor of 10x occurred after 14 days’ exposure to acephate at 0.007 and 

0.7 ppm. 

2.4.2 Environmental Transport Assessment 

Batch equilibrium studies 

Batch equilibrium studies using acephate and methamidophos were conducted using four 

soils ranging in texture from sand to clay loam. In three of the soils, acephate and 

methamidophos were not adsorbed in sufficient quantities to permit the calculation of 

Freundlich adsorption coefficients (Freundlich Kads). For the clay loam soil, the reported 

adsorption values for parent acephate and its degradate, methamidophos, are listed in the 

following table: 

Soil pH CEC 

(meq/100g) 

%clay %organic 

matter 

Acephate Methamidophos 

K 1/n r

2 K 1/n r2 Clay 

loam 

5.8 20.2 32 3.3 

0.090 1.06 0.96 0.029 0.64 0.93 

Calculated Koc for acephate and methamidophos in this clay loam soil were 2.7 and 0.9, 

respectively. Because of the minimal adsorption of the chemicals in the adsorption phase 

of the study, it was not possible to determine desorption values in the soils. Based on the 

values listed above, it appears that acephate and methamidophos are very mobile in soils. 

Volatility 

Based on the vapor pressure of acephate (pure active: 1.7 x 10-6mm Hg/Torr [MRID 

40390601]) and its calculated Henry’s constant (5.1 x 10-13 atm mole / m3

), it is not 

expected that acephate will volatilize from either soil or water in significant quantities. 

Therefore it is not expected that volatilization will be a significant route of dissipation for 

acephate. 

Long Range Transport 

Potential transport mechanisms generally include pesticide surface water runoff, spray 

drift, and secondary drift of volatilized or soil-bound residues leading to deposition onto 

nearby or more distant ecosystems. The magnitude of pesticide transport via secondary 

drift depends on the pesticide’s ability to be mobilized into air and its eventual removal 

through wet and dry deposition of gases/particles and photochemical reactions in the 

atmosphere. A number of studies have documented atmospheric transport and 

redeposition of pesticides from the Central Valley to the Sierra Nevada mountains 

18
(Fellers et al., 2004, Sparling et al., 2001, LeNoir et al., 1999, and McConnell et al., 

1998). Prevailing winds blow across the Central Valley eastward to the Sierra Nevada 

mountains, transporting airborne industrial and agricultural pollutants into Sierra Nevada 

ecosystems (Fellers et al., 2004, LeNoir et al., 1999, and McConnell et al., 1998). 

Therefore, physicochemical properties of the pesticide that describe its potential to enter 

the air from water or soil (e.g., Henry’s Law constant and vapor pressure), pesticide use, 

modeled estimated concentrations in water and air, and available air monitoring data 

from the Central Valley and the Sierra Nevadas are considered in evaluating the potential 

for atmospheric transport of acephate to habitat for the CRLF. 

In general, deposition of drifting or volatilized pesticides is expected to be greatest close 

to the site of application. Computer models of spray drift (AgDRIFT or AGDISP) are 

used to determine if the exposures to aquatic and terrestrial organisms are below the 

Agency’s Levels of Concern (LOCs). If the limit of exposure that is below the LOC can 

be determined using AgDRIFT or AGDISP, longer-range transport is not considered in 

defining the action area. For example, if a buffer zone <1,000 feet (the optimal range for 

AgDRIFT and AGDISP models) results in terrestrial and aquatic exposures that are 

below LOCs, no further drift analysis is required. If exposures exceeding LOCs are 

expected beyond the standard modeling range of AgDRIFT or AGDISP, the Gaussian 

extension feature of AGDISP may be used. In addition to the use of spray drift models to 

determine potential off-site transport of pesticides, other factors such as available air 

monitoring data and the physicochemical properties of the chemical are also considered. 

2.4.3 Mechanism of Action 

Some of the information for the mode of action below comes from Davies et. al., 1981. 

Organophosphate insecticides (such as acephate) act upon target pests through a 

neurotoxic action, which affects the central nervous system. Specifically, the mechanism 

of action is known to be acetylcholinesterase inhibition. The transmission of nerve 

impulses across synapses and the junctions between nerve and an organ (gland, muscle, 

nerve) is accomplished by the release of a chemical agent, acetylcholine. Acetylcholine 

must be rapidly destroyed or inactivated at or near the site of its release to continue 

transmission of new impulses. The destruction of acetylcholine at such sites is 

accomplished by an enzyme, acetylcholinesterase. Acetylcholinesterase is located at the 

neurosynaptic junctions and breaks the acetylcholine into acetyl and choline fragments. 

Acetylcholinesterase functions to increase the precision of nerve firing, enabling some 

nerve cells to fire as rapidly as 1,000 times per second without overlap of the of the 

neural impulses. Acetylcholinesterase inhibitors prevent the acetylcholinesterase from 

removing the acetylcholine and thereby causing disruption to the central nervous system. 

At a high enough concentration of the inhibitors, the muscles may not contract the 

diaphragm and breathing ceases and death results. 

Depending on the organophosphate involved, the dose received, and the duration of 

exposure; the period for regeneration of the acetylcholinesterase to occur varies among 

organisms. 

19
 2.4.4 Use Characterization

2.4.4.1 Use Profile 

Acephate is a an organophosphate insecticide currently registered for use on a variety of 

field, fruit, and vegetable crops; in food handling establishments; on ornamental plants 

both in greenhouses and outdoors (including lawns, turf, and cut flowers); and in and 

around the home. The use profile is based on the current, federally registered uses 

(Section 3 and 24c for California). There are well over 100 registered labels for 

acephate, with products ranging from 0.25% to 97.4% ai. Section 3 (nation-wide) and 

section 24(c) (California) registered uses for acephate are presented in Table 2.2. with the 

label maximum one time application, maximum annual application rate, and the 

minimum time between treatments. A complete list of product names and registration 

numbers is in Appendix G. 

Table 2.2. Labeled uses assessed in this document. 

Minimum 

Interval 

Between 

Retreatment 

(days) 

Seasonal 

Max 

Dose/Crop 

Cycle 

Max 

Application 

Rate Qty 

Max App 

Rate 

Unit/Area 

Registration 

Number Crop/Site (a) Equipment 

Bean and fruiting 

vegetable (1) 019713-00400 Aircraft; Ground 1.00 Lb ai /A 2.07 lb AN; 3 

Bermuda grass CA79013800 Aircraft; Ground 1.00 Lb ai /A NS NS 

Celery 019713-00400 Aircraft; Ground 1.00 Lb ai /A 2.07 lb NS; 3 

Christmas Tree 

Plantations 053883-00133 Aircraft; Ground 0.50 Lb ai /A NS NS; 28 

 066330-00354 Ground 0.50 lb/100 gal/A NS NS 

 066330-00354 Aircraft 0.50 lb/2 gal NS NS 

Citrus (non-bearing) 034704-00903 Ground 0.76 Lb ai/ A NS AN; 7 

 019713-00400 Sprinkler can 0.0012 gal mound NS 7 

lb ai/mound 

 059639-00026 Drencher 0.01 (4 ft diameter) NS NS 

Cole Crops (2) 019713-00400 Aircraft; Ground 1.00 Lb ai /A 2.07 lb AN; 7 

Cotton (Unspecified) 019713-00400 Aircraft; Ground 1.00 Lb ai /A 6 lb AN; 3 

Soil in-furrow 

 019713-00400 treatment 1.13 Lb ai /A NS an; 3 

 Seed treatment 

Hopper box; 

Slurry-type seed 

treater; Sprayer 

019713-00408 lb cwt (Lb 

 0.39 (0.117) ai/acre) NS NS 

Cranberry 059639-00026 Aircraft; Ground 1.00 Lb ai /A 0.9975 lb NS; 7 

Drainage Systems 019713-00400 Aircraft; Ground 0.25 Lb ai /A NS NS 

 070506-00001 Drencher 0.01 lb mound NS NS 

 070506-00002 Spoon 1.50 Tsp mound NS NS 

Outdoor Facilities/ 

Premises (3 and 12) 059639-00026 Aircraft; Ground 0.25 Lb ai /A NS AN 

 019713-00495 Sprinkler can 0.01 lb mound NS NS 

 019713-00495 Spoon 1.50 Tsp mound NS NS 

 070506-00001 Paintbrush 0.08 lb nest NS NS 

 070506-00001 Sprayer 0.08 lb nest NS NS 

20
Minimum 

Interval 

Between 

Retreatment 

(days) 

Seasonal 

Max 

Dose/Crop 

Cycle 

Max 

Application 

Rate Qty 

Max App 

Rate 

Unit/Area 

Registration 

Number Crop/Site (a) Equipment 

 081964-00002 Ground 0.25 lb/3.3 gal NS NS 

Aircraft; 

Ground; 

Hydraulic 

Sprayer 

Fencerows/ 

Hedgerows (4) 066330-00356 0.25 Lb ai /A NS NS 

 053883-00133 Spoon 0.002 gal mound NS NS 

 053883-00133 Sprinkler 0.01 lb mound NS NS 

 053883-00133 Spoon 1.50 Tsp mound NS NS 

Golf Course Turf 059639-00087 Ground 0.11 lb 1K sq.ft NS NS; 7 

066330-00356 Granule 

 073614-0001 applicator 5.0 Lb ai /A NS 7 

Lettuce 019713-00400 Aircraft; Ground 1.00 Lb ai /A 2.0 lb AN; 7 

Mint/Peppermint/Spe

armint 019713-00400 Aircraft; Ground 1.00 Lb ai /A 2.0 lb AN; 7 

Onion (24 C) CA87007100 Ground 1.00 Lb ai /A NS AN 

gal in. trunk 

Ornamentals (5) 070506-00001 Paintbrush 0.01 diam NS NS 

 059639-00087 Spoon 0.00015 gal pot NS NS 

 000239-02453 Shaker can 0.00011 gal sq.ft NS AN 

 070506-00008 Paintbrush 0.0038 gal tree NS NS 

 000499-00421 Aerosol can 0.50 lb 1K sq.ft NS NS 

 066330-00358 Sprayer 1.13 tbsp/1.5 gal NS 7 

oz 1.5K sq.ft 

 000499-00421 Aerosol can 4.00 (L) NS NS 

Mist blower; 

Hydraulic 

 019713-00544 sprayer 1.20 lb/100 gal/A NS NS 

 000239-02461 Sprayer 0.01 lb/1 gal NS 7 

 000239-02453 Shaker can 0.001 lb sq.ft NS 30 

Mist blower; 

Hydraulic 

 081964-00003 sprayer 1.20 Lb ai /A NS NS 

Aircraft; 

Ground; 

Hydraulic 

 059639-00033 Sprayer 0.45 Lb ai /A NS NS 

Hydraulic 

 059639-00028 sprayer 0.75 lb 1K sq.ft NS AN 

Granule 

 059639-00087 applicator 0.06 lb 1K sq.ft NS NS 

 000239-02472 Shaker can 0.00011 gal sq.ft NS AN 

 059639-00087 Spoon 0.00015 gal pot NS NS 

 059639-00033 Aircraft; Ground 1.00 Lb ai /A NS 3 

Peanuts 019713-00400 Aircraft; Ground 1.00 Lb ai /A 3.9975 lb AN; 7 

Hopper box; 

 059639-00033 Planter/seed box 0.20 lb cwt 3.9975 lb AN; 7 

Sewage Disposal 

Areas 019713-00544 Aircraft; Ground 0.13 Lb ai /A NS NS 

Soybeans 

(Unspecified) 059639-00026 Aircraft; Ground 1.00 Lb ai /A 1.5 lb 7 

21
Minimum 

Interval 

Between 

Retreatment 

(days) 

Seasonal 

Max 

Dose/Crop 

Cycle 

Max 

Application 

Rate Qty 

Max App 

Rate 

Unit/Area 

Registration 

Number Crop/Site (a) Equipment 

Stone Fruit (6) 059639-00091 Aircraft; Ground 0.97 Lb ai /A NS AN 

Pome Fruit (7) 059639-00091 Aircraft; Ground 0.97 Lb ai /A NS AN 

Tree Nut (8) 059639-00091 Aircraft; Ground 0.97 Lb ai /A NS AN 

Granule 

Turf (9) 000239-02632 applicator 4.95 Lb ai /A NS 7 

Hose-end 

 000239-02632 sprayer 6.75 tbsp 1K sq.ft NS AN 

 000239-02632 Sprayer 0.11 lb 1K sq.ft NS 14 

 059639-00026 Sprinkler can 0.01 lb mound NS 14 

 059639-00026 Sprinkler can 0.15 gal mound NS 14 

 059639-00026 Spoon 1.80 Tsp mound NS NS 

Uncultivated Land 

(10) 059639-00026 Aircraft; Ground 0.25 Lb ai /A NS NS 

Product 

 059639-00031 container 0.01 lb mound NS AN 

 059639-00031 Ground 1.80 Tsp mound NS NS 

 066330-00356 Spoon 0.00 gal mound NS NS 

 066330-00360 Ground 0.11 lb 1K sq.ft NS NS 

Vine Crop (11) 059639-00091 Aircraft; Ground 0.97 Lb ai /A NS AN 

Source: LUIS Report, Updated November 2006. 

(a) Similar use sites were combined into one category. 

(b) AN = as needed; NS = not specified 

1) Bean: Bean- Dried Type; Succulent-Lima; Succulent-Snap; Fruiting Vegetables; Pepper 

2) Cole: Brussels Sprouts; Cauliflower 

3) Outdoor Facilities/Premises: Commercial/Industrial/Industrial Premises/Equipment (Outdoor 

Household/Domestic Dwellings Outdoor Premises; Industrial Areas (Outdoor); Meat Processing Plant 

Premises (Nonfood Contact); Nonagricultural Outdoor Buildings/Structures; Paths/Patios; Paved Areas 

(Private Roads/Sidewalks); Refuse/Solid Waste Sites (Outdoor) 

4) Fencerows/Hedgerows: agricultural rights of way/fencerows/hedgerows; nonagricultural rights of 

way/fencerows/hedgerows 

5) Ornamentals: Crabapple; ornamental and/or shade trees; ornamental ground cover; ornamental herbaceous 

plants; ornamental nonflowering plants; ornamental woody shrubs and vines 

6) Stone Fruit: Apricot; Cherry; Plum; Prune 

7) Tree Fruit: Apple; Pear 

8) Tree Nut: Almond; Pistachio; Walnuts (English/Black) 

9) Turf: Commercial/industrial lawns; ornamental lawns and turf; ornamental sod farm (turf); recreation area 

lawns; residential lawns 

10) Uncultivated Land; agricultural fallow/idleland; agricultural uncultivated areas; nonagricultural uncultivated 

areas/soils; recreational areas 

11) Vine Crop: kiwi fruit; grapes 

12) The following uses do not result in exposure of the CRLF or essential habitat because they are solely indoor 

uses and associated maximum application rates are not included in this table : Commercial 

Storages/Warehouses Premises; Commercial Transportation Facilities-Nonfeed/Nonfood; 

Commercial/Industrial/Industrial Premises/Equipment (Indoor); ); Eating Establishments; Food Processing 

Plant Premises (Nonfood Contact); Food Stores/Markets/Supermarkets Premises; 

Food/Grocery/Marketing/Storage/Distribution Facility Premise; Hospitals/Medical Institutions Premises 

(Human/Veterinary); Household/Domestic Dwellings; Household/Domestic Dwelling Contents; 

Household/Domestic Dwellings Indoor Food Handling Areas; Household/Domestic Dwellings Indoor 

Premises; Poultry Processing Plant Premises (Nonfood Contact); Refuse Solid Waste Containers (Garbage 

Cans); Refuse/Solid Waste Sites (Indoor); 

22
Acephate was first registered in 1973 for ornamental uses, and in 1974 for food uses 

(agricultural crops). Use data from 1988 to 1997 indicate that approximately 4 to 5 

million pounds of active ingredient (ai) are used domestically each year (USEPA, 2001). 

Based on California Department of Pesticide Regulation (Cal DPR) Pesticide Use 

Reporting (PUR) annual reports, annual use of acephate in California ranged from 

approximately 194,000 to 259,000 pounds a.i. from 2001 through 2005 (Table 2.3)2. 

Table 2.3. Summary of annual acephate use in California from 2001 through 2005. 

Number of 

Year Pounds a.i. Applied Applications 

2001 240,109 21,098 

2002 258,955 20,177 

2003 223,749 18,676 

2004 201,816 18,624 

2005 194,365 16,009 

Source: CAL DPR PUR Annual Reports for 2001-2005, viewed April 2007 

Food: Acephate is registered for use on beans (green and lima), Brussels sprouts, 

cauliflower, celery, cottonseed, cranberries, lettuce, peanuts, peppermint, peppers (bell 

and non-bell), citrus, fruit trees, nut trees, soybeans, and spearmint. 

Other Agriculture, Non-food: Acephate is also registered for use on cotton, and as seed 

treatment on cotton and peanuts (seed for planting), on non-bearing fruit trees, such as 

ornamental citrus, and on tobacco. 

Residential: Acephate is registered for use outdoors around residential buildings, homes, 

and apartments, for the control of roaches, wasps, fire ants, and crickets, among other 

pests. It is also registered for outdoor use on home lawns, trees, shrubs and ornamentals. 

Public Health: Acephate is registered for use in and around industrial, institutional 

and commercial buildings, including restaurants, food handling establishments, 

warehouses, stores, hotels, manufacturing plants, and ships for the control of 

roaches and fire ants. 

Other Nonfood: Acephate is registered for use on sod, golf course turf, field borders, 

fence rows, roadsides, ditch banks, borrow pits, wasteland, and greenhouse and 

horticultural nursery floral and foliage plants. 

Target pests include: Armyworms, aphids, beetles, bollworms, borers, budworms, 

cankerworms, crickets, cutworms, fire ants, fleas, grasshoppers, leafhoppers, 

loopers, mealybugs, mites, moths, roaches, spiders, thrips, wasps, weevils, 

whiteflies, and others (EPA IRED, 2001). 

Formulation types: Wettable Powder, Soluble Powder, Soluble Extruded Pellets, 

Granular, and Liquid. All forms, except for granular, are mixed with water prior to 

application and are applied in a liquid form. 

 

2 http://www.cdpr.ca.gov/docs/pur/purmain.htm (April 2007) 

23
Analysis of labeled use information is the critical first step in evaluating the federal 

action. The current label for acephate represents the FIFRA regulatory action; therefore, 

labeled use and application rates specified on the label form the basis of this assessment. 

The assessment of use information is critical to the development of the action area and 

selection of appropriate modeling scenarios and inputs. 

Equipment for agriculture, greenhouse, nursery, and turf uses: Granular acephate can be 

applied by belly grinder, hand, tractor-drawn spreader, push-type spreader, and shaker 

can. Liquid acephate (formulated from soluble powders or soluble extruded pellets) may 

be applied by aircraft, airblast sprayer, backpack sprayer, chemigation, hydraulic 

sprayers, groundboom spray, handgun, high pressure sprayer, hopper box (seed 

treatment), low-pressure handwand, slurry (seed treatment), sprinkler can, transplanting 

in water (tobacco), or by an aerosol generator (greenhouses). 

Equipment for residential and public health uses: Residential applications can be made 

by aerosol can, backpack sprayer, hose-end sprayer, and low-pressure handwand. 

Residential granular applications can be made by shaker can or by hand. Residential 

soluble powder applications may be made by sprinkler can or compressed air sprayers. 

Method: Acephate may be applied on seed before planting, in-furrow at planting, or as a 

foliar spray, it may be applied to float beds, plant beds, or as a transplant (tobacco) 

treatment. For use against fire ants it may be applied directly on their soil mound (drench 

and dry methods). Acephate is also used indoors as spot, crack and crevice, and bait 

treatments. 

Rates: Rates vary according to method of application and pest. The highest registered 

maximum one time application rate is 5 lbs ai/A on commercial/residential turf. The 

highest seasonal application rate is 6 lb ai/A/year (1 lb ai/A at 6 applications per season) 

for cotton. 

2.4.4.2. Use and Useage in California, 2000-2005 

The Agency’s Biological and Economic Analysis Division (BEAD) provides an analysis 

of both national- and county-level usage information using state-level usage data 

obtained from USDA-NASS3

, Doane (www.doane.com; the full dataset is not provided 

due to its proprietary nature), and the California’s Department of Pesticide Regulation 

Pesticide Use Reporting (CDPR PUR) database4

 . CDPR PUR is considered a more 

comprehensive source of usage data than USDA-NASS or EPA proprietary databases, 

and thus the usage data reported for acephate by county in this California-specific 

assessment were generated using CDPR PUR data. Usage data are averaged together 

 

3

 United States Depart of Agriculture (USDA), National Agricultural Statistics Service (NASS) Chemical 

Use Reports provide summary pesticide usage statistics for select agricultural use sites by chemical, crop 

and state. See http://www.usda.gov/nass/pubs/estindx1.htm#agchem. 4

 The California Department of Pesticide Regulation’s Pesticide Use Reporting database provides a census 

of pesticide applications in the state. See http://www.cdpr.ca.gov/docs/pur/purmain.htm. 

24
25

over the years 2002 to 2005 to calculate average annual usage statistics by county and 

crop for acephate, including pounds of active ingredient applied (Table 2.4). California 

State law requires that every pesticide application be reported to the state and made 

available to the public. 

Table 2.4. Average Annual Pounds of Acephate Applied in California by Counties and 

Uses. Only the major uses are included. (2002-2005). Information on the remaining uses 

and counties reported in California can be found in Appendix J County Average Annual Pounds Applied Lettuce Cotton Bean Celery Non Food Outdoor Structural Pest Control 

 Pepper 

 Landscape 

Maintenance 

 Greenhouse 

 Cauliflower 

 Mint 

 Citrus 

Average 

Annual 

Pounds 

Applied 211,133 83,897 50,215 16,409 15,438 14,329 10,979 5,866 5,038 3,961 1,662 1,370 906 

Monterey 61,958 50,144 NR 1,280 7,146 310 45 319 707 770 1,188 NR NR 

Fresno 34,299 14,427 16,893 1,301 NR 75 169 1,302 2 63 4 NR 28 

Kern 14,189 129 10,949 996 NR 72 1,031 648 6 1 NR NR 233 

Imperial 14,021 5,450 8,287 19 39 50 37 27 NR NR 40 NR 6 

Santa 

Barbara 11,014 6,087 NR 32 1,691 2,414 98 105 33 172 358 NR NR 

Ventura 10,894 99 NR 1,153 4,991 1,285 2,738 103 56 204 NR NR NR 

Kings 6,616 226 6,309 13 NR NR 29 NR NR NR NR NR NR 

Riverside 6,190 76 4,156 36 93 426 337 81 794 58 7 NR 40 

Stanislaus 4,839 NR NR 1,982 NR 2,633 60 36 53 51 NR NR NR 

San Benito 4,586 1,984 NR NR 632 62 6 1,858 4 11 5 NR NR 

San Diego 4,444 NR NR 49 NR 1,858 697 44 749 918 NR 2 NR 

San Luis 

Obispo 4,314 3,149 NR 5 714 166 7 60 87 101 25 NR NR 

Merced 3,692 NR 2,220 1,405 NR 2 36 NR 0 9 19 NR NR 

Santa 

Clara 3,247 382 NR 7 61 109 430 1,249 713 273 NR NR NR 

San 

Joaquin 3,156 NR 139 2,011 NR 890 19 NR 6 77 NR NR NR 

Sutter 2,798 NR 11 2,705 NR 69 8 NR 0 NR NR NR NR 

Los 

Angeles 2,774 NR NR NR NR 769 1,369 NR 516 86 NR NR NR 

Santa 

Cruz 2,488 1,731 NR 4 72 164 30 NR 39 442 3 NR NR 

Orange 2,175 NR NR 14 NR 1,322 400 16 271 151 NR NR NR 

Source: CDPR PUR 2007 

The uses considered in this risk assessment represent all currently registered uses 

according to a review of all current labels. No other uses are relevant to this assessment. 

Any reported use, such as may be seen in the CDPR PUR database, represent either 

historic uses that have been canceled, mis-reported uses, or mis-use. Historical uses, misreported uses, and misuse are not considered part of the federal action and, therefore, are 

not considered in this assessment. 
29%

15%

7%

7%

5%

5%

3%

3%

3%

2%

2%

2%2%

2%

1%1%

1%

1%

1%

0%

5%

10%

15%

20%

25%

30%

MONTEREY

FRESNO

KERN

IMPERIAL

VENTURA

SANTA BARBARA

KINGS

RIVERSIDE

STANISLAUS

SAN BENITO

SAN DIEGO

SAN LUIS OBISPO

MERCED

SANTA CLARA

SAN JOAQUIN

SUTTER

LOS ANGELES

SANTA CRUZ

ORANGE

County

% Contribution to Total Pounds of Chemical Applied Annually

Throughout all 58 Counties in California

Figure 2.A. Percent Contribution of Each County to the Total Pounds Acephate Applied 

Annually in California Source: CDPR PUR 2007 

Figure 2.A shows useage for the top eight use sites, limited to the top nineteen counties, 

in terms of pounds applied in 2004. Monterey county is a major use area for acephate. 

Note that the following counties not included in the figure each contributed less than 1% 

of the total annual acephate use in California, between 2003-2005: Tulare, San Francisco, 

Colusa, Madera, Glenn, Solano, San Mateo, Yolo, Contra Costa, Siskiyou, San 

Bernardino, Sacramento, Sonoma, Butte, Alameda, Modoc, Shasta, Lassen, Marin, 

Tehama, Placer, Humboldt, Napa, Plumas, Del Norte, Sierra, Tuolumne, Nevada, 

Mendocino, El Dorado, Mono, Lake, Yuba, Mariposa, Inyo, Amador, Calaveras, Trinity. 

Figure 2B shows the data for 2003-2005 broken down by month of application, for the 

top eight uses.

26
0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

January

February

March

April

May

June

July

August

September

October

November

December

Month

Pounds Applie

d

2003

2004

2005

Figure 2.B. Pounds of A.I. applied per month to the following sites in 2003, 2004, 2005: Bean, Celery, 

Citrus, Cotton, Landscape Maintenance, Lettuce, Pepper, Pistachio. Source: CDPR PUR 2007 

 

 

Figure 2.C – CRLF Reproductive Events by Month

Light Blue = Breeding/Egg Masses

Green = Tadpoles (except those that over-winter) 

Orange = Young Juveniles

Adults and juveniles can be present all year 

Above, Figure 2.C represents the CRLF reproductive cycle over time, and is presented in 

parallel to Figure 2.B, depicting the timing of acephate application to illustrate the 

temporal co-occurrence of reproductive events with acephate usage. The months when 

there are egg masses and tadpoles present (November to May) correspond to high usage 

on lettuce, celery and cotton. While this does not account for spatial differences between 

the location of the frog habitat and timing of reproductive events, it does show that there 

is general overlap between the timing of acephate applications and CRLF reproduction. 

 

J F M A M J J A S O N D 

27
2.5 Assessed Species 

The CRLF was federally listed as a threatened species by USFWS effective June 24, 

1996 (USFWS 1996). It is one of two subspecies of the red-legged frog and is the largest 

native frog in the western United States (USFWS 2002). A brief summary of information 

regarding CRLF distribution, reproduction, diet, and habitat requirements is provided in 

Sections 2.5.1 through 2.5.4, respectively. Further information on the status, distribution, 

and life history of and specific threats to the CRLF is provided in Attachment 1. 

Final critical habitat for the CRLF was designated by USFWS on April 13, 2006 

(USFWS 2006; 71 FR 19244-19346). Further information on designated critical habitat 

for the CRLF is provided in Section 2.6. 

2.5.1 Distribution 

The CRLF is endemic to California and Baja California (Mexico) and historically 

inhabited 46 counties in California including the Central Valley and both coastal and 

interior mountain ranges (USFWS 1996). Its range has been reduced by about 70%, and 

the species currently resides in 22 counties in California (USFWS 1996). The species has 

an elevation range of near sea level to 1,500 meters (5,200 feet) (Jennings and Hayes 

1994); however, nearly all of the known CRLF populations have been documented below 

1,050 meters (3,500 feet) (USFWS 2002). 

Populations currently exist along the northern California coast, northern Transverse 

Ranges (USFWS 2002), foothills of the Sierra Nevada (5-6 populations), and in southern 

California south of Santa Barbara (two populations) (Fellers 2005a). Relatively larger 

numbers of CRLFs are located between Marin and Santa Barbara Counties (Jennings and 

Hayes 1994). A total of 243 streams or drainages are believed to be currently occupied 

by the species, with the greatest numbers in Monterey, San Luis Obispo, and Santa 

Barbara counties (USFWS 1996). Occupied drainages or watersheds include all bodies 

of water that support CRLFs (i.e., streams, creeks, tributaries, associated natural and 

artificial ponds, and adjacent drainages), and habitats through which CRLFs can move 

(i.e., riparian vegetation, uplands) (USFWS 2002). 

The distribution of CRLFs within California is addressed in this assessment using four 

categories of location including recovery units, core areas, designated critical habitat, and 

known occurrences of the CRLF reported in the California Natural Diversity Database 

(CNDDB) that are not included within core areas and/or designated critical habitat (see 

Figure 2.D). Recovery units, core areas, and other known occurrences of the CRLF from 

the CNDDB are described in further detail in this section, and designated critical habitat 

is addressed in Section 2.6. Recovery units are large areas defined at the watershed level 

that have similar conservation needs and management strategies. The recovery unit is 

primarily an administrative designation, and land area within the recovery unit boundary 

is not exclusively CRLF habitat. Core areas are smaller areas within the recovery units 

that comprise portions of the species’ historic and current range and have been 

determined by USFWS to be important in the preservation of the species. Designated 

28
critical habitat is generally contained within the core areas, although a number of critical 

habitat units are outside the boundaries of core areas, but within the boundaries of the 

recovery units. Additional information on CRLF occurrences from the CNDDB is used 

to cover the current range of the species not included in core areas and/or designated 

critical habitat, but within the recovery units. 

Recovery Units 

Eight recovery units have been established by USFWS for the CRLF. These areas are 

considered essential to the recovery of the species, and the status of the CRLF “may be 

considered within the smaller scale of the recovery units, as opposed to the statewide 

range” (USFWS 2002). Recovery units reflect areas with similar conservation needs and 

population status, and therefore, similar recovery goals. The eight units described for the 

CRLF are delineated by watershed boundaries defined by US Geological Survey 

hydrologic units and are limited to the elevational maximum for the species of 1,500 m 

above sea level. The eight recovery units for the CRLF are listed in Table 2.5 and shown 

in Figure 2.D. 

Core Areas 

USFWS has designated 35 core areas across the eight recovery units to focus their 

recovery efforts for the CRLF (see Figure 2.D). Table 2.5 summarizes the geographical 

relationship among recovery units, core areas, and designated critical habitat. The core 

areas, which are distributed throughout portions of the historic and current range of the 

species, represent areas that allow for long-term viability of existing populations and 

reestablishment of populations within historic range. These areas were selected because 

they: 1) contain existing viable populations; or 2) they contribute to the connectivity of 

other habitat areas (USFWS 2002). Core area protection and enhancement are vital for 

maintenance and expansion of the CRLF’s distribution and population throughout its 

range. 

For purposes of this assessment, designated critical habitat, currently occupied (post1985) core areas, and additional known occurrences of the CRLF from the CNDDB are 

considered. Each type of locational information is evaluated within the broader context 

of recovery units. For example, if no labeled uses of acephate occur (or if labeled uses 

occur at predicted exposures less than the Agency’s LOCs) within an entire recovery unit, 

a “no effect” determination would be made for all designated critical habitat, currently 

occupied core areas, and other known CNDDB occurrences within that recovery unit. 

Historically occupied sections of the core areas are not evaluated as part of this 

assessment because the USFWS Recovery Plan (USFWS 2002) indicates that CRLFs are 

extirpated from these areas. A summary of currently and historically occupied core areas 

is provided in Table 2.5 (currently occupied core areas are bolded). While core areas are 

considered essential for recovery of the CRLF, core areas are not federally-designated 

critical habitat, although designated critical habitat is generally contained within these 

core recovery areas. It should be noted, however, that several critical habitat units are 

located outside of the core areas, but within the recovery units. The focus of this 

29
assessment is currently occupied core areas, designated critical habitat, and other known 

CNDDB CRLF occurrences within the recovery units. Federally-designated critical 

habitat for the CRLF is further explained in Section 2.6. 

Table 2.5. California Red-legged Frog Recovery Units with Overlapping Core 

Areas and Designated Critical Habitat 

Currently 

Occupied 

(post-1985) 

1 Critical Habitat 

Units 

Recovery Unit 

(Figure 2.a) 

Historically 

Occupied Core Areas 2,7 (Figure 2.a) 3 4

4

Cottonwood Creek (partial) 9

(8) -- 

Feather River (1) BUT-1A-B 9

Yuba River-S. Fork Feather 9

River (2) YUB-1 

-- NEV-16

Traverse Creek/Middle Fork 

American River/Rubicon (3) -- 9

Sierra Nevada 

Foothills and Central 

Valley (1) 

Consumnes River (4) ELD-1 9

S. Fork Calaveras River (5) -- 9

(eastern boundary is 

the 1,500m elevation 

line) 

Tuolumne River (6) -- 9

Piney Creek (7) -- 9

East San Francisco Bay 9

(partial)(16) -- 

Cottonwood Creek (8) -- 9

Putah Creek-Cache Creek (9) -- 9

Jameson Canyon – Lower 

Napa Valley (partial) (15) -- 9

Belvedere Lagoon (partial) 

(14) -- 9

North Coast Range 

Foothills and 

Western Sacramento 

River Valley (2) 

Pt. Reyes Peninsula (partial) 9

(13) -- 

Putah Creek-Cache Creek 9

(partial) (9) -- 

Lake Berryessa Tributaries 9

(10) NAP-1 

Upper Sonoma Creek (11) -- 9

Petaluma Creek-Sonoma 

Creek (12) -- 9

Pt. Reyes Peninsula (13) MRN-1, MRN-2 9

North Coast and 

North San Francisco 

Bay (3) 

Belvedere Lagoon (14) -- 9

Jameson Canyon-Lower 9

Napa River (15) SOL-1 

-- CCS-1A6

East San Francisco Bay 9

(partial) (16) 

ALA-1A, ALA1B, STC-1B 

-- STC-1A6

South and East San 

Francisco Bay (4) 

South San Francisco Bay 9

(partial) (18) SNM-1A 

South San Francisco Bay 9

(partial) (18) 

SNM-1A, SNM2C, SCZ-1 Central Coast (5) 

30
Table 2.5. California Red-legged Frog Recovery Units with Overlapping Core 

Areas and Designated Critical Habitat 

Currently 

Occupied 

(post-1985) 

1 Critical Habitat 

Units 

Recovery Unit 

(Figure 2.a) 

Historically 

Occupied Core Areas 2,7 (Figure 2.a) 3 4

4

Watsonville Slough- Elkhorn 9

Slough (partial) (19) SCZ-2 5

Carmel River-Santa Lucia 9

(20) MNT-2 

Estero Bay (22) -- 9

-- SLO-86

Arroyo Grande Creek (23) -- 9

Santa Maria River-Santa 9

Ynez River (24) -- 

East San Francisco Bay 9

(partial) (16) 

MER-1A-B, 

STC-1B 

-- SNB-16, SNB-26

Santa Clara Valley (17) -- 9

Watsonville Slough- Elkhorn 

Slough (partial)(19) MNT-1 Diablo Range and 9

Salinas Valley (6) 

Carmel River-Santa Lucia 9

(partial)(20) -- 

Gablan Range (21) SNB-3 9

Estrella River (28) SLO-1A-B 9

-- SLO-86

Santa Maria River-Santa 9

Ynez River (24) 

STB-4, STB-5, 

STB-7 

Sisquoc River (25) STB-1, STB-3 9

Ventura River-Santa Clara 

River (26) 

VEN-1, VEN-2, 

VEN-3 

9

Northern Transverse 

Ranges and 

Tehachapi Mountains 

(7) 

-- LOS-16

Santa Monica Bay-Ventura 9

Coastal Streams (27) -- 

San Gabriel Mountain (29) -- 9

Forks of the Mojave (30) -- 9

Santa Ana Mountain (31) -- 9

Santa Rosa Plateau (32) -- 9

San Luis Rey (33) -- 9

Southern Transverse 

and Peninsular 

Ranges (8) 

Sweetwater (34) -- 9

Laguna Mountain (35) -- 9 1

 Recovery units designated by the USFWS (USFWS 2000, pg 49). 

2

 Core areas designated by the USFWS (USFWS 2000, pg 51). 

3

 Critical habitat units designated by the USFWS on April 13, 2006 (USFWS 2006, 71 FR 19244-19346). 

4

 Currently occupied (post-1985) and historically occupied core areas as designated by the USFWS 

(USFWS 2002, pg 54). 

5

 Critical habitat unit where identified threats specifically included pesticides or agricultural runoff 

(USFWS 2002). 

6

 Critical habitat units that are outside of core areas, but within recovery units. 7

 Currently occupied core areas that are included in this effects determination are bolded. 

31
Core Areas 

Figure 2.D Recovery Unit and Core Area Designations for CRLF 

32
Other Known Occurrences from the CNDBB 

The CNDDB provides location and natural history information on species found in 

California. The CNDDB serves as a repository for historical and current species location 

sightings. Information regarding known occurrences of CRLFs outside of the currently 

occupied core areas and designated critical habitat is considered in defining the current 

range of the CRLF. See: http://www.dfg.ca.gov/bdb/html/cnddb_info.html for additional 

information on the CNDDB. 

2.5.2 Reproduction 

CRLFs breed primarily in ponds; however, they may also breed in quiescent streams, 

marshes, and lagoons (Fellers 2005a). According to the Recovery Plan (USFWS 2002), 

CRLFs breed from November through late April. Peaks in spawning activity vary 

geographically; Fellers (2005b) reports peak spawning as early as January in parts of 

coastal central California. Eggs are fertilized as they are being laid. Egg masses are 

typically attached to emergent vegetation, such as bulrushes (Scirpus spp.) and cattails 

(Typha spp.) or roots and twigs, and float on or near the surface of the water (Hayes and 

Miyamoto 1984). Egg masses contain approximately 2000 to 6000 eggs ranging in size 

between 2 and 2.8 mm (Jennings and Hayes 1994). Embryos hatch 10 to 14 days after 

fertilization (Fellers 2005a) depending on water temperature. Egg predation is reported 

to be infrequent and most mortality is associated with the larval stage (particularly 

through predation by fish); however, predation on eggs by newts has also been reported 

(Rathburn 1998). Tadpoles require 11 to 28 weeks to metamorphose into juveniles 

(terrestrial-phase), typically between May and September (Jennings and Hayes 1994, 

USFWS 2002); tadpoles have been observed to over-winter (delay metamorphosis until 

the following year) (Fellers 2005b, USFWS 2002). Males reach sexual maturity at 2 

years, and females reach sexual maturity at 3 years of age; adults have been reported to 

live 8 to 10 years (USFWS 2002). Figure 2.E depicts CRLF annual reproductive timing. 

Figure 2.E – CRLF Reproductive Events by Month 

J F M A M J J A S O N D 

Light Blue = Breeding/Egg Masses

Green = Tadpoles (except those that over-winter) 

Orange = Young Juveniles

Adults and juveniles can be present all year 

2.5.3 Diet 

Although the diet of CRLF aquatic-phase larvae (tadpoles) has not been studied 

specifically, it is assumed that their diet is similar to that of other frog species, with the 

aquatic phase feeding exclusively in water and consuming diatoms, algae, and detritus 

(USFWS 2002). Tadpoles filter and entrap suspended algae (Seale and Beckvar, 1980) 

33
via mouthparts designed for effective grazing of periphyton (Wassersug, 1984, 

Kupferberg et al.; 1994; Kupferberg, 1997; Altig and McDiarmid, 1999). 

Juvenile and adult CRLFs forage in aquatic and terrestrial habitats, and their diet differs 

greatly from that of larvae. The main food source for juvenile aquatic- and terrestrialphase CRLFs is thought to be aquatic and terrestrial invertebrates found along the 

shoreline and on the water surface. Hayes and Tennant (1985) report, based on a study 

examining the gut content of 35 juvenile and adult CRLFs, that the species feeds on as 

many as 42 different invertebrate taxa, including Arachnida, Amphipoda, Isopoda, 

Insecta, and Mollusca. The most commonly observed prey species were larval alderflies 

(Sialis cf. californica), pillbugs (Armadilliadrium vulgare), and water striders (Gerris sp). 

The preferred prey species, however, was the sowbug (Hayes and Tennant, 1985). This 

study suggests that CRLFs forage primarily above water, although the authors note other 

data reporting that adults also feed under water, are cannibalistic, and consume fish. For 

larger CRLFs, over 50% of the prey mass may consists of vertebrates such as mice, frogs, 

and fish, although aquatic and terrestrial invertebrates were the most numerous food 

items (Hayes and Tennant 1985). For adults, feeding activity takes place primarily at 

night; for juveniles feeding occurs during the day and at night (Hayes and Tennant 1985). 

2.5.4 Habitat 

CRLFs require aquatic habitat for breeding, but also use other habitat types including 

riparian and upland areas throughout their life cycle. CRLF use of their environment 

varies; they may complete their entire life cycle in a particular habitat or they may utilize 

multiple habitat types. Overall, populations are most likely to exist where multiple 

breeding areas are embedded within varying habitats used for dispersal (USFWS 2002). 

Generally, CRLFs utilize habitat with perennial or near-perennial water (Jennings et al. 

1997). Dense vegetation, shading water of moderate depth is a habitat feature that 

appears especially important for CRLF (Hayes and Jennings 1988). Breeding sites 

include streams, deep pools, backwaters within streams and creeks, ponds, marshes, sag 

ponds (land depressions between fault zones that have filled with water), dune ponds, and 

lagoons. Breeding adults have been found near deep (0.7 m) still or slow moving water 

surrounded by dense vegetation (USFWS 2002); however, the largest number of tadpoles 

have been found in shallower pools (0.26 – 0.5 m) (Reis, 1999). Data indicate that 

CRLFs do not frequently inhabit vernal pools, as conditions in these habitats generally 

are not suitable (Hayes and Jennings 1988). 

CRLFs also frequently breed in artificial impoundments such as stock ponds, although 

additional research is needed to identify habitat requirements within artificial ponds 

(USFWS 2002). Adult CRLFs use dense, shrubby, or emergent vegetation closely 

associated with deep-water pools bordered with cattails and dense stands of overhanging 

vegetation (http://www.fws.gov/endangered/features/rl_frog/rlfrog.html#where). 

In general, dispersal and habitat use depends on climatic conditions, habitat suitability, 

and life stage. Adults rely on riparian vegetation for resting, feeding, and dispersal. The 

foraging quality of the riparian habitat depends on moisture, composition of the plant 

34
community, and presence of pools and backwater aquatic areas for breeding. CRLFs can 

be found living within streams at distances up to 3 km (2 miles) from their breeding site 

and have been found up to 30 m (100 feet) from water in dense riparian vegetation for up 

to 77 days (USFWS 2002). 

During dry periods, the CRLF is rarely found far from water, although it will sometimes 

disperse from its breeding habitat to forage and seek other suitable habitat under downed 

trees or logs, industrial debris, and agricultural features (USFWS 2002). According to 

Jennings and Hayes (1994), CRLFs also use small mammal burrows and moist leaf litter 

as habitat. In addition, CRLFs may also use large cracks in the bottom of dried ponds as 

refugia; these cracks may provide moisture for individuals avoiding predation and solar 

exposure (Alvarez 2000). 

2.6 Designated Critical Habitat 

In a final rule published on April 13, 2006, 34 separate units of critical habitat were 

designated for the CRLF by USFWS (USFWS 2006; FR 51 19244-19346). A summary 

of the 34 critical habitat units relative to USFWS-designated recovery units and core 

areas (previously discussed in Section 2.5.1) is provided in Table 2.D. 

‘Critical habitat’ is defined in the ESA as the geographic area occupied by the species at 

the time of the listing where the physical and biological features necessary for the 

conservation of the species exist, and there is a need for special management to protect 

the listed species. It may also include areas outside the occupied area at the time of 

listing if such areas are ‘essential to the conservation of the species.’ All designated 

critical habitat for the CRLF was occupied at the time of listing. Critical habitat receives 

protection under Section 7 of the ESA through prohibition against destruction or adverse 

modification with regard to actions carried out, funded, or authorized by a federal 

Agency. Section 7 requires consultation on federal actions that are likely to result in the 

destruction or adverse modification of critical habitat. 

To be included in a critical habitat designation, the habitat must be ‘essential to the 

conservation of the species.’ Critical habitat designations identify, to the extent known 

using the best scientific and commercial data available, habitat areas that provide 

essential life cycle needs of the species or areas that contain certain primary constituent 

elements (PCEs) (as defined in 50 CFR 414.12(b)). PCEs include, but are not limited to, 

space for individual and population growth and for normal behavior; food, water, air, 

light, minerals, or other nutritional or physiological requirements; cover or shelter; sites 

for breeding, reproduction, rearing (or development) of offspring; and habitats that are 

protected from disturbance or are representative of the historic geographical and 

ecological distributions of a species. The designated critical habitat areas for the CRLF 

are considered to have the following PCEs that justify critical habitat designation: 

• Breeding aquatic habitat; 

• Non-breeding aquatic habitat; 

• Upland habitat; and 

35
• Dispersal habitat. 

Please note that a more complete description of these habitat types is provided in 

Attachment 1. 

Occupied habitat may be included in the critical habitat only if essential features within 

the habitat may require special management or protection. Therefore, USFWS does not 

include areas where existing management is sufficient to conserve the species. Critical 

habitat is designated outside the geographic area presently occupied by the species only 

when a designation limited to its present range would be inadequate to ensure the 

conservation of the species. For the CRLF, all designated critical habitat units contain all 

four of the PCEs, and were occupied by the CRLF at the time of FR listing notice in 

April 2006. The FR notice designating critical habitat for the CRLF includes a special 

rule exempting routine ranching activities associated with livestock ranching from 

incidental take prohibitions. The purpose of this exemption is to promote the 

conservation of rangelands, which could be beneficial to the CRLF, and to reduce the rate 

of conversion to other land uses that are incompatible with CRLF conservation. Please 

see Attachment 1. for a full explanation on this special rule. 

USFWS has established adverse modification standards for designated critical habitat 

(USFWS 2006). Activities that may destroy or adversely modify critical habitat are those 

that alter the PCEs and jeopardize the continued existence of the species. Evaluation of 

actions related to use of acephate that may alter the PCEs of the CRLF’s critical habitat 

form the basis of the critical habitat impact analysis. According to USFWS (2006), 

activities that may affect critical habitat and therefore may result in adverse effects to the 

CRLF include, but are not limited to the following: 

(1) Significant alteration of water chemistry or temperature to levels beyond the 

tolerances of the CRLF that result in direct or cumulative adverse effects to 

individuals and their life-cycles. 

(2) Significant increase in sediment deposition within the stream channel or pond or 

disturbance of upland foraging and dispersal habitat that could result in 

elimination or reduction of habitat necessary for the growth and reproduction of 

the CRLF by increasing the sediment deposition to levels that would adversely 

affect their ability to complete their life cycles. 

(3) Significant alteration of channel/pond morphology or geometry that may lead to 

changes to the hydrologic functioning of the stream or pond and alter the timing, 

duration, water flows, and levels that would degrade or eliminate the CRLF 

and/or its habitat. Such an effect could also lead to increased sedimentation and 

degradation in water quality to levels that are beyond the CRLF’s tolerances. 

(4) Elimination of upland foraging and/or aestivating habitat or dispersal habitat. 

(5) Introduction, spread, or augmentation of non-native aquatic species in stream 

segments or ponds used by the CRLF. 

(6) Alteration or elimination of the CRLF’s food sources or prey base (also 

evaluated as indirect effects to the CRLF). 

36
As previously noted in Section 2.1, the Agency believes that the analysis of direct and 

indirect effects to listed species provides the basis for an analysis of potential effects on 

the designated critical habitat. Because acephate is expected to directly impact living 

organisms within the action area, critical habitat analysis for acephate is limited in a 

practical sense to those PCEs of critical habitat that are biological or that can be 

reasonably linked to biologically mediated processes. 

 2.6.1. Special Rule Exemption for Routine Ranching Activities 

As part of the critical habitat designation, the Service promulgated a special rule 

exemption regarding routine ranching activities where there is no Federal nexus from 

take prohibitions under Section 9 of the ESA. (USFWS 2006, 71 FR 19285-19290). The 

Service’s reasoning behind this exemption is that managed livestock activities, especially 

the creation of stock ponds, provide habitat for the CRLF. Maintenance of these areas as 

rangelands, rather than conversion to other uses should ranching prove to be 

economically infeasible is, overall, of net benefit to the species. 

Several of the specific activities exempted include situations where pesticides may be 

used in accordance with labeled instructions. In this risk assessment, the Agency has 

assessed the risk associated with these practices using the standard assessment 

methodologies. Specific exemptions, and the reasoning behind each of the exemptions is 

provided below. The rule provides recommended best management practices, but does 

not require adherence to these practices by the landowner. 

1. Stock Pond Management and Maintenance 

a. Chemical control of aquatic vegetation. These applications are allowed 

primarily because the Service felt “it is unlikely that vegetation control 

would be needed during the breeding period, as the primary time for 

explosive vegetation control is during the warm summer months.” The 

Service recommends chemical control measures be used only “outside of 

the general breeding season (November through April) and juvenile stage 

(April through September) of the CRLF.” Mechanical means are the 

preferred method of control. 

b. Pesticide applications for mosquito control. These applications are 

allowed because of concerns associated with human and livestock health. 

Alternative mosquito control methods, primarily introduction of nonnative 

fish species, are deemed potentially more detrimental to the CRLF than 

chemical or bacterial larvicides. The Service believes “it unlikely that 

[mosquito] control would be necessary during much of the CRLF breeding 

season,” and that a combination of management methods, such as 

manipulation of water levels, and/or use of a bacterial larvicide will 

prevent or minimize incidental take. 

2. Rodent Control. The Service notes “we believe the use of rodenticides present a 

low risk to CRLF conservation.” In large part, this is due to the fact that “it is 

37
unknown the extent to which small mammal burrows are essential for the 

conservation of CRLF.” 

a. Toxicant-treated grains. No data were available to evaluate the potential 

effects of these compounds (primarily anti-coagulants) on the CRLF. 

Grain is not a typical food item for the frog, but individuals may be 

indirectly exposed by consuming invertebrates which have ingested 

treated grain. There is a possibility of dermal contact, especially when the 

grain is placed in the burrows. Placing treated grain into the burrows is 

not prohibited, but should this method of rodent control be used, the 

Service recommends bait-station or broadcast application methods to 

reduce the probability of exposure. 

b. Burrow fumigants. Use of burrow fumigants is not prohibited, but the 

Service recommends “not using burrow fumigants within 0.7 mi (1.2 km) 

in any direction from a water body” suitable as CRLF habitat. 

2.7 Action Area

For listed species assessment purposes, the action area is considered to be the area 

affected directly or indirectly by the federal action and not merely the immediate area 

involved in the action (50 CFR 402.02). It is recognized that the overall action area for 

the national registration of acephate is likely to encompass considerable portions of the 

United States based on the large array of agricultural uses and non-agricultural uses. 

However, the scope of this assessment limits consideration of the overall action area to 

those portions that may be applicable to the protection of the CRLF and its designated 

critical habitat within the state of California. Deriving the geographical extent of this 

portion of the action area is the product of consideration of the types of effects that 

acephate may be expected to have on the environment, the exposure levels to acephate 

that are associated with those effects, and the best available information concerning the 

use of acephate and its fate and transport within the state of California. 

The definition of action area requires a stepwise approach that begins with an 

understanding of the federal action. The federal action is defined by the currently labeled 

uses for acephate. An analysis of labeled uses and review of available product labels was 

completed. This analysis indicates that, for acephate, the following uses are considered 

as part of the federal action evaluated in this assessment: 

All outdoor uses that result in spray drift or run-off exposure are included in the initial 

area of concern. Indoor uses are not considered part of the Action Area since exposure of 

the CRLF is unlikely. 

After a determination of which uses will be assessed, an evaluation of the potential 

“footprint” of the use pattern should be determined. This “footprint” represents the initial 

area of concern and is based on available land cover data for labeled outdoor uses. Local 

land cover data available for the state of California were analyzed to refine the 

understanding of potential acephate use. The initial area of concern is defined as all land 

38
cover types that represent the labeled uses described above. A map representing all the 

land cover types that make up the initial area of concern is presented in Figure 2.F. 

39
Figure 2.F. Initial Area of Concern 

40
Once the initial area of concern is defined, the next step is to compare the extent of that 

area with the results of the screening level risk assessment. The screening level risk 

assessment will define which taxa, if any, are predicted to be exposed at concentrations 

above the Agency’s Levels of Concern (LOC). The screening level assessment includes 

an evaluation of the environmental fate properties of acephate to determine which routes 

of transport are likely to have an impact on the CRLF. 

LOC exceedances are used to describe how far effects may be seen from the initial area 

of concern. Factors considered include: spray drift, downstream run-off, atmospheric 

transport, etc. This information is incorporated into GIS and a map of the action area is 

created. 

Based on the environmental fate assessment, the dominant routes of exposure to acephate 

and its degradate methamidophos are believed to be run-off and spray drift. Transport 

through groundwater is not considered to be a route of exposure due to the low 

persistence of both compounds. Volatilization and long-range transport are not 

considered to be a route of exposure due to the low vapor pressure and Henry’s Law 

constant of both compounds. Based on its low log Kow (-0.85) bioaccumulation is not 

expected to be a concern. 

41
Figure 2.G Acephate Action Area 

42
Subsequent to defining the action area, an evaluation of usage information was conducted 

to determine area where use of acephate may impact the CRLF. This analysis is used to 

characterize where predicted exposures are most likely to occur but does not preclude use 

in other portions of the action area. A more detailed review of the county-level use 

information was also completed. These data suggest that, as presented in Figures 2A, 

exposure is likely to be greatest in the counties of highest reported usage. Most of the 

acephate usage in California is concentrated in a few counties. 

Action Area Calculation 

The Action Area for acephate will be dominated by its effects on terrestrial species, due 

to the much higher RQ values in the terrestrial analysis. The highest risk quotient for any 

animal was for birds (avian) chronic risk based on dietary analysis (RQ = 167). 

The dose (lb/acre) that results in an RQ below the chronic level of concern is 0.006 lb 

ai/A (1 lb/acre÷167). No adjustment is needed for LOC, since it is 1 for chronic risk. 

The AgDISP model with the far-field Gaussian extension was used to calculate the spray 

drift buffer needed to reduce exposures to below 0.006 lb/acre. The following inputs 

were used; all other inputs were default values. This analysis indicates that the required 

spray drift buffer needed to define the Action Area for terrestrial effects is 2,913 feet 

(about 0.55 mile). 

Table 2.6. AgDISP Input Parameters 

Input parameter Value 

Release height 15 feet 

Wind Speed 15 mph 

Drop Size Distribution ASAE Very fine to Fine 

Spray volume rate 5 gallons per acre 

Non-volatile fraction 0.032 (1.33 lb product in 5 gal = 42 lb 

water) 

Active Fraction 0.024 (nonvol frac x % a.i. = 75%) 

Canopy None 

Specific gravity (Carrier) 1 

Initial Average Deposition 0.006 lb/acre 

43
2.8 Assessment Endpoints and Measures of Ecological Effect 

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

value that is to be protected.”5 Selection of the assessment endpoints is based on valued 

entities (e.g., CRLF, organisms important in the life cycle of the CRLF, and the PCEs of 

its designated critical habitat), the ecosystems potentially at risk (e.g,. waterbodies, 

riparian vegetation, and upland and dispersal habitats), the migration pathways of 

acephate (e.g., runoff, spray drift, etc.), and the routes by which ecological receptors are 

exposed to acephate-related contamination (e.g., direct contact, etc). 

 2.8.1. Assessment Endpoints for the CRLF 

Assessment endpoints for the CRLF include direct toxic effects on the survival, 

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

the prey base and/or modification of its habitat. In addition, potential destruction and/or 

adverse modification of critical habitat is assessed by evaluating potential effects to 

PCEs, which are components of the habitat areas that provide essential life cycle needs of 

the CRLF. 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 are also 

considered. 

A complete discussion of all the toxicity data available for this risk assessment, including 

resulting measures of ecological effect selected for each taxonomic group of concern, is 

included in Section 4 of this document. A summary of the assessment endpoints and 

measures of ecological effect selected to characterize potential assessed direct and 

indirect CRLF risks associated with exposure to acephate is provided in Table 2.7. 

Table 2.7 Summary of Assessment Endpoints and Measures of Ecological Effects for Direct 

and Indirect Effects of acephate on the California Red-legged Frog

Toxicity Endpoint (see effects 

table for endpoint selection, 

Section 4) 

6 Assessment Endpoint Measures of Ecological Effects

Aquatic Phase 

(eggs, larvae, tadpoles, juveniles, and adults)a

1a. Rainbow trout acute 96-hr LC50

832 ppm ai 1a. Most sensitive fish acute LC50 1. Survival, growth, and 

reproduction of CRLF 

individuals via direct 

effects on aquatic phases 

1b. Most sensitive fish chronic 

NOAEC 1b. none available 1c. Most sensitive fish early-life stage 

NOAEC 1c. Rainbow trout 0.215 ppm ai 

 

5

 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001. 6

 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in 

Appendix A. 

44
Table 2.7 Summary of Assessment Endpoints and Measures of Ecological Effects for Direct 

and Indirect Effects of acephate on the California Red-legged Frog

Toxicity Endpoint (see effects 

table for endpoint selection, 

Section 4) 

6 Assessment Endpoint Measures of Ecological Effects

(Acute-Chronic-Ratio) 

2a1. Rainbow trout acute 96-hr LC50

832 ppm ai 2. Survival, growth, and 

reproduction of CRLF 

individuals via effects to 

food supply (i.e.,

freshwater invertebrates, 

non-vascular plants) 

2a. Most sensitive fish (1), aquatic 

invertebrate (2), and aquatic plant (3-) 

EC

2a2. -Daphnia magna acute 48-hr 

EC50 = 1.1 ppm ai 50 or LC50 (guideline) 2a3. Skeletonema costatum algae 5-

day EC 2b. Most sensitive aquatic invertebrate 

(1-) and fish (2) chronic NOAEC 

(guideline or ECOTOX) 

50 >50 ppm ai 

2b1. Daphnia magna NOAEC = 

0.015 ppm ai 

2b2. none available 

3a. Vascular plant EC 3. Survival, growth, and 

reproduction of CRLF 

individuals via indirect 

effects on habitat, cover, 

and/or primary 

productivity (i.e., aquatic 

plant community) 

50 (duckweed 

guideline test or ECOTOX vascular 

plant) 

3a. none available 

3b. Non-vascular plant EC50 () 3b. Skeletonema costatum algae 5-

day EC50 >50 ppm ai 

4. Survival, growth, and 

reproduction of CRLF 

individuals via effects to 

riparian vegetation, 

required to maintain 

acceptable water quality 

and habitat in ponds and 

streams comprising the 

species’ current range. 

4a. Distribution of EC25 values for 

monocots (seedling emergence, 

vegetative vigor, or ECOTOX) 

4a. EC25 and NOEC values are 

greater than 4.5 lb/acre 

4b. Distribution of EC25 values for 

dicots (seedling emergence, vegetative 

vigor, or ECOTOX)7 4b. EC25 and NOEC values are 

greater than 4.5 lb/acre 

Terrestrial Phase (Juveniles and adults) 

5. Survival, growth, and 

reproduction of CRLF 

individuals via direct 

effects on terrestrial 

phase adults and 

juveniles 

5a. Most sensitive birdb () or terrestrial- 5a. Dark eyed junko acute oral LD

phase amphibian acute LC 50

50 or LD50 = 106 mg ai/kg-bw 

(guideline) 

5b. Most sensitive birdb 5b. Mallard duck Reproductive 

study NOEL = 5 ppm ai 

 () or 

terrestrial-phase amphibian chronic 

NOAEC (guideline or ECOTOX) 

6a1. Honey bee acute contact LD50

= 1.20 ug ai/bee 6. Survival, growth, and 

reproduction of CRLF 

individuals via effects on 

prey (i.e., terrestrial 

invertebrates, small 

terrestrial vertebrates, 

including mammals and 

terrestrial phase 

amphibians) 

6a. Most sensitive terrestrial 

invertebrate (1-) and vertebrate (2-) 

acute EC 6a2. Rat Acute oral LD50 = 866 mg 

ai/kg bw 50 or LC50 (guideline or 

ECOTOX)c

6b1. None available 6b. Most sensitive terrestrial 

invertebrate(1) and vertebrate(2-) 

chronic NOAEC (guideline or 

ECOTOX) 

6b2. Rat 3- generation reproductive 

study NOAEL = 50 mg ai/kg bwday diet4

7. Survival, growth, and 

reproduction of CRLF 

individuals via indirect 

7a. Distribution of EC25 for monocots 

(seedling emergence, vegetative vigor, 

or ECOTOX 

7a. All monocot EC25 and NOEC 

values are greater than 4.5 lb/acre 

 

7

 The available information indicates that the California red-legged frog does not have any obligate 

relationships. 

45
Table 2.7 Summary of Assessment Endpoints and Measures of Ecological Effects for Direct 

and Indirect Effects of acephate on the California Red-legged Frog

Toxicity Endpoint (see effects 

table for endpoint selection, 

Section 4) 

6 Assessment Endpoint Measures of Ecological Effects

effects on habitat (i.e.,

riparian vegetation) 

7b. Distribution of EC25 for dicots 

(seedling emergence, vegetative vigor, 

or ECOTOX)

7b. All dicot EC25 and NOEC 

values are greater than 4.5 lb/acre 5

a

 Adult frogs are no longer in the “aquatic phase” of the amphibian life cycle; however, submerged adult 

frogs are considered “aquatic” for the purposes of this assessment because exposure pathways in the water 

are considerably different that exposure pathways on land. 

b

 Birds are used as surrogates for terrestrial phase amphibians. c

 Although the most sensitive toxicity value is initially used to evaluate potential indirect effects, sensitivity 

distribution is used (if sufficient data are available) to evaluate the potential impact to food items of the 

CRLF.

2.8.2. Assessment Endpoints for Designated Critical Habitat 

As previously discussed, designated critical habitat is assessed to evaluate actions related 

to the use of acephate that may alter the PCEs of the CRLF’s critical habitat. PCEs for 

the CRLF were previously described in Section 2.6. Actions that may destroy or 

adversely modify critical habitat are those that alter the PCEs and may jeopardize the 

continued existence of the CRLF. Therefore, these actions are identified as assessment 

endpoints. It should be noted that evaluation of PCEs as assessment endpoints is limited 

to those of a biological nature (i.e., the biological resource requirements for the listed 

species associated with the critical habitat) and those for which acephate effects data are 

available. 

Assessment endpoints and measures of ecological effect selected to characterize potential 

modification to designated critical habitat associated with exposure to acephate are 

provided in Table 2.7. Adverse modification to the critical habitat of the CRLF includes 

the following, as specified by USFWS (2006) and previously discussed in Section 2.6: 

1. Alteration of water chemistry/quality including temperature, turbidity, and 

oxygen content necessary for normal growth and viability of juvenile and 

adult CRLFs. 

2. Alteration of chemical characteristics necessary for normal growth and 

viability of juvenile and adult CRLFs. 

3. Significant increase in sediment deposition within the stream channel or pond 

or disturbance of upland foraging and dispersal habitat. 

4. Significant alteration of channel/pond morphology or geometry. 

5. Elimination of upland foraging and/or aestivating habitat, as well as dispersal 

habitat. 

6. Introduction, spread, or augmentation of non-native aquatic species in stream 

segments or ponds used by the CRLF. 

7. Alteration or elimination of the CRLF’s food sources or prey base. 

46
Measures of such possible effects by labeled use of acephate on critical habitat of the 

CRLF are described in Table 2.7. Some components of these PCEs are associated with 

physical abiotic features (e.g., presence and/or depth of a water body, or distance between 

two sites), which are not expected to be measurably altered by use of pesticides. 

Assessment endpoints used for the analysis of designated critical habitat are based on the 

adverse modification standard established by USFWS (2006). 

47
Table 2.8. Summary of Assessment Endpoints and Measures of Ecological Effect for Primary 

Constituent Elements of Designated Critical Habitat 

Assessment Endpoint Measures of Ecological Effect8

Aquatic Phase PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat) 

a. Most sensitive aquatic plant 

Alteration of channel/pond morphology or geometry 

and/or increase in sediment deposition within the 

stream channel or pond: aquatic habitat (including 

riparian vegetation) provides for shelter, foraging, 

predator avoidance, and aquatic dispersal for juvenile 

and adult CRLFs. 

EC50 (guideline or a. Skeletonema costatum 

ECOTOX) algae 5-day EC50 >50 ppm ai 

b. Distribution of EC25 values 

for terrestrial monocots b. All monocot EC25 and 

(seedling emergence, NOEC values are greater than 

vegetative vigor, or 4.5 lb/acre 

ECOTOX) 

c. Distribution of EC25 values c. All dicot EC25 and NOEC 

for terrestrial dicots (seedling values are greater than 4.5 

emergence, vegetative vigor, lb/acre 

or ECOTOX) 

a. Most sensitive EC

Alteration in water chemistry/quality including 

temperature, turbidity, and oxygen content necessary 

for normal growth and viability of juvenile and adult 

CRLFs and their food source.9

50 values 

for aquatic plants (guideline or a. Skeletonema costatum 

ECOTOX) algae 5-day EC50 >50 ppm ai 

b. Distribution of EC25 values 

for terrestrial monocots b. All monocot EC25 and 

(seedling emergence or NOEC values are greater than 

vegetative vigor, or 4.5 lb/acre 

ECOTOX) 

c. Distribution of EC25 values c. All dicot EC25 and NOEC 

for terrestrial dicots (seedling values are greater than 4.5 

emergence, vegetative vigor, lb/acre

or ECOTOX) 

a. Most sensitive EC

Alteration of other chemical characteristics necessary 

for normal growth and viability of CRLFs and their 

food source. 

50 or LC50 a. Skeletonema costatum values for fish or aquatic- algae 5-day EC50 >50 ppm ai phase amphibians and aquatic 

invertebrates (guideline or b. All monocot EC25 and ECOTOX) NOEC values are greater than b. Most sensitive NOAEC 4.5 lb/acre values for fish or aquaticphase amphibians and aquatic c. All dicot EC25 and NOEC invertebrates (guideline or values are greater than 4.5 ECOTOX) lb/acre

Reduction and/or modification of aquatic-based food 

sources for pre-metamorphs (e.g., algae) 

a. Most sensitive aquatic plant a. Skeletonema costatum 

EC50 (guideline or algae 5-day EC50 >50 ppm ai 

ECOTOX) 

Terrestrial Phase PCEs 

(Upland Habitat and Dispersal Habitat) 

Elimination and/or disturbance of upland habitat; 

ability of habitat to support food source of CRLFs: 

Upland areas within 200 ft of the edge of the riparian 

vegetation or dripline surrounding aquatic and riparian 

a. Distribution of EC25 values a. Skeletonema costatum 

for monocots (seedling algae 5-day EC50 >50 ppm ai 

emergence, vegetative vigor, 

or ECOTOX) b. All monocot EC25 and 

 

8

 All toxicity data reviewed for this assessment are included in Appendix A. 

9

 Physico-chemical water quality parameters such as salinity, pH, and hardness are not evaluated because 

these processes are not biologically mediated and, therefore, are not relevant to the endpoints included in 

this assessment. 

48
habitat that are comprised of grasslands, woodlands, 

and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance 

Elimination and/or disturbance of dispersal habitat: 

Upland or riparian dispersal habitat within designated 

units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites 

including both natural and altered sites which do not 

contain barriers to dispersal 

Reduction and/or modification of food sources for 

terrestrial phase juveniles and adults

b. Distribution of EC25 values 

for dicots (seedling 

emergence, vegetative vigor, NOEC values are greater than or ECOTOX) 4.5 lb/acre c. Most sensitive food source 

acute EC50/LC50 and NOAEC c. All dicot EC25 and NOEC values for terrestrial values are greater than 4.5 vertebrates (mammals) and lb/acre invertebrates, birds or 

terrestrial-phase amphibians, Alteration of chemical characteristics necessary for and freshwater fish. 

normal growth and viability of juvenile and adult 

CRLFs and their food source. 

 

2.9 Conceptual Model 

 2.9.1 Risk Hypotheses 

Risk hypotheses are specific assumptions about potential adverse effects (i.e., changes in 

assessment endpoints) and may be based on theory and logic, empirical data, 

mathematical models, or probability models (U.S. EPA, 1998). For this assessment, the 

risk is stressor-linked, where the stressor is the release of acephate to the environment. 

The stressor is defined to be acephate and its degradate methamidophos. The following 

risk hypotheses are presumed for this endangered species assessment: 

 

• Labeled uses of acephate within the action area may directly affect the CRLF by 

causing mortality or by adversely affecting growth or fecundity; 

• Labeled uses of acephate within the action area may indirectly affect the CRLF by 

reducing or changing the composition of food supply; 

• Labeled uses of acephate within the action area may indirectly affect the CRLF 

and/or adversely modify designated critical habitat by reducing or changing the 

composition of the aquatic plant community in the ponds and streams comprising the 

species’ current range and designated critical habitat, thus affecting primary productivity 

and/or cover; 

• Labeled uses of acephate within the action area may indirectly affect the CRLF 

and/or adversely modify designated critical habitat by reducing or changing the 

composition of the terrestrial plant community (i.e., riparian habitat) required to maintain 

acceptable water quality and habitat in the ponds and streams comprising the species’ 

current range and designated critical habitat; 

Based on the results of the submitted terrestrial plant toxicity tests, it appears that 

seedlings and emerged plants may not be sensitive to acephate. 

There are two Tier II multiple dose phytotoxicity tests for acephate (seedling emergence 

and vegetative vigor) that were submitted by the registrant. The EC25 is greater than 4.5 

lb ai/A and the NOEC is 4.5 lb ai/A. A typical application rate for acephate is 1.0 lb/A. 

49
Based on the results of the submitted terrestrial plant toxicity tests, it appears that 

seedlings and emerged plants are not sensitive to acephate and therefore acepahte No 

Effect on the CRLF based on these endpoints. 

• Labeled uses of acephate within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing breeding and nonbreeding aquatic habitat (via modification of water quality parameters, habitat 

morphology, and/or sedimentation); 

• Labeled uses of acephate within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing the food supply required for normal 

growth and viability of juvenile and adult CRLFs; 

• Labeled uses of acephate within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing upland habitat within 

200 ft of the edge of the riparian vegetation necessary for shelter, foraging, and predator 

avoidance. 

• Labeled uses of acephate within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing dispersal habitat within 

designated units and between occupied locations within 0.7 mi of each other that allow 

for movement between sites including both natural and altered sites which do not contain 

barriers to dispersal. 

• Labeled uses of acephate within the action area may adversely modify the 

designated critical habitat of the CRLF by altering chemical characteristics necessary for 

normal growth and viability of juvenile and adult CRLFs. 

 2.9.2 Diagram 

The conceptual model is a graphic representation of the structure of the risk assessment. 

It specifies the stressor (acephate and methamidophos), release mechanisms, biological 

receptor types, and effects endpoints of potential concern. The conceptual models for 

aquatic and terrestrial phases of the CRLF are shown in Figures 2.H and 2.I, and the 

conceptual models for the aquatic and terrestrial PCE components of critical habitat are 

shown in Figures 2.J and 2.K. Exposure routes shown in dashed lines are not 

quantitatively considered because the resulting exposures are expected to be so low as not 

to cause adverse effects to the CRLF. 

Long-range atmospheric transport is not expected due to the non-volatility and nonpersistent nature of acephate. Likewise, groundwater transport is considered unlikely due 

to the non-persistence of acephate, even when its mobility in soil is considered. The 

operative routes of exposure will be spray drift at the time of application, and run-off due 

to precipitation within a few days of application. 

50
Stressor

Source

Attribute

Change

Habitat 

PCEs

Acephate applied to use site

Direct

application Spray drift

Red-legged Frog

Juvenile

Adult

Terrestrial 

insects

Food resources

Reduction in food 

sources

Elimination and/or disturbance of 

upland or dispersal habitat

Reduction in primary productivity

Reduced shelter

Restrict movement

Terrestrial plants

grasses/forbs, fruit, 

seeds (trees, shrubs)

Runoff

Mammals

Exposure

Media and

Receptors

Soil

Ingestion

Ingestion

Ingestion

Ingestion

Dermal uptake/Ingestion

Long range 

atmospheric 

transport

Root uptake

Wet/dry deposition

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Other chemical 

characteristics

Adversely modified 

chemical characteristics

Population

Reduced survival

Reduced growth

Reduced reproduction

Community

Reduced seedling emergence 

or vegetative vigor 

(Distribution)

Figure 2.H. Conceptual Diagram for Terrestrial Phase Effects on CRLF 

Stressor

Source

Receptors

Attribute

Change

Acephate applied to use site

Spray drift

Red-legged Frog

Eggs Juveniles

Larvae Adult

Tadpoles

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Food sources

Reduction in algae

Reduction in prey

Habitat quality and channel/pond 

morphology or geometry

Adverse water quality changes

Increased sedimentation

Reduced shelter

Surface water/

Sediment

Runoff

Aquatic Animals

Invertebrates

Vertebrates

Exposure

Media

Uptake/gills 

or integument

Ingestion Ingestion

Long range 

atmospheric 

transport

Wet/dry deposition

Soil Groundwater

Uptake/gills 

or integument

Aquatic Plants

Non-vascular

Vascular

Uptake/cell, 

roots, leaves

Riparian and 

Upland plants

terrestrial exposure 

pathways and PCEs

see Figure 2.d

Community

Reduced seedling 

emergence or vegetative 

vigor (Distribution)

Habitat 

PCEs

Other chemical 

characteristics

Adversely modified 

chemical characteristics

Population 

Yield

Reduced yield

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Figure 2.I . Conceptual Diagram for Effects on Aquatic Phase CRLF 

51
Stressor

Source

Attribute

Change

Habitat 

PCEs

Acephate applied to use site

Direct

application Spray drift

Red-legged Frog

Juvenile

Adult

Terrestrial 

insects

Food resources

Reduction in food 

sources

Elimination and/or disturbance of 

upland or dispersal habitat

Reduction in primary productivity

Reduced shelter

Restrict movement

Terrestrial plants

grasses/forbs, fruit, 

seeds (trees, shrubs)

Runoff

Mammals

Exposure

Media and

Receptors

Soil

Ingestion

Ingestion

Ingestion

Ingestion

Dermal uptake/Ingestion

Long range 

atmospheric 

transport

Root uptake

Wet/dry deposition

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Other chemical 

characteristics

Adversely modified 

chemical characteristics

Population

Reduced survival

Reduced growth

Reduced reproduction

Community

Reduced seedling emergence 

or vegetative vigor 

(Distribution)

Figure 2.J . Conceptual Diagram for Effects Terrestrial Critical Habitat 

Stressor

Source

Receptors

Attribute

Change

Acephate applied to use site

Spray drift

Red-legged Frog

Eggs Juveniles

Larvae Adult

Tadpoles

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Food sources

Reduction in algae

Reduction in prey

Habitat quality and channel/pond 

morphology or geometry

Adverse water quality changes

Increased sedimentation

Reduced shelter

Surface water/

Sediment

Runoff

Aquatic Animals

Invertebrates

Vertebrates

Exposure

Media

Uptake/gills 

or integument

Ingestion Ingestion

Long range 

atmospheric 

transport

Wet/dry deposition

Soil Groundwater

Uptake/gills 

or integument

Aquatic Plants

Non-vascular

Vascular

Uptake/cell, 

roots, leaves

Riparian and 

Upland plants

terrestrial exposure 

pathways and PCEs

see Figure 2.d

Community

Reduced seedling 

emergence or vegetative 

vigor (Distribution)

Habitat 

PCEs

Other chemical 

characteristics

Adversely modified 

chemical characteristics

Population 

Yield

Reduced yield

Individual organisms

Reduced survival

Reduced growth

Reduced reproduction

Figure 2.K . Conceptual Diagram for Effects on Aquatic Critical Habitat 

52
2.10 Analysis Plan 

Analysis of risks to the California Red-Legged Frog (both direct and indirect) and to its 

critical habitat will be assessed consistent with the Overview Document (USEPA, 2004) 

and Agency guidance for ecological risk assessments (USEPA 1998). 

There are a number of labeled uses for acephate for indoor applications. Indoor uses 

include: Commercial Storages/Warehouses Premises; Commercial Transportation 

Facilities-Nonfeed/Nonfood; Commercial/Industrial/Industrial Premises/Equipment 

(Indoor); Eating Establishments; Food Processing Plant Premises (Nonfood Contact); 

Food Stores/Markets/Supermarkets Premises; Food/Grocery/Marketing/Storage/ 

Distribution Facility Premise; Hospitals/Medical Institutions Premises 

(Human/Veterinary); Household/Domestic Dwellings; Household/Domestic Dwelling 

Contents; Household/Domestic Dwellings Indoor Food Handling Areas; 

Household/Domestic Dwellings Indoor Premises; Poultry Processing Plant Premises 

(Nonfood Contact); Refuse Solid Waste Containers (Garbage Cans); Refuse/Solid Waste 

Sites (Indoor). 

These applications have been considered. There is no exposure pathway from indoor 

applications to the CRLF or its habitat and therefore, indoor applications are determined 

to have No Effect on the CRLF. 

 

2.10.1 Exposure Analysis 

Direct effects to the aquatic phase CRLF will be assessed by comparing modeled surface 

water exposure concentrations of acephate and its degradate methamidophos to acute and 

chronic (early life stage hatching success and growth) effect concentrations for aquatic 

phase amphibians (surrogate freshwater fish) from laboratory studies (see the Effects 

Analysis section below). Effects to aquatic dietary food resources (aquatic invertebrates, 

algae) of the aquatic phase CRLF or effects to aquatic habitat that support the CRLF will 

also be assessed by comparing modeled surface water exposure concentrations of total 

acephate residues to laboratory established effect levels appropriate for the taxa. 

A Tier 1 analysis (GENEEC 2.0) will be done first, since the toxicity endpoints for 

acephate are all above 1 ppm, and it is not anticipated that any LOCs will be exceeded, 

with the possible exception of invertebrates. As a refinement step, surface water 

concentrations of acephate and methamidophos will be quantified using the Tier 2 model, 

PRZM-EXAMS, if any LOCs are exceeded at the Tier 1 level. The definitive Tier 2 

assessment of methamidophos is given in the methamidophos CRLF assessment 

document. 

For the screening assessment, the standard EXAMS water body of 2 meters maximum 

depth, and 20,000 cubic meters volume, will be used. Since acephate is applied by 

numerous application methods, the model accounts for loading of acephate into the 

surface water via spray drift, run-off and erosion (Figure 2.J). Agricultural scenarios 

appropriate for labeled acephate uses will be used to account for local soils, weather and 

53
growing practices which impact the magnitude and frequency of acephate loading to the 

surface water. Maximum labeled application rates, with maximum number of 

applications and shortest intervals, will be used to help define (1) the Action Area within 

California for the Federal Action and (2) for evaluating effects to the CRLF. 

Methamidophos is a major degradate of acephate. Methamidophos is also a registered 

organophosphate insecticide, and is registered for use in the U.S. on cotton, potatoes, and 

tomatoes and alfalfa grown for seed. Risks from acephate use, which includes its major 

degradate methamidophos, are assessed in this effects determination. The Agency is 

evaluating the risks to the CRLF posed by registered methamidophos uses separately. 

Concentrations of acephate estimated by PRZM-EXAMS represent acephate loading in 

water bodies adjacent to any treated field and assume that the concentration applies to 

any water body within the treated area. 

Risks to the terrestrial phase CRLF will be assessed by comparing modeled exposure to 

effect concentrations from laboratory studies. Exposure in the terrestrial phase will be 

quantified using the TREX model, which automates the calculation of dietary exposure 

according to the Hoerger-Kenaga nomogram, as modified by Fletcher (1994). The 

nomogram tabulates the 90th and 50th percentile exposure expected on various classes of 

food items, and scales the exposure (in dietary terms) to the size and daily food intake of 

several size classes of birds and mammals. Birds are also used as surrogates to represent 

reptiles and terrestrial-phase amphibians. A foliar decay half-life of 8.2 days, the 

maximum for acephate found in Willis and McDowell (1987) will be substituted for the 

default 35-day value. Effects from methamidophos are not considered quantitatively as 

LOC is expected to be exceeded for parent acephate. 

 2.10.2 Effects Analysis 

As previously discussed in Section 2.8.1 and 2.8.2, assessment endpoints for the frog 

include direct toxic effects on survival, reproduction, and growth of the species itself, as 

well as indirect effects, such as reduction of the prey base and/or modification of CRLF 

habitat. Direct effects to the CRLF are based on toxicity information for freshwater fish 

and birds, which are generally used as a surrogate for aquatic and terrestrial phase 

amphibians, respectively. The open literature will be screened also for available frog 

toxicity data. Indirect effects to the CRLF are assessed by looking at available toxicity 

information relative to the frog’s prey items and habitat requirements (freshwater 

invertebrates, freshwater vertebrates, aquatic plants, terrestrial invertebrates, terrestrial 

vertebrates, and terrestrial plants). Both guideline and open literature toxicity data will 

be identified and evaluated for use in determining RQ values. 

Acephate’s toxicity dataset is incomplete; chronic fish studies are lacking. Other 

organophosphates will be screened for available chronic fish data that can be used to 

derive acute to chronic ratio (ACR) for acephate. 

54
Toxicity studies for acephate degradates (where available) will be discussed for exposure 

to the aquatic phase of the CRLF and incorporated into this risk assessment. 

 2.10.3 Action Area Analysis 

The Action Area for the federal action is the geographic extent of exceedence of Listed 

species Levels of Concern (LOC) for any taxon or effect (plant or animal, acute or 

chronic, direct or indirect) resulting from the maximum label-allowed use of acephate. 

To define the extent of the Action Area, the following exposure assessment tools will be 

used: PRZM-EXAMS, TREX, AgDrift, and ArcGIS, a geographic information system 

(GIS) program. Other tools may be used as required if these are inadequate to define the 

maximum extent of the Action Area. 

To determine the downstream extent of the Action area for any aquatic effects, acephate 

residues are also estimated for downstream from the treated areas by assuming dilution 

with stream water (derived from land area) from unaffected sources propagating 

downstream, until a point is reached beyond which there are no relevant LOC 

exceedances. Once the distribution of predicted stream water concentrations is obtained, 

it is further processed using a model that calculates expected dilution in the stream 

according to contributing land area. As the land area surrounding the field on which 

acephate is applied is enlarged, it encompasses a progressively greater drainage area; in 

effect, a progressively larger ‘sub-watershed’ is created, with a concomitant increase in 

dilution at the drainage point. This drainage point moves down-gradient along the stream 

channel as the sub-watershed is expanded. At a certain point the predicted stream 

concentrations will fall below the LOC. The area below this point is then assumed not to 

be at risk, with the upstream areas (up to the initial application area) assumed to present 

the potential for (direct and indirect) impact on the RLF. Additional acephate inputs 

within the same watershed will cause the area bounded by (that is, within) the LOC to 

increase, extending the length of stream that is likely to be impacted. 

In order to determine the extent of the action area downstream from the initial area of 

concern, the Agency will need to complete the screening level risk assessment. Once all 

aquatic risk quotients (RQs) are calculated, the Agency determines which RQ to level of 

concern (LOC) ratio is greatest for all aquatic organisms (plant and animal). For 

example, if both fish and aquatic plants have the same RQ of 1, the fish RQ to LOC ratio 

(1/0.05) would be greater than for plants (1/1). Therefore, the Agency would identify all 

stream reaches downstream from the initial area of concern where the PCA (percent 

cropped area) for the land uses identified for acephate are greater than 1/20, or 5%. All 

streams identified as draining upstream catchments greater than 5% of the landclass of 

concern, will be considered part of the action area. 

55
3. Exposure Assessment 

3.1 Label Application Rates and Intervals 

The registered uses of acephate in California include cotton, lettuce, citrus, celery, 

peppers, beans, mint, Bermuda grass for seed, landscape maintenance, pistachio, 

structural pest control, greenhouses, plants in containers, transplants, flowers, and others 

(see Table 2.2). 

 

The application rates, intervals, and frequency are summarized in Table 3.1. 

Table 3.1. Label Use Rates for Acephate in California 

Use Application 

Rate (lb 

ai/acre) 

Number of 

applications 

allowed 

Application 

Interval 

Application 

Type 

Beans & Lima 

beans – dry 

and succulent 

forms 

 1 lb a.i. 2 7 days air or ground 

Celery 1 lb a.i. 2 3 days Air or ground 

Cole Crops – 

Brussels 

sprouts and 

cauliflower 

1 lb a.i. 2 not specified 

Cotton 1 lb a.i. 6 3, 5, 7 days or 

as necessary 

Air, ground, infurrow 

Head lettuce – 

crisphead type 

1 lb a.i. 2 Not specified Air or ground 

Mint – 

peppermint and 

spearmint 

1 lb a.i. 2 10 days Air or ground 

Non-bearing 

Citrus 

0.75 lb a.i. Not specified 7 days or as 

necessary 

Ground or soil 

drench 

Non-crop areas 

– field borders, 

fencerows, 

roadsides, 

ditchbanks, 

borrowpits 

0.25 lb a.i. Not specified Not specified Ground or air 

Non-crop area 

– wastelands 

0.125 lb a.i. 1 Not applicable Ground or air 

Peanuts 1 lb a.i. 4 Not specified Air or ground 

Peppers – bell 1 lb a.i. 2 As necessary Air or ground 

Bermuda grass 

for seed 

1 lb a.i. Not specified Not specified Air or ground 

Nursery stock, 

non-bearing 

deciduous fruit 

trees, nut trees, 

and vines in 

nursery fields 

or non-baring 

orchards 

1 lb a.i. Not specified As needed Air or ground 

56
The largest number of applications with the shortest interval between applications will be used whenever 

the label does not specify the number of applications or application interval. 

3.2 Aquatic Exposure Assessment 

As discussed in section 2.5, the CRLF occupies a variety of shallow, static and flowing 

aquatic habitats in the aquatic phase of its life cycle (egg to tadpole). The current range 

of the CRLF is represented by the core areas, critical habitat and occurrence sections in 

Figure 2.D. 

 3.2.1. Conceptual Model of Exposure 

Aquatic exposure of the CRLF within the action area is estimated with the PRZMEXAMS model (EPA, 2004). Screening-level exposures (estimated environmental 

concentrations, EEC) are produced using the standard farm pond of 20,000 cubic meters 

volume. Watersheds where acephate is used are assumed to have 100% cropped area. 

The downstream extent of streams with exposures above the Level of Concern (LOC) is 

estimated (using GIS methods) by expanding the watershed considered until 

uncontaminated stream flow dilutes the initial pond concentration to below the LOC. 

Standard assumptions of 1% spray drift for ground application and 5% drift for aerial 

application are used. If the pond concentration from PRZM-EXAMS exceeds the LOC, a 

spray drift buffer is calculated (using AgDrift model) that will reduce the pond 

concentration to below the LOC. If a spray drift buffer cannot be used to reduce the pond 

concentration to below the LOC, then a separate spray drift buffer (neglecting run-off) is 

calculated with AgDrift to ensure that pond concentrations are below the LOC (see 

section 2.10.3 above). 

 3.2.2 Existing Monitoring Data

There is very little useful water monitoring data for acephate, due to its non-persistent 

nature. There were no data for acephate or methamidophos in the California surface 

water database or in the USGS NAWQA surface water monitoring program. The 

assessment will therefore be based on modeled concentrations as described in section 

3.2.1. 

 3.2.3 Modeling Approach 

Use sites and the PRZM scenarios used to represent them are given in Table 3.4. 

Risk quotients (RQs) were initially based on EECs derived using the Pesticide Root Zone 

Model/Exposure Analysis Modeling System (PRZM/EXAMS) standard ecological pond 

scenario according to the methodology specified in the Overview Document (U.S. EPA, 

2004). Where LOCs for direct/indirect effects and/or adverse habitat modification are 

exceeded based on the modeled EEC using the static water body (i.e., “may affect”), 

refined modeling may be used to differentiate “may affect, but not likely to adversely 

57
affect” from “may affect and likely to adversely affect” determinations for the CRLF and 

its designated critical habitat. 

The general conceptual model of exposure for this assessment is that the highest 

exposures are expected to occur in the headwater streams adjacent to agricultural fields. 

Many of the streams and rivers within the action area defined for this assessment are in 

close proximity to agricultural use sites. 

Twenty-six California scenarios were developed for the CRLF assessment. Each 

scenario is intended to represent a high-end exposure setting for a particular crop. Each 

scenario location is selected based on various factors including crop acreage, runoff and 

erosion potential, climate, and agronomic practices. Once a location is selected, a 

scenario is developed using locally specific soil, climatic, and agronomic data. Each 

PRZM scenario is assigned a specific climatic weather station providing 30 years of daily 

weather values. 

Specific California PRZM scenarios were chosen for this assessment, including citrus, 

lettuce, row crop (representing beans, celery, and peppers), cotton, turf (representing 

bermudagrass for seed and landscape maintenance), almond (representing nut trees), 

fruit (representing various fruit trees) and cole crops (broccoli, cauliflower). Non-crop 

areas were not modeled because the application rates are lower than the agricultural uses, 

and agricultural scenarios are believed to represent non-agricultural exposures 

adequately. Structural pest control was not modeled due to lack of an appropriate PRZM 

scenario, and the low likelihood of exposure. All scenarios were used within the standard 

framework of PRZM/EXAMS modeling using the standard graphical user interface 

(GUI) shell, PE4v01.pl. 

3.2.3.1 Model Inputs 

A Tier 1 assessment (GENEEC 2.0) was run because the toxicity endpoints for acephate 

are in the part-per-million range, and it was not deemed likely that any LOCs would be 

exceeded. Tier 2 analysis (PRZM-EXAMS) was run if any LOCs were exceeded at the 

Tier 1 level. 

The estimated water concentrations from surface water sources were calculated using 

Tier 2 PRZM (Pesticide Root Zone Model) and EXAMS (Exposure Analysis Modeling 

System). PRZM is used to simulate pesticide transport as a result of runoff and erosion 

from a standardized watershed, and EXAMS estimates environmental fate and transport 

of pesticides in surface waters. The linkage program shell (PE4v01.pl) that incorporates 

the site-specific scenarios was used to run these models. 

The PRZM/EXAMS model was used to calculate concentrations using the standard 

ecological water body scenario in EXAMS. Weather and agricultural practices were 

simulated over 30 years so that the 1 in 10 year exceedance probability at the site was 

estimated for the standard ecological water body. 

58
Models to estimate the effect of setbacks on load reduction for runoff are not currently 

available. It is well documented that vegetated setbacks can result in a substantial 

reduction in pesticide load to surface water (USDA, NRCS, 2000). Therefore, the 

aquatic EECs presented in this assessment are likely to over-estimate exposure in areas 

with well-vegetated setbacks. While the extent of load reduction cannot be accurately 

predicted through each relevant stream reach in the action area, data from USDA (USDA, 

2000) suggest reductions could range from 11 to 100%. 

The date of first application was set at March 1, because most uses for which there are 

data (PUR) show use in California in most months of the year, and March corresponds to 

both a rainy part of the year (thereby capturing higher run-off values), and the 

reproductive season of the frog. 

The appropriate PRZM input parameters were selected from the environmental fate data 

submitted by the registrant and in accordance with US EPA-OPP EFED water model 

parameter selection guidelines, Guidance for Selecting Input Parameters in Modeling the 

Environmental Fate and Transport of Pesticides, Version 2.3, February 28, 2002. 

Exposures for the toxic degradate methamidophos were calculated by applying two 

correction factors: 0.77 for the molecular weight difference, and 0.23 for the maximum 

amount of methamidophos formed in an acephate soil metabolism study (MRID 

0014991). Thus, methamidophos exposures are (0.77)*(0.23) = 0.18 times the acephate 

exposures. 

 

Table 3.2. Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Acephate CRLF Assessment 

Fate Property Value MRID (or source) Comments 

calculated from 

structure 

none Molecular Weight 183.16 

Calculated from vapor 

pressure and solubility Henry’s constant 5.1 E-13 atm-m3/mole Acephate IRED 

MRID 40390601; Vapor Pressure 1.7 E-6 torr Acephate IRED 40645901 

MRID 40390601 Solubility in Water 801000 mg/L Acephate IRED 

MRID 41081603 Photolysis in Water Stable Acephate IRED 

MRID 00014991

Aerobic Soil Metabolism 

Half-lives 

3 times single value of 

14 hours, as per Input 

Parameter Guidance 

2.3 days Acephate IRED 

MRID 41081603 Hydrolysis 163 days Acephate IRED 

Aerobic Aquatic 

Metabolism (water 

column) 

2 time soil input value 

as per Input Parameter 

Guidance 

4.6 days Acephate IRED 

2 time soil input value 

as per Input Parameter 

Guidance 

Anaerobic Aquatic 

Metabolism (benthic) 4.6 days Acephate IRED 

Kd 0.09 Acephate IRED MRID 40504811 

59
Fate Property Value MRID (or source) Comments 

95 % for aerial as per Input Parameter 

Guidance Application Efficiency Default valuec

99 % for ground 

as per Input Parameter 

Guidance for ecological 

assessments 

5 % for aerial Spray Drift Fractionb Default value 1 % for ground 

Application method 

(CAM) 2 Foliar spray 

none 

none Incorporation depth 0 cm Foliar spray 

Inputs determined in accordance with EFED “Guidance for Chemistry and Management Practice Input Parameters for 

Use in Modeling the Environmental Fate and Transport of Pesticides” dated February 28, 2002.

 3.2.4. Aquatic EEC Results 

The tables (3.3, 3.4) below present the results of the GENEEC and PRZM-EXAMS 

modeling. Details of the results are found in Appendix B. 

Table 3.3. GENEEC Modeled Exposure to Acephate for Maximum Use Rate (Cotton) 

Crop/Use Peak, ppb 21-day average, ppb 60-day average, ppb 

Cotton, aerial, 6 

applications of 1 lb/acre 

spaced at 3 days 

85.4 43.6 18.4 

Table 3.4. PRZM-EXAMS Modeled Exposures to Acephate 

PRZM Scenario 

(Crop/Use) 

Air or 

Ground 

Application 

Rate (lb/acre) 

Number of 

Applications @ 

interval (day) 

Peak 

EEC 

(ppb) 

21-day avg 

EEC ( ppb) 

60-day avg 

(ppb) 

Citrus G 0.5 1 0.55 0.33 0.20 

Cotton A 1 6 @ 3 19.2 12.8 6.7 

G 1 6 @ 3 13.4 7.5 3.5 

Lettuce A 1 2 @ 7 16.7 10.7 5.0 

G 1 2 @ 7 15.0 9.0 4.1 

Row Crop (beans, A 1 2 @ 3 9.7 5.3 2.4 

celery, peppers) G 1 2 @ 3 5.9 3.3 1.5 

Turf A 1 1 4.5 2.7 1.3 

(Bermudagrass 

for seed) 

G 1 1 2.5 1.6 0.76 

Almond A 1 1 18.1 13.6 7.5 

(pistachio) G 1 1 12.9 8.1 4.0 

Fruit trees A 1 1 14.0 9.7 5.0 

G 1 1 7.3 4.6 2.1 

Cole crops A 1 2 @ 7 13.4 9.0 4.4 

G 1 2 @ 7 11.3 7.1 3.4 

3.3. Terrestrial Exposure Assessment

As discussed in section 2.5, adult CRLF occupy a variety of terrestrial dispersal habitats. 

The current range of the CRLF is represented by the core areas, critical habitat and 

occurrence sections in Figure 2.D. 

60
 3.3.1 Conceptual Model of Exposure 

Terrestrial exposure of the CRLF on agricultural fields within the Action Area is 

estimated with the TREX model, which automates exposure analysis according to the 

Hoerger-Kenaga nomogram. Exposure to animals off the field is estimated with the 

AgDrift and AgDISP models. 

Selection of Foliar Half-life Value for T-REX

Willis and McDowell (1987) was consulted for data on acephate persistence on foliage to 

replace the default value of 35 days. The default value was not believed to be reasonable 

for a non-persistent pesticide like acephate. Table III (p. 35) of this reference gives eight 

values for acephate, five of which are for dislodgeable residues (range 0.7 to 8.2 days), 

and three of which are for total residues (range 2.8 to 3.5 days). Normally, total residue 

values would be used for acephate, since it has a low Koc and is taken up through the 

roots (i.e., acts systemically). This rule is applied because it is believed that residues will 

be higher and more persistent if the pesticide is taken up into the plant, rather than just 

being on the surface of the foliage (which is measured by dislodgeable residue). Of the 

eight values, only one was measured on a crop in California (lemons), and it was 

measured as dislodgeable residue. This value was also the longest, and therefore most 

conservative of the values, at 8.2 days. The next longest value was 3.5 days (total 

residue) on citrus in Florida. Since the crops are similar (lemon and citrus) and the 

dislodgeable value exceeds the total value, contrary to what is expected, the value of 8.2 

days was selected as the input to T-REX. 

 3.3.2. Modeling Approach 

On-field exposure of the CRLF and its prey was estimated with TREX, using both 

maximum label rates of 1 lb/acre, 6 applications spaced at 3 day intervals (cotton). A 

low-rate scenario (0.25 lb/acre) was also done to bound possible risks. The decay rate 

used on foliage and other food items was 8.2 days (Willis & McDowell, 1987, p. 35), 

which was measured for acephate on lemons in California. Direct risk to the CRLF was 

bounded using 20-gram and 100-gram avian weight classes, since the weight of young 

adult frogs falls in this range. The CRLF was assumed to consume the broadleaf 

plant/small insect food category, since the bulk of its diet is invertebrates, and the small 

insect food category provides a higher dose. In addition, large CRLF also consumes 

other frogs and mice. 

The T-HERPS model was used to characterize risk to the CRLF, by applying food intake 

rates and prey items appropriate to frogs, in place of the bird food intake rates assumed in 

T-REX. 

Indirect risks to the CRLF through effects on its prey base were estimated in two ways. 

First, indirect effects via larger prey (Pacific tree frog and California mouse) were 

estimated conservatively using the 20-gram weight class for the Pacific tree frog and the 

61
15-gram weight class for the mouse. The short-grass food category was used since it 

provides the highest dose and is eaten by the mouse. 

 

Indirect effects via smaller prey (terrestrial invertebrates) were estimated using the LD50 

data for the honey bee, and an assumed body weight of 0.128 grams. The dose was 

calculated as the large insect EEC in ppm (avian, dose-based, 20-gram animal), divided 

by the body weight of the bee. The LD50 (ppm) was calculated as the LD50 

(micrograms per bee) divided by the body weight. The RQ was then the dose divided by 

the LD50 (ppm). 

To define the Action Area, the dose (in lb/acre) needed to bring all RQs below their 

respective LOC (0.1 for acute, non-endangered birds and mammals, and 1.0 for chronic) 

was calculated by dividing the LOC by the RQ, and multiplying the result by the single 

application rate (1 lb/acre): 

Dose below LOC (lb/acre) = (LOC/RQ)*(application rate, lb/acre). 

The AgDrift or AgDISP model was then used to calculate the buffer distance needed to 

reduce the dose to below the LOC. If the result was beyond the range of these models, 

then the Gaussian extension to AgDISP was used. 

 3.3.3. Model Inputs 

TREX and T-HERPS model inputs included application rate (1 lb/acre) number of 

applications (6), application interval (3 days), and foliar decay rate (8.2 days). 

 3.3.4 Results 

See Appendix F and Appendix F1 for T-REX and T-Herps details of EEC calculations. 

Summaries are given here. 

3.3.4.1 EECs for Direct Effects to CRLF 

Tables 3.5 to 3.6. present the results of the TREX analysis. The EECs in Table 3.5 are 

based on the maximum exposure (cotton scenario) of 6 applications of 1 lb a.i./acre, 

spaced at 3 day intervals. The EECs in Table 3.6. are based on the lowest labeled 

application rate of 0.13 lb a.i./acre (for sewage disposal areas) and an assumption of one 

application. 

Table 3.5. Summary of EEC for Direct Effects to CRLF (Maximum exposure) 

Frog EEC, ppm 

size, g Small 

insects 

Large 

Insects 

20 536.5 59.6 

100 306 34 

62
Table 3.6. Summary of EEC Direct Effects to CRLF (lowest exposure) 

Frog EEC, ppm 

size, g Small 

insects 

Large 

Insects 

20 20 2.2 

100 11.4 1.3 

3.3.4.2. EECs for Indirect Effects to CRLF 

The EECs in Table 3.7. are based on the maximum exposure (cotton scenario) of 6 

applications of 1 lb a.i./acre, spaced at 3 day intervals, and those in Table 3.8. on the low 

exposure scenario of 0.13 lb a.i./acre. The lowest weight classes (15-20 g) and highest 

residue (short grass) categories were used to provide a protective assessment of exposure. 

Table 3.7. Summary of EEC for Indirect Effects on CRLF (Maximum exposure) 

Avian, 20 gram body weight

Food Item Category Dose-Based EEC, mg/kg-bw 

Short Grass 954 

Tall Grass 437 

Broadleaf Plants/small Insects 536 

Fruits/pods/seeds/large insects 59.6 

Mammal, 15-gram Body Weight 

Food Item Category Dose-Based EEC, mg/kg-bw 

Short Grass 798 

Tall Grass 366 

Broadleaf Plants/small Insects 449 

Fruits/pods/seeds/large insects 49.9 

Table 3.8. Summary of EEC for Indirect Effects on CRLF (lowest exposure) 

Food Item Category Dose-Based EEC, mg/kg-bw 

Avian, 20-gram body weight 

Short Grass 35.5 

Tall Grass 16.3 

Broadleaf Plants/small Insects 20 

Fruits/pods/seeds/large insects 2.2 

Mammal, 15-gram Body Weight 

Food Item Category Dose-Based EEC, mg/kg-bw 

Short Grass 29.75 

Tall Grass 13.6 

Broadleaf Plants/small Insects 16.7 

Fruits/pods/seeds/large insects 1.9 

63
4. Effects Assessment 

This assessment evaluates the potential for acephate to adversely affect the California 

Red-Legged Frog (CRLF). As previously discussed in Section 2.7, selected assessment 

endpoints for the CRLF include assessment of direct toxic effects on the survival, 

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

the prey base and/or modification of its habitat (Tables 2.7 and 2.8). Taxa selected as 

measurement endpoints include freshwater fish as a prey item and also as a surrogate for 

aquatic phase of CRLF, if no amphibian toxicity data are available; freshwater aquatic 

invertebrates (prey item); birds as surrogates for terrestrial phase of CRLF and other 

amphibians (prey item); small mammals (prey item); terrestrial invertebrates (prey item); 

aquatic plants, and terrestrial plants (Tables 2.7 and 2.8). Toxicity data for freshwater 

fish and birds are used as surrogate data for aquatic-phase and terrestrial-phase 

amphibians (U.S. EPA, 2004). 

Information on the toxicity of acephate to selected taxa is characterized based on 

registrant-submitted studies and a comprehensive review of the open literature on 

acephate. Values used for each measurement endpoint identified in Tables 2.7 and 2.8 

are selected from this data. Currently, no FIFRA data requirements exist for aquaticphase or terrestrial-phase frogs and are therefore not part of typical registrant submitted 

data packages. However, some aquatic-phase frog survival data for acephate are 

available from open literature (Table 4.1), these data were reviewed for use in the risk 

determination. A summary of the available ecotoxicity information; the selected 

individual, population, and community-level endpoints for characterizing risks; and 

interpretation of the LOC, in terms of the probability of an individual effect based on 

probit dose response relationship are provided in Sections 4.1 through 4.3, respectively. 

In addition, toxicity data on acephate’s relevant degradate methamidophos, are discussed 

briefly and cross-referenced to the methamidophos effects determination document for 

the CRLF (USEPA, 2007). 

4.1 Evaluation of Aquatic and Terrestrial Ecotoxicity Studies 

Toxicity measurement endpoints are selected from data from guideline studies submitted 

by the registrant, and from open literature studies that meet the criteria for inclusion into 

the ECOTOX database maintained by EPA/Office of Research and Development (ORD) 

(U.S. EPA, 2004). Open literature data presented in this assessment were obtained from 

a search of the ECOTOX database (12/29/2006). Table 4.1 summarizes the most 

sensitive results for each measurement endpoint, based on an evaluation of both the 

submitted studies and the open literature, as previously discussed. A brief summary of 

submitted and open literature data considered relevant to this ecological risk assessment 

is presented below. Additional information is provided in Appendix A. 

In order to be included in the ECOTOX database, papers must meet the following 

minimum criteria: 

(1) the toxic effects are related to single chemical exposure; 

64
(2) the toxic effects are on an aquatic or terrestrial plant or animal species; 

(3) there is a biological effect on live, whole organisms; 

(4) a concurrent environmental chemical concentration/dose or application 

rate is reported; and 

(5) there is an explicit duration of exposure. 

Data that pass the ECOTOX screen are further evaluated for use in the assessment along 

with the registrant-submitted data, and may be incorporated qualitatively or quantitatively 

into this endangered species assessment. In general, effects data in the open literature, 

matching measurement endpoints listed in Tables 2.7 and 2.8, that are more conservative 

than the registrant-submitted data and that are found to be scientifically sound based on a 

review of the paper are used quantitatively. In addition, effects data for taxa that are 

directly relevant to the California Red-Legged Frog (i.e., aquatic-phase and terrestrialphase amphibian data) were also considered over the use of surrogate taxa effects data, if 

available. The degree to which open literature data are used quantitatively or 

qualitatively is dependent on whether the information is scientifically sound and whether 

it is quantitatively linked to the assessment endpoints (e.g., maintenance of California 

Red-Legged Frog survival, reproduction, and growth) identified in Section 2.7 (Tables 

2.7 and 2.8). For example, endpoints such as behavior modifications are likely to be 

qualitatively evaluated, because quantitative relationships between degree and type of 

behavior modifications and reduction in species survival, reproduction, and/or growth are 

usually not available. 

65
Table 4.1. Acephate measurement endpoints and values selected for use in RQ 

calculations for the effects determination. 

Assessment 

Endpoint (a)

Measures of 

Effect 

Species Toxicity 

Value 

Study classification Reference 

(Selection basis) 

Freshwater fish 

acute 96-hr LC

Rainbow trout 832 ppm ai Supplemental MRID 

40098001 

(Mayer, 1986) 

as calculated 

using 

TOXANAL 

50 (most sensitive) 

Survival and 

reproduction of 

individuals and 

communities of 

freshwater fish in 

close proximity to 

sites 

Freshwater fish 

early life-stage 

NOAEC 

Rainbow trout 5.76 ppm ai Extrapolated using 

most sensitive acute 

96-h LC

Section 

4.1.1.2. 

50 for 

Rainbow trout (832 

ai) divided by 144 

(highest rainbow trout 

ACR for 

organophosphates) 

Daphnia 

magna 

Freshwater 

invertebrate acute 

96-h LC

Supplemental MRID 

47116601 

(McCann, 

1978) 

acute 48-hr 

EC

50 (for 

copepods 48-h 

LC50 or EC50

where the effect 

measured is 

surrogate)

50 = 1.1 

ppm ai 

(Most sensitive) 

Survival and 

reproduction of 

individuals and 

communities of 

freshwater 

invertebrates in close 

proximity to sites Daphnia 

magna 

Freshwater 

invertebrate 

reproductive 

NOAEC

 0.15 ppm ai Supplemental MRID 

44466601 

(McCann, 

1978) 

(Most sensitive) 

Freshwater green 

algae, 

cyanobacteria or 

diatom 96-h IC50

for biomass. 

Skeletonema 

costatum 

diatom 

5-day EC

Standing crop or 

biomass and growth 

of aquatic plants in 

close proximity to 

sites 

Freshwater green 

algae, 

cyanobacteria or 

diatom 96-h 

NOAEC (or EC05) 

for biomass 

50

>50 ppm ai 

Supplemental MRID 

40228401 

(Mayer, 1986)

(Most sensitive)

NOEC = 5.0 

ppm ai

Avian (single 

dose) acute oral 

LD

Dark eyed 

junco 

106 mg 

ai/kg-bw 

Supplemental MRID 

00093911 

(Zinkl, 1981) 

 (Most sensitive) 

50

Avian subacute 

5-day dietary LC50

Japanese quail dietary subacute LC50 = 

718 ppm ai 

Supplemental 

 (Most sensitive) 

Smith, G.J., 

1987. 

Abundance (i.e., 

survival, 

reproduction, and 

growth) of 

individuals and 

populations of birds 

in close proximity to 

sites. (b) 

Avian reproduction 

NOAEL 

Mallard duck Reproductive 

study NOEL 

= 5 ppm ai 

Acceptable MRID 

00029691 

(Beavers, 

1979) 

 (Most sensitive) 

(e)

66
Assessment 

Endpoint (a)

Measures of 

Effect 

Species Toxicity 

Value 

Study classification Reference 

(Selection basis) 

Mammalian acute 

oral (single dose) 

LD

Meadow Vole Acute oral 

weight 

adjusted 

LD

Rattner and 

Hoffman, 

1984, see 

section 4.1.5.1 

for details on 

tox value 

derivation 

Acceptable 

(c ) (Most sensitive 

50

50 = 180. 

5 mg ai/kg 

bw 

Abundance (i.e., 

survival, 

reproduction, and 

growth) of 

individuals and 

populations of 

mammals in close 

proximity to sites 

Acceptable MRID 

40323401, 

40605701 

Mammalian 

reproductive 

NOAEC or 

NOAEL 

Rat 3- generation 

reproductive 

study 

NOAEL = 50 

mg ai/kg bwday diet 

(Most sensitive) 

(d) 

Honey bee acute 

contact LD

Acceptable MRID 

00014714, 

44038201 

(Atkins et al, 

1971) 

Honey bee acute contact 

LD Survival of beneficial 

insect populations in 

close proximity to 

sites 

50 50 = 1.20 

ug ai/bee 

(Most sensitive) 

6a. Seedling 

emergence EC25

>3.96 lb ai/A Acceptable Porch, J.R., et 

al., 2003; 

MRID 

46173203 

6b. Seedling 

emergence 

NOAEC 

3.96 lb ai/A 

6c. Vegetative 

vigor EC25

>3.96 lb ai/A

Onion, 

ryegrass, corn, 

wheat, 

buckwheat, 

soybean, 

lettuce, flax, 

tomato, radish 

Survival and growth 

of terrestrial plants in 

close proximity to 

sites Acceptable Porch, J.R., et 

al., 2003; 

MRID 

46173204 

6d. Vegetative 

vigor NOAEC 

3.96 lb ai/A 

(a) Assessment endpoints and measures of effect from Table 2.7 and 2.8. 

 (b) Note: Since, acute toxicity studies in birds demonstrated acute toxicity (LC50 or LD50) values higher than 

the highest concentrations tested and resulted in no mortalities, acute risk quotients will not be derived for 

birds. 

(c) As compared to other mammalian toxicity values when adjusted for body weight using allometric 

equations.

(d) Parental and pup weight, food consumption, litter size, mating performance and viability are all most 

sensitive measured effects. 

(e) Based on reduced number viable embryos, live 3-week embryos.

67
Table 4.2. Methamidophos measurement endpoints and values selected for use in 

RQ calculations in this effects determination. 

Assessment 

Endpoint (a)

Measures of 

Effect 

Species Toxicity Value Study Reference 

classification 

(Selection basis) 

Freshwater fish 

acute 96-hr LC

Rainbow trout 25 ppm ai Supplemental MRID 

00041312 

(Nelson & 

Roney, 1979) 

50 (most sensitive) 

Survival and 

reproduction of 

individuals and 

communities of 

freshwater fish in 

close proximity to 

sites 

Freshwater fish 

early life-stage 

NOAEC 

Rainbow trout 0.1736 ppm ai Extrapolated using 

most sensitive 

acute 96-h LC

Section 

4.1.1.2. 

50 for 

Rainbow trout (25 

ppm ai) divided by 

144 (highest 

rainbow trout ACR 

for 

organophosphates) 

Daphnia 

magna 

Freshwater 

invertebrate acute 

96-h LC50 (for 

copepods 48-h 

LC50 or EC50

where the effect 

measured is 

surrogate)

acute 48-hr EC50 = 

0.026 ppm ai 

Supplemental MRID 

00041311 

(Nelson & 

Roney 1979) 

(Most sensitive) 

Survival and 

reproduction of 

individuals and 

communities of 

freshwater 

invertebrates in close 

proximity to sites Daphnia 

magna 

Freshwater 

invertebrate 

reproductive 

NOAEC

0.0045 ppm ai Supplemental MRID 

46554501 

(Kern et. al., 

2005) 

(Most sensitive) 

Freshwater green 

algae, 

cyanobacteria or 

diatom 96-h IC50

for biomass. 

Skeletonema 

costatum 

diatom 

5-day EC

Standing crop or 

biomass and growth 

of aquatic plants in 

close proximity to 

sites 

Freshwater green 

algae, 

cyanobacteria or 

diatom 96-h 

NOAEC (or EC05) 

for biomass 

50 >50 

ppm ai 

Supplemental MRID 

40228401 

(Mayer, 

1986)

(Most sensitive)

1 NOEC = 5.0 ppm 

ai

Avian (single 

dose) acute oral 

LD

Common 

grackle 

4.1 mg ai/kg-bw Supplemental MRID 

00144428 

(Lamb, 1972) 

 (Most sensitive) 

50

Avian subacute 

5-day dietary LC50

Bobwhite 

quail 

dietary sub-acute 

LC50 = 42 ppm ai 

Supplemental 

 (Most sensitive) 

MRID 

00093904 

(Beavers & 

Fink,1979) 

Abundance (i.e., 

survival, 

reproduction, and 

growth) of 

individuals and 

populations of birds 

in close proximity to 

sites. (b) 

Avian reproduction 

NOAEL 

Mallard duck Reproductive study 

NOEL = 3 ppm ai

Acceptable MRID 

00014114 

(Beavers & 

Fink, 1978) 

3 (Most sensitive) 

68
Assessment 

Endpoint (a)

Measures of 

Effect 

Species Toxicity Value Study Reference 

classification 

(Selection basis) 

Mammalian acute 

oral (single dose) 

LD50

mouse Acute oral LD50 = 

16.2 mg ai/kg bw 

Acceptable 

(Most sensitive (c )

MRID 

00014047 

(1968) 

Abundance (i.e., 

survival, 

reproduction, and 

growth) of 

individuals and 

populations of 

mammals in close 

proximity to sites 

Mammalian 

reproductive 

NOAEC or 

NOAEL 

Rat 3- generation 

reproductive study 

NOAEL = 0.5 

mg/kg bw 

Acceptable MRID 

00148455, 

41234301 

(1984) 

(Most sensitive) 

5

 (10 

ppm) 

Survival of beneficial 

insect populations in 

close proximity to 

sites 

Honey bee acute 

contact LD

Honey bee acute contact LD

50

50

= 1.37 ug ai/bee 

Acceptable MRID 

00036935 

(Atkins et al, 

1975) 

(Most sensitive) 

6a. Seedling 

emergence EC25

>4.0 lb ai/A 

6b. Seedling 

emergence 

NOAEC 

4.0 lb ai/A 

Acceptable MRID 

46655802 

Christ and 

Lam, 2005 

6c. Vegetative 

vigor EC25

>4.0 lb ai/A 

Onion, 

ryegrass, corn, 

wheat, 

buckwheat, 

soybean, 

lettuce, flax, 

tomato, radish

Survival and growth 

of terrestrial plants in 

close proximity to 

sites Acceptable MRID 

46655802 

Christ and 

Lam, 2005 

6d. Vegetative 

vigor NOAEC 

4.0 lb ai/A 

1

 Most sensitive measure of effect in study that NOAEC is based on 

2

 Most sensitive measure of effect in study that NOAEC is based on. 3

 Most sensitive measure of effect in study that NOAEC is based on. 4

 Since there are no aquatic plant studies for methamidophos, acephate RED was used to provide information on 

aquatic plant endpoint. 

5

 Decrease in number of births, pup viability and body weight. There does not appear to be a palatability problem in 

the studies (personal communication Nancy McCarroll, HED, 2/10/98). 

4.1.1 Toxicity to Freshwater Aquatic Animals 

In the following sections relative acute toxicity of acephate and its major degradate, 

methamidophos, to aquatic animals is categorized using the scheme listed in Table 4.3. 

Table 4.3. Categories of Toxicity for Aquatic Organisms 

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 

4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies 

Acute fish testing with acephate fulfilled data requirements (§72-1). There are no data 

from fish early life stage chronic testing (§72-4). 

69
Parent acephate 

Acephate technical grade active ingredient (TGAI) acute toxicity results exist for several 

cold water and warm water freshwater fish species, including Rainbow Trout, Bluegill 

Sunfish, Brook Trout, Atlantic Salmon, Cutthroat Trout, Yellow Perch, Channel Catfish, 

and Fathead Minnow. A complete list of all the acute freshwater fish toxicity data for 

acephate is provided in Appendix A. For twelve studies, the acute freshwater fish 96-h 

LC50 values for technical grade acephate range from >50 to >1,000 ppm ai (Appendix A) 

and of these twelve studies only one had definitive 96-h LC50 values (Rainbow trout 

values 96-h LC50 = 832 ppm (Mayer, 1986)). Data from the Mayer study was reviewed 

and the LC50 was calculated. There is another more sensitive LC50 for trout from 

Mayer, but a definitive 96-h LC50 value above 100 ppm ai for a pesticide is not required 

unless exposure concentrations above 100 ppm are expected to occur under actual use 

conditions, which it does not for acephate labeled uses (Section 3.2.3). Based on this 

data, acephate is categorized at most as slightly toxic acutely to freshwater fish to 

practically non-toxic (Table 4.3). The most sensitive freshwater fish acute 96-h LC50

value of 832 ppm ai with rainbow trout (Oncorhynchus mykiss) (MRID 40098001, 

Mayer, 1986) was selected as the measurement endpoint for characterizing acute risks to 

freshwater vertebrate prey of the CRLF and for characterizing acute direct risks to the 

CRLF aquatic-phase (Table 4.1). No sublethal effects were reported as part of this study. 

This study is considered to be supplemental because of a lack of raw data to run a 

statistical analysis and only 5 fishes per concentration level was tested (no replicates). 

Acephate formulation (75% wettable powder) acute toxicity test results were also 

available for several cold water and warm water freshwater species including Rainbow 

Trout, Bluegill Sunfish, Brook Trout, Largemouth Bass, Cutthroat Trout, Goldfish, 

Yellow Perch, Channel Catfish, Fathead Minnow, and Mosquito Fish (Appendix A). For 

these fourteen studies the 96-h LC50 values range from >100 to 6,000 ppm ai. Like the 

studies with acephate TGAI, most of these LC50 values were not definitive values. 

However, based on the limited data it does not appear that acephate as the 75% wettable 

powder formulation is more toxic than the TGAI. 

Methamidophos, major degradate 

There is only a single acute 96-h LC50 study with a freshwater fish and the major 

degradate, methamidophos TGAI, which was with a warm water carp (Cyprinus carpio) 

(Appendix A). The definitive 96-h LC50 value of 68 ppm methamidophos for the carp is 

more toxic than the definitive 96-h LC50 value observed for acephate freshwater fish 

(1,100 ppm ai). Acute test results of formulations with methamidophos are also more 

toxic than acephate TGAI or acephate formulations. Methamidophos 96-h LC50 values 

for formulations ranged from 12 to 38 ppm for two species, Rainbow Trout and Bluegill 

Sunfish, whereas for these same species the acephate TGAI results ranged from >50 to 

1,100 ppm ai and for acephate formulations >150 to 2,050 ppm ai. 

70
4.1.1.2 Freshwater Fish: Chronic Exposure (Growth/Reproduction) Studies

Parent Acephate 

Similar to the acute data, chronic freshwater fish toxicity studies would be used to assess 

potential direct effects to the CRLF because direct chronic toxicity guideline data for 

frogs do not exist. Since there are no chronic data for freshwater fish, an acute to chronic 

ratio (ACR) was determined. Acephate is an organophosphate insecticide. The EFED 

toxicity database was accessed to derive an acute to chronic ratio of all organophosphate 

insecticides that have an acute LC50 and an early life stage fish study for rainbow trout. 

Rainbow trout is usually the most sensitive fish species among pesticides and is the most 

sensitive fish acute endpoint for acephate. Nineteen organophosphates were found that 

have both an acute and chronic study for rainbow trout. The ACR ranged from 5.4 for 

Terbufos to 144.0 for Dichlorvos. In order to provide the most conservative estimate for 

the chronic freshwater fish NOEC for acephate, the ACR of 144 will be used to estimate 

the NOEC for rainbow trout. The estimated chronic NOEC for rainbow trout as derived 

from and ACR of 144 and a LC50 of 832 ppm is 5.76 ppm or 5760 ppb. 

The following section presents the methodology used in deriving an avian ACR for 

organophosphates, the group to which acephate belongs, that was used to extrapolate a 

chronic fish NOAEC for acephate. The resulting early life stage for freshwater fish 

NOAEL was used as a surrogate for the aquatic-phase amphibian (U.S. EPA 2006). Of 

the organophosphates, 12 were evaluated for this extrapolation Table 4.4. The EFED 

toxicity database was accessed to derive an acute to chronic ratio of all organophosphate 

insecticides that have an acute LC50, an early life stage fish study for rainbow trout, and 

have been reviewed previously for scientific soundness. Rainbow trout is usually the 

most sensitive fish species among pesticides and is the most sensitive fish acute endpoint 

for acephate. A species and chemical specific ACR would ideally be determined which 

will then be used in the final organophosphate ACR derivation. 

The estimated fish (aquatic phase amphibians) chronic NOAEC for acephate is derived as 

follows. The (acephate) rainbow trout LC50 used in this assessment is 832 ppm ai. The 

largest acute-to-chronic ratio from the organophosphates is 144 for Dichlorvos. This 

ratio is used to calculate the final NOEC for acephate. 

750(acute)/5.2(chronic) = 144 = ACR ratio for Dichlorvos 

Estimated NOEC for acephate = LC50 = 832 ppm = 144 

 NOEC est. NOEC 

 Estimated Trout NOEC for acephate = 832/144 = 5.8 ppm ai

The table below (4.4) shows the inputs for the organophosphates that were considered for 

the acephate ACR. 

71
Methamidophos degradate

As with acephate, methamidophos does not have a chronic fish study. Therefore an ACR 

was also done to determine the estimated Trout NOEC for methamidophos. 

Estimated NOEC for methamidophos = LC50 = 25 ppm = 144 

 NOEC est. NOEC 

 Estimated Trout NOEC for methamidophos = 25/144 = 0.1736 ppm ai

The table below shows the inputs for the organophosphates that were considered for the 

acephate ACR. 

Acute to Chronic Table for Organophosphates 

Table 4.4. Acephate Acute to Chronic Ratio for Rainbow Trout NOEC 

Chemical 

96-hr LC50 

(ppm ai) MRIDs 

NOAEC

(ppm ai) MRIDs 

Acephate

NOEC 

ACR (PPM ai) 

Azinphos methyl 0.0088 03125193 0.00029 00145592 30.344 27.41 

Coumaphos 0.890 40098001 0.0117 43066301 76.068 10.93 

Dichlorvos 0.750 43284702 0.0052 43788001 144.23 5.76 

Dimethoate 7.500 TN 

1069* 

0.430 43106303 

17.441 47.70 

Disulfoton 1.850 40098001 0.220 41935801 8.4090 98.94 

Fenamiphos 0.068 40799701 0.0038 41064301 17.894 46.49 

Fenitrothion 2.000 40098001 0.046 40891201 43.478 19.13 

Fenthion 0.830 40214201 0.0075 40564102 110.66 7.518 

Fonofos 0.050 00090820 0.0047 40375001 10.638 78.20 

Isofenphos 1.800 00096659 0.153 00126777 11.764 70.72 

Phosmet 0.105 40098001 0.0032 40938701 32.812 25.35 

terbufos 0.0076 40098001 0.0014 41475801 5.4285 153.26 

* TN 1069 is test number for EPA’s Animal Biology Lab, McCann, 1977 

4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open Literature 

Information

In addition to submitted studies, data were located in the open literature that report 

sublethal effect levels to freshwater fish that are less than the selected measures of effect 

summarized in Table 4.1. 

Some sublethal effects to fish were found in open literature for acephate. 

One study (Zinkl, 1987) found that the percentage of ChE inhibition that suggests 

poisoning by acephate or methamidophos is greater than 70% since brain ChE inhibition 

is at least this much in some trout that did not die. There is persistent ChE depression 

72
(Brain ChE activity remains depressed 8 days after a 24-hour exposure to 25 mg/L of 

methamidophos and 15 days after exposure to 400 mg/L of acephate) which suggests 

sublethal effects such as inability to sustain physical activity in search of food, eluding 

predators, and maintaining position in flowing water would occur. However, additional 

studies are needed to conclude whether these sublethal effects do occur. 

Several studies (Geen, 1981; Schoettger,1976, MRID 14861; Boscor, 1975, MRID 

14637; Rabeni, 1979, MRID 14547) indicate no significant adverse effects on fish and 

benthic invertebrates from acephate applications. 

The above field studies and laboratory data suggest that acephate and other 

organophosphate insecticides may not directly cause mortality to fish or benthic 

invertebrates. The amount of ChE inhibition that may cause mortality to fish (excess of 

70% inhibition) suggests fish species may be somewhat resistant to adverse effects from 

acephate. Data are inconclusive as to whether there are behavior modifications to aquatic 

organisms from acephate or other organophosphate exposure. 

4.1.2 Toxicity to Amphibians – Aquatic Phase 

Amphibian toxicity data were used to assess potential direct and indirect effects to the 

CRLF. Direct effects to amphibians other than CRLF resulting from exposure to 

acephate may indirectly affect the CRLF via reduction in available food. 

A summary of acute and chronic amphibian data, including published data in ECOTOX 

is provided below in Sections 4.1.3.1 through 4.1.3.3. 

4.1.2.1 Amphibians: Acute Exposure Studies

The most sensitive study (MRIDs 00093943, 05019255, Lyons, 1976) found the Green 

Frog larvae/tadpole (Rani clamitans) 24 hr. LC50 to be 6433 ppm (5857-6775). This 

study is categorized as supplemental because of lack of raw data, dose-response data was 

not reported, ten tadpoles per treatment level was tested, with only one test vessel per 

treatment level (no replicates), and this study being a non-guideline study. Although the 

study was run for 96-hour period, only a 24-hour toxicity endpoint was derived because a 

linear dose-response pattern was not obtained. A behavior bioassay suggested that 

concentrations up to 500 ppm produced no observable differences between the treatment 

and control groups. 

Another study of green frog larvae/tadpole was tested with acephate (MRID 4404290, 

Hall, 1980) up to 5 ppm for bio-concentration. Neither bio-accumulation nor toxicity 

was noted at 5 ppm concentration level. 

A study (ECOTOX 11134, Geen, 1984) tested an amphibian, salamander, with acephate 

and found a 96-hour LC50 to be 8816 ppm. Exposure of egg masses to acephate 

concentrations of 798 ppm did not show any significant differences with the control to 

the time of hatching. 

73
Acephate is classified as practically non-toxic to aquatic-phase amphibians on an acute 

basis. 

Additional information can be found in Appendix A. 

 4.1.3 Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of acephate to the CRLF. Direct effects to freshwater invertebrates resulting from 

exposure to acephate may indirectly affect the CRLF via reduction in available food. As 

discussed in Section A.5.1 of Attachment A, CRLF aquatic-phase is presumed to be algae 

grazers but there is some uncertainty in that assumption. Therefore, aquatic invertebrates 

are also assumed to be a food source for CRLF aquatic-phase. 

A summary of acute and chronic freshwater invertebrate data, including published data in 

ECOTOX is provided below in Sections 4.1.3.1 through 4.1.3.3. 

4.1.3.1 Freshwater Invertebrates: Acute Exposure Studies 

The most sensitive acceptable study (MRID 47116601, McCann, 1978) found the 

Daphnia magna LC50 to be 1.11 ppm ai (0.65-1.88). The probit slope is 1.62. The 

range of LC50 toxicity for freshwater invertebrates is 1.11 to >1,000 ppm. One other 

Daphnia magna was tested and the LC50 is 71.8 ppm. 

A complete list of all the acute freshwater invertebrate toxicity data for acephate is 

provided in Appendix A. 

Acephate classification ranges from moderately toxic to practically non-toxic to 

freshwater invertebrates on an acute basis. 

4.1.3.2. Freshwater Invertebrates: Chronic Exposure Studies 

A submitted freshwater invertebrate life-cycle study (MRID 44466601, McCann, 1978) 

using Daphnia magna was reviewed. The control had 35% mortality of the adults and 

the treatments range from 10% to 35% mortality for adults with the highest concentration 

level having 10% mortality. Since this is a 21 day static test, it is assumed that the 

mortalities come from handling the organisms. There is a dose response trend of 

offspring per adult per day. With the dose response trend and because methamidophos 

daphnia life study has a more sensitive NOEC of 4.5 ppb, it was decided to make this 

study supplemental and not invalid since there is some useful information in this study. 

The NOEC is found to be 150 μg ai/L (0.150 ppm) for 21-day test. The NOEC is based 

on reduction in numbers of young at 375 ppb and higher. 

4.1.3.3. Freshwater Invertebrates: Open Literature Data 

74
In addition to submitted studies, data were located in the open literature that report 

sublethal effect levels to freshwater invertebrates; however, they are less sensitive than 

the selected measures of effect summarized in Table 4.1 and 4.2. 

 4.1.4. Toxicity to Birds 

As previously discussed, no guideline tests exist for frogs; therefore, birds are used as 

surrogate species for amphibians including frogs (U.S. EPA, 2004). The available open 

literature has no information on acephate toxicity to terrestrial-phase amphibians. Avian 

toxicity from open literature shows that acute and chronic ecotoxicity endpoints are 

generally less sensitive than the registrant submitted avian studies. A summary of acute 

and chronic avian data, including sublethal effects, is provided below. 

4.1.4.1 Birds: Acute Exposure (Mortality) Studies 

Avian acute toxicity studies were used to assess potential direct effects to the CRLF 

because direct acute toxicity guideline data on frogs are unavailable. Acephate toxicity 

has been evaluated in some avian species, including mallard duck, bobwhite quail, darkeyed junco, common grackle, starling, redwing blackbird, and Japanese quail and the 

results of these studies demonstrate a moderate range of sensitivity. The range of acute 

oral LD50 values for acephate ranges from 106 mg a.i./kg-bw to 350 mg a.i./kg bw. The 

range of subacute dietary LC50 is from 718 ppm to >5000ppm; therefore, acephate is 

categorized as moderately toxic to avian species on an acute oral basis to birds and as 

practically non-toxic to moderately toxic to avian species on a subacute dietary basis. 

Acute Oral LD50 and Avian sub acute dietary endpoint analysis

The most sensitive acute oral LD50 value is 106 mg/kg-bw for the dark eyed junco 

(MRID 00093911, Zinkl, 1981). However, there are uncertainties in using this value for 

risk assessment for the California Red-Legged Frog (CRLF). There were 5 dose groups 

with a geometric progression of 1.4X (EPA recommends 2X progression between doses). 

Only 4 birds were tested in each dose group (EPA recommends 10 birds per dose group). 

The 106 mg/kg-bw dose group had 2 birds out of 4 that died. No confidence interval and 

no probit slope were calculated. This study compared the LD50 value of birds fed larvae 

laced with acephate with birds that were given acephate by gavage. The birds initially 

refused to ingest larvae that contained 16 μg acephate/larvae; however, the birds were 

willing to consume larvae containing five μg acephate. The study found that acephate 

given by gavage without larvae produced more inhibition than the larvae-fed birds. The 

study also concludes that the higher the dose, the more ChE inhibition is found in the 

birds. Increased time of exposure may prolong the time for recovery from ChE 

inhibition. Feeding the birds larvae containing acephate may decrease the activity of the 

acephate when compared to the gavage. The birds fed for five days recovered in 12 to 22 

days. 

75
The next lowest LD50 value is 109 mg/kg-bw (86-139 mg ai/kg-bw)for bobwhite quail 

(MRID 43939301, Campbell, 1992). This study was conducted with a granular 

formulation (15% ai). The probit slope is 5.4. The formulation LD50 = 734 mg/kg (86-

139 mg/kg formulation). This study followed EPA guidelines and is an acceptable study 

for the formulation. The LD50 value (109 mg/kg-bw) from this bobwhite study will be 

used in the T-REX model for assessing effects to CRLF (terrestrial-phase). 

There are a total of six acute oral studies with acephate using four different species, with 

the range of LD50 being from 106 mg/kg-bw to 350 mg/kg-bw. Additional information 

may be found in Appendix A. 

 4.1.4.2. Birds: Chronic Exposure (Reproduction) Studies 

Avian reproduction studies indicate that when parents are fed between 5 and 20 ppm 

technical grade acephate, the survival of embryos and chicks are adversely affected. 

Effects seen in a study on northern bobwhite quail at 80 ppm include reduced body 

weight, number of eggs laid, eggs set, viable embryos, number of embryos alive at 3 

weeks, number of normal hatchlings, and 14-day old survivors. The NOEL is 20 ppm for 

the bobwhite quail (MRID 00029692, Beavers, 1979). 

Effects seen in a study on mallard ducks at 20 ppm include a reduced number of viable 

embryos and live 3 week embryos. The NOEL for the mallard is 5 ppm (MRID 

00029691, Beavers, 1979). 

4.1.4.3. Avians: Sublethal Effects and Additional Open Literature 

Information 

Vyas (ECOTOX 40313) reported that acephate (representing all organophosphates) 

affected adult migratory white-throated sparrows (Zonotrichia albicollis). Adult birds 

exposed to 256 ppm acephate a.i. were not able to establish a preferred migratory 

orientation and exhibited random activity. All juvenile treatment groups displayed a 

seasonally correct southward migratory orientation. The author hypothesized that 

acephate may have produced aberrant migratory behavior by affecting the memory of the 

adult’s migratory route and wintering ground. The “experiment reveals that an 

environmentally relevant concentration” (similar to 0.5 lb ai/A application) of an OP 

such as acephate “can alter migratory orientation, but its effect is markedly different 

between adult and juvenile sparrows. Results suggest that the survival of free-flying 

adult passerine migrants may be compromised following organophosphorus pesticide 

exposure.” Although birds are surrogate species for frogs, it is uncertain as to whether 

this aberrant behavior in birds can translate over to another aberrant behavior with frogs 

in the absence of additional data. 

ECOTOX 40343, Vyas, 1996. The effects of a 14-day dietary exposure of acephate on 

cholinesterase activity in three regions; basal ganglia, hippocampus, and hypothamulus 

were examined in the brain of the white-throated sparrow, Zonotrichia albicollis. All 

76
three regions experienced depressed cholinesterase activity between 0.5-2 ppm ai 

acephate. The regions exhibited cholinesterase recovery at 2-16 ppm ai acephate; 

however, cholinerase activity dropped and showed no recovery at higher dietary levels (> 

16 ppm acephate) which suggests that each region maintains its own ChE activity level 

integrity until the brain is saturated so that the differences of the regions is nil. Each 

region of the brain is responsible for different survival areas such as a foraging and 

escaping predators, memory and spatial orientation, food and water intake, reproduction 

and several others. Evidence indicated that the recovery is initiated by the magnitude of 

depression, not the duration. In general, as acephate concentration increased, depression 

in ChE activity among brain regions increased and differences of ChE activity among the 

three brain regions decreased. The pattern of ChE depression in different regions of the 

brain following low level exposure may prove to be a critical factor in the survival of the 

bird. The authors hypothesized that adverse effects to birds in the field may occur at 

pesticide exposure levels customarily considered negligible. 

Zinkl, 1978. Several large acreages of forest were sprayed with acephate at 0.5, 1.0 or 

2.0 lb. ai/A application rates. There was no brain ChE inhibition on day zero after 

application. Birds collected from the 2 lb ai/A plots from day one thru six post spray 

showed ChE inhibition. Brain ChE inhibition was shown in birds 33 days after treatment 

but not 89 days after treatment. Birds seemed to have more inhibition of ChE in summer 

application when compared to the fall application in the 1 lb. ai/A plots (30-50% and 25-

40% depression, respectively). The greatest ChE inhibition occurred in dark-eyed juncos 

(65%) collected 15 days after treatment. In the 2 lb. ai/A plots, dark-eyed juncos and 

golden-crowned kinglets had 54% ChE inhibition. Of the 14 species collected, only pine 

siskins (Siinus pinus) did not show any ChE inhibition. Symptoms of organophosate 

poisoning were observed such as a warbling vireo salivating profusely, an American 

robin having difficulty maintaining a perching position, and a mountain chickadee having 

visible tremors. All of these observations were made in the 1 lb. ai/A plots. The authors 

concluded that since marked ChE inhibition did not occur on day zero, but was evident 

up to 33 days after application, there was either an accumulative effect that was detected 

later or acephate was converted to a more potent ChE inhibitor such as methamidophos. 

Spraying the forest with 0.5, 1.0 or 2.0 lb. ai/A caused marked and widespread, and 

prolonged ChE depression in passerine birds. 

ECOTOX 39518, MRID 40329701. Zinkl, 1980, Zinkl, 1979. Acephate was sprayed in 

a forest at 0.5 lb ai/A. Eleven species of birds had ChE inhibition that ranged on average 

from 20 to 40%. The maximum depression of ChE found in chipping sparrows was 57% 

at day six. Western tanager species was found to have significant inhibition up to 26 

days after application. Brain residue analysis of a western tanager collected on day three 

contained 0.318 ppm of acephate and 0.055 ppm of methamidophos. 

MRID 05014922 and 00163173. (Bart, 1979). Acephate was applied in this study on 

June 13 at 0.55 kg/ha (0.5 lb ai/A) on two 200 hectare plots. Authors measured the 

presence of the red-eyed vireos by the number of their particular songs. Significant 

(P<0.05) decline in number of red-eyed vireos was observed. The decline was 

concentrated in the interior of the treated plots rather than spread throughout. This would 

77
conclude that acephate affected the decline of the number of red-eyed vireos. No birds 

were tested and it is not known whether the declines in vireos were due to direct effects 

or indirect effects such as killing off the food items. 

MRID 00141694. (Rudolph, 1984) Kestrels were dosed with 50 mg/kg of 75% acephate 

formulation. Serum ChE was 37% inhibited and returned to predosed levels eight days 

later. Then the birds were dosed again and the serum ChE activity was inhibited at 42%; 

brain ChE was at 26% inhibition. The kestrel prey-catching activity was not altered from 

the acephate at 50 mg/kg-bw dose level. 

MRID 40644802. (Richmond, 1979) Site: Wallowa-Whitman National Forest. 

Applications of 1.12 (1.0 lb ai/A) and 2.24 (2.0 lb ai/A) kg/ha were made on forest plots 

in Oregon. Extensive inhibition of brain ChE activity (commonly at 30-50%) for up to 

33 days for 11 of the 12 species of birds that were collected was observed. The highest 

frequency of ChE inhibition was observed on day two post spray. Two species of birds 

had observable population decreases. Some birds on the plots treated with 1.12 kg/ha had 

65% ChE inhibition which is thought to be fatal. At both plots, birds were found with 

coordination problems, salivating profusely, and inability to fly. These behaviors were 

observed up to 20 days after application in the 2.24 kg/ha plot. It was also observed that 

breeding pairs for the warbling vireo and yellow-rumpled warbler were decreased. The 

authors hypothesized that application of acephate at rates of 1.12 and 2.24 kg/ha can 

cause sickness and death to forest birds. 

MRID 00093909. (McEwen, 1981) Site:WY, UT and AZ rangeland. In 1979 and 1980, 

the birds and small mammals collected up to 24 days after application had reduced ChE 

activity. Reduction of 20% or more is indicative of exposure to brain ChE inhibitor. Of 

the birds collected in AZ, 24.5% had reduced ChE activity >20%. The birds with the 

most ChE inhibition were the last ones collected (21-24 days post treatment). In 1981, 

horned larks and lark buntings were collected in WY on a 12,000 acre plot that was 

treated with acephate at the rate of 0.105 kg/ha. More than 20% ChE inhibition was 

found in 19% of the horned larks and 25% of the lark buntings. Deer mice were also 

collected in WY. They were found to have ChE inhibition that ranged from 12.7% to 

14.6%. 

4.1.5. Toxicity to Mammals 

Rat or mouse toxicity values are obtained from the Agency's Health Effects Division 

(HED) as substitute for wild mammal testing. Toxicity data on small mammals are used 

in this assessment to assess the effect of acephate exposure on their availability as food 

items for the CRLF. While the relative percent composition of mammal in the frog’s diet 

is uncertain, gut content studies have found the CRLF gut to contain 50% mammal 

content (USFWS, 2002). It is necessary to consider the affect of acephate on this 

potentially significant food source, as population level effects to mammals may result in 

indirect effects to the CRLF. Additional information can be found in Appendix A. 

4.1.5.1. Mammals: Acute Exposure (Mortality) Studies 

78
Mammalian toxicity studies reviewed by the Agency indicate that acephate is 

characterized as moderately toxic to small mammals on an acute oral basis (LD50=866 

mg/kg bw, rat, MRID 00014675; LD50=321 mg/kg bw, meadow vole, Rattner and 

Hoffman, 1984). However, for the degradate, toxicity studies indicate that 

methamidophos is highly toxic to small mammals on an acute oral and dermal basis 

(MRID 00014044, 00014047, 00014048). 

The meadow vole appears (by numbers alone) to be more sensitive than the rat. In order 

to provide the vole toxicity input into the T-REX exposure model, the LD50 value needs 

to be adjusted. Below are the steps to compute the adjusted value. 

In order for the terrestrial exposure and the LD50 toxicity value for mammals to be 

imputed into the T-REX model for determination of RQ to mammals, the LD50 value of 

the meadow vole must be converted to an adjusted LD50. The dose-based LD50 (mg/kgbw) or NOAEL (mg/kg-bw) values from acceptable or supplemental toxicity studies are 

adjusted for the size of the animal tested compared with the size of the animal being 

assessed (e.g., 350-gram rat) are relative to the animal’s body weight (mg residue/kg bw) 

because consumption of the same mass of pesticide residue results in a higher body 

burden in smaller animals compared with larger animals. Adjusted mammalian LD50s 

(mg/kg-bw) are used to calculate dose-based acute risk quotients for 15-, 35-, and 1000-

gram mammals. The following equations are used for the adjustment (U.S. EPA 1993): 

Adjusted mammalian LD50 where: 

Adj NOAEL or LD NOAEL or LD

TW

AW .

(. )

50 50

0 25

= ⎛

⎝

⎜ ⎞

⎠

⎟

Adj. LD50 = adjusted NOAEL or LD50 (mg/kg-bw) 

LD50 = endpoint reported from mammal study (mg/kg-bw) 

TW = body weight of tested animal (35g vole) 

AW = body weight of assessed animal (350 g rat) 

Thus, the meadow vole LD50 of 321 mg/kg-bw x (35g/350)0.25

 = 180.5 mg/kg-bw = 

adjusted LD50 for meadow vole input for T-Rex exposure model. 

The meadow vole adjusted LD50 is more sensitive than the rat LD50 (866 mg/kg-bw). 

Therefore the meadow vole LD50 will be the value use in the T-REX terrestrial exposure 

model. 

4.1.5.2. Mammals: Chronic Exposure (Reproduction) Studies 

Laboratory data indicate that acephate and its degradate, methamidophos, may pose 

chronic risk to mammals by affecting the reproductive capacity of mammals. Acephate 

fed to female rats at 500 ppm were found to have significant adverse effects when 

compared to controls of parental and pup body weight, food consumption, litter size, and 

mating performance and viability. The NOEL for the rat reproductive study is 50 ppm 

(MRID 40323401, MRID 40605701). 

79
4.1.5.3 Mammals: Sublethal Effects and Additional Open Literature 

Information 

ECOTOX 39518, MRID 40329701. (Zinkl, 1980,). There is a marked inhibition of brain 

ChE activity in squirrels after aerial treatment of forests at rates of 0.57 kg/ha (0.51 lb/A) 

of Orthene but no mortality noted. 

ECOTOX 35459, Stehn, 1976. Increased ingestion of arthropods by insectivorous 

mammals has been reported following acephate application. This signifies a direct 

pathway for substantial exposure to acephate due to consumption of dead and dying 

insects. 

4.1.6 Toxicity to Insects 

Toxicity data on insects are used in this assessment to assess the effect of acephate 

exposure on their availability as food items for the CRLF. Insect toxicity from open 

literature shows that acute ecotoxicity endpoints are generally less sensitive than the 

registrant submitted bee studies. Additional information can be found in Appendix A. 

4.1.6.1. Acute toxicity to bees 

Analysis of the results of honey bee acute contact studies indicate that acephate is highly 

toxic to bees and beneficial insects on an acute contact basis (MRID 00014714, MRID 

44038201). The study indicated an LD50 of 1.2 ug/honey bee. Further studies indicated 

that acephate is highly toxic to bees from two hours to 96 hours after foliar application at 

rates of 1 lb/A and from 2 hours to 24 hours at 0.5 lb ai/A rate (Appendix A).

EPA also reviewed a study (MRID 05004012) that tried to determine a toxicity ratio of 

acephate. By comparing the sensitivity of beneficial predator insects to that of the pest 

tobacco budworm, one would be able to determine the selectivity of toxicity to the 

beneficial or pest insect. The ratio is calculated using the LC50 values for the pest divided 

by the LC50 values for the beneficial insect; a ratio greater than 1 represents that acephate 

is more toxic to the predator than to the pest. Green lacewing had a calculated ratio of 

6.4 and the ratio for the parasitic wasp was 10.0. Acephate is more toxic to these two 

beneficial predators than it is to the pest. 

An acute contact toxicity study for methamidophos, a degradate of acephate, on bees 

indicates that methamidophos is highly toxic to bees on an acute contact basis (MRID 

00036935). The LD50 was 1.37 ug/bee. 

4.1.6.2 Insects: Sublethal Effects and Additional Open Literature 

Information 

80
ECOTOX 35475. Stoner,1985. All bee colonies that were fed 10 ppm acephate lost 

queens early in the study and the affected colonies were unable to rear new queens. 

Acephate appears to be systemic in nurse bees, causing glandular secretions fed to queens 

to be toxic. The study implied infrequent encounters by honey bee foragers with 

acephate on crops at levels of 1 ppm (1 ppm is NOAEC level) or less should be harmless. 

However, foragers may be expected to encounter levels greater than 1 ppm in the field 

because of 6-9 day residue persistence and residual systemic activity of acephate in plants 

for up to 15 days. Acephate fed to worker bees via sugar syrup showed up in the royal 

jelly for the queen, indicating that acephate is systemic to bees. Although these 

concentrations of 1 ppm or less were harmless to the worker bees, levels at 0.1 ppm 

showed significant reduction of the surviving brood. Consequently, the study concluded 

that acephate is a hazard to honey bees because of its high contact toxicity, and because 

of its systemic nature. 

MRID 00099762. (Johansen, 1977). Orthene was found to be more detrimental to honey 

bee populations than carbaryl. Brood cycles of some colonies were found to be 

permanently broken, and all of the bees were dead within 45-48 days after exposure. 

Depression in the numbers of wild foraging bees at all treated plots was apparent. 

Measured seed and fruit production of northern bluebells (Mertensia paniculata) were 

significantly reduced from lack of pollination due to acephate when compared to control. 

MRID 00099763. (Johansen, 1977). Severe impacts on yellow jacket wasps and ants at 

rates of application of 1 and 2 lb ai/A sprayed on forest. Temperature seems to affect the 

exposure of wasps in that cooler temperature (39o

F) causes wasps not to forage out of 

nests and therefore not be exposed as much, whereas warmer temperatures (59o

F) 

increases the activity of wasps and the exposure to acephate. 

4.1.7. Summary of Effects Assessment 

Of the numerous studies evaluated, the lowest, and therefore most sensitive, toxicity 

endpoint was chosen to assess the possibility of direct or indirect effect to the frog, as 

determined by calculation of the RQ. 

4.1.7.1 Direct Effects 

The acute mortality endpoint for the rainbow trout, with a 96-hr LC50 = 110ppm ai, is the 

most sensitive value of the available data, and will be used as a surrogate (measurement 

endpoint) for calculating direct effects (risk) to the aquatic-phase frog. Because there 

were no data available for chronic toxicity associated with long term (relative to lifecycle) exposure, an acute to chronic ratio was calculated using the relationship between 

the acute and chronic toxicity of other organophosphate pesticides, as described above. 

The chronic exposure endpoint for the aquatic phase of the frog life-cycle is estimated to 

be 0.150 ppm for the rainbow trout. 

81
There are three avian toxicity tests used to calculate the risk to the terrestrial phase of the 

CRLF. For acute exposure, the mortality endpoint for dark eyed junco, LD50=106 

mg/kg-bw was used. Dietary effects were calculated based on data from tests with the 

Japanese quail, LC50=718 ppm. Finally, risks due to chronic exposure were calculated 

using the exposure level where no effects to embryo mortality were observed for chronic 

exposure of the mallard duck to acephate, NOAEL= 5 ppm. 

4.1.7.2. Indirect Effects 

Based on the life history of the frog, impacts to CRLF prey or habitat could indirectly 

affect the CRLF. To assess the risk of indirect effects from prey, the toxicity endpoints 

for small mammals, birds (other frogs as food item), terrestrial arthropods, and aquatic 

invertebrates were used. 

The most sensitive acute mammal endpoint is mortality of the rat upon acute exposure to 

acephate, LD50=866 mg/kg-bw. The most sensitive mammalian reproduction endpoint is 

adverse affects to parental and pup weight, food consumption, litter size, mating 

performance and viability upon chronic exposure of a rat to acephate, NOAEL=50 

mg/kg-bw. 

The most sensitive acute endpoint is mortality of the dark eyed junco, LD50=106 mg/kgbw was used. Dietary effects were calculated based on data from tests with the Japanese 

quail, LC50=718 ppm. Finally, risks due to chronic exposure were calculated using the 

exposure level where no effects to embryo mortality were observed for chronic exposure 

of the mallard duck to acephate, NOAEL= 5 ppm. 

The most sensitive acute aquatic invertebrate endpoint is the daphnid 48-hr EC50=1.1 

ppm a.i. and the chronic endpoint used to estimate risk to aquatic prey items is the 

daphnid NOAEC= 0.015 ppm. 

Acephate is highly toxic to bees up to 96 hours after foliar application. Comparative 

toxicity between beneficial and non-beneficial insects shows that Acephate may be more 

harmful to beneficial insects than insect pests. 

4.1.7.3. Use of Probit Slope Response Relationship to Provide Information on the 

Endangered Species Levels of Concern 

The Agency uses the probit dose response relationship as a tool for providing additional 

information on the potential for acute direct effects to individual listed species and 

aquatic animals that may indirectly affect the listed species of concern (U.S. EPA, 2004). 

As part of the risk characterization, an interpretation of acute RQ for listed species is 

discussed. This interpretation is presented in terms of the chance of an individual event 

(i.e., mortality or immobilization) should exposure at the EEC actually occur for a species 

with sensitivity to acephate on par with the acute toxicity endpoint selected for RQ 

calculation. To accomplish this interpretation, the Agency uses the slope of the dose 

response relationship available from the toxicity study used to establish the acute toxicity 

82
measures of effect for each taxonomic group that is relevant to this assessment (i.e., 

freshwater fish used as a surrogate for aquatic-phase amphibians and freshwater 

invertebrates). The individual effects probability associated with the acute RQ is based 

on the mean estimate of the slope and an assumption of a probit dose response 

relationship. In addition to a single effects probability estimate based on the mean, upper 

and lower estimates of the effects probability are also provided to account for variance in 

the slope, if available. The upper and lower bounds of the effects probability are based 

on available information on the 95% confidence interval of the slope. A statement 

regarding the confidence in the estimated event probabilities is also included. Studies 

with good probit fit characteristics (i.e., statistically appropriate for the data set) are 

associated with a high degree of confidence. Conversely, a low degree of confidence is 

associated with data from studies that do not statistically support a probit dose response 

relationship. In addition, confidence in the data set may be reduced by high variance in 

the slope (i.e., large 95% confidence intervals), despite good probit fit characteristics. 

 

Individual effect probabilities are calculated based on an Excel spreadsheet tool IECV1.1 

(Individual Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP, 

Environmental Fate and Effects Division (June 22, 2004). The model allows for such 

calculations by entering the mean slope estimate (and the 95% confidence bounds of that 

estimate) as the slope parameter for the spreadsheet. In addition, the acute RQ is entered 

as the desired threshold. 

4.1.7.4. Review of Ecological Incident Information System (EIIS) 

In general, although there are many reported incidents of toxic effects to non-target plants 

and animals from acephate, the majority of these reports are not clearly documented or 

else acephate was applied in combination with other pesticides and it is not possible to 

determine which pesticide primarily caused the undesirable effect. A more detailed 

account of these reports can be found in Appendix C. The majority of acephate specific 

incidents reported were associated with bee kills. Some reports were also associated with 

bird and fish kills, and damage to plants, but the exact causes of the reported incidents are 

uncertain. 

The EIIS database show the following reported incidents that are associated with 

acephate use. More details of the incidents can be found in Appendix C. The two avian 

incidents have a probable certainty incident. The plant incidents are localized residential 

uses with a mixture of other active ingredients and in an aerosol container. Fish incidents 

are not reported below but can be found in Appendix C. In each of the incidents, the fish 

kills resulted from a mixture of other active ingredients that are known to be more toxic 

to fish than acephate. 

Avians – 

 

1998 in SC – 24 dead boat-tailed grackles collected and methamidophos residues found 

within them attributed to acephate use on fire ants 

83
2005 in GA - 50 boat-tailed grackles were found dead. Acephate residues found within 

some of them. Acephate was used to control fire ants. 

Plant Injury – 

There are no data or information available to ascertain the extent of the damage or the 

type of damage for the plants. Thus it is not known whether acephate caused the damage. 

There are some alleged mortalities for plants from the use of a rose and spray mixture. It 

is not certain as to whether the mixture caused the mortalities since the data reported to 

EPA is very sparse. 

1994 in PA - Orthenex Rose and Flower Spray (aerosol) is alleged to have cause damage 

to ornamentals and/or flowers. 

1998 in FL - There was an allegation of plant damage from the use of Ortho Systemic 

Rose and Floral Spray on ornamentals. 

1998 in PA - There was an allegation of plant damage from the use of Ortho OrthenexTM

Insect and Disease Control Formula III on ornamentals. 

1999 in DC - There was an allegation of plant damage from the use of Isotox Insect 

Killer Formula IV. Product was sprayed on a dwarf Alberta pine with the results that the 

pine is dying. 

1999 in IN - There was an allegation of plant damage from the use of Ortho OrthenexTM

Insect and Disease Control Formula III on ornamentals. The report indicated that the 

flowering almond and hibiscus were dying. 

1999 in TX - There was an allegation of plant damage from the use of Ortho OrthenexTM

Insect and Disease Control Formula III on ornamentals. There was an allegation of plant 

damage from the use of Ortho OrthenexTM Insect and Disease Control Formula III on 

ornamentals. The report indicated that the homeowner applied this product on 40 – 50 

bushes used as hedge per recommendation of county extension agent. About 95% of the 

bushes died. 

1999 in GA - There was an allegation of plant damage from the use of Ant-Stop 

OrtheneTM Fire Ant Kill. Product was applied on spots of the lawn resulting in “burnt 

spots”. 

Bee Kills

Washington State reported 7 incidents of bee kills from 1992 to 2002. Most of the 

incidents indicate that 40 to 60 colonies were killed off per incident. 

 

84
85
5. Risk Characterization

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

determine the potential ecological risk from varying acephate use scenarios within the 

action area and likelihood of direct and indirect effects on the California Red Legged 

frog. The risk characterization provides estimation and description of the likelihood of 

adverse effects; articulates risk assessment assumptions, limitations, and uncertainties; 

and synthesizes an overall conclusion regarding the effects determination (i.e., “no 

effect,” “likely to adversely affect,” or “may affect, but not likely to adversely affect”) for 

the California Red Legged frog.

5.1 Risk Estimation 

Risk was estimated by calculating the ratio of estimated environmental concentrations 

(EECs; see Tables 3.3 through 3.8) and the appropriate toxicity endpoint (see Tables 2.7, 

4.1, 4.2). This ratio is the risk quotient (RQ), which is then compared to pre-established 

acute and chronic levels of concern (LOCs) for each category evaluated (Table 5.1.). 

Appendix E describes the categories of toxicity. 

Table 5.1. Levels of Concern for Terrestrial and Aquatic Organisms 

Taxa Acute LOC Chronic LOC 

Avian1 (terrestrial phase amphibians) 0.1 1 2 Mammalian 0.1 1 

Terrestrial plants3 1 4 Aquatic Animals (aquatic phase 

amphibians) 

0.05 1 

Insects 0.05 5 1 

Used in RQ calculations: 

1

 LD50 and estimated NOEL 2

 LD50 and NOEC 3

 EC25 

4

 LC/EC50 and estimated and reproductive NOEC 5

 LD50 per EFED’s CRLF Steering Committee 

Aquatic screening level RQs are based on the most sensitive endpoints and modeled 

surface water concentrations from the following scenarios of acephate: citrus, cotton, 

lettuce, row crops (beans, celery, peppers), turf (landscape maintenance), almond 

(surrogate for pistachio), fruit trees, and cole crops (broccoli, cauliflower, Brussels 

sprouts). 

5.1.1 Aquatic Direct and Indirect Effects 

5.1.1.2 Direct Effects 

There is no direct acute risk to CRLF aquatic-phase from the use of acephate. Tier 1 

EECs (Table 3.3) were below the LOC. The very low RQs (Tables 5.2, 5.3) for direct 

effects to fish (surrogate for CRLF) indicate that direct effects are not expected even for 

86
applications much higher than the maximum labeled rate. Therefore, a Tier 2 analysis 

was not conducted for fish. 

5.1.1.3 Indirect Effects 

Indirect risk to aquatic phase CRLF is driven by acephate use on cotton and almond with 

an acute RQ of 0.13 for prey items (aquatic invertebrates, Table 5.5). 

The only acute RQs above the LOC in the aquatic phase were for invertebrates exposed 

to methamidophos as a degradate of acephate (estimated as 18% of the modeled acephate 

EEC). This suggests that the CRLF may be at indirect risk in the aquatic phase due to 

reduction in prey base (invertebrates). Aquatic plant RQ were also below LOC, therefore 

no indirect effects mediated via aquatic plants are expected, either as they affect food 

supply or habitat. 

Chronic effects to all aquatic taxa were below LOC for both parent acephate and 

its degradate methamidophos. 

Table 5.2. Tier 1 Acute Risk Quotients for Acephate 

RQs Direct 

Effects, Green 

frog LD

Use EEC RQs for Direct 

and Indirect 

Effects, 

Rainbow trout, 

LC

RQs Indirect 

Effects, Prey 

Item: Daphnia 

magna, EC

50 = 

50 = 6,433,000 ppb

50 = 832,000 1,110 ppb

ppb

Cotton, Aerial, 

6 applications 

of 1 lb/acre 

spaced at 3 

days 

Peak, 85.4 ppb 

< 0.05 0.08 * < 0.05 

* Exceeds LOC of 0.05 

Table 5.3. Tier 1 Chronic Risk Quotients for Acephate 

Use Chronic RQ 

RQs for Direct and 

Indirect Effects, 

Rainbow trout, NOAEC 

= 5,800 ppb

RQs Indirect Effects, 

Prey Item: Daphnia 

magna, NOAEC = 150 

ppb

Cotton, Aerial, 6 EEC 

applications of 1 lb/acre 

spaced at 3 days 

21-day, 43.6 ppb < 0.05 2.9 * 

60-day, 18.4 ppb 

* Exceeds LOC of 1.0 

87
 Table 5.4. Risk Quotients: Acute Aquatic Acephate Exposure (Tier 2) 

RQs 

Direct 

Effects, 

Green frog 

LD

Application 

Method (Air/ 

Acephate 

Peak 

EEC 

(ppb)

RQs for 

Direct and 

Indirect 

Effects, 

Rainbow 

trout, LC

RQs 

Indirect 

Effects, 

Prey Item: 

Daphnia 

magna, EC

Crop/Use 

Ground)

50 = 

6,433,000 

ppb 

50

= 832,000 

ppb 

50

= 1,110 ppb 

Citrus G 0.55 

Cotton A 19.2 

G 13.4 

Lettuce A 16.7 

G 15.0 

Row Crop (beans, A 9.7 

celery, peppers) G 5.9 

A 4.5 Turf 

(Bermudagrass 

for seed) 

< 0.05 < 0.05 < 0.05 

G 2.5 

Almond A 18.1 

(pistachio) G 12.9 

Fruit trees A 14.0 

G 7.3 

Cole Crops A 13.4 

G 11.3 

 Table 5.5. Risk Quotients: Acute Aquatic Methamidophos (Acephate degradate) 

 Exposure (Tier 2) 

Acephate 

Peak 

EEC 

(ppb) 

Methamidoph

as Adjusted 

Peak EEC 

(ppb) 

RQ for 

Indirect 

Effects, 

Prey Item: 

Daphnia 

magna(EC

RQ for Direct 

and Indirect 

Effects, Aquatic 

Life Phase: 

Rainbow trout 

(LC

Crop/Use Applicati

on 

Method 

(Air/ 

Ground) 

50

= 26 ppb) 50 = 25,000 ppb) 

Citrus G 0.55 0.099 < 0.05 

Cotton A 19.2 3.46 0.13 

G 13.4 2.41 0.09 

Lettuce A 16.7 3.00 0.12 

G 15.0 2.70 0.10 

Row Crop (beans, A 9.7 1.75 0.067 

celery, peppers) G 5.9 1.06 < 0.05 

Turf A 4.5 0.81 < 0.05 

(Bermudagrass for 

seed) G 2.5 0.45 < 0.05 

< 0.05

Almond A 18.1 3.26 0.13 

(pistachio) G 12.9 2.32 0.089 

Fruit trees A 14.0 2.52 0.097 

G 7.3 1.31 0.0504 

Cole crops A 13.4 2.41 0.093 

G 11.3 2.03 0.078 

EEC=Expected Environmental Concentration; ppb=Parts per billion; Risk Quotients (RQ) in bold type exceed LOC 

88
 Table 5.6. Risk Quotients: Chronic Aquatic Exposure to Acephate and 

 Methamidophos (Tier 2) 

Acephate 

EEC (ppb) 

Methamidophos 

EEC (ppb) 

RQs for Direct Effects 

Rainbow Trout 

RQs for Indirect Effects 

Daphnia 

Crop/Use Application 

Method (Air/ 

Ground) 21-

day 

60-

day 

21-day 60-day Acephate 

NOAEC 

=5,800 

(ppb)

Meth. 

NOAEC 

=0.1736 

(ppb)

Acephate 

NOAEC 

=150 

(ppb)

Meth. 

NOAEC 

=4.5 (ppb)

Citrus G 0.33 0.20 0.06 0.04 

Cotton A 12.8 6.7 2.30 1.21 

G 7.5 3.5 1.35 0.63 

Lettuce A 10.7 5.0 1.93 0.9 

G 9.0 4.1 1.62 0.74 

Row A 5.3 2.4 0.95 0.43 

Crop G 3.3 1.5 0.59 0.27 

Turf A 2.7 1.3 0.49 0.23 

G 1.6 0.76 0.29 0.13 

< 1 < 1 < 1 < 1 

Almond A 13.6 7.5 2.45 1.35 

G 8.1 4.0 1.46 0.72 

Fruit A 9.7 5.0 1.75 0.9 

Trees G 4.6 2.1 0.83 0.38 

Cole A 9.0 4.4 1.62 0.79 

Crops G 7.1 3.4 1.28 0.61 

5.1.2 Terrestrial Phase Direct Effects 

 Maximum usage of acephate (cotton) 

Direct acute risk to terrestrial-phase of CRLF is driven by acephate use on cotton with an 

acute RQ of 0.34 to 6.82 (dose-based) and 0.07 to 0.66 (dietary based, Table 5.7). Direct 

chronic risk to CRLF terrestrial-phase is driven by acephate use on cotton with RQs 

ranging from 10 to 94. 

Exposures and RQ values for terrestrial organisms are given in the T-REX output below. 

The CRLF is represented by a bird (“avian”) of 20 or 100 grams body weight, which 

consumes small insects or large insects. Acute, dose-based RQs are reported by the size 

of the animal and their diet. The acute RQs ranges from 0.34 for a 100-gram frog 

consuming large insects to 6.82 for a 20-gram frog consuming small insects, respectively. 

These RQs are above the Listed Species LOC, 0.1. Dietary-based acute RQs are above 

the LOC for small insects only (RQ = 0.66). 

Chronic risk to the CRLF is also represented by the avian taxa. Chronic RQ for birds 

eating small insects and large insects range from 10 to 94, well above the LOC (1). 

89
 Minimum usage of acephate (sewage disposal)

For the minimum exposure of CRLF, (sewage disposal areas, Table 5.8), dose-based 

acute RQs are above LOC (0.1) for consumption of small insects. The chronic dietary 

RQ is also above LOC for the CRLF. 

Tables 5.7 and 5.8 provide the RQs for direct and indirect effects to the CRLF. 

Table 5.7. T-REX Inputs for Analysis of Direct and Indirect Effects to CRLF (Maximum 

Use, Cotton) 

Chemical Name: Acephate 

 Use Cotton 

 Formulation Orthene 75 

Application Rate 1 lbs a.i./acre 

Half-life 8.2 days 

Application Interval 3 days 

Maximum # 

Apps./Year 6 

Length of Simulation 1 year 

Kenaga Table 5.7a Dietary-based EECs (ppm) Values 

Short Grass 837.5 

Tall Grass 383.9 

Broadleaf plants/sm Insects 471.1 

Fruits/pods/seeds/lg insects 52.3 

Table 5.7b 

RQs associated with Direct Effect as Represented by Avian species, as surrogate for the CRLF 

 20 g Acute 

Dose-based 

100 g Acute 

Dose-based 

Acute 

Dietary Chronic Dietary 

Broadleaf plants/sm insects 6.8 3.1 0.66 94.2 

Fruits/pods/seeds/lg insects 0.76 0.34 0.07 10.5 

Table 5.7c Indirect Effects as Represented by Prey Item - RQs

15 g 

Mammal 

Dose-based 

Acute 

15 g 

Mammal 

Dose-based 

Chronic 

Dietarybased 

Mammal 

Chronic RQ 

20 g Avian 

Acute Dosebased 

Avian 

Acute 

Dietary 

Avian 

Chronic 

 Dietary 

Short Grass 2.01 145.3 16.8 12.15 1.17 167.5 

Tall Grass 0.92 66.6 7.7 5.57 0.53 76.77 

Broadleaf 

plants/sm insects 

1.13 

81.7 9.4 6.83 0.66 94.22 

Fruits/pods/lg 

insects 

0.13 

9.08 1.1 0.76 0.07 10.47 

90
Table 5.8. 

T-REX Inputs for Analysis of Direct and Indirect Effects to CRLF (Minimum Use, 

Sewage Disposal) 

Chemical Name: Acephate 

 Use Sewage disposal 

 Formulation Orthene 75 

Application Rate 0.13 lbs a.i./acre 

Half-life 8.2 days 

Application Interval na 

Maximum # 

Apps./Year 1 

Length of Simulation 1 year 

Kenaga Table 5.8a Dietary-based EECs (ppm) Values 

Short Grass 31.2 

Tall Grass 14.3 

Broadleaf plants/sm Insects 17.55 

Fruits/pods/seeds/lg insects 1.95 

Table 5.8b 

RQs associated with Direct Effect as Represented by Avian species, as surrogate for the CRLF 

 20 g Acute 

Dose-based 

100 g Acute 

Dose-based 

Acute 

Dietary Chronic Dietary 

Broadleaf plants/sm insects 0.25 0.11 0.02 3.5 

Fruits/pods/seeds/lg insects 0.03 0.01 <0.01 0.39 

Table 5.8c Indirect Effects as Represented by Prey Item - RQs

15 g 

Mammal 

Dose-based 

Acute 

15 g 

Mammal 

Dose-based 

Chronic 

Dietarybased 

Mammal 

Chronic RQ 

20 g Avian 

Acute Dosebased 

Avian 

Acute 

Dietary 

Avian 

Chronic 

 Dietary 

Short Grass 0.07 5.41 0.62 0.45 0.04 6.24 

Tall Grass 0.03 2.48 0.29 0.21 0.02 2.86 

Broadleaf 

plants/sm insects 

0.04 

3.05 0.35 0.25 0.02 3.51 

Fruits/pods/lg 

insects 

<0.01 

0.34 0.04 0.03 <0.01 0.39 

5.1.3. Individual Effects Calculation for Direct Acute Effect on CRLF 

The risk of mortality to the CRLF is based on the avian (bird) taxon in T-REX, where the 

CRLF is represented by a 20-gram or 100-gram bird that consumes small insects or large 

insects. The individual chance of effects is calculated from the probit-slope response 

relationship, using an Excel spreadsheet (IEC v1.1, June 22, 2004). 

The acute toxicity data used is for the bobwhite quail. The bobwhite LD50 is 109 mg/kgbody weight. Individual effect probabilities were calculated for the cotton scenario (six 

91
applications of 1 lb a.i./acre spaced at 3 day intervals). A lower application rate, as is 

applied to sewage disposal areas, was modeled at a rate of 0.13 lb/A. The results are 

shown in Table 5.9. 

Table 5.9a. Individual Effects Chance for Mortality of CRLF 

Exposure scenario TREX risk quotient Chance of effect (1-in-...) 

Listed Species Threshold (LOC, 0.1) 9.38 E+11 

Cotton, 20-g bird, small 

insects (max exposure) 

6.8 Approaching 1 

Cotton, 100-g bird, small 

insects (max exposure) 

3.1 Approaching 1 

Cotton, 20-g bird, large 

insects (max exposure) 

0.76 3.38 

Cotton, 100-g bird, large 

insects (max exposure) 

0.34 57 

0.13 lb/A, 20-g bird, small 

insects (min. exposure) 

0.25 297 

0.13 lb/A, 100-g bird, large 

insects 

0.03 2.76E+11 

0.13 lb/A, 20-g bird, small 

insects 

0.11 1.25E+5 

0.13 lb/A, 100-g bird, large 

insects 

0.01 8.86E+18 

Table 5.9b. Individual Effects Probability Calculation for Prey Items 

Organism slope Threshold (LOC or 

RQ) 

Chance of Effect, 1 

in ... 

Daphnia magna at 

LOC 

0.05 (LOC) 4.18E+8 

Daphnia magna at 

maximum RQ 

0.13 (RQ) 2.99E+4 

Honey bee at LOC 0.05 (LOC) 4.18E+8 

 4.5 (default) 

Honey bee, Large 

insect RQ (cotton) 

5.5 (RQ) 1 

Honey bee, Small 

insect RQ (cotton) 

50 (RQ) 1 

Honey bee, Large 

insect RQ (sewage) 

0.2 1,210 

Honey bee, Small 

insect RQ (sewage) 

1.86 1.13 

15 gm mammal at 

LOC 

0.1 (LOC) 2.94E+5 

15 gm mammal at 

RQ 

1.13 (RQ) 1.68 

92
These results show that the risk to an individual CRLF is high (approaching 100% 

mortality) if exposed to the maximum cotton exposure scenario, and consuming a diet of 

small insects. Risks are also large for a diet of large insects (1-in-3.38, or 30% for the 

20-g bird, and 1-in-57, or 1.75% for the 100-g bird). 

For the low exposure scenario (0.13 lb/acre, applied once) the risk is appreciable for a 20-

g bird consuming small insects (1-in-297, or 0.34%). 

The lowest RQ that gives an individual chance of effect approaching 100% is 3.1. The 

RQ for 20-g birds consuming small insects, if exposure is 2 applications of 1 lb a.i./acre, 

spaced at 3 day intervals, is 3.42. 

These results indicate that the chance of individual mortality for a CRLF, as represented 

by 20-g and 100-g birds, is considerable even for low application rates (0.34%) and 

approaches 100% at exposures well below the maximum allowed on the label. 

5.1.4. Indirect Effects, Terrestrial Phase 

Indirect effects on the CRLF in the terrestrial phase of its life cycle might be due to loss 

of prey (insects, small mammals, small frogs, small birds) or effects on plants that 

provide habitat. Because no adverse effects on terrestrial plants are expected, no indirect 

effects on the CRLF mediated via plants are expected. 

Small mammals and frogs that might be eaten by the CRLF are represented in the TREX 

analysis as the 15-gram mammal and the 20-g bird, respectively. Both prey items are 

assumed to eat short-grass food items, as this provides the highest dose, and therefore the 

most protective assessment. 

5.1.4.1 Acute Effects 

The small mammal RQ is 0.49 (Table 5.7), and the small bird RQ is 12.1 (Table 5.7), 

both above the LOC of 0.1 for Listed species. The dietary-based RQ (Table 5.7) is 1.2 

for birds (above LOC), and was 16.7 for mammals (Table 5.9). 

Acute effects on insects are calculated (Appendix L) using the acute contact LD50 for the 

honey bee (1.2 micrograms per bee) divided by 0.128 grams body weight, to obtain an 

LD50 in ppm. Exposures are the small insect and large insect from the TREX dietary 

analysis. The LD50 is then (1.2 micrograms)/(0.128 grams) = 9.4 ppm. The dietarybased EECs are 471 ppm (small insect) and 52 ppm (large insect). The resulting RQ 

values are 50 and 5.5, respectively, and are well above the LOC for terrestrial 

invertebrates (0.05). 

93
5.1.4.1. Chronic Effects 

The chronic avian RQ is 167 (Table 5.7) , and the chronic mammal RQ is 145 (dosebased, Table 5.9) or 17 (dietary-based, Table 5.9). All of the RQs are above the LOC (1). 

Based on these RQ values, it is presumed that the CRLF will be indirectly affected by 

adverse effects (both acute and chronic) on animals in its prey base. 

5.2 Risk Description 

The risk description synthesizes an overall conclusion regarding the likelihood of adverse 

impacts leading to an effects determination (i.e., “no effect,” “may affect, but not likely 

to adversely affect,” or “likely to adversely affect”) for the California Red Legged frog. 

If the RQs presented in the Risk Estimation (Section 5.1.2) show no indirect effects, and 

LOCs for the CRLF are not exceeded for direct effects (Section 5.1.1), a “no effect” 

determination is made based on acephate’s use within the action area. If, however, 

indirect effects are anticipated and/or exposure exceeds the LOCs for direct effects, the 

Agency concludes a preliminary “may affect” determination for the CRLF. Following a 

“may affect” determination, additional information is considered to refine the potential 

for exposure at the predicted levels based on the life history characteristics (i.e., habitat 

range, feeding preferences, etc) of the CRLF and potential community-level effects to 

aquatic plants and terrestrial plants growing in semi-aquatic areas. Based on the best 

available information, the Agency uses the refined evaluation to distinguish those actions 

that “may affect, but are not likely to adversely affect” from those actions that are “likely 

to adversely affect” the CRLF. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the CRLF include the following: 

● Significance of Effect: Insignificant effects are those that cannot be 

meaningfully measured, detected, or evaluated in the context of a level of effect 

where “take” occurs for even a single individual. “Take” in this context means to 

harass or harm, defined as the following: 

• Harm includes significant habitat modification or degradation that 

results in death or injury to listed species by significantly impairing 

behavioral patterns such as breeding, feeding, or sheltering. 

• Harass is defined as actions that create the likelihood of injury to listed 

species to such an extent as to significantly disrupt normal behavior 

patterns which include, but are not limited to, breeding, feeding, or 

sheltering. 

94
● Likelihood of the Effect Occurring: Discountable effects are those that are 

extremely unlikely to occur. For example, use of dose-response information to 

estimate the likelihood of effects can inform the evaluation of some discountable 

effects. 

● Adverse Nature of Effect: Effects that are wholly beneficial without any 

adverse effects are not considered adverse. 

 

A description of the risk and effects determination for each of the established assessment 

endpoints for the CRLF is provided in Sections 5.2.1 through 5.2.3. 

 5.2.1 Direct Effects to the California Red Legged Frog 

The federal action is all labeled uses. In order to compare the location of the labeled uses 

with the areas important to the frog, the potential use areas in California were over laid 

with the core areas, critical habitat and known occurrence areas of the CRLF. The result 

of this layering is the ability to discern areas of overlap between potential use and the 

CRLF life-cycle. 

5.2.1.1. Aquatic Phase 

Risk Quotients for freshwater fish (surrogates for the CRLF) are below LOC for both 

acute and chronic effects (Tables 5.2 thru 5.6). 

5.2.1.2. Terrestrial Phase (Direct Effects) 

Risk Quotients for terrestrial-phase CRLF, as represented by 20-gram and 100-gram 

birds, exceed LOC for both acute and chronic (reproductive) effects (Table 5.7 and 5.8). 

Acute RQs range from 0.34 to 6.8 for CRLF for maximum exposure from cotton (1 lb 

ai/A applied 6 times with 3 day interval) and from 0.11 to 0.25 (small insect food source) 

for a minimum exposure of 0.13 lb ai/A applied once onto sewage disposal areas. RQs 

for large insect food source are below LOC (0.1). Chronic RQs range from 10 to 94 for 

CRLF for maximum exposure from cotton (1 lb ai/A applied 6 times with 3 day interval) 

and from 0.39 to 3.5 for a minimum exposure of 0.13 lb ai/A applied once onto sewage 

disposal areas. Both mortality and adverse reproductive effects to the CRLF (from a 

small insect diet) are anticipated based on labeled uses of acephate and risk quotients. 

RQs for CRLF from large insect diet are below acute and chronic LOC. 

Refinement of RQ for CRLF terrestrial phase 

Birds are currently used as surrogates for reptiles and terrestrial-phase amphibians. 

However, reptiles and amphibians are poikilotherms (body temperature varies with 

environmental temperature) while birds are homeotherms (temperature is regulated, 

constant, and largely independent of environmental temperatures). Therefore, reptiles 

and amphibians (collectively referred to as herptiles in this guidance) tend to have much 

95
lower metabolic rates and lower caloric intake requirements than birds or mammals. As a 

consequence, birds are likely to consume more food than amphibians or reptiles on a 

daily dietary intake basis, assuming similar caloric content of the food items. This can be 

seen when comparing the estimated caloric requirements for free living iguanid lizards 

(Iguanidae) (EQ 1) to passerines (song birds) (EQ 2) (U.S. EPA, 1993): 

0.799 iguanid FMR (kcal/day)= 0.0535 *(bw in g) (EQ 1) 

 

0.749 passerine FMR (kcal/day) = 2.123 *(bw in g) (EQ 2) 

 

With relatively comparable exponents (slopes) to the allometric functions, one can see 

that, given a comparable body weight, the free living metabolic rate of birds can be 40 

times higher than reptiles, though the requirement differences narrow with high body 

weights. Consequently, use of avian food intake allometric equation as a surrogate to 

herptiles is likely to result in an over-estimation of exposure for reptiles and terrestrialphase amphibians. 

There is a current need to evaluate dietary exposure to terrestrial-phase amphibian species 

(e.g., California Red-Legged Frog, CRLF) and an anticipated need to evaluate dietary 

exposure for amphibians and reptiles in the future for the purpose of conducting 

endangered species effects determinations. Therefore, T-REX (version 1.3.1.) has been 

altered to allow for an estimation of food intake for herptiles (T-HERPS) using the same 

basic procedure that T-REX uses to estimate avian food intake. 

A comparison is made between the T-REX model which uses the bird as a surrogate for 

the CRLF and the T-HERPS model which calculates the allometric functions for 

amphibians. 

Cotton (maximum exposure) Discussion 

T-REX model shows that the ranges of direct affects to birds as surrogate for CRLF is 

from 0.34 to 6.8 for dose-based acute, from 0.07 to 0.66 (LOC for listed terrestrial 

animals) for dietary acute, and from 10 to 94 for chronic dietary. 

T-HERPS model show the ranges of RQ for amphibians that was corrected for body 

weight, metabolic rates and caloric intake requirements from avian data. The ranges of 

RQ for T-HERPS is from <0.01 to 4.8 (LOC for listed terrestrial animals) for the dosebased acute, 0.02 to 1.17 for dietary acute, and from 8.3 to 168 for chronic dietary. 

The refinement of cotton models show a slight decrease in RQs in T-HERPS. All the 

chronic LOCs for CRLF is exceeded and many of the acute LOCs are exceeded (see 

Tables 5-10 and 5-11). 

96
 Sewage Disposal Areas (minimum exposure) Discussion

T-REX model shows that the ranges of direct affects to birds as surrogate for CRLF is 

from 0.11 to 0.25 (small insect diet) for dose-based acute, <0.05 (LOC for listed 

terrestrial animals) for dietary acute, and from 0.39 to 3.5 for chronic dietary. 

T-HERPS model show the ranges of RQ for amphibians that was corrected for body 

weight, metabolic rates and caloric intake requirements from avian data. The only the 

small herbivore mammal diet RQ for T-HERPS exceeds the LOC with an RQ of 0.18, 

from <0.01 to 4.8 (LOC for listed terrestrial animals) for the dose-based acute, all of 

dietary acute RQs is below LOC, and from 4 of the 7 food items are above LOC for 

chronic dietary. 

The refinement of sewage disposal areas models show a slight decrease in RQs in THERPS. For four out of seven food items, the chronic LOCs for CRLF is exceeded 

and only the small herbivore mammal diet RQ exceeds the acute LOC (see Tables 5-

10 and 5-11). 

Results of the T-HERPS model are below: 

97
Table 5-10 Summary of T-HERPS Risk Quotient Calculations Based on Upper 

Bound Kenaga EECs for cotton (Maximum Exposure) 

Acute and Chronic RQs are based on the Upper Bound 

Kenaga Residues.

Chemical Name: The maximum single day residue estimation is used for 

 Use both the acute and reproduction RQs.

 Formulation

Application Rate 1 lbs a.i./acre RQs reported as "0.00" in the RQ tables below should be noted a

Half-life 8.2 days <0.01 in your assessment. This is due to rounding and significa

Application Interval 3 days figure issues in Excel.

Maximum # Apps./Year 6

Length of Simulation 1 year

Bobwhite quail LD50 (mg/kg-bw) 109.00

Japanese Quail LC50 (mg/kg-diet) 718.00

Bobwhite quail NOAEL(mg/kg-bw) 0.00

Mallard duck NOAEC (mg/kg-diet) 5.00

Kenaga

Values

Short Grass 837.49

Tall Grass 383.85

Broadleaf plants/sm Insects 471.09

Fruits/pods/seeds/lg insects 52.34

Small herbivore mammals 551.86

Small insectivore mammals 34.49

Small terrestrial phase amphibians 16.35

Terrestrial Herpetofauna Results

Weight Body Ingestion (Fdry) Ingestion (Fwet) % body wgt FI

Class Weight (g) (g bw/day) (g/day) consumed (kg-diet/day)

Small 1.4 0.017 0.1 3.9 5.44E-05

Mid 37 0.212 1.4 3.8 1.41E-03

Large 238 0.893 6.0 2.5 5.96E-03

Body Adjusted LD50

Weight (g) (mg/kg-bw)

1.4 109.00

37 109.00

238 109.00

small (g) mid (g) large (g)

1.4 37 238

Short Grass 32.54 31.98 20.96

Tall Grass 14.91 14.66 9.61

Broadleaf plants/sm Insects 18.30 17.99 11.79

Fruits/pods/seeds/lg insects 2.03 2.00 1.31

Small herbivore mammals N/A 522.03 81.16

Small insectivore mammals N/A 32.63 5.07

Small terrestrial phase amphibian N/A 0.62 0.41

1.4 37 238

Short Grass 0.30 0.29 0.19

Tall Grass 0.14 0.13 0.09

Broadleaf plants/sm insects 0.17 0.17 0.11

Fruits/pods/seeds/lg insects 0.02 0.02 0.01

Small herbivore mammals N/A 4.79 0.74

Small insectivore mammals N/A 0.30 0.05

Small terrestrial phase amphibian N/A 0.01 0.00

Acute Chronic

Short Grass 1.17 167.50

Tall Grass 0.53 76.77

Broadleaf plants/sm Insects 0.66 94.22

Fruits/pods/seeds/lg insects 0.07 10.47

Small herbivore mammals 0.77 110.37

Small insectivore mammals 0.05 6.90

Small terrestrial phase amphibian 0.02 3.27

Note: To provide risk management with the maximum possible information,

it is recommended that both the dose-based and concentration-based 

RQs be calculated when data are available

Cotton

Orthene

Herpetofaunal Size Classes and Body Weights

 Upper Bound Kenaga Residues For RQ Calculation

Dietary-based EECs (ppm)

Acephate

Endpoints

Avian

Dietary-based RQs RQs

(Dietary-based EEC/LC50 or 

Amphibian/Reptile Acute RQs for Small, Medium, and Dose-based RQs Large Species (grams)

(Dose-based EEC/adjusted LD50)

Dose-based EECs 

(mg/kg-bw) 

98
Table 5-11 Summary of T-HERPS Risk Quotient Calculations Based on Upper 

Bound Kenaga EECs for sewage disposal areas (Minimum Exposure)

Acute and Chronic RQs are based on the Upper Bound 

Kenaga Residues.

Chemical Name: The maximum single day residue estimation is used for 

 Use both the acute and reproduction RQs.

 Formulation

Application Rate 0.13 lbs a.i./acre RQs reported as "0.00" in the RQ tables below should be noted as

Half-life 8.2 days <0.01 in your assessment. This is due to rounding and significant

Application Interval 0 days figure issues in Excel.

Maximum # Apps./Year 1

Length of Simulation 1 year

Bobwhite quail LD50 (mg/kg-bw) 109.00

Japanese Quail LC50 (mg/kg-diet) 718.00

Bobwhite quail NOAEL(mg/kg-bw) 0.00

Mallard duck NOAEC (mg/kg-diet) 5.00

Kenaga

Values

Short Grass 31.20

Tall Grass 14.30

Broadleaf plants/sm Insects 17.55

Fruits/pods/seeds/lg insects 1.95

Small herbivore mammals 20.56

Small insectivore mammals 1.28

Small terrestrial phase amphibians 0.61

Terrestrial Herpetofauna Results

Weight Body Ingestion (Fdry) Ingestion (Fwet) % body wgt FI

Class Weight (g) (g bw/day) (g/day) consumed (kg-diet/day)

Small 1.4 0.017 0.1 3.9 5.44E-05

Mid 37 0.212 1.4 3.8 1.41E-03

Large 238 0.893 6.0 2.5 5.96E-03

Body Adjusted LD50

Weight (g) (mg/kg-bw)

1.4 109.00

37 109.00

238 109.00

small (g) mid (g) large (g)

1.4 37 238

Short Grass 1.21 1.19 0.78

Tall Grass 0.56 0.55 0.36

Broadleaf plants/sm Insects 0.68 0.67 0.44

Fruits/pods/seeds/lg insects 0.08 0.07 0.05

Small herbivore mammals N/A 19.45 3.02

Small insectivore mammals N/A 1.22 0.19

Small terrestrial phase amphibian N/A 0.02 0.02

1.4 37 238

Short Grass 0.01 0.01 0.01

Tall Grass 0.01 0.01 0.00

Broadleaf plants/sm insects 0.01 0.01 0.00

Fruits/pods/seeds/lg insects 0.00 0.00 0.00

Small herbivore mammals N/A 0.18 0.03

Small insectivore mammals N/A 0.01 0.00

Small terrestrial phase amphibian N/A 0.00 0.00

Acute Chronic

Short Grass 0.04 6.24

Tall Grass 0.02 2.86

Broadleaf plants/sm Insects 0.02 3.51

Fruits/pods/seeds/lg insects 0.00 0.39

Small herbivore mammals 0.03 4.11

Small insectivore mammals 0.00 0.26

Small terrestrial phase amphibian 0.00 0.12

Note: To provide risk management with the maximum possible information,

it is recommended that both the dose-based and concentration-based 

RQs be calculated when data are available

Avian

Dietary-based RQs RQs

(Dietary-based EEC/LC50 or 

Amphibian/Reptile Acute RQs for Small, Medium, and Dose-based RQs Large Species (grams)

(Dose-based EEC/adjusted LD50)

Dose-based EECs 

(mg/kg-bw) 

 Upper Bound Kenaga Residues For RQ Calculation

Dietary-based EECs (ppm)

Acephate

Endpoints

seagwe diposal

Orthene

Herpetofaunal Size Classes and Body Weights

99
5.2.2. Indirect Effects to the CRLF 

5.2.2.1. Aquatic Phase 

Sub-adult and adult CRLF consume invertebrates. Since acute RQs for freshwater 

invertebrates range up to 0.13 (Table 5.5), there is a “May Affect” finding. However, 

since the RQ is below the Acute Risk LOC (0.5), other factors must be considered in 

determining if this constitutes a “Likely to Adversely Affect” or “Not Likely to 

Adversely Affect” finding, as explained below in section 5.4.2. Based on the likelihood 

of individual effects on aquatic invertebrates (Table 5.9b above), indirect risk to the 

CRLF via effects on aquatic invertebrates is considered “NLAA.” 

5.2.2.2. Terrestrial Phase 

Cotton (maximum exposure) Discussion

Risk quotients for two common prey animals (frog and small mammal and bird) greatly 

exceed both acute and chronic LOC (Table 5-8). These prey animals are anticipated to 

suffer adverse effects (mortality and reproductive effects) from labeled acephate uses. 

The acute RQ for a terrestrial invertebrate (honey bee), representing the bulk of the 

terrestrial phase CRLF diet, ranges from 5.5 to 50. Thus, adverse indirect effects to the 

CRLF, mediated via reduction in prey base, are anticipated. 

The terrestrial-phase CRLF uses small mammal burrows for shelter. If populations of 

small mammals are reduced, as is anticipated from the acute RQs for individual effects 

on them, then there may be fewer burrows for the CRLF to exploit. In addition, there are 

chronic effects that may reduce the population of mammalian food item for CRLF. Thus, 

there may be an indirect effect on the CRLF through adverse effects to terrestrial phase 

habitat. 

Sewage Disposal Areas (minimum exposure) Discussion

Risk quotients for common prey animals (frog and small mammal and bird) do exceed 

some of dosed-based acute but are discountable due to the low likelihood of effects to 

individual prey animals. Chronic RQs for birds and mammals are above LOC for all 

food items except for large insects (Table 5.8). These prey animals are anticipated to 

suffer adverse reproductive effects from labeled acephate uses. The acute RQ for a 

terrestrial invertebrate (honey bee), representing the bulk of the terrestrial phase CRLF 

diet, ranges from 0.20 to 1.86. Thus, adverse indirect acute effects “May Affect” the 

CRLF, mediated via reduction in prey base. Chronic effects (reproductive effects) “May 

Affect” CRLF and are not discountable. 

5.3 Action Area 

100
The Action Area for endangered species from the labeled use of a pesticide is defined by 

exceedence of the Level of Concern for any Listed species. Risk Quotients from the 

screening risk assessment are compared to the Listed Species LOCs for all taxa to 

determine the geographic extent of the Action Area. 

If necessary, standard modeling assumptions are changed to determine the limits of LOC 

exceedence. For example, the spray drift assumption for aerial application can be 

lowered from the standard 5% until LOC is no longer exceeded, and that spray drift 

amount entered into AgDrift or AgDISP to determine the distance from the sprayed field 

to the standard pond that will lower RQ to below LOC. That distance around the sprayed 

field then determines the Action Area (assuming no secondary poisoning effects from 

movement of contaminated animals). 

5.3.1. Aquatic Phase 

The Action Area for effects on aquatic species consists of two parts. One is a spray drift 

perimeter around the use site, and the other is a downstream dilution factor. Both parts 

are intended to find the geographic extent of Listed species LOC exceedance. 

5.3.1.1 Spray Perimeter. 

The Action Area for effects on aquatic species was based on acute effects to Listed 

aquatic invertebrates, since these were the only LOCs exceeded (Tables 5-6 and 5-7). To 

be below the LOC for Listed aquatic invertebrates (0.05), the peak concentration in the 

EXAMS pond would need to (0.05)*(26 ppb) = 1.3 ppb, where 26 ppb is the EC50 for 

Daphnia magna for methamidophos, a degradate of acephate. 

Tier 1 and Tier 2 exposure analyses showed that no LOCs are exceeded from parent 

acephate for invertebrates. Tier 2 exposure analysis for the degradate methamidophos 

showed RQs as high as 0.13 (Table 5.5). The likelihood of an individual effect on 

Daphnia at this RQ is very low (Table 5.9b), thus this effect is discountable, and no spray 

perimeter for indirect effects is needed. 

5.3.1.2 Downstream Dilution 

The downstream dilution analysis calculates how far downstream the EEC remains above 

the Listed species LOC, given flow contributions from both contaminated and 

uncontaminated streams in the watersheds of potential acephate use. The initial area of 

concern was defined by Figure 2.E., which shows all agricultural land in all counties in 

California where acephate is used. Flow contributions from streams in the corresponding 

watersheds are included in a GIS (Geographic Information System) analysis, until the 

pesticide concentrations (initially the EXAMS pond peak EEC) from contaminated and 

uncontaminated streams, weighted for flow, fall below the Listed species LOC. 

101
The downstream dilution factor that must be achieved is defined by the maximum ratio 

between an RQ and its corresponding LOC. In the case of methamidophos as a degradate 

of acephate, this is the acute RQ for aquatic invertebrates from aerial application to 

cotton and almond (0.13), divided by into LOC (0.05) for a factor of 0.38. See Table 5.6. 

5.3.2. Terrestrial Phase 

The Action Area due to effects on Listed species is also defined by the geographic extent 

of LOC exceedence. Quantitative estimates of exposure of avian (including reptiles and 

terrestrial amphibians) and mammal species is done with the TREX model, which 

automates exposure analysis according to the Hoerger-Kenaga nomogram, as modified 

by Fletcher (1994). 

For acephate, the Action Area was calculated on the basis of the smallest avian (20-gram 

body weight) or mammal (15-gram), consuming the most highly contaminated food 

category (short grass). This results in the highest RQs, and thus the most conservative 

estimate of the Action Area. 

The lowest ratio between the LOC for Listed terrestrial avian and mammalian species 

(0.1 for acute effects and 1.0 for chronic effects) and the RQ, times the maximum single 

application rate, is used to determine the exposure (in lb/acre) that is below LOC, as 

shown in Table 5-8. 

Exposure below LOC = (LOC/RQ )*(1 lb/acre). 

In the case of methamidophos, the target exposure is 0.006 lb/acre, due to chronic effects 

on avian species (chronic RQ = 167). 

The distance from the use site (sprayed field) needed to achieve the target exposure of 

0.006 lb/acre was calculated with the Gaussian Far-Field extension of the AgDISP model. 

The input parameters for AgDISP are given below (Table 5-12); all other parameters 

were the default values. 

Table 5-12. Input Parameters for AgDISP Gaussian Far-Field Extension Analysis 

Input Parameter Value 

Release Height 15 feet 

Wind Speed 15 mph 

Spray Quality ASAE very fine to fine 

Non-Volatile fraction 0.032 (1.33 lb product 

in 5 gal = 42 lb water) 

Active fraction 0.024 (nonvol frac x % 

a.i. = 75%) 

Surface Canopy None 

Specific Gravity, Carrier 1 

Deposition type Terrestrial point 

Initial Average Deposition 0.006 lb/acre 

102
The result of this analysis is that a perimeter of 2,913 (0.55 miles) feet from the edge of 

the sprayed field is needed to bring the chronic avian RQ to below the LOC of 1. Thus, 

the Action Area extends to a distance of 2,913 feet from the edge of fields sprayed with 

acephate. 

Figure 5A shows the full extent of the Action Area, based on the terrestrial effects 

distance of 2,913 feet and the downstream dilution factor of 0.38. 

103
 

Figure 5A Action Area for Acephate 

104
5.4 Listed Species Effect Determination for the California Red-Legged Frog 

5.4.1. “May Affect” Determination 

When the action area overlaps (spatially) the designated Core Areas and Critical Habitats 

a “may affect” determination is made. If there is no overlap, and thus no expected 

exposure, a “no effect” determination is made. Upon a “may affect” determination the 

probability of effect is considered and a “Likely to Adversely Affect” or “Not Likely to 

Adversely Affect” determination is made. 

Based on the action area for acephate use in California, the use of acephate “May Effect” 

the CRLF. Table 5.13 displays the proportion of the core area within each recovery unit 

that overlaps with the potential use areas. 

Table 5.13 Terrestrial spatial summary results for acephate uses with 2,913-ft buffer. 

Measure RU1 RU2 RU3 RU4 RU5 RU6 RU7 RU8 Total 

Initial Area 

of Concern 

(no buffer) 

67,491 sq km 

Established 

species 

range area 

(sq km) 

3654 2742 1323 3279 3650 5306 4917 3326 28,197 

Overlapping 

area (sq km) 

1615 1855 983 2323 2918 4243 3576 2662 20,175 

Percent 

area 

affected 

44 68 74 71 80 80 73 80 72 

1 # 

Occurrence 

Sections 

9 3 64 270 272 108 89 24 865

1

26 occurrence sections occur outside of Recovery Units. 

5.4.2 “Adverse Effect” Determination 

Risk Quotients for direct, acute and chronic effects to the terrestrial-phase CRLF (Tables 

5.4, 5.5, 5.10 and 5.11) are well above their respective LOCs for the high exposure 

(cotton). 

At low exposure (sewage disposal areas) the direct, acute and chronic LOC is exceeded 

for small insect food items. Chronic Risk quotients for animals (mammals, birds and 

amphibians) that may serve as prey for the CRLF are also well above LOCs for three of 

four food categories. The risk quotient for a terrestrial invertebrate (honey bee), 

representing the bulk of the CRLF diet, is 5.5 to 50 for high exposure (cotton), well 

105
above the LOC of 0.05. For the low exposure (sewage disposal areas), the RQs are 0.2 

and 1.86 which also provide an individual chance of effect of one in 1,210 and 1.13, 

respectively. Thus, both direct and indirect adverse effects to the terrestrial-phase 

CRLF and its critical habitat are anticipated. 

Risk quotients for direct, acute and chronic effects to the aquatic-phase CRLF, as 

represented by freshwater fish, are below the LOC (Tables 5.2 and 5.3). Acute and 

chronic effects on the aquatic phase CRLF and its critical habitat are not anticipated. 

Aquatic invertebrate acute RQs (tables 5.6 and 5.7) are below the acute LOC (0.5) for all 

uses, and the likelihood of individual effects is low (Table 5.9b). Thus adverse indirect 

effects on the CRLF due to loss of prey items are discountable, and therefore NLAA. 

Acephate is not toxic to aquatic plants, so no indirect effects to the CRLF via reduction in 

primary production as a food source are anticipated. 

Based on this analysis, it is concluded that the labeled uses of acephate in California 

“may affect, and are likely to adversely effect” the California Red-Legged Frog, where 

the Action area overlaps its habitat, due to terrestrial effects. 

Table 5.12 Acephate Effects Determination Summary 

Assessment Effects Basis for Determination Endpoint determination

Aquatic Phase 

(Eggs, larvae, tadpoles, juveniles, and adults) 

Direct Effects and Critical Habitat Effects 

1. Survival, growth, All Acute and Chronic RQ are below the listed LOC for and reproduction of No Effect surrogate species (rainbow trout) CRLF 

Indirect Effects 

2. Reduction or 

modification of 

aquatic prey base 

May Affect, Acute LOC is exceeded for aquatic invertebrates, 

Not Likely to however effect is considered discountable based on 

Adversely Affect low likelihood of individual effect. 

3. Reduction or 

modification of No Effect No LOC Exceedences for any plant species aquatic plant 

community 

4. Degradation of 

riparian vegetation 

No LOC Exceedences for any plant species. No 

No Effect adverse aquatic critical habitat modification is 

expected. 

Terrestrial Phase 

(Juveniles and Adults)

Direct Effects 

5. Survival, growth, 

and reproduction of 

CRLF 

Acute and Chronic LOC exceedences for birds, the 

surrogate species for direct effects to frogs. Initial May Affect, Area of Concern overlaps habitat. Use is widespread Likely to (nearly all counties). Use is documented in all months. Adversely Affect Probability of effect approaches 100% at calculated 

RQs. 

106
Assessment Effects Basis for Determination Endpoint determination

Indirect Effects and Critical Habitat Effects 

6. Reduction or 

modification of 

terrestrial prey base 

Acute and Chronic LOC exceedences for multiple 

components of CRLF prey base (mammals, birds, and 

May Affect, terrestrial invertebrates). LAA to terrestrial phase 

Likely to CRLF and its critical habitat based on acute RQs 

Adversely Affect exceeding 0.5 and chronic RQs over LOC for 

mammals, insects, birds. Adverse terrestrial critical 

habitat modification is expected. 

7. Degradation of No Effect No plant LOC exceedences. riparian vegetation 

When evaluating the significance of this risk assessment’s direct/indirect and adverse 

habitat modification effects determinations, it is important to note that pesticide 

exposures and predicted risks to the species and its resources (i.e., food and habitat) are 

not expected to be uniform across the action area. In fact, given the assumptions of drift 

and downstream transport (i.e., attenuation with distance), pesticide exposure and 

associated risks to the species and its resources are expected to decrease with increasing 

distance away from the treated field or site of application. Evaluation of the implication 

of this non-uniform distribution of risk to the species would require information and 

assessment techniques that are not currently available. Examples of such information and 

methodology required for this type of analysis would include the following: 

• Enhanced information on the density and distribution of CRLF life stages 

within specific recovery units and/or designated critical habitat within the 

action area. This information would allow for quantitative extrapolation 

of the present risk assessment’s predictions of individual effects to the 

proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

• Quantitative information on prey base requirements for individual aquaticand terrestrial-phase frogs. While existing information provides a 

preliminary picture of the types of food sources utilized by the frog, it 

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

biologically relevant thresholds of effects on the prey base, and ultimately 

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

107
repeated exposure events and the inherent demographic characteristics of 

the prey population play into the extent to which prey resources may 

recover is not predictable. An enhanced understanding of long-term prey 

responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and 

together with the information described above, a more complete prediction 

of effects to individual frogs and potential adverse modification to critical 

habitat. 

5.5 Risk Hypotheses Revisited 

Table 5.13 below revisits the risk hypotheses presented in section 2.9.1. The risk 

hypotheses were accepted or rejected in accordance with the “No Effect,” “May Affect,” 

and “Likely to Adversely Affect,” or “Not Likely to Adversely Affect” findings in this 

assessment. 

Table 5.13 Risk Hypotheses Revisited 

Risk Hypothesis Conclusions 

Labeled uses of acephate within the action 

area may directly affect the CRLF by 

causing mortality or by adversely affecting 

growth or fecundity 

Rejected for Aquatic exposure. “No 

Effect” finding. 

Accepted for Terrestrial exposure. “LAA” 

finding. 

Labeled uses of acephate within the action 

area may indirectly affect the CRLF by 

reducing or changing the composition of 

food supply 

Accepted for Terrestrial exposure. “LAA” 

finding. 

Rejected for Aquatic exposure. “NLAA” 

finding. 

Labeled uses of acephate within the action 

area may indirectly affect the CRLF and/or 

adversely modify designated critical habitat 

by reducing or changing the composition of 

the aquatic plant community in the ponds 

and streams comprising the species’ current 

range and designated critical habitat, thus 

affecting primary productivity and/or cover 

Rejected. “No Effect” finding for aquatic 

plants. 

Labeled uses of acephate within the action 

area may indirectly affect the CRLF and/or 

adversely modify designated critical habitat 

by reducing or changing the composition of 

the terrestrial plant community (i.e., 

riparian habitat) required to maintain 

acceptable water quality and habitat in the 

ponds and streams comprising the species’ 

current range and designated critical habitat

Rejected. “No Effect” finding for 

terrestrial plants. 

Labeled uses of acephate within the action Rejected. “No Effect” for aquatic plants 

108
area may adversely modify the designated 

critical habitat of the CRLF by reducing or 

changing breeding and non-breeding 

aquatic habitat (via modification of water 

quality parameters, habitat morphology, 

and/or sedimentation) 

and “NLAA” for indirect effects via 

invertebrates. 

Labeled uses of acephate within the action 

area may adversely modify the designated 

critical habitat of the CRLF by reducing the 

food supply required for normal growth 

and viability of juvenile and adult CRLFs 

Accepted for Terrestrial exposure. “LAA” 

finding via effects on vertebrate and 

invertebrate food items. 

Rejected for Aquatic exposure. “NLAA” 

finding for aquatic invertebrates, “No 

Effect” for aquatic plants. 

Labeled uses of acephate within the action 

area may adversely modify the designated 

critical habitat of the CRLF by reducing or 

changing upland habitat within 200 ft of 

the edge of the riparian vegetation 

necessary for shelter, foraging, and 

predator avoidance 

Accepted. Effects on small mammals may 

reduce number of burrows used for shelter. 

Labeled uses of acephate within the action 

area may adversely modify the designated 

critical habitat of the CRLF by reducing or 

changing dispersal habitat within 

designated units and between occupied 

locations within 0.7 mi of each other that 

allow for movement between sites 

including both natural and altered sites 

which do not contain barriers to dispersal 

Accepted. Effects on small mammals may 

reduce number of burrows used for shelter. 

Labeled uses of acephate within the action 

area may adversely modify the designated 

critical habitat of the CRLF by altering 

chemical characteristics necessary for 

normal growth and viability of juvenile and 

adult CRLFs 

Accepted. Presence of acephate in 

terrestrial habitat is believed to have direct 

and indirect effects on CRLF. 

109
6. Uncertainties 

6.1.1 Maximum Use Scenario

The screening-level risk assessment focuses on characterizing potential ecological risks 

resulting from a maximum use scenario, which is determined from labeled statements of 

maximum application rate and number of applications with the shortest time interval 

between applications. The frequency at which actual uses approach this maximum use 

scenario may be dependant on insecticide resistance, timing of applications, cultural 

practices, and market forces. 

6.1.2 Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Four years of data (2002 – 

2005) were included in this analysis because statistical methodology for identifying 

outliers, in terms of area treated and pounds applied, was provided by CDPR for these 

years only. No methodology for removing outliers was provided by CDPR for 2001 and 

earlier pesticide data; therefore, this information was not included in the analysis because 

it may misrepresent actual usage patterns. CDPR PUR documentation indicates that 

errors in the data may include the following: a misplaced decimal; incorrect measures, 

area treated, or units; and reports of diluted pesticide concentrations. In addition, it is 

possible that the data may contain reports for pesticide uses that have been cancelled. 

The CPDR PUR data does not include home owner applied pesticides; therefore, 

residential uses are not likely to be reported. As with all pesticide use data, there may be 

instances of misuse and misreporting. The Agency made use of the most current, 

verifiable information; in cases where there were discrepancies, the most conservative 

information was used. 

 

6.2. Exposure Assessment Uncertainties 

Due to lack of appropriate PRZM scenarios for California, not all labeled uses were 

modeled for aquatic exposure. It is likely that the cotton use, at 6 lb a.i. per acre, 

provides the highest aquatic exposure estimate, including those not modeled, most of 

which have maximum rates of 1 to 2 lb a.i. per acre. 

Landscape maintenance is known to be a major use of acephate. This exposure is 

described for the aquatic environment using the PRZM turf scenario. However, there are 

a number of application techniques, such as those described as gallons per pot or 

teaspoons per mound, for which it is difficult to compare exposure potential to 

agricultural uses that are given in pounds per acre. It is assumed that the greatest aquatic 

and terrestrial exposure potential is for agricultural uses that allow aerial application. 

 

110
All exposure estimates were done with maximum application rates, minimum intervals, 

and maximum number of applications, to define the Action Area for the Federal action. 

Actual exposures will depend on actual use rates, which may be lower. 

Aquatic exposure modeling inputs were based on the available guideline data. Some 

inputs (e.g., soil metabolism half-life = 2.3 days) were based on a single value, which by 

EFED policy is multiplied by 3 to account for uncertainty. The aquatic metabolism rates 

(both aerobic and anaerobic) were set by policy at 2 times the soil input value. The 

partition coefficient (Kd) used was the highest and only quantified value obtained (0.09). 

The use of values for the other soils (essentially, Kd = 0) would have resulted in 

somewhat higher exposure estimates. 

Spray drift estimates were set at 1% for ground application and 5% for aerial application, 

per EFED policy. Actual spray drift from aerial application may be higher. 

The decay half-life of acephate on foliage and other food items for the TREX analysis 

was set at 8.2 days, rather than the default value of 35 days. This value was obtained 

from Willis & McDowell (1987) from a field experiment on lemons in California; this is 

the same reference used to obtain the default value of 35 days. The value of 8.2 days was 

the highest of the half-lives for acephate, so it is the most protective of the measured 

values. 

Methamidophos exposures in the aquatic environment were estimated as 18% of the 

modeled acephate concentrations. This is based on a maximum 23% formation of 

methamidophos in a Fresno loam soil, and a molecular weight conversion factor of 0.77. 

This approach is considered reasonable, as the fate and transport properties of acephate 

and methamidophos are very similar (very mobile and non-persistent). The use of more 

sophisticated techniques (e.g., parent-daughter kinetics modeling ans use of 

methamidophos-specific fate inputs) is unlikely to provide more defensible exposure 

estimates. 

 6.2.1 PRZM Modeling Inputs and Predicted Aquatic Concentrations 

The standard ecological water body scenario (EXAMS pond) used to calculate potential 

aquatic exposure to pesticides is intended to represent conservative estimates, and to 

avoid underestimations of the actual exposure. The standard scenario consists of 

application to a 10-hectare field bordering a 1-hectare, 2-meter deep (20,000 m3) pond 

with no outlet. Exposure estimates generated using the EXAMS pond are intended to 

represent a wide variety of vulnerable water bodies that occur at the top of watersheds 

including prairie pot holes, playa lakes, wetlands, vernal pools, man-made and natural 

ponds, and intermittent and lower order streams. As a group, there are factors that make 

these water bodies more or less vulnerable than the EXAMS pond. Static water bodies 

that have larger ratios of pesticide-treated drainage area to water body volume would be 

expected to have higher peak EECs than the EXAMS pond. These water bodies will be 

either smaller in size or have larger drainage areas. Smaller water bodies have limited 

111
storage capacity and thus may overflow and carry pesticide in the discharge, whereas the 

EXAMS pond has no discharge. As watershed size increases beyond 10-hectares, it 

becomes increasingly unlikely that the entire watershed is planted with a single crop that 

is all treated simultaneously with the pesticide. Headwater streams can also have peak 

concentrations higher than the EXAMS pond, but they likely persist for only short 

periods of time and are then carried and dissipated downstream. 

The Agency acknowledges that there are some unique aquatic habitats that are not 

accurately captured by this modeling scenario and modeling results may, therefore, 

under- or over-estimate exposure, depending on a number of variables. For example, 

aquatic-phase CRLFs may inhabit water bodies of different size and depth and/or are 

located adjacent to larger or smaller drainage areas than the EXAMS pond. The Agency 

does not currently have sufficient information regarding the hydrology of these aquatic 

habitats to develop a specific alternate scenario for the CRLF. As previously discussed in 

Section 2.X and Attachment 1, CRLFs prefer habitat with perennial (present year-round) 

or near-perennial water and do not frequently inhabit vernal (temporary) pools because 

conditions in these habitats are generally not suitable (Hayes and Jennings 1988). 

Therefore, the EXAMS pond is assumed to be representative of exposure to aquaticphase CRLFs. In addition, the Services agree that the existing EXAMS pond represents 

the best currently available approach for estimating aquatic exposure to pesticides 

(USFWS/NMFS 2004). 

 6.2.2 Aquatic Exposure Estimates 

In general, the linked PRZM/EXAMS model produces estimated aquatic concentrations 

that are expected to be exceeded once within a ten-year period. The Pesticide Root Zone 

Model is a process or “simulation” model that calculates what happens to a pesticide in a 

farmer’s field on a day-to-day basis. It considers factors such as rainfall and plant 

transpiration of water, as well as how and when the pesticide is applied. It has two major 

components: hydrology and chemical transport. Water movement is simulated by the use 

of generalized soil parameters, including field capacity, wilting point, and saturation 

water content. The chemical transport component can simulate pesticide application on 

the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in 

the soil are estimated by simultaneously considering the processes of pesticide uptake by 

plants, surface runoff, erosion, decay, volatilization, foliar wash-off, advection, 

dispersion, and retardation. 

Uncertainties associated with each of these individual components add to the overall 

uncertainty of the modeled concentrations. Additionally, model inputs from the 

environmental fate degradation studies are chosen to represent the upper confidence 

bound on the mean values that are not expected to be exceeded in the environment 

approximately 90 percent of the time. Mobility input values are chosen to be 

representative of conditions in the environment. The natural variation in soils adds to the 

uncertainty of modeled values. Factors such as application date, crop emergence date, 

and canopy cover can also affect estimated concentrations, adding to the uncertainty of 

modeled values. Factors within the ambient environment such as soil temperatures, 

112
sunlight intensity, antecedent soil moisture, and surface water temperatures can cause 

actual aquatic concentrations to differ for the modeled values. 

Unlike spray drift, tools are currently not available to evaluate the effectiveness of a 

vegetative setback on runoff and loadings. The effectiveness of vegetative setbacks is 

highly dependent on the condition of the vegetative strip. For example, a wellestablished, healthy vegetative setback can be a very effective means of reducing runoff 

and erosion from agricultural fields. Alternatively, a setback of poor vegetative quality 

or a setback that is channelized can be ineffective at reducing loadings. Until such time 

as a quantitative method to estimate the effect of vegetative setbacks on various 

conditions on pesticide loadings becomes available, the aquatic exposure predictions are 

likely to overestimate exposure where healthy vegetative setbacks exist and 

underestimate exposure where poorly developed, channelized, or bare setbacks exist. 

 6.3.3 Residue Levels Selection

The Agency relies on the work of Fletcher et al. (1994) for setting the assumed pesticide 

residues in wildlife dietary items. These residue assumptions are believed to reflect a 

realistic upper-bound residue estimate, although the degree to which this assumption 

reflects a specific percentile estimate is difficult to quantify. It is important to note that 

the field measurement efforts used to develop the Fletcher estimates of exposure involve 

highly varied sampling techniques. It is entirely possible that much of these data reflect 

residues averaged over entire above ground plants in the case of grass and forage 

sampling. 

 6.3.4 Dietary Intake

It was assumed that ingestion of food items in the field occurs at rates commensurate 

with those in the laboratory. Although the screening assessment process adjusts dryweight estimates of food intake to reflect the increased mass in fresh-weight wildlife food 

intake estimates, it does not allow for gross energy differences. Direct comparison of a 

laboratory dietary concentration- based effects threshold to a fresh-weight pesticide 

residue estimate would result in an underestimation of field exposure by food 

consumption by a factor of 1.25 – 2.5 for most food items. 

Differences in assimilative efficiency between laboratory and wild diets suggest that 

current screening assessment methods do not account for a potentially important aspect of 

food requirements. Depending upon species and dietary matrix, bird assimilation of wild 

diet energy ranges from 23 – 80%, and mammal’s assimilation ranges from 41 – 85% 

(U.S. Environmental Protection Agency, 1993). If it is assumed that laboratory chow is 

formulated to maximize assimilative efficiency (e.g., a value of 85%), a potential for 

underestimation of exposure may exist by assuming that consumption of food in the wild 

is comparable with consumption during laboratory testing. In the screening process, 

exposure may be underestimated because metabolic rates are not related to food 

consumption. 

113
6.4 Effects Assessment Uncertainties 

 6.4.1 Age Class and Sensitivity of Effects Thresholds 

It is generally recognized that test organism age may have a significant impact on the 

observed sensitivity to a toxicant. The acute toxicity data for fish are collected on 

juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on 

recommended immature age classes (e.g., first instar for daphnids, second instar for 

amphipods, stoneflies, mayflies, and third instar for midges). 

Testing of juveniles may overestimate toxicity at older age classes for pesticide active 

ingredients, such as acephate, that act directly without metabolic transformation because 

younger age classes may not have the enzymatic systems associated with detoxifying 

xenobiotics. In so far as the available toxicity data may provide ranges of sensitivity 

information with respect to age class, this assessment uses the most sensitive life-stage 

information as measures of effect for surrogate aquatic animals, and is therefore, 

considered as protective of the California Red Legged Frog. 

6.4.2 Extrapolation of Long-term Environmental Effects from Short-Term 

Laboratory Tests 

The influence of length of exposure and concurrent environmental stressors to the 

California Red Legged Frog (i.e., urban expansion, habitat modification, decreased 

quantity and quality of water in CRLF habitat, predators, etc.) will likely affect the 

species’ response to acephate. Additional environmental stressors may decrease the 

CRLF’s sensitivity to the insecticide, although there is the possibility of 

additive/synergistic reactions. Timing, peak concentration, and duration of exposure are 

critical in terms of evaluating effects, and these factors will vary both temporally and 

spatially within the action area. Overall, the effect of this variability may result in either 

an overestimation or underestimation of risk. However, as previously discussed, the 

Agency’s LOCs are intentionally set very low, and conservative estimates are made in the 

screening level risk assessment to account for these uncertainties. 

 6.4.3 Sublethal Effects 

For an acute risk assessment, the screening risk assessment relies on the acute mortality 

endpoint as well as a suite of sublethal responses to the pesticide, as determined by the 

testing of species response to chronic exposure conditions and subsequent chronic risk 

assessment. Consideration of additional sublethal data in the assessment is exercised on a 

case-by-case basis and only after careful consideration of the nature of the sublethal 

effect measured and the extent and quality of available data to support establishing a 

plausible relationship between the measure of effect (sublethal endpoint) and the 

assessment endpoints. 

114
 6.4.4 Location of Wildlife Species 

For this baseline terrestrial risk assessment, a generic bird or mammal was assumed to 

occupy either the treated field or adjacent areas receiving a treatment rate on the field. 

Actual habitat requirements of any particular terrestrial species were not considered, and 

it was assumed that species occupy, exclusively and permanently, the modeled treatment 

area. Spray drift model predictions suggest that this assumption leads to an 

overestimation of exposure to species that do not occupy the treated field exclusively and 

permanently. 

6.5. Use of avian data as surrogate for amphibian data. 

Toxicity data for terrestrial phase amphibians was not available for use in this 

assessment. Therefore, avian toxicity data were used as a surrogate for risk estimation. 

There is uncertainty regarding the relative sensitivity of herptiles and birds to acephate. 

If birds are substantially more or less sensitive than the California red legged frog, then 

risk would be over or under estimated, respectively. 

6.6. Assumptions Associated with the Acute LOCs 

The risk characterization section of this endangered species assessment includes an 

evaluation of the potential for individual effects. The individual effects probability 

associated with the acute RQ is based on the mean estimate of the slope and an 

assumption of a probit dose response relationship for the effects study corresponding to 

the taxonomic group for which the LOCs are exceeded. 

 6.8. Action Area 

An example of an important simplifying assumption that may require future refinement is 

the assumption of uniform runoff characteristics throughout a landscape. It is well 

documented that runoff characteristics are highly non-uniform and anisotropic, and 

become increasingly so as the area under consideration becomes larger. The assumption 

made for estimating the aquatic Action Area (based on predicted in-stream dilution) was 

that the entire landscape exhibited runoff properties identical to those commonly found in 

agricultural lands in this region. However, considering the vastly different runoff 

characteristics of: a) undeveloped (especially forested) areas, which exhibit the least 

amount of surface runoff but the greatest amount of groundwater recharge; b) 

suburban/residential areas, which are dominated by the relationship between 

impermeable surfaces (roads, lots) and grassed/other areas (lawns) plus local drainage 

management; c) urban areas, that are dominated by managed storm drainage and 

impermeable surfaces; and d) agricultural areas dominated by Hortonian and focused 

runoff (especially with row crops), a refined assessment should incorporate these 

differences for modeled stream flow generation. As the zone around the immediate 

(application) target area expands, there will be greater variability in the landscape; in the 

context of a risk assessment, the runoff potential that is assumed for the expanding area 

will be a crucial variable (since dilution at the outflow point is determined by the size of 

115
the expanding area). Thus, it important to know at least some approximate estimate of 

types of land use within that region. Runoff from forested areas ranges from 45 – 

2,700% less than from agricultural areas; in most studies, runoff was 2.5 to 7 times higher 

in agricultural areas (e.g., Okisaka et al., 1997; Karvonen et al., 1999; McDonald et al., 

2002; Phuong and van Dam 2002). Differences in runoff potential between 

urban/suburban areas and agricultural areas are generally less than between agricultural 

and forested areas. In terms of likely runoff potential (other variables – such as 

topography and rainfall – being equal), the relationship is generally as follows (going 

from lowest to highest runoff potential): 

Three-tiered forest < agroforestry < suburban < row-crop agriculture < urban. 

There are, however, other uncertainties that should serve to counteract the effects of the 

aforementioned issue. For example, the dilution model considers that 100% of the 

agricultural area has the chemical applied, which is almost certainly a gross overestimation. Thus, there will be assumed chemical contributions from agricultural areas 

that will actually be contributing only runoff water (dilutant); so some contributions to 

total contaminant load will really serve to lessen rather than increase aquatic 

concentrations. In light of these (and other) confounding factors, Agency believes that 

this model gives us the best available estimates under current circumstances. 

116
7. References 

Altig, R. and R.W. McDiarmid. 1999. Body Plan: Development and Morphology. InR.W. 

McDiarmid and R. Altig (Eds.), Tadpoles: The Biology of Anuran Larvae.University of 

Chicago Press, Chicago. pp. 24-51. 

Alvarez, J. 2000. Letter to the U.S. Fish and Wildlife Service providing comments on 

the Draft California Red-legged Frog Recovery Plan. 

Bart, J. et. al., 1975. Environmental Impact Study of Aerially Applied Orthene on a 

Forest and Aquatic Ecosystems: Impact on Birds. MRID 05014922, MRID 00014639. 

Bocsor & O'Connor, 1975. Environmental Impact Study of Aerially Applied Orthene on 

a Forest and Aquatic Ecosystem: Impact on Aquatic Ecosystem. MRID 00014637 

Clarke Jr., D.R., B.A. Rattner. 1987. OrtheneR Toxicity to Little Brown Bats (Myotis 

lucifugus): Acetochlorinesterase Inhibition, Coordination Loss, and Mortality. Environ. 

Toxicol. and Chem. Vol 6 pp. 705-708. 

Crawshaw, G.J. 2000. Diseases and Pathology of Amphibians and Reptiles in:

Ecotoxicology of Amphibians and Reptiles; ed: Sparling, D.W., G. Linder, and C.A. 

Bishop. SETAC Publication Series, Columbia, MO. 

Davies, J.E. and V. H. Freed, editors. 1981. An agromedical approach to pesticide 

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