Risks of Methidathion Use to Federally 

Threatened California Red-legged Frog 

(Rana aurora draytonii) 

Pesticide Effects Determination 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

Washington, D.C. 20460 

February 18, 2009 
Primary Authors: 

Colleen Flaherty, Biologist 

Keara Moore, Chemist 

Pamela Hurley, Toxicologist 

Secondary Review: 

Brian Anderson, Biologist 

James Hetrick, Senior Scientist 

Branch Chief, Environmental Risk Assessment Branch 3: 

Dana Spatz

2 
Table of Contents

1. Executive Summary...............................................................................................8 

2. Problem Formulation ..........................................................................................16 

2.1 Purpose..........................................................................................................................16 

2.2 Scope..............................................................................................................................18 

2.3 Previous Assessments...................................................................................................18 

2.3.1 Methidathion Interim Registration Eligibility Decision, 2002................................18 

2.3.2. Organophosphate Cumulative Assessment, and Methidathion 

Reregistration Eligibility Decision, 2006.........................................................20 

2.4 Stressor Source and Distribution ...............................................................................20 

2.4.1 Environmental Fate and Transport...................................................................20 

2.4.2 Mechanism of Action...........................................................................................25 

2.4.3 Use Characterization ...........................................................................................25 

2.5 Assessed Species................................................................................................................29 

2.5.1 Distribution...........................................................................................................29 

2.5.2 Reproduction........................................................................................................32 

2.5.3 Diet ........................................................................................................................32 

2.5.4 Habitat ..................................................................................................................33 

2.6 Designated Critical Habitat ........................................................................................34 

2.7 Action Area...................................................................................................................36 

2.8 Assessment Endpoints and Measures of Ecological Effect ......................................39 

2.8.1. Assessment Endpoints for the CRLF.................................................................39 

2.8.2 Assessment Endpoints for Designated Critical Habitat...................................40 

2.9 Conceptual Model........................................................................................................43 

2.9.1 Risk Hypotheses...................................................................................................43 

2.9.2 Diagram ................................................................................................................43 

2.10 Analysis Plan ................................................................................................................45 

2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model................46 

2.10.2 Data Gaps .............................................................................................................49 

3.1 Label Application Rates and Intervals ......................................................................50 

3.2 Aquatic Exposure Assessment....................................................................................51 

3.2.1 Modeling Approach .............................................................................................51 

3.2.2 Model Inputs.........................................................................................................51 

3.2.3 Results...................................................................................................................54 

3.2.4 Existing Monitoring Data....................................................................................55 

3.3 Terrestrial Animal Exposure Assessment .................................................................59 

3.3.1 Modeling Approach .............................................................................................59 

3.4 Terrestrial Plant Exposure Assessment.....................................................................62 

4. Effects Assessment...............................................................................................62 

4.1 Toxicity of Methidathion to Aquatic Organisms ......................................................63 

4.1.1 Toxicity to Freshwater Fish ................................................................................64 

4.1.2 Toxicity to Freshwater Invertebrates.................................................................67 

4.1.3 Toxicity to Aquatic Plants...................................................................................68 

4.2 Toxicity of Methidathion to Terrestrial Organisms.................................................68 

4.2.1 Toxicity to Birds...................................................................................................69 

3
4.2.2 Toxicity to Mammals...........................................................................................71 

4.2.3 Toxicity to Terrestrial Invertebrates .................................................................72 

4.2.4 Toxicity to Terrestrial Plants..............................................................................72 

4.3 Use of Probit Slope Response Relationship to Provide Information on the 

Endangered Species Levels of Concern .................................................................................73 

4.4 Incident Database Review...........................................................................................73 

4.4.1 Terrestrial Incidents............................................................................................74 

4.4.2 Aquatic Incidents.................................................................................................75 

5. Risk Characterization..........................................................................................75 

5.1 Risk Estimation............................................................................................................75 

5.1.1 Exposures in the Aquatic Habitat ......................................................................76 

5.1.2 Exposures in the Terrestrial Habitat .................................................................80 

5.1.3 Primary Constituent Elements of Designated Critical Habitat.......................87 

5.1.4 Spatial Extent of Potential Effects......................................................................89 

5.2 Risk Description...........................................................................................................93 

5.2.1 Direct Effects........................................................................................................97 

5.2.2 Indirect Effects (via Reductions in Prey Base)................................................105 

5.2.3 Indirect Effects (via Habitat Effects) ...............................................................109 

5.2.4 Modification to Designated Critical Habitat...................................................110 

6. Uncertainties.......................................................................................................112 

6.1 Exposure Assessment Uncertainties.........................................................................112 

6.1.1 Maximum Use Scenario.....................................................................................112 

6.1.2 Aquatic Exposure Modeling of Methidathion.................................................112 

6.1.3 Potential Groundwater Contributions to Surface Water Chemical 

Concentrations ................................................................................................114 

6.1.4 Action Area Uncertainties.................................................................................114 

6.1.5 Usage Uncertainties ...........................................................................................115 

6.1.6 Terrestrial Exposure Modeling of Methidathion............................................116 

6.1.7 Spray Drift Modeling.........................................................................................117 

6.2 Effects Assessment Uncertainties .............................................................................117 

6.2.1 Age Class and Sensitivity of Effects Thresholds.............................................117 

6.2.2 Use of Surrogate Species Effects Data .............................................................118 

6.2.3 Sublethal Effects.................................................................................................118 

6.2.4 Location of Wildlife Species..............................................................................119 

7. Risk Conclusions................................................................................................119 

8. References...........................................................................................................123 

4
Appendices 

Attachment 1 CRLF Life History 

Attachment 2 Final CRLF Baseline Status and Cumulative Effects 

Appendix A Bibliography of submitted fate studies 

Appendix B Chemical Names and Structures 

Appendix C California PUR Data 

Appendix D CRLF Spatial Summary 

Appendix E Risk Quotient Method and Levels of Concern 

Appendix F PRZM/EXAMS Model Outputs 

Appendix G T-REX Example Output 

Appendix H Bibliography of ECOTOX Papers 

Appendix I Accepted ECOTOX Data Table (sorted by effect) 

Appendix J Ecological Effects 

Appendix K Data Table for Mammalian Studies from Health Effects Division 

Appendix L Incident Data 

Appendix M T-HERPS Example Output 

Appendix N Bibliography of Submitted Ecotoxicity Studies 

List of Tables 

Table 1.1 Effects Determination Summary for Methidathion Use and the CRLF .......................12 

Table 1.2 Effects Determination Summary for Methidathion Use and CRLF Critical 

Habitat Impact Analysis.........................................................................................13 

Table 1.3 Methidathion Use-specific Direct Effects Determinations1 for the CRLF...................13 

Table 1.4 Methidathion Use-specific Indirect Effects Determinations1

 Based on Effects to 

Prey ........................................................................................................................14 

Table 2.1. Summary of Methidathion Environmental Fate Properties.........................................22 

Table 2.2 Methidathion Uses Assessed for the CRLF.................................................................28 

Table 2.3 Assessment Endpoints and Measures of Ecological Effects ........................................40 

Table 2.4 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Primary Constituent Elements of Designated Critical Habitata

.............................42 

Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to 

Methidathion..........................................................................................................51 

Table 3.2. PRZM/EXAMS Crop-Specific Inputs for Aquatic Exposure to Methidathion...........53 

Table 3.3 Aquatic EECs (μg/L) for Methidathion Uses in California...........................................54 

Table 3.4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for 

Methidathion with T-REX.....................................................................................60 

Table 3.5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Methidathion ...........................................61 

Table 3.6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to 

Terrestrial Invertebrate Prey Items ........................................................................61 

5
Table 4.1 Freshwater Aquatic Toxicity Profile for Methidathion ................................................64 

Table 4.2 Categories of Acute Toxicity for Aquatic Organisms..................................................64 

Table 4.3 Terrestrial Toxicity Profile for Methidathion ...............................................................68 

Table 4.4 Categories of Acute Toxicity for Avian and Mammalian Studies.................................69 

Table 5.1 Summary of Acute Direct Effect RQs for the Aquatic-phase CRLFa ..........................77 

Table 5.2 Summary of Chronic Direct Effect RQs for the Aquatic-phase CRLFa........................78 

Table 5.3 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via 

Direct Effects on Aquatic Invertebrates as Dietary Food Items ............................79 

Table 5.4 Summary of Chronic RQs Used to Estimate Indirect Effects to the CRLF via 

Direct Effects on Aquatic Invertebrates as Dietary Food Items ............................80 

Table 5.5. Summary of Direct Effect Acute Dose-Based RQs for the Terrestrial-phase 

CRLF for Spray Applications of Methidathion .....................................................81 

Table 5.6. Summary of Direct Effect Acute Dietary-Based RQs for the Terrestrial-phase 

CRLF for Spray Applications of Methidathion .....................................................82 

Table 5.7. Summary of Direct Effect Chronic RQs for the Terrestrial-phase CRLF for 

Spray Applications of Methidathion......................................................................83 

Table 5.8 Summary of RQs For Indirect Effects to the Terrestrial-phase CRLF via Direct 

Effects on Terrestrial Invertebrates as Dietary Food Items...................................84 

Table 5.9. Summary of Acute RQs For Indirect Effects to the Terrestrial-phase CRLF via 

Direct Effects on Small Mammals as Dietary Food Items ....................................85 

Table 5.10. Summary of Chronic RQs For Indirect Effects to the Terrestrial-phase CRLF 

via Direct Effects on Small Mammals as Dietary Food Items...............................86 

Table 5.11. Input parameters for simulation of methidathion in spray drift using AgDisp 

(v. 8.15) with Gaussian far field extension............................................................90 

Table 5.12 Summary of maximum predicted distances for potential spray drift effects...............91 

Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to 

CRLF......................................................................................................................94 

Table 5.14 Risk Estimation Summary for Methidathion – PCEs of Designated Critical 

Habitat for the CRLF .............................................................................................95 

Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients..............................................................................................................100 

Table 5.16. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients..............................................................................................................107 

Table 7.1 Effects Determination Summary for Methidathion Use and the CRLF .....................121 

Table 7.2 Effects Determination Summary for Methidathion Use and CRLF Critical 

Habitat Impact Analysis.......................................................................................122 

6
List of Figures 

Figure 2.1. Map of Estimated Annual Agricultural Use of Methidathion in 2002 ................26 

Figure 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence 

Designations for CRLF........................................................................................31 

Figure 2.3 – CRLF Reproductive Events by Month.................................................................32 

Figure 2.4 Initial area of concern, or “footprint” of potential use, for methidathion ..........38 

Figure 2.5 Conceptual Model for Methidathion Effects on Terrestrial Phase of the 

CRLF.....................................................................................................................44 

Figure 2.6 Conceptual Model for Methidathion Effects on Aquatic Phase of the 

CRLF.....................................................................................................................45 

Figure 5.1. Overlap Map: CRLF Habitat and Methidathion Initial Area of Concern ........93 

7
1. Executive Summary 

The purpose of this assessment is to evaluate potential direct and indirect effects on the 

California red-legged frog (Rana aurora draytonii) (CRLF) arising from FIFRA 

regulatory actions regarding use of methidathion on agricultural and non-agricultural 

sites. In addition, this assessment evaluates whether these actions can be expected to 

result in modification of the species’ designated critical habitat. This assessment was 

completed in accordance with the U.S. Fish and Wildlife Service (USFWS) and National 

Marine Fisheries Service (NMFS) Endangered Species Consultation Handbook

(USFWS/NMFS, 1998 and procedures outlined in the Agency’s Overview Document 

(U.S. EPA, 2004). 

The CRLF was listed as a threatened species by USFWS in 1996. The species is endemic 

to California and Baja California (Mexico) and inhabits both coastal and interior 

mountain ranges. 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) in California. 

 

Methidathion is a non-systemic organophosphate insecticide/acaricide registered for use 

to control a wide range of sucking, leaf-eating, and scale insects. Currently labeled uses 

of methidathion considered as part of the federal action evaluated in this assessment 

include: Alfalfa, almond, apple, apricot, artichoke, calemondin, cherry, citron, citrus, 

clover, cotton, grapefruit, kiwi, kumquat, lemon, lime, mango, nectarine, olive, peach, 

pear, plum, prune, pummelo, safflower, sunflower, tangelo, tangerine, timothy, walnut, 

and nursery stock. Methidathion is formulated as an emulsifiable concentrate or a 

wettable powder. Applications can be applied by ground, airblast, or aerial spray with 

annual application rates that range from 1 lb active ingredient/A to 10 lb a.i./A. Label 

requirements include restrictions on spray heights and buffer widths in order to reduce 

risk from spray drift. 

Based on laboratory studies, methidathion has generally low persistence in soil, with 

aerobic and anaerobic soil metabolism half-lives of less than two weeks. Dissipation 

rates appear to slow over time, however, so there is a possibility that in some conditions 

low levels may persist. Terrestrial field dissipation studies found half-lives of less than 

30 days. In water, methidathion has limited hydrolysis except in alkaline conditions, but 

it is subject to aquatic photolysis with a half-life of 10 days. There are no aquatic 

metabolism data available. Soil adsorption studies indicate that methidathion is 

moderately mobile, but it has not been detected below 18 inches in any terrestrial field 

dissipation studies. There is some potential for long range atmospheric transport. 

Although the physical properties suggest that volatilization would be limited, monitoring 

studies have found methidathion residues at more than 20 km from any use site. 

Soil metabolism of methidathion leads primarily to carbon dioxide and bound residues, 

but one unidentified degradate was formed at up to 13% and appeared to be persistent. 

Other major degradates include S-129561, formed through hydrolysis and through aquatic 

 

1

 5-methyl-1-3,4-thiadazol-2 (3H)-one 

8
and soil photolysis, and des-methyl GS-130072

 and PAMMTO3, formed through 

hydrolysis in some conditions. Methidathion oxon4

 is formed only as a minor degradate, 

but it is included as a stressor of concern in this assessment because, based on data for 

other organophosphate pesiticides, oxon degradates have the potential to be much more 

toxic than the parent. Laboratory studies demonstrate that at least 4.9% of applied 

methidathion can be transformed to methidathion oxon through soil photolysis, but no 

data are available regarding photo-oxidation in air, which could also be an important 

route of transformation. 

Since CRLFs exist within aquatic and terrestrial habitats, exposure of the CRLF, its prey 

and its habitats to methidathion are assessed separately for the two habitats. Tier-II 

aquatic exposure models are used to estimate high-end exposures of methidathion in 

aquatic habitats resulting from runoff and spray drift from different uses. Peak modelestimated aquatic environmental concentrations resulting from different methidathion 

uses range from 7.3 to 114.7 μg/L. These estimates are supplemented with analysis of 

available California surface water monitoring data from U. S. Geological Survey’s 

National Water Quality Assessment (NAWQA) program and the California Department 

of Pesticide Regulation. The maximum concentration of methidathion reported by 

NAWQA for California surface waters from 15 sites is 0.31 μg/L. The California 

Department of Pesticide Regulation surface water database included more than 4,000 

samples from 135 locations in agricultural areas and reported a maximum measured 

concentration of 15.1 μg/L, which is in the range of peak modeled values for the different 

uses of methidathion. Ground water per se is not evaluated herein, but it is nonetheless 

significant because discharging groundwater is likely to support low-order streams, 

wetlands, and intermittent ponds – environments that are favorable to CRLFs. Long-term 

chronic concentrations derived from the PRZM-EXAMS model could reflect background 

concentrations that might be found in discharged groundwater/stream baseflow. 

NAWQA monitoring data did not detect methidathion in ground water sampling at more 

than 200 sites, most of which were collected in counties with high methidathion use in 

areas with land cover classified as agricultural or mixed use. 

To estimate methidathion exposures to the terrestrial-phase CRLF, and its potential prey 

resulting from uses involving methidathion applications, the T-REX model is used for 

foliar uses. AgDRIFT is used to estimate deposition of methidathion on terrestrial and 

aquatic habitats from spray drift. The TerrPlant model is used to estimate methidathion 

exposures to terrestrial-phase CRLF habitat, including plants inhabiting semi-aquatic and 

dry areas, resulting from uses involving foliar methidathion applications. The T-HERPS 

model is used to allow for further characterization of dietary exposures of terrestrialphase CRLFs relative to birds. 

There are also atmospheric monitoring data which demonstrate long range transport of 

both methidathion and methidathion oxon. Both species have been detected in air, fog, 

and/or rain at local, intermediate, and long-range locations. In the monitoring study with 

 

2

 des-methyl S-[(5-methoxy-2-oxo-1, 3, 4-thiadiazol-3 (2-1)-yl-methyl O,O-dimethylphosphorothioate] 

3

 phosphorthioic acid, 4-(mercaptomethy1)-2-meth-oxy-A2-,13 ,4-thiadiazolin-5-one 

4

 S-[(5-methione-2-oxo-1,3,4-thiodiazol-3(2H)-yl methyl o,o dimethyl phosphorothioate; (GS-13007) 

9
data at the furthest range, methidathion was detected in air at 22 km from the nearest use 

site and methidathion oxon was detected at a site another 10 km away. Methidathion 

oxon was also detected on pine needles at the 22 km site. 

The effects determination assessment endpoints for the CRLF include direct toxic effects 

on the survival, reproduction, and growth of the CRLF itself, as well as indirect effects, 

such as reduction of the prey base or modification of its habitat. Direct effects to the 

CRLF in the aquatic habitat are based on toxicity information for freshwater fish, which 

are generally used as a surrogate for aquatic-phase amphibians. In the terrestrial habitat, 

direct effects are based on toxicity information for birds, which are used as a surrogate 

for terrestrial-phase amphibians. Given that the CRLF’s prey items and designated 

critical habitat requirements in the aquatic habitat are dependant on the availability of 

freshwater aquatic invertebrates and aquatic plants, toxicity information for these 

taxonomic groups is also discussed. In the terrestrial habitat, indirect effects due to 

depletion of prey are assessed by considering effects to terrestrial insects, small terrestrial 

mammals, and frogs. Indirect effects due to modification of the terrestrial habitat are 

characterized by available data for terrestrial monocots and dicots. 

This assessment primarily considers effects of exposures to methidathion only but 

quantitative characterization is provided for methidathion oxon, the one known degradate 

of concern. Although no toxicity data are available for methidathion oxon, it is presumed 

to be of concern because toxicity data for oxon transformation products of similar 

pesticides in the organophospate class suggest they will be of equal or greater toxicity 

than the parent. There are other degradates suggested by the degradation pathway but not 

tested for in laboratory studies that may also be of concern. No data are available 

regarding the formation and decline or the toxicity of methidathion oxon or of any of the 

other potential OP or oxon degradates. Given that acute and chronic risk quotients are 

exceeded for the parent compound alone, any contribution in toxicity from any of the 

major degradates or from methidathion oxon or other OP degradates would increase the 

risk estimates. 

Risk quotients (RQs) are derived as quantitative estimates of potential high-end risk. 

Acute and chronic RQs are compared to the Agency’s levels of concern (LOCs) to 

identify instances where methidathion use within the action area has the potential to 

adversely affect the CRLF and its designated critical habitat via direct toxicity or 

indirectly based on direct effects to its food supply (i.e., freshwater invertebrates, algae, 

fish, frogs, terrestrial invertebrates, and mammals) or habitat (i.e., aquatic plants and 

terrestrial upland and riparian vegetation). When RQs for each particular type of effect 

are below LOCs, the pesticide is determined to have “no effect” on the CRLF. Where 

RQs exceed LOCs, a potential to cause adverse effects is identified, leading to a 

conclusion of “may affect.” If a determination is made that use of methidathion use 

within the action area “may affect” the CRLF and its designated critical habitat, 

additional information is considered to refine the potential for exposure and effects, and 

the best available information is used to distinguish those actions that “may affect, but are 

not likely to adversely affect” (NLAA) from those actions that are “likely to adversely 

affect” (LAA) the CRLF and its critical habitat. 

10
Based on the best available information, the Agency makes a Likely to Adversely Affect 

determination for the CRLF from the use of methidathion. Additionally, the Agency has 

determined that there is the potential for modification of CRLF designated critical habitat 

from the use of the chemical. For direct effects, the acute listed species LOCs for 

freshwater fish and for birds (surrogate to the aquatic-and terrestrial-phase CRLF, 

respectively) are exceeded for all uses. The chronic LOC for both freshwater fish and 

birds is also exceeded for all uses. For indirect effects, with the exception of mangos on 

an acute dietary basis for non-listed species, the acute dose-based, acute dietary-based 

and chronic dietary-based RQs for birds (terrestrial-phase amphibians) exceed both the 

acute non-listed species LOC and/or the chronic LOC for all of the assessed uses. For 

aquatic and terrestrial invertebrates and mammals, the acute RQs exceed the acute nonlisted LOC for aquatic and/or terrestrial animals for all uses. The chronic LOC for 

aquatic and/or terrestrial animals is also exceeded for aquatic invertebrates, mammals 

and fish/aquatic-phase amphibians (all uses). No studies are available for aquatic 

vascular and non-vascular and terrestrial plants; however, the weight of the evidence 

indicates an effect determination of may affect, not likely to adversely affect (NLAA). 

A summary of the risk conclusions and effects determinations for the CRLF and its 

critical habitat is presented in Tables 1.1 and 1.2. Use-specific determinations for direct 

and indirect effects to the CRLF are provided in Tables 1.3 and 1.4. Further 

information on the results of the effects determination is included as part of the Risk 

Description in Section 5.2. Given the LAA determination for the CRLF and potential 

modification of designated critical habitat, a description of the baseline status and 

cumulative effects for the CRLF is provided in Attachment 2.

11
Table 1.1 Effects Determination Summary for Methidathion Use and the CRLF

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

Aquatic-phase (Eggs, Larvae, and Adults): 

Highly toxic to freshwater fish. Acute listed species LOC exceeded for all uses 

for three tested surrogate species. Chronic effects based on survival and growth. 

Chronic LOC exceeded for all uses. Probability of an individual effect on an 

acute basis is high, both at the acute LOC and at the RQ levels. Incident data 

also indicate freshwater fish vulnerability, and monitoring has detected surface 

water concentrations above the acute endpoint. Significant overlap expected to 

exist between use sites and CRLF habitat, particularly when the spraydrift and 

downstream dilution buffers are added to the methidathion use area. Spraydrift 

distances range from 23 to over 1000 feet and the downstream dilution buffer is 

285 km. 

Terrestrial-phase (Juveniles and Adults): 

Moderately toxic to very highly toxic to avian species. Acute dose-based, acute 

dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF, 

using avian data as a surrogate, exceed acute listed species LOC and chronic 

LOC for all of the assessed uses. Chronic effects based on reduction in number 

of normal hatchlings/live 3-week embryos. Limited data indicate that on a dose 

basis, the acute LOC is exceeded for all uses, even with the least sensitive 

species at the lowest application rate (mangos). On a dietary basis, the same is 

true for all uses except mangos. Probability of an individual effect on an acute 

basis is high at the RQ levels. Incident data indicate that birds are vulnerable. 

Significant overlap exists between use sites and CRLF habitat, particularly when 

the spraydrift buffers are applied to the methidathion use area. Spraydrift 

distances range from 194 to over 1000 feet. 

Potential for Indirect Effects

Aquatic prey items, aquatic habitat, cover and/or primary productivity 

Very highly toxic to freshwater invertebrates. Acute and chronic RQs exceed 

acute LOC for non-listed species and chronic LOC, respectively for all assessed 

uses. The percentage effect to the aquatic invertebrate prey base for all uses is 

very high. For fish (aquatic-phase amphibians), acute non-listed species LOC 

exceeded for all uses. Chronic LOC exceeded for all uses. Percentage effect to 

the freshwater fish/aquatic-phase amphibian prey base is very high. No studies 

are available for aquatic vascular and non-vascular plants. 

Survival, growth, 

and/or reproduction 

of CRLF 

individuals 

LAA1

Terrestrial prey items, riparian habitat 

RQs for both small and large insects exceed acute LOC for non-listed species for 

all methidathion uses, even with only a single use. Insufficient studies to 

conduct a sensitivity analysis; however, lowest RQ, using highest terrestrial 

invertebrate endpoint still exceeds the acute list species LOC. Percentage effect 

to the terrestrial invertebrate prey base for all uses is very high. For terrestrialphase amphibians using avian data as a surrogate, with the exception of mangos 

on an acute dietary basis, the acute dose-based, acute dietary-based and chronic 

dietary-based RQs exceed the acute non-listed species LOC and the chronic LOC 

for all of the assessed methidathion uses, respectively. Percentage effect to the 

avian/terrestrial-phase amphibian prey base is very high. For mammals, the 

acute and chronic RQs exceed the non-listed species acute LOC and the chronic 

LOC, respectively for all uses. At the lowest RQ, the percentage effect to the 

mammalian prey base is very high. No studies are available for terrestrial plants. 1

 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely 

affect (LAA)

12
13

Table 1.2 Effects Determination Summary for Methidathion Use and CRLF Critical Habitat 

Impact Analysis

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase PCE No studies available for aquatic vascular and non-vascular plants and 

terrestrial plants. 

For the aquatic-phase CRLF, acute non-listed species LOC for freshwater 

fish (aquatic-phase amphibians) exceeded for all uses. Chronic LOC for 

freshwater fish exceeded for all uses. Probability of an individual effect and 

percentage effect to the freshwater fish/aquatic-phase amphibian prey base 

on an acute basis at the RQ are high. Incident data support freshwater fish 

vulnerability. For freshwater invertebrates, acute and chronic RQs exceed 

acute LOC (non-listed species) and chronic LOC, respectively for all 

assessed uses. Again, the percentage effect to the aquatic invertebrate prey 

base is very high. Significant overlap expected between use sites and CRLF 

habitat, particularly when the spraydrift and downstream dilution buffers are 

added to the use area. Spraydrift distances range from 23 (mangos) to over 

1000 feet and the downstream dilution buffer is 285 km. 

Modification of 

terrestrial-phase 

PCE 

Habitat 

Modification1

No studies are available for terrestrial plants. 

For the terrestrial-phase CRLF, using avian data as a surrogate, with the 

exception of mangos on a dietary basis for non-listed species, the acute dosebased, acute dietary-based and chronic dietary-based RQs for the terrestrialphase CRLF, exceed both the acute listed and non-listed species LOC and 

chronic LOC for all of the assessed uses. Probability of an individual effect 

on an acute basis at the lowest RQ level and percentage effect to the 

avian/terrestrial-phase amphibian prey base are very high. Incident data 

indicate that birds are vulnerable. RQs for both small and large insects 

exceed acute LOC for non-listed species for all methidathion uses, even with 

only a single use. Percentage effect to terrestrial invertebrate prey base for 

all uses are very high. For mammals, the acute and chronic RQs exceed the 

non-listed species acute LOC and the chronic LOC, respectively for all uses. 

The percentage effect to the mammalian prey base is very high for all uses. 

Significant overlap exists between use sites and CRLF habitat, particularly 

when the spraydrift buffers are applied to the methidathion use area. 

Spraydrift distances range from 194 (mangos) to over 1000 feet. 1

 Habitat Modification or No effect (NE)

Table 1.3 Methidathion Use-specific Direct Effects Determinations1 for the CRLF

Aquatic Habitat Terrestrial Habitat Use(s) Acute Chronic Acute Chronic 

All uses LAA LAA LAA LAA 1 NE = No effect; NLAA = May affect, but not likely to adversely affect; LAA = Likely to adversely affect 
14

Table 1.4 Methidathion Use-specific Indirect Effects Determinations1 Based on Effects to P

Aquatic 

Invertebrates 

Aquatic-phase 

frogs and fish 

Terrestrial-phas

frogs 

Use(s) Algae 

Acute Chronic 

Terrestrial 

Invertebrates 

(Acute) Acute Chronic Acute Chronic 

All uses NLAA LAA LAA LAA LAA LAA LAA LAA 

1 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect 
Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

and Wildlife Service under Section 7 of the Endangered Species Act should be initiated. 

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 

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 modification to critical habitat. 

15
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 

methidathion on various agricultural and non-agricultural uses. In addition, this 

assessment evaluates whether use on these crops is expected to result in modification of 

the species’ designated critical habitat. This ecological risk assessment has been 

prepared consistent with a settlement agreement in the case Center for Biological 

Diversity (CBD) vs. EPA et al. (Case No. 02-1580-JSW(JL)) entered in Federal District 

Court for the Northern District of California on October 20, 2006. 

In this assessment, direct and indirect effects to the CRLF and potential modification to 

its designated critical habitat are evaluated in accordance with the methods described in 

the Agency’s Overview Document (U.S. EPA 2004). Screening level methods include 

use of standard models such as PRZM-EXAMS, T-REX, and AgDRIFT, all of which are 

described at length in the Overview Document. Use of such information is consistent 

with the methodology described in the Overview Document (U.S. EPA 2004), which 

specifies that “the assessment process may, on a case-by-case basis, incorporate 

additional methods, models, and lines of evidence that EPA finds technically appropriate 

for risk management objectives” (Section V, page 31 of 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 methidathion is based on an action area. The action area is the area 

directly or indirectly affected by the federal action, as indicated by the exceedence of the 

Agency’s Levels of Concern (LOCs). It is acknowledged that the action area for a 

national-level FIFRA regulatory decision associated with a use of methidathion 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 use of methidathion in accordance with current labels: 

16
• “No effect”; 

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

• “May affect and likely to adversely affect”. 

Designated 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. 

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 use of methidathion as 

it relates to this species and its designated critical habitat. If, however, potential direct or 

indirect effects to individual CRLFs are anticipated 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 methidathion. 

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

with the CRLF “may affect” this species 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 geographical proximity of CRLF 

habitat and methidathion use sites) and further evaluation of the potential impact of 

methidathion on the PCEs is also used to determine whether modification of 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 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 methidathion is expected to directly impact living organisms within the action 

area (defined in Section 2.7), critical habitat analysis for methidathion 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 modify critical habitat are those that alter the PCEs and 

appreciably diminish the value of the habitat. Evaluation of actions related to use of 

methidathion that may alter 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. 

17
2.2 Scope 

Methidathion is used as an insecticide on a variety of terrestrial food and feed crops and 

terrestrial non-food crops. Based on usage data provided by the Biological and Economic 

Analysis Division (BEAD), on average, roughly 110,000 pounds of methidathion are 

applied annually to agricultural crops. Methidathion usage is highest on almonds and 

oranges, with annual average applications of 20,000 lbs. a.i. applied. The crop with the 

highest average percent crop treated with methidathion is artichokes at 60% (10,000 lbs. 

a.i. applied). 

The end result of the EPA pesticide registration process (i.e., 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 methidathion in accordance with the approved product labels for 

California is “the action” relevant to this ecological risk assessment. 

Although current registrations of methidathion allow for use nationwide, this ecological 

risk assessment and effects determination addresses currently registered uses of 

methidathion 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. 

Methidathion degrades into one known degradate of concern, methidathion oxon. 

Although no toxicity data are available for methidathion oxon, it is presumed to be of 

concern because toxicity data for oxon transformation products of similar pesticides in 

the organophospate class suggest they will be of equal or greater toxicity than the parent. 

Methidathion oxon is the only degradate considered in this analysis. There are no 

toxicity data for the four major degradates, but these compounds no longer contain the 

OP moiety and so are assumed to be of lower toxicity than the parent. 

Methidathion has no registered products that contain multiple active ingredients. 

2.3 Previous Assessments 

2.3.1 Methidathion Interim Registration Eligibility Decision, 2002 

The Agency completed a screening-level ecological risk assessment (USEPA, 1999) in 

support of the Interim Reregistration Eligibility Decision (IRED) for methidathion 

(USEPA 2002). Completion of the organophosphate (OP) cumulative assessment 

(USEPA 2006b) resulted in finalization of the IRED as a Reregistration Eligibility 

Decision (RED) (USEPA 2006a), which is summarized below. 

The Agency’s ecological risk assessment was based principally on methidathion’s use 

on cotton, citrus, stone fruits, nut crops, and artichokes. The assessment was based on 

data collected in the laboratory and in the field to characterize the fate and 

18
ecotoxicological effects of methidathion. Data sources used in this assessment 

included: 1) registrant submissions in support of reregistration, 2) publicly available 

literature on ecological effects, and 3) incident reports of adverse effects on aquatic and 

terrestrial organisms associated with the use of methidathion. 

The RED concluded that methidathion poses a risk to ecosystems in use areas. The levels 

of concern were exceeded for both acute and chronic effects to mammals, birds, fish, and 

aquatic invertebrates. In addition, risk to terrestrial invertebrates was presumed. The 

presumption of risk to non-target aquatic and terrestrial animals was supported by field 

studies and adverse ecological incidents. The potential risk to terrestrial plants was not 

assessed due to a lack of toxicity data. Toxicity data available for degradates of 

methidathion, including methidathion oxon, were not available for consideration in this 

assessment. 

To address the presumed risk to birds, the Agency required the additional dilution of 

methidathion products (a minimum of 500 gallons of water per acre) in an attempt to 

reduce the amount of methidathion on food items. In addition, precautionary statements 

regarding the risk to birds were required to be added to the label in order to protect avian 

species. 

To address the presumed risk to fish and aquatic invertebrates, the following risk 

mitigation strategies were required: 

For all applications applied at rates greater than 3.0s a.i./A: 

o Do not apply within 50 feet of lakes, reservoirs, rivers, permanent streams, 

natural ponds, marshes or estuaries 

- For all applications applied at rates of 3.0s a.i./A or less: 

o Do not apply within 25 feet of lakes, reservoirs, rivers, permanent streams, 

natural ponds, marshes or estuaries 

- For ground applications: 

o Shut off sprayer when turning at end rows 

o Do not apply when gusts or sustained winds exceed 12 mph 

- For air blast application: 

o Adjust deflectors and aiming devices so that spray is only directed into the 

canopy 

o Block off upward pointed nozzles when there is no overhanging canopy 

o Use only enough air volume to penetrate the canopy and provide good 

coverage 

o Do not allow spray to go beyond the edge of the cultivated area. Spray the 

outside row only from outside the planting 

- For aerial application: 

o Do not apply within 150 feet of water 

o Do not apply when gusts or sustained winds exceed 8 mph 

In addition, to mitigate the risk to aquatic animals, a label amendment was required to 

include a surface water advisory statement. 

19
The Agency was also concerned about the risk to beneficial insects from the use of 

methidathion. To reduce the likelihood for significant mortality to bees, precautionary 

labeling was required. 

2.3.2. Organophosphate Cumulative Assessment, and Methidathion 

Reregistration Eligibility Decision, 2006 

Because the Agency determined that methidathion shares a common mechanism of 

toxicity with the structurally-related organophosphate insecticides, a cumulative human 

health risk assessment for the organophosphate (OP) pesticides was necessary before the 

Agency could make a final determination of reregistration eligibility of methidathion. 

This cumulative assessment was finalized in 2006 (USEPA 2006b). The results of the 

Agency’s ecological assessments for methidathion are discussed in the July 31, 2006, 

final Reregistration Eligibility Decision (RED) (USEPA 2006a). 

The OP cumulative relied on a combined assessment methodology of modeling and 

monitoring data for human health exposure via drinking water. Unlike other assessments, 

the cumulative approach focused on regions of high OP use. No ecological risks were 

evaluated in the OP cumulative process, but its analysis of methidathion fate properties 

and its modeling approach remain relevant to this assessment. Additionally, the Agency 

requested toxicity data on methidathion oxon to further refine estimates. 

2.3.3 Assessment of Potential Effects to 26 Evolutionarily Significant Units 

of Pacific Salmon and Steelhead 

In April 2004, an assessment of methidathion uses in the Pacific northwest and California 

was conducted to determine potential effects to 26 evolutionarily significant units (ESUs) 

of Pacific salmon and steelhead. Based on toxicity and fate data, the usage information, 

habitat information and other factors, that assessment determined that uses of 

methidathion would have no effect on 7 ESUs, was not likely to adversely affect 9 ESUs 

and may affect the remaining 10 ESUs. 

2.4 Stressor Source and Distribution 

 2.4.1 Environmental Fate and Transport 

 2.4.1.1. Physical and Chemical Properties 

Chemical name: 3-Dimethoxyphosphinothioylthiomethyl-5-methoxy-1,3,4- 

 thiadiazol-2(3H)-one. 

Chemical structure: 

20
 

Molecular formula: C6H11N2O4PS3. 

Molecular weight: 302.3 g/mol 

Physical state: Colorless crystals. 

Melting point: 39-40°C. 

Solubility (20°C): 250 mg/L water; 

Vapor pressure (20°C): 2.48 x 10-6 mm Hg 

Henry's Law Constant: 3.97 x 10-9 atm m3

/mol 

Octanol/Water Partition 295 at pH 6.1 

 Coefficient: 

 2.4.1.2. Environmental Fate Properties 

Methidathion is classified as moderately mobile, and through most routes of 

transformation for which data are available, initial degradation is rapid and occurs with 

half-lives of less than two weeks, although in some cases, transformation slows and lower 

levels can persist for a longer time. Table 2.1 summarizes the environmental fate 

properties of methidathion, along with the major and minor degradates detected in the 

submitted environmental fate and transport studies. See Appendix A for a bibliography 

of these submitted studies. 

21
Table 2.1. Summary of Methidathion Environmental Fate Properties 

Study Value (units) Major Degradatesa

Minor Degradates MRID # Study Status 

Hydrolysis 

half life 

pH 5: 37, 27d 

pH 7: 48, 25d 

pH 9: 13, 8d 

GS-12956 

Des-methyl GS-13007 

42037701 

44554501 

Acceptable 

Acceptable 

Direct Aqueous 

Photolysis 

10.0 d b GS-12956 42081709 Supplemental 

Soil Photolysis 40 d c GS-12956 

Methidathion oxon (4.9%) 

GS-28307 

42081710 Supplemental 

Air Photolysis No data -- 42215101 Unacceptable 

Aerobic Soil 

Metabolism 

11.3 d (sandy loam) 

8.5 d (sandy loam) d

Unidentified e

Methidathion oxon (0.2%) 

GS-12956, GS-28369 

GS-28370, GS-20865

44545101 

42262501 

Acceptable 

Supplemental 

Anaerobic Soil 

Metabolism 

DT90 < 30 d 

(System) 

GS-28369, GS-12956, 

Two unidentified 

42262501 Supplemental 

Anaerobic Aquatic 

Metabolism 

No data -- No study 

submitted 

Aerobic Aquatic 

Metabolism 

No data -- No study 

submitted 

Mobility Kd: 2.5 to 14.8 mL/g 

 Avg = 6.5 mL/g 

Koc: 194 to 589 mL/goc 

 Avg = 364 mL/goc

n/a 00158529 Acceptable 

Laboratory Volatility No data -- 42098801 Unacceptable 

Terrestrial Field 

Dissipation 

CA: 5-30 d (4 plots) 

NE: <15 d (1 plot) 

GS-12956 

methidathion oxon

40094103 

41924401 

41924402 

Supplemental 

Supplemental 

Supplemental 

a

 Major degradates are those formed at >10% of the applied. Although a minor degradate, methidathion oxon is of toxicological 

concern and so the maximum percentage detected is reported here. Other oxon degradates that may be of toxicological concern may 

not have been detected because available studies did not include radiolabeling on the phosphate ester side chain of the parent 

compound. 

b

 The RED reported half-life of 11.6 days was not corrected for sunlight or for results from the dark control. c

 Problems with the study suggest that the measured half-life of 40 days is not representative of actual behavior. An assessment by the 

CDPR reports a soil photolysis half-life of 1.5 days. 

d

 Previous assessments reported this value as 3 days. That is in fact the DT50 and so this was updated to represent the linear first 

order half-life. 

e The degradate was unidentified but was tentatively characterized as a cyclic structure resulting from the reaction of carbazic acid and 

cysteine. 

Both aerobic and anaerobic soil metabolism show initially rapid degradation. In two 

aerobic soil metabolism studies, DT50s were 3 to 7 days and linear first order half-lives 

were less than 2 weeks, although one study showed some slowing over time, indicating 

that methidathion may persist at lower levels for several months. One anaerobic study 

22
was conducted but sampling was insufficient to determine a half-life. At the time of 

flooding, methidathion represented 41% of the initially applied radioactivity and by the 

first sampling point at 30 days post-flooding, it had been reduced to 6%. However, by 30 

days later metabolism had not progressed, with 5% of the initially applied radioactivity 

remaining as methidathion. On the soil surface, photodegradation of methidathion may 

occur, but there is some uncertainty regarding the rate. In the submitted laboratory study, 

photolysis was slow with a half-life of 40.6 days, but problems in the study suggest this 

may underestimate the actual rate. An assessment by the California Department of 

Pesticide Regulation (CDPR) reports a much more rapid soil photolysis half-life of 1.5 

days (Washburne, 2003). 

Methidathion may reach water bodies through spray drift or runoff, in the dissolved phase 

or adsorbed to eroding soil. In water, methidathion is only moderately susceptible to 

abiotic hydrolysis in acidic and neutral conditions with half-lives in two studies ranging 

from 25 to 48 days. Dissipation rates increase in alkaline conditions with half-lives of 8 

to 13 days at pH 9. In clear, shallow water, transformation may be more rapid due to 

photodegradation. A laboratory aquatic photolysis study measured a half life of 10.0 d, 

corrected for the dark control and natural sunlight. There are no data available for 

metabolism rates in aquatic environments. 

In supplemental terrestrial field dissipation studies on four plots in California, 

methidathion had reported DT50s in the top 6 inches of 5 to 30 days, and in one plot in 

Nebraska, the DT50 was <15 days. Methidathion was not detected at depths below 18 

inches in the field studies. There were several detections of methidathion in samples 

collected from the 12-18 inch cores, but these detections occurred in samples collected 

before any irrigation or rainfall event occurred and so appear to have been due to 

contamination during sample collection. 

Dissipation of dislodgeable residues on foliar surfaces was found to occur with half-lives 

of ≤3 days based on the results of six studies, and two studies found half-lives for 

dissipation of total residues in foliage to be 3.5 and 5 days. Additionally, methidathion is 

not expected to bioaccumulate. 

 2.4.1.3. Transformation Products 

Soil metabolism of methidathion leads primarily to carbon dioxide and bound residues. 

Other major degradates include GS-12956, formed through hydrolysis and through 

aquatic and soil photolysis (maxima from 18% to 66% of the applied radioactivity); desmethyl GS-13007 and PAMMTO, formed through hydrolysis at up to 21% and 18%, 

respectively; and an unidentified compound formed through aerobic soil metabolism and 

tentatively characterized as a cyclic structure resulting from the reaction of carbazic acid 

and cysteine. This compound appears to be persistent, as it reaches its maximum level of 

13% of the applied radioactivity on day 11 but remained at 10% by the end of the study 

(day 263). Multiple minor degradates were formed, not all of them identified. See 

Appendix B for chemical names and structures of all identified degradates. None of the 

23
major degradates contain the OP moiety and so they are likely to be less toxic than the 

parent. 

The only detected degradate of toxicological concern is minor degradate methidathion 

oxon. Many of the OP pesticides can undergo oxidative desulfonation (cleavage of P=S 

bond to form P=O bond) to form oxons, either through photo-oxidation or chemical 

oxidation in the presence of oxidizing agents, and oxons have the potential to be 

considerably more toxic than the parent compounds. In submitted laboratory studies, 

methidathion oxon has been documented to form via chlorination, soil photolysis (up to 

4.9% of the applied), and aerobic soil metabolism (0.2%), and the oxon was also detected 

in studies of terrestrial field dissipation and of dislodgeable foliar residues. The results of 

the soil photolysis study may underestimate oxon formation, because the maximum level 

was measured in the final sampling period and so oxon levels may continue to increase, 

and also because the study review concluded that due to a study deficiency, the rate of 

transformation observed may have been too low, supported by the more rapid 

transformation cited by CDPR (Washburn, 2003). In the dislodgeable residue study, 

methidathion was detected in citrus at up to 15 ng/cm2

 on leaves, with detectable residues 

through 7 days after application (MRID 00131096). No data are available regarding 

photo-oxidation in air of volatilized methidathion to its oxon forms, which could be an 

important route of transformation, as demonstrated by air monitoring data that find both 

methidathion and methidathion oxon at more that 20 km away from any use location. 

There are no data to characterize the fate of methidathion oxon. 

It should be noted that due to an issue in the laboratory study design, there could be 

additional degradates of toxicological concern that were not observed. In laboratory 

transformation studies, only the thiadiazole ring of the compound had radioactive 

labeling and so degradates containing the phosphate ester portion of the parent compound 

may not have been observed. In particular, major degradate GS-12956 is formed through 

cleavage of the C-N bond. Therefore, GS-12956 does not contain the OP moiety but the 

cleavage would result in a corresponding phosphate ester (phophorodithioic acid, o-o-strimethyl ester) which retains the OP moiety and so can be assumed to be toxic. Given 

that GS-12956 is formed in amounts up to 66% of the applied radioactivity, it is likely 

that the corresponding phosphate ester is formed as well. Additionally, this phosphate 

ester degradate could also undergo oxidative desulfonation to lead to additional oxon 

transformation products that would not have been detected in laboratory studies and may 

be of toxicological concern as well. 

 2.3.1.4. Environmental Transport Properties 

The Koc model appears to be appropriate to describe methidathion adsorption. Based on 

measured Koc values of 113 to 342 mL/goc, methidathion is classified as moderately 

mobile according to the FAO classification scheme. 

A number of studies have documented atmospheric transport and re-deposition of 

pesticides from the Central Valley to the Sierra Nevada Mountains (Fellers et al., 2004, 

Sparling et al., 2001, LeNoir et al., 1999, and McConnell et al., 1998). Prevailing winds 

24
blow across the Central Valley eastward to the Sierra Nevada Mountains, transporting 

airborne industrial and agricultural pollutants into the Sierra Nevada ecosystems (Fellers 

et al., 2004, LeNoir et al., 1999, and McConnell et al., 1998). Several sections of critical 

habitat for the CLRF are located east of the Central Valley. The magnitude of transport 

via secondary drift depends on methidathion’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. The vapor pressure of methidathion (2.48 x 10-6 mm Hg) 

and its Henry’s Law constant (3.97 x 10-9 atm m3/mol) would suggest that it has a low 

potential to volatize from dry or moist soils or from water, but monitoring studies have 

detected methidathion and its oxon in air at distances intermediate and up to 20 km away 

from any use sites (Royce et al, 1993; Aston and Seiber, 1997; Majewski, 2006). Given 

the potential toxicity of these residues, this route of transport could be significant for 

assessing the extent of ecological effects. 

2.4.2 Mechanism of Action 

Methidathion is an insecticide belonging to the organophosphate class of pesticides. The 

pesticide acts through inhibition of acetylcholinesterase and is used to kill a broad range 

of insects and mites. Organophosphate toxicity is based on the inhibition of the enzyme 

acetylcholinesterase which cleaves the neurotransmitter acetylcholine. Inhibition of 

acetylcholinesterase by organophosphate insecticides, such as methidathion, interferes 

with proper neurotransmission in cholinergic synapses and neuromuscular junctions 

(USEPA 2000). 

Target pests include the peach twig borer, scale insects, artichoke plume moth, 

leafminers, spider mites, boll weevils, bollworms, lygus bug, whitefly, aphid, pear psylla, 

mealybugs, thrips, sunflower stern weevil, sunflower moth, sunflower seed weevil, 

sunflower midge, Banks grass mite, flea beetle, hornworm, tobacco budworm, codling 

moth and hickory shuckworm. 

2.4.3 Use Characterization 

 2.4.3.1 Use Statistics 

Methidathion is used as an insecticide on a variety of terrestrial food and feed crops and 

terrestrial non-food crops. As shown in Figure 2.1, U.S. Geological Survey (USGS) 

National Water Quality Assessment Program (NAWQA) data indicate that in 2002, 

methidathion was used on agricultural crops predominantly in California, Washington, 

southern Texas, Pennsylvania, and New Jersey. At that time, the use of methidathion on 

citrus fruit represented about 30% of the national use. Based on national usage data 

compiled by the Biological and Economic Analysis Division (BEAD) primarily from 

2001 to 2006, on average, roughly 110,000 pounds of methidathion are applied annually 

to agricultural crops, 95% of it in California. These data show that usage is highest on 

almonds and oranges, with annual average applications to each of 20,000 lbs. a.i. The 

crop with the highest average percent crop treated with methidathion is artichokes (60%). 

25
Figure 2.1. Map of Estimated Annual Agricultural Use of Methidathion in 2002. 

Source: http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m6037

Use data specific to California are available from the California Department of Pesticide 

Regulation’s (CDPR) Pesticide Use Reporting (PUR) database, which includes every 

pesticide application made by professional applicators. The Biological and Economic 

Analysis Division (BEAD) summarized these data, from 1999 to 2006, to the county 

level by site, pesticide, and unit treated. Based on this analysis, an average of 82,031 lbs 

of methidathion was applied in California to an average of 52,823 acres per year. Use 

was at a maximum of 177,105 lbs in 1999 and then dropped by nearly half the following 

year, to 98,129 lbs, and remained relatively stable between 2003 and 2006 with average 

applications ranging from around 50,000 lb/year to 60,000 lb/year. From 1999-2006, 

methidathion was used in a total of 34 counties involving 41 different uses. Four 

counties accounted for 70% of the total lbs applied on average per county [Kern (25%), 

Tulare (20%), Monterey (14%), Fresno (11%)] (see Figure 2.2). Fruit orchards, 

including apple, apricot, cherry, nectarine, peach, pear, plum, and prune, accounted for 

approximately 30% of the total lbs applied per year in CA on average. Other major crops 

include almonds (23%), oranges (17%), and artichokes (14%). This analysis may not be 

entirely representative of current use patterns because labeled uses may have changed 

since these data were collected, and because it may also include misreporting. Complete 

data from the BEAD analysis of the CDPR PUR database are presented in Appendix C. 

26
Figure 2.2 Average Pounds of Methidathion Applied Per County from 1996 - 2005 

27
 2.4.3.2 Application Rates and Methods 

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

action. The current label for methidathion 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. Table 2.2 presents the 

uses and corresponding application rates considered in this assessment, based on a 

verification memorandum from SRRD (Dirk Helder, September 2, 2008). 

Methidathion is formulated as a wettable powder in water-soluble bags (25% active 

ingredient) or as an emulsifiable concentrate (22% - 24% active ingredient). Application 

rates range from 0.25 to 5.0 lbs active ingredient/acre. Application is via foliar treatment 

and application equipment includes fixed wing aircraft, ground boom, air blast, lowpressure handwand or backpack sprayer, although based on application instructions for 

citrus, olives, and nursery stock, these uses are assumed to be applied through ground 

methods only. Labels have additional requirements in order to reduce spray drift. These 

include the buffers and other mitigation actions listed in Section 2.3.1 as well as a 

requirement for a spray with medium to coarse droplet size applied at a maximum boom 

height of 4 ft about the ground or canopy for ground spray and of 10 ft for aerial spray.

Table 2.2 Methidathion Uses Assessed for the CRLF 

Use

Max. Single 

Appl. Rate (lb 

ai/A) 

Max. Number of 

Applications 

Number of 

Crop Cycles Per 

Year 

Minimum 

Application 

Interval (Days) 

NON-FOOD/NON-FEED USES 

alfalfa 1 NS 2-9 a NS 

nursery stock b 

(includes ornamental 

herbaceous plants and 

ornamental woody 

shrubs and vines) 

10 c 1/cc d 1e NA 

FOOD/FEED USES 

alfalfa 1 1/cc 2-9 a NA 

almond 3 1/cc 1e EC: 14 

WP: NS 

apple 3 1/cc 1 a NA 

apricot 3 1/cc 1 e NA 

artichoke 1 8/cc 1 e 14 

calamondin b 4 1/cc 1 e NA 

cherry 3 1/cc 1 e NA 

citron (citrus) b 4 1/cc 1 e NA 

citrus b 5 2/cc 1 e 45 

citrus hybrids other 

than tangelo 

4 1/cc 1 e NA 

clover 1 2/cc NS 

cotton (unspecified) 1 4/cc 1 a 5 

28
Table 2.2 Methidathion Uses Assessed for the CRLF 

Use

Max. Single 

Appl. Rate (lb 

ai/A) 

Max. Number of 

Applications 

Number of Minimum 

Application 

Interval (Days) 

Crop Cycles Per 

Year 

grapefruit 5 2/cc 1 a 45 

kiwi fruit 2 1/1 yr NA NA 

kumquat b 4 1/cc 1 e NA 

lemon b 5 2/cc 1 a 45 

lime b 4 1/cc 1 e NA 

mango b 0.25 5/cc 1 e 21 

nectarine 3 1/cc 1 a NA 

olive b 3 1/cc 1 e NA 

orange 5 2/cc 1 a 45 

peach 3 1/cc 1 a NA 

pear 3 1/cc 1 a NA 

plum 3 1/cc 1 e NA 

prune 3 1/cc 1 e NA 

pummelo (shaddock) 4 1/cc 1 e NA 

safflower (unspecified) 0.5 3/1 yr NA 7 

sunflower 0.5 3/cc 1 e 7 

tangelo b 4 1/cc 1 a NA 

tangerines b 4 1/cc 1 a NA 

timothy 1 NS 1 e NA 

walnut (english/black) EC: 2 

WP: 2 

EC: 2/cc 

WP: 3/cc 

1 e EC: 2 

WP: 2 

NA = Not Applicable NS = Not Specified on the label 

a

 U.S. EPA. 2007. Memo from Monisha Kaul (BEAD) to Melissa Panger (EFED). Subject: Maximum Number of Crop Cycles Per 

Year in California for Methomyl Use Sites. Dated February 28. 

b

 Ground spray only 

c

 The nursery stock application rate is reported on labels as 0.5 lb a.i./100 gal spray with no instructions for application on areal basis. 

Therefore, this rate is based on the general labels statement that methidathion should not be applied at greater than 10 lb a.i./A. d

 cc = crop cycle 

e

 Number of crop cycles per year as assumed by EFED. 

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 

29
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 elevational 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 2.2). 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 

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. 

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. 

30
Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

3. Traverse Creek/ Middle Fork/ American R. Rubicon

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

* Core areas that were historically occupied by the California red-legged frog are not included in the map

Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

3. Traverse Creek/ Middle Fork/ American R. Rubicon

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

* Core areas that were historically occupied by the California red-legged frog are not included in the map

Figure 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence 

Designations for CRLF 

31
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 2.3 depicts CRLF annual reproductive timing. 

Figure 2.3 – 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) 

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, 

32
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 close to water, shading, and water of moderate depth are habitat 

features that appear 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 

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 (UWFWS 2002). According to 

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

33
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 Attachment 1. 

‘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 

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 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 

• Dispersal habitat. 

Further 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 

34
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 modification standards for designated critical habitat (USFWS 

2006). Activities that may destroy or modify critical habitat are those that alter the PCEs 

and jeopardize the continued existence of the species. Evaluation of actions related to 

use of methidathion 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 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) 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 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. 

(4) 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. 

(5) Elimination of upland foraging and/or aestivating habitat or 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 (also 

evaluated as indirect effects to the CRLF). 

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 methidathion is expected to directly impact living 

organisms within the action area, critical habitat analysis for methidathion 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. 

35
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 methidathion is likely to encompass considerable portions of 

the United States based on the large array of 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. The Agency’s approach to defining the action area under the provisions of 

the Overview Document (USEPA 2004) considers the results of the risk assessment 

process to establish boundaries for that action area with the understanding that exposures 

below the Agency’s defined Levels of Concern (LOCs) constitute a no-effect threshold. 

For the purposes of this assessment, attention will be focused on the footprint of the 

action (i.e., the area where pesticide application occurs), plus all areas where offsite 

transport (i.e., spray drift, downstream dilution, etc.) may result in potential exposure 

within the state of California that exceeds the Agency’s LOCs. 

Deriving the geographical extent of this portion of the action area is based on 

consideration of the types of effects that methidathion may be expected to have on the 

environment, the exposure levels to methidathion that are associated with those effects, 

and the best available information concerning the use of methidathion and its fate and 

transport within the state of California. Specific measures of ecological effect for the 

CRLF that define the action area include any direct and indirect toxic effect to the CRLF 

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

growth, and fecundity as well as the full suite of sublethal effects available in the effects 

literature. Therefore, the action area extends to a point where environmental exposures 

are below any measured lethal or sublethal effect threshold for any biological entity at the 

whole organism, organ, tissue, and cellular level of organization. In situations where it is 

not possible to determine the threshold for an observed effect, the action area is not 

spatially limited and is assumed to be the entire 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 methidathion. An analysis of labeled uses and review of available product labels 

was completed. Several of the currently labeled uses are special local needs (SLN) uses 

or are restricted to specific states and are excluded from this assessment. In addition, a 

distinction has been made between food use crops and those that are non-food/nonagricultural uses. For those uses relevant to the CRLF, the analysis indicates that, for 

methidathion, the following agricultural uses are considered as part of the federal action 

evaluated in this assessment: 

• Alfalfa, almond, apple, apricot, artichoke, calemondin, cherry, citron, citrus, 

clover, cotton, grapefruit, kiwi, kumquat, lemon, lime, mango, nectarine, olive, 

oranges, peach, pear, plum, prune, pummelo, safflower, sunflower, tangelo, 

tangerine, timothy, and walnut. 

36
In addition, the following non-food and non-agricultural uses are considered: 

• Nursery stock, ornamental and herbaceous plants, woody shrubs and vines 

Following a determination of the assessed uses, an evaluation of the potential “footprint” 

of methidathion use patterns (i.e., the area where pesticide application occurs) is 

determined. This “footprint” represents the initial area of concern, based on an analysis 

of available land cover data for the state of California. The initial area of concern is 

defined as all land cover types and the stream reaches within the land cover areas that 

represent the labeled uses described above. A map representing all the land cover types 

that make up the initial area of concern for methidathion is presented in Figure 2.4. 

The following land cover types were used for methidathion: cultivated crops, developed 

(low, medium and high intensity and open space), forest, open water, orchards and 

vineyards, pasture/hay, wetlands, turf and rights-of-way. More information regarding 

which specific uses are represented for each land cover types can be found in Appendix 

D. 

37
Figure 2.4 Initial area of concern, or “footprint” of potential use, for methidathion 

Once the initial area of concern is defined, the next step is to define the potential 

boundaries of the action area by determining the extent of offsite transport via spray drift 

38
and runoff where exposure of one or more taxonomic groups to the pesticide exceeds the 

listed species LOCs. 

As previously discussed, the action area is defined by the most sensitive measure of 

direct and indirect ecological toxic effects including reduction in survival, growth, 

reproduction, and the entire suite of sublethal effects from valid, peer-reviewed studies. 

Due to the lack of a defined no effect concentration for a mammalian toxicity study 

(Yavuz et al., 2005; see Section 4.2.2 for study details), the spatial extent of the action 

area (i.e., the boundary where exposures and potential effects are less than the Agency’s 

LOC) for methidathion cannot be determined. Therefore, it is assumed that the action 

area encompasses the entire state of California, regardless of the spatial extent (i.e., initial 

area of concern or footprint) of the pesticide use(s). 

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 

methidathion (e.g., runoff, spray drift, etc.), and the routes by which ecological receptors 

are exposed to methidathion (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 or modification of its habitat. In addition, potential 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. It should be noted 

that assessment endpoints are limited to direct and indirect effects associated with 

survival, growth, and fecundity, and do not include the full suite of sublethal effects used 

to define the action area. According the Overview Document (USEPA 2004), the 

Agency relies on acute and chronic effects endpoints that are either direct measures of 

impairment of survival, growth, or fecundity or endpoints for which there is a 

scientifically robust, peer reviewed relationship that can quantify the impact of the 

measured effect endpoint on the assessment endpoints of survival, growth, and fecundity. 

 

5

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

39
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 methidathion is provided in Table 2.3. 

Table 2.3 Assessment Endpoints and Measures of Ecological Effects 

Assessment Endpoint Measures of Ecological Effects6

Aquatic-Phase CRLF 

(Eggs, larvae, juveniles, and adults)a

Direct Effects 

1. Survival, growth, and reproduction of CRLF 1a. Freshwater fish 96-hour acute LC50

1b. Freshwater fish chronic NOAEC

Indirect Effects and Critical Habitat Effects 

2. Survival, growth, and reproduction of CRLF 

individuals via indirect effects on aquatic prey food 

supply (i.e., fish, freshwater invertebrates, nonvascular plants) 

1a. Freshwater fish 96-hour acute LC50; freshwater 

invertebrate 48-hr EC50; aquatic non-vascular plant 

acute EC50

2b. Freshwater fish chronic NOAEC; freshwater 

invertebrate chronic NOAEC 

3. Survival, growth, and reproduction of CRLF 

individuals via indirect effects on habitat, cover, 

food supply, and/or primary productivity (i.e.,

aquatic plant community) 

3a. Vascular plant acute EC50 

3b. Non-vascular plant acute EC50 

4. Survival, growth, and reproduction of CRLF 

individuals via effects to riparian vegetation 

4a. Distribution of EC25’s for monocots 

4b. Distribution of EC25’s for dicots 

Terrestrial-Phase CRLF 

(Juveniles and adults) 

Direct Effects 

5. Survival, growth, and reproduction of CRLF 

individuals via direct effects on terrestrial phase 

adults and juveniles 

5a. Avian acute LD50 

5b. Avian chronic NOAEC 

Indirect Effects and Critical Habitat Effects 

6. Survival, growth, and reproduction of CRLF 

individuals via effects on terrestrial prey (i.e., 

terrestrial invertebrates, small mammals , and frogs) 

6a. Terrestrial invertebrate acute LD50; mammalian 

acute LD50

6b. Mammalian chronic NOAEC

7. Survival, growth, and reproduction of CRLF 

individuals via indirect effects on habitat (i.e.,

riparian and upland vegetation) 

7a. Distribution of EC25’s for monocots 

7b. Distribution of EC25’s for dicots 

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

 According to Mayer, D. & C. Johansen. 1990. Pollinator Protection: A Bee & Pesticide Handbook. Wicwas Press. Cheshire, Conn. 

p. 161 

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 methidathion 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 modify critical 

habitat are those that alter the PCEs and jeopardize the continued existence of the CRLF. 

 

6

 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in 

Appendix A. 

40
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 methidathion effects data are available. 

Modification to the critical habitat of the CRLF includes, but is not limited to, the 

following, as specified by USFWS (2006): 

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. 

Measures of such possible effects by labeled use of methidathion on critical habitat of the 

CRLF are described in Table 2.4. 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 

modification standard established by USFWS (2006). 

41
Table 2.4 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Primary Constituent Elements of Designated Critical Habitata

Assessment Endpoint Measures of Ecological Effect 

Aquatic-Phase CRLF PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)

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. 

a. Most sensitive aquatic plant EC50 

b. Distribution of EC25 values for terrestrial monocots 

c. Distribution of EC25 values for terrestrial dicots 

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.

a. Most sensitive EC50 values for aquatic plants 

b. Distribution of EC25 values for terrestrial monocots 

c. Distribution of EC25 values for terrestrial dicots 

Alteration of other chemical characteristics necessary 

for normal growth and viability of CRLFs and their 

food source. 

a. Freshwater fish 96-hour acute LC50; freshwater 

invertebrate 48-hr EC50; aquatic plant acute EC50

b. Freshwater fish chronic NOAEC; freshwater 

invertebrate chronic NOAEC 

Reduction and/or modification of aquatic-based food 

sources for pre-metamorphs (e.g., algae) a. Most sensitive aquatic plant EC50 

Terrestrial-Phase CRLF 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 

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 

a. Distribution of EC25 values for monocots 

b. Distribution of EC25 values for dicots 

c. Mammalian acute LD50; mammalian chronic NOAEC 

d. Freshwater fish 96-hour acute LC50; freshwater fish

chronic NOAEC

e. Avian acute LD50; avian acute LC50; avian chronic 

NOAEC 

f. Honey Bee acute LD50

Alteration of chemical characteristics necessary for 

normal growth and viability of juvenile and adult 

CRLFs and their food source. a 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. b

 According to Mayer, D. & C. Johansen. 1990. Pollinator Protection: A Bee & Pesticide Handbook. Wicwas Press. Cheshire, Conn. 

p. 161 

42
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 methidathion to the 

environment. The following risk hypotheses are presumed for this endangered species 

assessment: 

The labeled use of methidathion within the action area may: 

• directly affect the CRLF by causing mortality or by adversely affecting growth or 

fecundity; 

• indirectly affect the CRLF by reducing or changing the composition of food 

supply; 

• indirectly affect the CRLF or 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; 

• indirectly affect the CRLF or 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; 

• 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); 

• modify the designated critical habitat of the CRLF by reducing the food supply 

required for normal growth and viability of juvenile and adult CRLFs; 

• 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. 

• 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. 

• 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 methidathion and methidathion oxon release mechanisms, biological 

receptor types, and effects endpoints of potential concern. The conceptual models for 

terrestrial and aquatic exposures are shown in Figures 2.5 and 2.6, respectively, which 

43
include the conceptual models for the aquatic and terrestrial PCE components of critical 

habitat. Exposure routes shown in dashed lines are not quantitatively considered because 

the contribution of those potential exposure routes to potential risks to the CRLF and 

modification to designated critical habitat is expected to be negligible. 

Stressor 

Source 

Receptors 

Attribute 

Change 

Methidathion applied to use site and its degradate, methidathion oxon, 

formed after application 

Direct 

application Spray drift

Birds/terrestrialphase amphibians/ 

reptiles/mammals 

Terrestrial 

insects 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain

Reduction in prey 

Modification of PCEs 

related to prey availability 

Habitat integrity 

Reduction in primary productivity

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Terrestrial/riparian plants

grasses/forbs, fruit, seeds 

(trees, shrubs) 

Runoff 

Mammals/

birds 

Exposure 

Media 

Soil

Ingestion 

Ingestion

Ingestion 

Ingestion

Dermal uptake/Ingestion

Long range 

atmospheric 

transport 

Root uptake 

Wet/dry deposition 

Terrestrial-phase 

amphibians 

Ingestion 

Figure 2.5 Conceptual Model for Methidathion Effects on Terrestrial Phase of the 

CRLF 

44
Stressor

Source 

Receptors

Attribute 

Change 

Methidathion applied to use site and its degradate, methidathion oxon, 

formed after application 

Spray drift 

Fish/aquatic-phase 

amphibians 

**Piscivorous mammals 

 and birds 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain

Reduction in algae 

Reduction in prey 

Modification of PCEs 

 related to prey availability 

Habitat integrity 

Reduction in primary 

productivity

Reduced cover 

Community change 

Modification of PCEs related to 

 habitat 

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 plants

terrestrial 

exposure 

pathways see 

Figure 2.5 

Figure 2.6 Conceptual Model for Methidathion Effects on Aquatic Phase of the 

CRLF 

2.10 Analysis Plan 

In order to address the risk hypothesis, the potential for direct and indirect effects to the 

CRLF, its prey, and its habitat is estimated. In the following sections, the use, 

environmental fate, and ecological effects of methidathion are characterized and 

integrated to assess the risks. This is accomplished using a risk quotient (ratio of 

exposure concentration to effects concentration) approach. Although risk is often defined 

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

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

adverse effect. However, as outlined in the Overview Document (U.S. EPA, 2004), the 

likelihood of effects to individual organisms from particular uses of methidathion is 

estimated using the probit dose-response slope and either the level of concern (discussed 

below) or actual calculated risk quotient value. 

45
2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 

 2.10.1.1 Measures of Exposure 

The environmental fate properties of methidathion along with available monitoring data 

indicate that runoff and spray drift are the principle potential transport mechanisms of 

methidathion to the aquatic and terrestrial habitats of the CRLF, and monitoring data 

indicate that long range atmospheric transport is a concern as well. In this assessment, 

transport of methidathion through runoff and spray drift is considered in deriving 

quantitative estimates of methidathion exposure to CRLF, its prey and its habitats. Long 

range atmospheric transport is considered qualitatively. 

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

environmental concentrations (EECs) of methidathion using maximum labeled 

application rates and methods of application. The models used to predict aquatic EECs 

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

(PRZM/EXAMS). The model used to predict terrestrial EECs on food items is T-REX. 

The model used to derive EECs relevant to terrestrial and wetland plants is TerrPlant. 

These models are parameterized using relevant reviewed registrant-submitted 

environmental fate data. 

PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are screening 

simulation models coupled with the input shell pe5.pl (Aug 2007) to generate daily 

exposures and 1-in-10 year EECs of methidathion that may occur in surface water bodies 

adjacent to application sites receiving methidathion through runoff and spray drift. 

PRZM simulates pesticide application, movement and transformation on an agricultural 

field and the resultant pesticide loadings to a receiving water body via runoff, erosion and 

spray drift. EXAMS simulates the fate of the pesticide and resulting concentrations in 

the water body. The standard scenario used for ecological pesticide assessments assumes 

application to a 10-hectare agricultural field that drains into an adjacent 1-hectare water 

body, 2-meters deep (20,000 m3

 volume) with no outlet. PRZM/EXAMS was used to 

estimate screening-level exposure of aquatic organisms to methidathion. The measure of 

exposure for aquatic species is the 1-in-10 year return peak or rolling mean concentration. 

The 1-in-10 year peak is used for estimating acute exposures of direct effects to the 

CRLF, as well as indirect effects to the CRLF through effects to potential prey items, 

including: algae, aquatic invertebrates, fish and frogs. The 1-in-10-year 60-day mean is 

used for assessing chronic exposure to the CRLF and fish and frogs serving as prey 

items; the 1-in-10-year 21-day mean is used for assessing chronic exposure for aquatic 

invertebrates, which are also potential prey items. 

Exposure estimates for the terrestrial-phase CRLF and terrestrial invertebrates and 

mammals (serving as potential prey) assumed to be in the target area or in an area 

exposed to spray drift are derived using the T-REX model (version 1.3.1, 12/07/2006). 

This model incorporates the Kenega nomograph, as modified by Fletcher et al. (1994), 

which is based on a large set of actual field residue data. The upper limit values from the 

nomograph represented the 95th percentile of residue values from actual field 

46
measurements (Hoerger and Kenega, 1972). For modeling purposes, direct exposures of 

the CRLF to methidathion through contaminated food are estimated using the EECs for 

the small bird (20 g) which consumes small insects. Dietary-based and dose-based 

exposures of potential prey (small mammals) are assessed using the small mammal (15 g) 

which consumes short grass. The small bird (20g) consuming small insects and the small 

mammal (15g) consuming short grass are used because these categories represent the 

largest RQs of the size and dietary categories in T-REX that are appropriate surrogates 

for the CRLF and one of its prey items. Estimated exposures of terrestrial insects to 

methidathion are bound by using the dietary based EECs for small insects and large 

insects. 

Birds are currently used as surrogates for terrestrial-phase CRLF. However, amphibians 

are poikilotherms (body temperature varies with environmental temperature) while birds 

are homeotherms (temperature is regulated, constant, and largely independent of 

environmental temperatures). Therefore, amphibians tend to have much lower metabolic 

rates and lower caloric intake requirements than birds or mammals. As a consequence, 

birds are likely to consume more food than amphibians on a daily dietary intake basis, 

assuming similar caloric content of the food items. Therefore, the use of avian food 

intake allometric equation as a surrogate to amphibians is likely to result in an overestimation of exposure and risk for reptiles and terrestrial-phase amphibians. Therefore, 

T-REX (version 1.3.1) has been refined to the T-HERPS model (v. 1.0), which allows for 

an estimation of food intake for poikilotherms using the same basic procedure as T-REX 

to estimate avian food intake. 

EECs for terrestrial plants inhabiting dry and wetland areas are derived using TerrPlant 

(version 1.2.2, 12/26/2006). This model uses estimates of pesticides in runoff and in 

spray drift to calculate EECs. EECs are based upon solubility, application rate and 

minimum incorporation depth. 

The spray drift model AgDrift (v. 2.01) was used to assess exposures of terrestrial phase 

CRLF and its prey to methidathion deposited on terrestrial habitats by spray drift from 

ground and aerial applications. In addition to the buffered area from the spray drift 

analysis, the downstream extent of methidathion that exceeds the LOC for the effects 

determination is also considered. 

 2.10.1.2 Measures of Effect 

Data identified in Section 2.8 are used as measures of effect for direct and indirect effects 

to the CRLF. Data were obtained from registrant submitted studies or from literature 

studies identified by ECOTOX. The ECOTOXicology database (ECOTOX) was searched 

in order to provide more ecological effects data and in an attempt to bridge existing data 

gaps. ECOTOX is a source for locating single chemical toxicity data for aquatic life, 

terrestrial plants, and wildlife. ECOTOX was created and is maintained by the USEPA, 

Office of Research and Development, and the National Health and Environmental Effects 

Research Laboratory's Mid-Continent Ecology Division. 

47
The assessment of risk for direct effects to the terrestrial-phase CRLF makes the 

assumption that toxicity of methidathion to birds is similar to or less than the toxicity to 

the terrestrial-phase CRLF. The same assumption is made for fish and aquatic-phase 

CRLF. Algae, aquatic invertebrates, fish, and amphibians represent potential prey of the 

CRLF in the aquatic habitat. Terrestrial invertebrates, small mammals, and terrestrialphase amphibians represent potential prey of the CRLF in the terrestrial habitat. Aquatic, 

semi-aquatic, and terrestrial plants represent habitat of CRLF. 

The acute measures of effect used for animals in this screening level assessment are the 

LD50, LC50 and EC50. LD stands for "Lethal Dose", and LD50 is the amount of a material, 

given all at once, that is estimated to cause the death of 50% of the test organisms. LC 

stands for “Lethal Concentration” and LC50 is the concentration of a chemical that is 

estimated to kill 50% of the test organisms. EC stands for “Effective Concentration” and 

the EC50 is the concentration of a chemical that is estimated to produce a specific effect in 

50% of the test organisms. Endpoints for chronic measures of exposure for listed and 

non-listed animals are the NOAEL/NOAEC and NOEC. NOAEL stands for “No 

Observed-Adverse-Effect-Level” and refers to the highest tested dose of a substance that 

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

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

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

NOEC is the No-Observed-Effects-Concentration. For non-listed plants, only acute 

exposures are assessed (i.e., EC25 for terrestrial plants and EC50 for aquatic plants). 

It is important to note that the measures of effect for direct and indirect effects to the 

CRLF and its designated critical habitat are associated with impacts to survival, growth, 

and fecundity, and do not include the full suite of sublethal effects used to define the 

action area. According the Overview Document (USEPA 2004), the Agency relies on 

effects endpoints that are either direct measures of impairment of survival, growth, or 

fecundity or endpoints for which there is a scientifically robust, peer reviewed 

relationship that can quantify the impact of the measured effect endpoint on the 

assessment endpoints of survival, growth, and fecundity. 

 2.10.1.3 Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects characterization 

to determine the potential ecological risk from agricultural and non-agricultural uses of 

methidathion, and the likelihood of direct and indirect effects to CRLF in aquatic and 

terrestrial habitats. The exposure and toxicity effects data are integrated in order to 

evaluate the risks of adverse ecological effects on non-target species. For the assessment 

of methidathion risks, the risk quotient (RQ) method is used to compare exposure and 

measured toxicity values. EECs are divided by acute and chronic toxicity values. The 

resulting RQs are then compared to the Agency’s levels of concern (LOCs) (USEPA, 

2004) (see Appendix E). 

For this endangered species assessment, listed species LOCs are used for comparing RQ 

values for acute and chronic exposures of methidathion directly to the CRLF. If 

48
estimated exposures directly to the CRLF of methidathion resulting from a particular use 

are sufficient to exceed the listed species LOC, then the effects determination for that use 

is “may affect”. When considering indirect effects to the CRLF due to effects to animal 

prey (aquatic and terrestrial invertebrates, fish, frogs, and mice), the listed species LOCs 

are also used. If estimated exposures to CRLF prey of methidathion resulting from a 

particular use are sufficient to exceed the listed species LOC, then the effects 

determination for that use is a “may affect.” If the RQ being considered also exceeds the 

non-listed species acute risk LOC, then the effects determination is a LAA. If the acute 

RQ is between the listed species LOC and the non-listed acute risk species LOC, then 

further lines of evidence (i.e. probability of individual effects, species sensitivity 

distributions) are considered in distinguishing between a determination of NLAA and a 

LAA. When considering indirect effects to the CRLF due to effects to algae as dietary 

items or plants as habitat, the non-listed species LOC for plants is used because the CRLF 

does not have an obligate relationship with any particular aquatic and/or terrestrial plant. 

If the RQ being considered for a particular use exceeds the non-listed species LOC for 

plants, the effects determination is “may affect”. Further information on LOCs is 

provided in Appendix E. 

2.10.2 Data Gaps 

For environmental fate, there are no data gaps which will affect conclusions regarding 

potential environmental exposure to the parent compound, methidathion. Guideline 

aerobic and anaerobic aquatic metabolism studies have not been submitted, and the 

anaerobic soil metabolism study is not sufficient to determine a degradation half-life. 

These data would be useful to refine characterization of the environmental fate of 

methidathion, but they would not change quantitative exposure estimates. Submitted 

laboratory volatility and photodegradation in air were determined to be unacceptable. 

These studies would be necessary to quantitatively assess the potential for atmospheric 

transport of methidathion and methidathion oxon. 

Several fate studies do not provide adequate identification of potential degradates of 

methidathion. None of the available laboratory transformation studies include labeling of 

the phosphate ester portion of the methidathion compound and so potentially toxic 

degradates would not have been observed. This may lead to increased risk but cannot be 

characterized based on existing data and so is addressed as an uncertainty. 

For environmental effects, there are no aquatic or terrestrial plant data. Limited data 

from ECOTOX are discussed in the risk description section along with the risk 

conclusions for aquatic and terrestrial plants from assessments with other 

organophosphate pesticides. 

There are no toxicity data for methidathion oxon. Toxicity information for other 

organophosphate insecticides (e.g., diazinon, chlorpyrifos) indicate that oxon degradates 

may be considerably more toxic than the parent OP. Methidathion oxon has been 

detected in monitoring data and has been observed to form in field and laboratory studies. 

49
Without toxicity data, risk from this degradate of toxicological concern is calculated 

based on assumptions about formation and toxicity. 

Although not specifically a data gap, it is noted that the chronic NOAEC of 6.3 μg/L for 

the fathead minnow is similar to the acute 96-hour LC50 of several other freshwater fish 

species: rainbow trout (6.6 to 14 μg/L), bluegill sunfish (2.2 to 9 μg/L), and goldfish (6.8 

μg/L). A chronic NOAEC for any of these other species would likely be lower than the 

available fathead minnow NOAEC. Therefore, the estimation of risk to freshwater fish 

and aquatic-phase amphibians is based on the most conservative acute to chronic ratio 

from other organophosphates that have both an acute and chronic study for rainbow trout. 

3. Exposure Assessment 

Methidathion is formulated as a wettable powder in water-soluble bags (25% active 

ingredient) and emulsifiable concentrate (22% - 24% active ingredient). Application 

equipment includes fixed wing aircraft, ground boom, air blast, low-pressure 

handwand or backpack sprayer. Application is via foliar treatment, and annual 

application rates range from 1.0 to 10.0 lbs a.i./A. Risks from ground boom and aerial 

applications are expected to result in the highest off-target levels of methidathion due 

to generally higher spray drift levels. Ground boom and aerial modes of application 

tend to use lower volumes of application applied in finer sprays than applications 

coincident with sprayers and spreaders and thus have a higher potential for off-target 

movement via spray drift. 

3.1 Label Application Rates and Intervals 

Methidathion labels may be categorized into two types: labels for manufacturing uses 

(including technical grade methidathion and its formulated products) and end-use 

products. While technical products, which contain methidathion of high purity, are 

not used directly in the environment, they are used to make formulated products, 

which can be applied in specific areas to control insects. The formulated product 

labels legally limit methidathion’s potential use to only those sites that are specified 

on the labels. 

Currently registered agricultural and non-agricultural uses of methidathion within 

California are alfalfa, almond, apple, apricot, artichoke, cherry, citrus (calemondin, 

citron, grapefruit, kumquat, lemon, lime, orange, pummelo, tangelo, and tangerine), 

clover, cotton, kiwi, mango, nectarine, nursery stock (including ornamental herbaceous 

plans ant ornamental woody shrubs and vines), olive, peach, pear, plum, pruneo, 

safflower, sunflower, timothy, and walnut. Application rates and information for these 

uses were reported in Table 2.2. 

50
3.2 Aquatic Exposure Assessment 

3.2.1 Modeling Approach 

Surface water aquatic exposures for all assessed uses are quantitatively estimated using 

the Pesticide Root Zone Model coupled with the Exposure Analysis Model System 

(PRZM/EXAMS). These screening level models are operated based on scenarios that 

represent high exposure sites for methidathion use and represent a 10 hectare field that 

drains into a 1-hectare pond that is 2 meters deep and has no outlet. PRZM/EXAMS 

modeling does not account for transport to groundwater followed by discharge to surface 

water as a possible route of aquatic exposure. In this assessment, long-term chronic 

concentrations derived from the PRZM-EXAMS model are assumed to reflect 

background concentrations that might be found in discharged groundwater/stream 

baseflow, but this factor remains an uncertainty, and will be discussed as such in Section 

6.1.3. 

3.2.2 Model Inputs 

 3.2.2.1 Physical Properties and Environmental Fate Inputs 

Methidathion environmental fate data were discussed previously and are listed in Table 

2.1. Chemical-specific model input parameters for PRZM and EXAMS are based on 

these data and are listed in Table 3.1. 

Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to 

Methidathion 

Fate Property Value Comment 1 Source 

Molecular Weight 302.3 

Henry’s constant 3.97 x 10-9 atm m3/mol USEPA, 1999 

Vapor Pressure 2.5 x 10-6 mm Hg USEPA, 1999 

Solubility in Water 2500 mg/L Measured value x 10 USEPA, 1999 

Photolysis in Water 10 days MRID 42081709 

Aerobic Soil Metabolism 

Half-lives 17.5 days 

Upper 90% confidence 

bound on the mean of two 

values 

MRID 44545101, 

MRID 42262501 

Hydrolysis at pH 7 48 days Maximum of two values MRID 42037701 

Aerobic Aquatic 

Metabolism 

(water column) 

35 days Twice the aerobic soil 

metabolsim half-life. 

Anaerobic Aquatic 

Metabolism (benthic) Stable No data available

51
Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to 

Methidathion 

Fate Property Value Comment 1 Source 

Koc 364 mL/gOC Mean of four values MRID 00158529 

1 – 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 

 3.2.2.2 Use-specific Management Practices Inputs 

Crop-specific management practices for all of the assessed uses of methidathion were 

accounted for in aquatic exposure modeling. Application rates and intervals, application 

dates, and crop scenarios used as modeling inputs are listed in Table 3.2 and the 

justification for these inputs are described below. 

Use-specific parameters are input into modeling scenarios which have been developed to 

represent locally specific soil and climatic conditions in vulnerable use sites. Scenarios 

also include crop specific agronomic data and management practices such as planting and 

harvest date. The scenarios used in modeling were selected from the group of standard 

and CRLF-specific scenarios representing crops grown in California. For most 

methidathion uses, scenarios are available which were developed specifically for that use. 

In cases where a scenario does not exist for a specific use, it is necessary to assign a 

surrogate scenario. For sunflower, the California Corn scenario was selected because it 

represents a non-irrigated row crop. For safflower, the California Wheat scenario was 

used because it is an annual, broadleaf oilseed crop that can be grown in the small-grain 

production areas of California. For kiwi, the California Grape scenario was used because 

the kiwi is a woody vine grown primarily in the same counties as grapes. Some scenarios 

represent multiple uses, as listed in Table 3.2. For these uses, the application rate 

modeled is the highest from all uses within the group. Unless otherwise specified, the 

application method is aerial spray. 

Application dates are not specified on product labels. For modeling, an estimated 

application window was developed based on label instructions, crop profiles maintained 

by the USDA (http://www.ipmcenters.org/cropprofiles/GetCropProfiles.cfm; accessed 

12/08), historical use data from the California PUR dataset, and planting dates and 

precipitation data specific to each scenario. From within this window, multiple 

application dates were modeled using a multi-run tool in pe5 and the date leading to the 

most conservative EEC was used. For example, the distribution of applications seen in 

the CDPR PUR data indicates that methidathion is typically applied to olives between 

mid-October and mid-December, while the scenario precipitation data show that within 

this time period, late November has the highest precipitation and so the greatest 

likelihood of runoff. Using the pe5 multi-run tool for this window, an application date of 

December 1 was shown to lead to the most conservative aquatic exposure estimates. 

52
Table 3.2. PRZM/EXAMS Crop-Specific Inputs for Aquatic Exposure to 

Methidathion

Scenario Uses Represented 

by Scenario 

Date of first 

application 

Application 

Rate 

(lbs a.i./A)

Number of 

Applications 

Per Crop Cycle 

Application 

Interval 

(Days) 

CA Alfalfa Alfalfa, Clover 

Timothy 8/15 1 2 5a

CA Almond Almond, Walnut 1/7 2 3 2 

CA Citrus b

Calemondin, 

Citron, Citrus, 

Grapefruit, 

Kumquat, Lemon, 

Lime, Orange, 

Pummelo, Tangelo, 

Tangerine 

8/15 5 2 45 

CA Citrus c Mango 8/15 0.25 1 NA 

CA Corn Sunflower 1/7 0.5 3 7 

CA Cotton Cotton 7/10 1 4 5 

CA Grapes Kiwi 2/15 2 1 NA 

CA Tree Fruit 

Apple, Apricot, 

Cherry, Nectarine, 

Peach, Pear, Plum, 

Prune 

12/18 3 1 NA 

CA Olive b Olive 12/1 3 1 NA 

CA Row 

Crop Artichoke 7/25 1 8 14 

CA Wheat Safflower 5/25 0.5 3 7 

CA Nursery d Nursery 1/22 10 c 1 NA a

 The application interval is not specified on the label. A 5 day interval was assumed to be conservative because the 

target pests are similar to those for cotton. 

b

 Application through ground spray only. 

c

 Mango was modeled separately from the other citrus uses because it is the lowest application rate of all labeled 

uses and so EECs were needed to provide characterization of the full range of exposures. d

 The nursery stock application rate is reported on labels as 0.5 lb a.i./100 gal spray with no instructions for application 

on areal basis. Therefore, this rate is based on the general labels statement that methidathion should not be applied at 

greater than 10 lb a.i./A. 

For all uses, application was modeled as foliar applied (CAM = 2) with no removal of 

foliar pesticide after harvest (IPSCND = 3). Application efficiency inputs were set to the 

default values corresponding to each application method: 99% efficiency for ground 

applications (citrus, olives, and nursery stock) and 95% for all other aerial spray 

applications. Spray drift inputs were estimated using the AgDRIFT model in order to 

account for the effect of the labels’ mitigation requirements, which include buffer zones 

of 25 to 150 ft between cropped areas and water bodies (see Section 2.3.1) and a medium 

to coarse droplet size. Using these inputs, AgDRIFT estimated spray drift of 0.6% of the 

applied active ingredient for ground applications and of 2% for aerial uses. 

53
3.2.3 Results 

PRZM/EXAMS EECs representing 1-in-10 year peak, 21-day, and 60-day concentrations 

of total toxic residues of methidathion in the aquatic environment are located in Table 

3.5. All model outputs are included in Appendix F. Estimated aquatic exposures from 

terrestrial food and feed uses are highest for the CA Almond scenario, representing 

methidathion use on almonds and walnuts, with a peak EEC of 49.3 ug/L. Peak EECs for 

other agricultural crops ranged from 4.6 ug/L for alfalfa to 20.7 ug/L for sunflowers. 

The non-food use in ornamental nurseries resulted in a peak EEC of 115.6 ug/L. This 

result is based on an application rate of 10 lb a.i./A, the maximum application rate 

reported in the general instructions, because labeled application rates specific to the 

nursery use are reported in terms of product per foot of plant, rather than per unit area, as 

modeling requires. Aquatic EECs for the non-food uses on alfalfa, clover, and timothy 

have a peak EEC of 4.6 ug/L, but these results may underestimate exposure because the 

modeled use only includes a single crop cycle, while these crops can be grown at up to 

nine crop cycles per year. 

Table 3.3 Aquatic EECs (μg/L) for Methidathion Uses in California 

Scenario Crops Represented Peak EEC 21-day average 

EEC 

60-day average 

EEC 

CA Alfalfa Alfalfa, Clover 

Timothy 4.6 3.4 2.5 

CA Almond Almond, Walnut 49.3 40.2 26.8 

CA Citrus 

Calemondin, Citron, 

Citrus, Grapefruit, 

Kumquat, Lemon, Lime, 

Orange, Pummelo, 

Tangelo, Tangerine 

18.3 13.6 9.0 

CA Citrus Mango 0.45 0.34 0.22 

CA Corn Sunflower 20.7 16.2 10.9 

CA Cotton Cotton 13.0 9.3 6.4 

CA Grapes Kiwi 6.6 5.0 3.2 

CA Tree Fruit 

Apple, Apricot, Cherry, 

Nectarine, Peach, Pear, 

Plum, Prune 

15.6 9.2 5.0 

CA Olive Olive 15.2 11.7 7.6 

CA Row Crop Artichoke 16.0 12.1 8.7 

CA Wheat Safflower 13.6 10.4 6.6 

CA Nursery Nursery 115.6 84.8 54.3 

54
3.2.4 Existing Monitoring Data 

 3.2.4.1 USGS NAWQA Surface Water Data 

The USGS NAWQA surface water dataset in California includes 322 samples collected 

between 2001 and 2007 at 15 sites, all but one in the San Joaquin and Sacramento study 

areas. Six of the sites are located on Mustang Creek in Merced County in an area with 

landcover classified as “other, ” and the remaining sites represent a mix of landcover 

classes. Methidathion was detected 13 times at 7 sites, but more than half of the 

detections (8 detections, 0.01 to 0.04 ppb) were measured in one 24 hour period at two 

sites on Mustang Creek. At these and other Mustang Creek sites, sampling was 

conducted in winter months with multiple sampling per day, with the detections 

occurring in February, 2004. The other nine sites were generally sampled between 30 

and 60 times, biweekly or bimonthly for one to several years. Methidathion was detected 

at four of these sites at levels of 0.1 to 0.31 ppb. Limits of quantitation (LOQ) were 

0.006 ug/L to 0.009 ug/L. The sampling did not include analysis for any of the 

degradates of methidathion. 

 3.2.4.2 USGS NAWQA Groundwater Data 

The NAWQA groundwater California dataset included 271 samples from 223 wells 

analyzed for methidathion between 2001 and 2006, with no detections. Detection 

limits were 0.0058 to 0.0087 ug/L. Most of the samples (77%) were in the San Joaquin 

study area, primarily in Fresno, Merced, and Stanislaus Counties. The remaining sites 

were in the Sacramento study area with a few in the Santa Ana study area as well. 30 

sites in Sacramento county were classified as urban landcover, while the remaining 

samples were about half classified as agricultural landcover and half classified as 

mixed or other. No samples were analyzed for methidathion degradates. 

 3.2.4.3 California Department of Pesticide Regulation (CDPR) 

Surface Water Data 

CDPR’s monitoring dataset for methidathion includes 4,044 samples from 135 sites, most 

targeted to agricultural use areas, although specific land cover information is not 

available. The sampling was conducted for 19 different studies, primarily in the 

Sacramento and San Joaquin River drainages, although there was no sampling in the 

southern San Joaquin Valley (Fresno, Kern, and Tulare Counties), which is the area with 

the greatest methidathion use (see Figure 2.2). More than half of the samples (58%) were 

collected at five sites. They are primarily from two studies which were targeted to OP 

use areas and that had daily to weekly sampling over several years. These studies have 

implications more specific to methidathion and so the results will be discussed separately 

in more detail below. 

55
The remainder of the sampling included 1,694 samples from 130 sites, with 

approximately one-third of the samples (28%) from the Sacramento Valley and another 

44% in the northern San Joaquin Valley. LOQs were between 0.0028 ug/L to 0.10 ug/L. 

In the Sacramento Valley, methidathion was detected in 46 out of 472 samples, with a 

maximum detection of 15.1 ug/L in Sutter County, and an average across all detections of 

1.2 ug/L. In the San Joaquin Valley, there were 30 detections out of 738 samples, with 

the the maximum detection of 2.4 ug/L found in San Joaquin County and the average of 

0.3 ug/L. Additionally, there were 5 detections at two sites in Orange County at 

concentrations of 0.05 ug/L to 0.17 ug/L. Acute toxicity endpoints for fish and aquatic 

invertebrates are 2.2 ug/L and 3.0 ug/L, respectively, so monitoring data demonstrate the 

potential for surface water concentrations in excess of these endpoints. Although 

Monterey is one of the highest use counties, methidathion was not detected in any of the 

144 samples taken there, which were mostly from sites on waterways dominated by 

summer agricultural runoff. Overall, although only 35% of the total samples were 

collected in January or February, 90% of the detections (72 samples) were measured in 

those months. This reflects the fact that the typical use period of methidathion on fruit 

and nut orchards is in the winter dormant season. 

Two studies monitored for methidathion oxon in surface water as well as for the parent. 

There were no detections of the oxon in either study, even though in some cases 

methidathion had been detected in samples collected at the same time. In most cases, the 

limit of quantitation was 0.05 ug/L. One study included 450 samples, collected weekly to 

monthly at 7 sites, with no detections (Ganapathy, 1997). At one Merced County site in 

this study, methidathion had been detected in each of 4 samples collected weekly in 

January and February. The second methidathion monitoring study included 290 samples 

at 26 sites, primarily in Stanislaus and Merced County (Ross et al, 2000). Methidathion 

had been detected at 7 sites in this study area, at concentrations up to 12.4 ug/L. There 

were no detections of methidathion oxon sampled at the same times. 

Studies targeted to organophosphate pesticide use included extensive monitoring at 

several sites on Orestimba Creek, a tributary of the San Joaquin River located in 

Stanislaus County in an area expected to be vulnerable to runoff due to local soil and 

hydrology conditions. Agriculture in the area includes almond and walnut orchards as 

well as alfalfa. The sampling sites were selected primarily due to expected use of 

diazinon and chlorpyrifos, but methidathion is used in this area as well. In one study 

(Poletika & Robb, 1998), daily samples were taken at three sites on Orestimba Creek 

between June, 1996 to April, 1997. Methidathion was detected in 58 of 967 samples at 

levels from 0.001 ug/L to 0.331 ug/L, with an average across detected samples of 0.035, 

and based on an LOQ of 0.002 ug/L. There were 9 detections at the upstream site, with a 

maximum of 0.023 ug/L and mostly occurring in the first week of November, 1996. The 

midstream location had 22 detections, up to 0.078 ug/L, and the downstream site near the 

confluence with the San Joaquin River had 27 detections of up to 0.331 ug/L. Both sites 

had multiple detections in late July and early August. The highest peak occurred at the 

downstream site on March 20 and had decreased to undetectable levels by April 12. The 

study report indicates that, according to data from the county agricultural commisioner, 

five applications of methidathion were made in fields in the drainage during the sampling 

56
period. However, the timing of the detections did not correspond with any of the 

reported pesticide applications, so there were either unreported applications or alternative 

sources of runoff. Additional monitoring at the downstream site on Orestimba Creek was 

conducted by CDPR, with 136 samples collected, primarily during winter months in 1997 

to 2002. There were four detections of 0.06 ug/L to 2.14 ug/L (CDPR, metadata 

reference 10). 

Another study conducted by the USGS included long term monitoring at two sites 

draining to the San Francisco Bay (Maccoy, et al, 1995). Unlike the previously discussed 

study, which was in a small water body local to use areas, the sites sampled here are near 

outlets of large agricultural watersheds, one site on the Sacramento River and 

downstream of most other riverine inputs in the Sacramento Valley, and one on the San 

Joaquin River and downstream of most other rivierine inputs in the San Joaquin Valley. 

The San Joaquin Valley is the area of highest methidathion use and the Sacramento 

Valley has substantial use as well, so these locations represent high potential for 

methidathion exposure. On the San Joaquin River near Vernalis, sampling went from 

January, 1991 to April, 1994, and methidathion was detected 49 times out of 515 

samples. All detections occurred in January or February at concentrations of 0.028 ug/L 

to 0.802 ug/L. On the Sacramento River, sampling began in May, 1991 and methidathion 

was detected in 23 of the 562 samples taken, all in February. In 1993, methidathion was 

detected in 16 of 23 samples collected in February at levels from 0.03 ug/L to 0.21 ug/L, 

and in 1994, 7 of the 17 samples collected in February had methidathion, concentrations 

of 0.04 ug/L to 0.7 ug/L. 

 3.2.4.4 Atmospheric Monitoring Data 

There are several studies available which monitor for methidathion and methidathion 

oxon in air, fog, and/or rain onsite and at intermediate and long range distances from 

treated fields. A literature review by Majewski (1996) found three studies which tested 

air and fog and two studies which tested fog only for a total of 11 sites tested in 

California, both in the San Joaquin Valley and on the coast near Monterey. At eight of 

the sites, monitoring was also conducted for methidathion oxon. Methidathion was 

detected in both matrices at all 11 locations in all samples tested, while methidathion 

oxon was detected at 7 locations (all but one of those monitored) and in most of the 

samples tested. Detected concentrations of methidathion in air ranged from 0.01 ng/m3

 to 

23.8 ng/m3

 and in fog from 4 x 10-5 ug/L (0.04 ng/L) to 15.5 ug/L. This maximum 

concentration in fog exceeds the effects levels for fish and aquatic invertebrates. 

Concentrations for methidathion oxon were not reported. Majewski attributes the high 

detection frequency to the fact that most sampling was done in or near orchards, and the 

fact that most samples were collected in January, during the typical use period. It is 

likely, then, that at least some of these detections were the result of spray drift rather than 

atmospheric transport. The study near Monterey, however, was conducted in September 

and states that at least one of the three sites was in a non-agricultural area, so there is the 

possibility that longer range transport occurred. The frequency of detection of the oxon 

indicates that oxidation was occurring, either in the air directly or in soil or on plant 

surfaces followed by volatilization. 

57
Another air monitoring study was conducted for the California DPR in support of their 

Toxic Air Contaminant Program (Ross, 1991). In Tulare County, an area with high 

methidathion use on oranges, air sampling was conducted at five sites, four within a 

quarter mile of orange groves. The fifth site was intended to be used as background and 

was located in a downtown location with no orange groves in the immediate area. At 

each site, 17 samples per site were taken from June 27, 1991 to July 25, 1991, the time of 

peak use locally. Samplers were placed on the roof of a building or in an open area and 

collection occurred over a 23 to 25 hour period. All sites, including the intended 

background site, had samples with measured concentrations above the level of detection 

(LOD) for both methidathion (16 samples; LOD = 10 ng/m3) and methidathion oxon (27 

samples; LOD = 30 ng/m3). Methidathion was detected above the LOQ in 7 samples (32 

to 78 ng/m3) at two sites and methidathion oxon was detected in 7 samples (11 to 120 

ng/m3) at 4 sites, with the lowest detection found at the background site. Methidathion 

oxon was detected more frequently and at higher concentrations than the parent 

compound, including detections at a site with no use in the immediate area, indicating 

that transformation occurs and that there is a potential for off-site atmospheric transport. 

Vogel et al. (2008) monitored for methidathion in rain at a local and intermediate scale at 

a site in the Merced River basin where a variety of crops, including grapes, almonds, and 

corn, were grown. Between February and April of 2003 and December 2003 and April 

2004, methidathion was measured for in 23 single event rain samples at lower, middle, 

and upper positions in the watershed. Detections above the limit of quantitation of 0.006 

ug/L were found in 11 samples, with the highest detection of 0.69 ug/L found on 

December 16, 2003. Seven samples had detections between 0.01 ug/L and 0.1 ug/L and 

the other three were <0.01 ug/L. The detections were presumed to be the result of 

atmospheric transport because while the highest detection occurred before spraying began 

in local orchards, during that time methidathion was used on crops at the intermediate 

and broad scales. No sampling for methidathion oxon was conducted. 

Another study by Aston and Seiber (1997) showed the most clear evidence of long range 

transport of both methidathion and its oxon degradate with detections in air at up to 30 

km from any agricultural site and on pine needles 22 km away from use areas. In May 

through October of 1994, Aston and Seiber (1997) monitored for methidathion and 

methidathion oxon at three sites near Sequoia National Park. Biweekly samples of air 

and of pine needles were taken at three sites; the first was located in Tulare County next 

to a citrus grove treated with methidathion and within an area of high citrus production, 

the second was at a site within Sequoia National Park that was 22 km east of the nearest 

agricultural spraying, and the third site at an elevation on 1920 m was another 10 km 

away to the northeast. Each air sample represented air collected over a 24 h period, and 

pine needles were collected from potted pines placed at each site. The flow of air from 

the agricultural areas of the Central Valley to the two long range sites was reported to be 

unobstructed. 

Methidathion was detected in air in every sample at the citrus grove location and 

concentrations and detection frequency decreased with distance. Methidathion oxon was 

also detected in all samples at the citrus grove and it was more persistent with distance 

58
than the parent. One week following spraying in the onsite grove, methidathion was 

detected in air at the highest concentration of 17.0 ng/m3

. Concentrations at the four 

sampling events prior to spraying ranged from 1.1 ng/m3 to 15.0 ng/m3 and in the four 

following sampling events air concentrations ranged from 0.4 ng/m3

 to 2.7 ng/m3. Over 

the sampling period, methidathion oxon was detected onsite at levels from 0.28 ng/m3 to 

10.0 ng/m3, with the highest concentration at the initial sampling period, followed by 

steady decline interrupted by a peak after spraying. At the intermediate site, 

methidathion was detected only in the initial sampling, June 7, at 0.23 ng/m3

. The oxon 

degradate was detected on the first date as well, at 0.66 ng/ng/m3

, and declined over the 

next month when it was detected a third time, at 0.21 ng/m3

. At all but one of the 5 final 

sampling events, methidathion oxon was detected but it was either below the limit of 

quantitation (0.17 ng/m3

) or it was not quantifiable due to inconsistencies in duplicate 

samples. At the far site, methidathion was reported as being unquantifiable in all samples 

and methidathion oxon was detected at 0.21 ng/m3 on June 20 and was undetected or 

unquantifiable in all other samples. 

In pine needles, both methidathion and methidathion oxon were detected in all samples at 

the citrus grove site but only the oxon degradate was detected at the first site in Sequoia 

National Park and neither was detected at the further site. Next to the citrus grove, 

methidathion concentrations ranged from approximately 50 ng/g to 65 ng/g and 

methidathion oxon from approximately 15 ng/g to 150 ng/g (values estimated based on 

charts presented in the study report). The highest methidathion concentration was 

measured a week after spraying. Local spray drift is unlikely to be the cause because the 

pots had been removed before spraying and were not returned for 48 hours, although 

spray drift from other fields may be a possibility if they were treated at the same time. At 

this site, methidathion oxon was highest on September 18. At the intermediate site, there 

were no detections of methidathion, but methidathion oxon was detected at the three 

sampling events between July 11 and August 8 at levels from approximately 8 ng/g to 50 

ng/g. There were no detections of either compound on pine needles at the furthest site. 

3.3 Terrestrial Animal Exposure Assessment 

 3.3.1 Modeling Approach 

T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of methidathion 

for the CRLF and its potential prey (e.g. small mammals and terrestrial insects) 

inhabiting terrestrial areas. EECs used to represent the CRLF are also used to represent 

exposure values for frogs serving as potential prey of CRLF adults. T-REX simulates a 1-

year time period. For this assessment, spray applications of methidathion are considered, 

as discussed in below. 

Terrestrial EECs for foliar formulations of methidathion were derived for the uses 

summarized in Table 3.4. In an effort to be consistent with the aquatic exposure 

assessment, the same uses and rates were modeled for terrestrial exposure estimation (see 

Table 3.2). For terrestrial exposure estimation purposes, a half-life of 6.6 days was used 

as an input parameter based the 90% upper bound of the total foliar dissipation half-life 

59
values, 3.5 and 5.0 days, on alfalfa (Willis and McDowell, 1987). Use specific input 

values, including number of applications, application rate and application interval are 

provided in Table 3.4. An example output from T-REX is available in Appendix G. 

Table 3.4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs 

for Methidathion with T-REX 

Uses Application Rate 

(lbs a.i./A)

Number of Applications Per 

Crop Cycle 

Application Interval 

(Days) 

Artichoke 1 8 14 

Citrus 5 2 45 

Clover 1 2 7 (assumed) 

Cotton 1 4 5 

Kiwi 2 1 NA 

Nursery 10 1 NA 

Safflower, Sunflower 0.5 3 7 

Tree Fruit, Olives, 

Almonds 

3 1 NA 

Walnut 2 3 2

T-REX is also used to calculate EECs for terrestrial insects exposed to methidathion. 

Dietary-based EECs calculated by T-REX for small and large insects (units of a.i./g) are 

used to bound an estimate of exposure to terrestrial invertebrates. Available acute contact 

toxicity data for bees exposed to methidathion (in units of μg a.i./bee), are converted to 

μg a.i./g (of bee) by multiplying by 1 bee/0.128 g. The EECs are later compared to the 

adjusted acute contact toxicity data for bees in order to derive RQs. 

For modeling purposes, exposures of the CRLF to methidathion through contaminated 

food are estimated using the EECs for the small bird (20 g) which consumes small 

insects. Dietary-based and dose-based exposures of potential mammalian prey are 

assessed using the small mammal (15 g) which consumes short grass. Upper-bound 

Kenega nomogram values reported by T-REX for these two organism types are used for 

derivation of EECs for the CRLF and its potential prey (Table 3.5). Dietary-based EECs 

for small and large insects reported by T-REX as well as the resulting adjusted EECs are 

available in Table 3.6. An example output from T-REX v. 1.3.1 is available in Appendix 

G. 

60
Table 3.5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Methidathion 

EECs for CRLF EECs for Prey 

(small mammals) 

Use Dietary-based 

EEC (ppm) 

Dose-based EEC 

(mg/kg-bw) 

Dietary-based 

EEC (ppm) 

Dose-based EEC 

(mg/kg-bw) 

Artichoke 175 200 312 297 

Citrus 681 776 1211 1154 

Clover 200 227 355 339 

Cotton 290 330 516 492 

Kiwi 270 308 480 458 

Nursery 68 77 120 114 

Safflower, Sunflower 115 131 205 196 

Tree Fruit, Olive, Almonds 405 461 720 686 

Walnut 666 759 1184 1129 

Table 3.6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase 

CRLF via Effects to Terrestrial Invertebrate Prey Items 

Use Small Insect Large Insect 

Artichoke 175 19 

Citrus 681 76 

Clover 200 22 

Cotton 290 32 

Kiwi 270 30 

Nursery 68 8 

Safflower, Sunflower 115 13 

Tree Fruit, Olive, Almonds 405 45 

Walnut 666 74 

3.3.2 Field Studies 

In a field study on California citrus (Brewer et al., 1998; MRID 44806601), methidathion 

was applied to foliage by air blast spray equipment for one application at 10.0 lb ai/A. 

Immediately after application the residues on hulled millet seed ranged from 248 to 305 

(mean 276) ppm, and residues on crickets ranged from 22 to 59 (mean 40) ppm. T-REX 

model estimates – 150 ppm for seeds, 1350 ppm for small insects, and 150 ppm for large 

insects – are similar to actual field residues, and in the case of residues on seeds, the 

model may underestimate exposure. It is noted, however, that although it may 

underestimate potential risks to seed eaters, this dietary food item is not used in the 

assessment, and the foliage data did not suggest an underestimation of exposures. 

61
3.4 Terrestrial Plant Exposure Assessment 

Since there are no terrestrial plant toxicity data available, exposures were not 

quantitatively estimated. See Section 5.2 for a qualitative discussion regarding the 

potential effects of methidathion on CRLF via effects to terrestrial plants. 

4. Effects Assessment 

This assessment evaluates the potential for methidathion to directly or indirectly affect 

the CRLF or modify its designated critical habitat. As previously discussed in Section 

2.7, assessment endpoints for the CRLF effects determination include direct toxic effects 

on the survival, reproduction, and growth of CRLF, as well as indirect effects, such as 

reduction of the prey base or modification of its habitat. In addition, potential 

modification of critical habitat is assessed by evaluating effects to the PCEs, which are 

components of the critical habitat areas that provide essential life cycle needs of the 

CRLF. Direct effects to the aquatic-phase of the CRLF are based on toxicity information 

for freshwater fish, while terrestrial-phase effects are based on avian toxicity data, given 

that birds are generally used as a surrogate for terrestrial-phase amphibians. Because the 

frog’s prey items and habitat requirements are dependent on the availability of freshwater 

fish and invertebrates, small mammals, terrestrial invertebrates, and aquatic and 

terrestrial plants, toxicity information for these taxa are also discussed. Acute (shortterm) and chronic (long-term) toxicity information is characterized based on registrantsubmitted studies and a comprehensive review of the open literature on methidathion. 

As described in the Agency’s Overview Document (U.S. EPA, 2004), the most sensitive 

endpoint for each taxon is used for risk estimation. For this assessment, evaluated taxa 

include aquatic-phase amphibians, freshwater fish, freshwater invertebrates, aquatic 

plants, birds (surrogate for terrestrial-phase amphibians), mammals, terrestrial 

invertebrates, and terrestrial plants. 

Toxicity endpoints are established based on data generated 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 ECOTOX information obtained on 24 August 2008. 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; 

(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 evaluated along with the registrant-submitted 

data, and may be incorporated qualitatively or quantitatively into this endangered species 

62
assessment. In general, effects data in the open literature that are more conservative than 

the registrant-submitted data are considered. The degree to which open literature data are 

quantitatively or qualitatively characterized for the effects determination is dependent on 

whether the information is relevant to the assessment endpoints (i.e., maintenance of 

CRLF survival, reproduction, and growth) identified in Section 2.8. For example, 

endpoints such as behavior modifications are likely to be qualitatively evaluated, because 

quantitative relationships between modifications and reduction in species survival, 

reproduction, and/or growth are not available. Although the effects determination relies 

on endpoints that are relevant to the assessment endpoints of survival, growth, or 

reproduction, it is important to note that the full suite of sublethal endpoints potentially 

available in the effects literature (regardless of their significance to the assessment 

endpoints) are considered to define the action area for methidathion. 

Citations of all open literature not considered as part of this assessment because they 

were either rejected by the ECOTOX screen or accepted by ECOTOX but not used (e.g., 

the endpoint is less sensitive) are included in Appendix H. Appendix H also includes a 

rationale for rejection of those studies that did not pass the ECOTOX screen and those 

that were not evaluated as part of this endangered species risk assessment. 

A detailed spreadsheet of the available ECOTOX open literature data, including the full 

suite of lethal and sublethal endpoints is presented in Appendix I. Appendix I also 

includes a summary of the human health effects data for methidathion. 

In addition to registrant-submitted and open literature toxicity information, other sources 

of information, including use of the acute probit dose response relationship to establish 

the probability of an individual effect and reviews of the Ecological Incident Information 

System (EIIS), are conducted to further refine the characterization of potential ecological 

effects associated with exposure to methidathion. A summary of the available aquatic 

and terrestrial ecotoxicity information, use of the probit dose response relationship, and 

the incident information for methidathion are provided in Sections 4.1 through 4.4, 

respectively. 

There are no toxicity data available for methidathion oxon, a degradate of the parent 

methidathion; however, based on toxicity information for other organophosphate 

insecticides (e.g., diazinon, chlorpyrifos), methidathion oxon may be considerably more 

toxic than methidathion. The potential for additional risk to the CRLF from exposure to 

methidathion oxon as a transformation product of applied methidathion will be 

characterized in the risk description (Section 5.2). 

A detailed summary of the available ecotoxicity information for all methidathion 

degradates and formulated products is presented in Appendix A. 

4.1 Toxicity of Methidathion to Aquatic Organisms 

Table 4.1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of both the submitted studies and the open literature, as previously 

63
discussed. A brief summary of submitted and open literature data considered relevant to 

this ecological risk assessment for the CRLF is presented below. Additional information 

is provided in Appendix J. 

Table 4.1 Freshwater Aquatic Toxicity Profile for Methidathion

Assessment Endpoint Species Toxicity Value Used 

in Risk Assessment 

MRID Study 

Classification 

Acute Direct Toxicity to 

Aquatic-Phase CRLF 

Bluegill Sunfish 

Lepomis macrochirus LC50 = 2.2 μg/L 00011841 Supplemental 

Chronic Direct Toxicity 

to Aquatic-Phase CRLF Fathead minnow 

Pimephales promelas

NOAEC = 6.3 μg/L 

LOAEC = 12.0 μg/L 

post-hatch survival; 

growth 

00015735 Acceptable 

Indirect Toxicity to 

Aquatic-Phase CRLF via 

Acute Toxicity to 

Freshwater Invertebrates 

(i.e. prey items) 

Water Flea 

Daphnia magna LC50 = 3.0 μg/L 42081704 Acceptable 

Indirect Toxicity to 

Aquatic-Phase CRLF via 

Chronic Toxicity to 

Freshwater Invertebrates 

(i.e. prey items) 

Water Flea 

Daphnia magna

NOAEC = 0.66 μg/L 

LOAEC = 1.13 μg/L 

survival; number of 

young per female per 

reproductive day 

42081707 Acceptable 

Indirect Toxicity to 

Aquatic-Phase CRLF via 

Toxicity to Non-vascular 

Aquatic Plants 

No data available 

Indirect Toxicity to 

Aquatic-Phase CRLF via 

Toxicity to Vascular 

Aquatic Plants 

No data available 

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

4.2 (U.S. EPA, 2004). Toxicity categories for aquatic plants have not been defined. 

Table 4.2 Categories of Acute 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 Toxicity to Freshwater Fish 

Given that no methidathion toxicity data are available for aquatic-phase amphibians, 

freshwater fish data were used as a surrogate to estimate direct acute and chronic risks to 

the CRLF. Freshwater fish toxicity data were also used to assess potential indirect effects 

of methidathion to the CRLF. Effects to freshwater fish resulting from exposure to 

methidathion could indirectly affect the CRLF via reduction in available food. As 

64
discussed in Section 2.5.3, over 50% of the prey mass of the CRLF may consist of 

vertebrates such as mice, frogs, and fish (Hayes and Tennant, 1985). 

A summary of acute and chronic freshwater fish data, including data from the open 

literature, is provided below in Sections 4.1.1.1 through 4.1.1.3. 

 4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies 

Available data indicate that methidathion is very highly toxic on an acute basis to three 

surrogate freshwater fish species (Appendix J). The acute 96-hour LC50s for bluegill 

sunfish, rainbow trout, and goldfish range from 2.2 to 14 μg/L. The most sensitive 

endpoint, the bluegill sunfish 96-hour LC50 of 2.2 (0.9 – 5.1) μg a.i./L (MRID 00011841), 

will be used to calculate RQs for direct effects to the aquatic-phase CRLF. The probit 

dose-response slope is 2.9 (1.9 – 4.0) for this study. 

Two acute toxicity studies with the methidathion formulation 2E (25.2% a.i.) are also 

available for consideration in this risk assessment. These studies suggest that the tested 

formulation and technical grade methidathion are similarly toxic to bluegill sunfish and 

rainbow trout on an acute basis. 

 4.1.1.2 Freshwater Fish: Chronic Exposure 

(Growth/Reproduction) Studies 

Chronic fish toxicity data for methidathion are limited to one early life stage study with 

fathead minnow as the test organism (MRID 00015730 or 45822701) (Appendix J). The 

35-day flow-through study investigated the chronic effects following mean-measured 

concentration levels up to 12 μg ai/L. For survival, total length, and wet weight, the 

NOAEC, LOAEC, and EC50 were 6.3, 12, and >12 μg ai/L respectively. It is important 

to note that the chronic NOAEC of 6.3 μg/L for the fathead minnow is similar to the 

acute 96-hour LC50 of several other test species: rainbow trout (6.6 to 14 μg/L), bluegill 

sunfish (2.2 to 9 μg/L), and goldfish (6.8 μg/L). A chronic NOAEC for any of these other 

species would likely be lower than the available fathead minnow NOAEC. Therefore, 

instead of using the chronic NOAEC for fathead minnows, this assessment estimates a 

chronic NOAEC for rainbow trout using an acute to chronic ratio (ACR) calculation. 

Methidathion is an organophosphate insecticide. The EFED toxicity database, which 

contains the results from submitted studies that have been previously reviewed for 

scientific soundness, 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 (the species which would most likely have chronic data). Nineteen 

organophosphates were found that have both an acute and chronic study for rainbow 

trout. The ACRs ranged from 5.4 for Terbufos to 140 for Dichlorvos. Using this range of 

ACRs, the estimated chronic NOAECs for rainbow trout tested with methidathion would 

range from 0.07 to 1.9 ppb. In order to provide the most conservative estimate for the 

chronic freshwater fish NOAEC for methidathion, the ACR of 140 is utilized. Using that 

value and an LC50 of 10 ppb for rainbow trout tested with methidathion (MRID 

00011841), the estimated chronic NOAEC for rainbow trout is 0.07 ppb. 

65
The following section presents a modification (for methidathion) of the methodology 

used in deriving a freshwater fish ACR for the acephate CRLF assessment 

(http://www.epa.gov/espp/litstatus/effects/redleg-frog/acephate/analysis_acephate.pdf). 

Of the 19 organophosphates found to have both acute and chronic rainbow trout data, 12 

were evaluated for the ACR extrapolation Table 4.3. 

The estimated fish (aquatic phase amphibians) chronic NOAEC for methidathion is 

derived as follows. The (methidathion) rainbow trout LC50 used for the ACR calculation 

is 10 ppb ai. The largest acute-to-chronic ratio from the organophosphates is 140 for 

Dichlorvos. This ratio is used to calculate the final NOAEC for methidathion. 

ACR ratio for Dichlorvos: 0.75 ppm (acute LC50)/0.0052 ppm (chronic NOAEC) = 140 

Estimated NOAEC for methidathion = LC50 = 10 ppb = 140 

 NOAEC est. NOAEC 

Estimated Trout NOAEC for methidathion = 10/140 = 0.07 ppb ai 

The table below (4.3) shows the inputs for the organophosphates that were considered for 

the methidathion ACR. 

Acute to Chronic Table for Organophosphates 

Table 4.3. Methidathion Acute to Chronic Ratio for Rainbow Trout NOAEC 

Chemical

96-hr LC

50

(ppm ai)a MRIDs

NOAEC 

(ppm ai)a MRIDs ACRa

Methidathion 

NOAEC 

(ppb ai)a

Azinphos methyl 0.0088 03125193 0.00029 00145592 30 0.33 

Coumaphos 0.89 40098001 0.012 43066301 76 0.13 

Dichlorvos 0.75 43284702 0.0052 43788001 140 0.07 

Dimethoate 7.5 TN 1069b 0.43 43106303 17 0.59 

Disulfoton 1.9 40098001 0.22 41935801 8.4 1.2 

Fenamiphos 0.07 40799701 0.0038 41064301 18 0.56 

Fenitrothion 2.0 40098001 0.046 40891201 43 0.23 

Fenthion 0.83 40214201 0.0075 40564102 110 0.09 

Fonofos 0.05 00090820 0.0047 40375001 11 0.91 

Isofenphos 1.8 00096659 0.15 00126777 12 0.83 

Phosmet 0.11 40098001 0.0032 40938701 33 0.30 

Terbufos 0.0076 40098001 0.0014 41475801 5.4 1.9 a

 Rounded to two significant figures 

b

 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 the bluegill sunfish acute toxicity study (MRID 00011841) used to calculate RQs, it 

was reported that test organisms in the highest exposure group (1000 μg/L) exhibited 

slight spastic motions and swam on their sides. 

66
In the chronic fish early life stage study for methidathion (MRID 00015730 or 

45822701), clinical signs of toxicity during the definitive toxicity test were not provided 

in the raw data. Total length and wet weight were the most sensitive endpoints. There 

were statistically significant reductions detected for both endpoints at the highest 

measured treatment level (12 μg ai/L) relative to the pooled control. The NOAEC and 

LOAEC for both endpoints were 6.3 and 12 μg ai/L, respectively. 

There is one additional fish toxicity study available in the open literature for 

methidathion. The test organism, the common eel (Anguilla anguilla), appears to be 

about 3 orders of magnitude less sensitive than the bluegill sunfish, the surrogate 

organism used here to assess direct effects to the aquatic-phase CRLF. See Appendix I

for more details. 

4.1.2 Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of methidathion to the CRLF. Effects to freshwater invertebrates resulting from exposure 

to methidathion could indirectly affect the CRLF via reduction in available food items. 

As discussed in Section 2.5.3, the main food source for juvenile aquatic- and terrestrialphase CRLFs is thought to be aquatic invertebrates found along the shoreline and on the 

water surface, including aquatic sowbugs, larval alderflies and water striders. 

A summary of acute and chronic freshwater invertebrate data, including data published in 

the open literature, is provided below in Sections 4.1.2.1 through 4.1.2.3. 

4.1.2.1 Freshwater Invertebrates: Acute Exposure Studies 

Available data indicate that methidathion is very highly toxic on an acute basis to a 

surrogate freshwater invertebrate species, Daphnia magna (Appendix J). A daphnid 

acute toxicity study is available for methidathion technical grade active ingredient 

(TGAI) and for the methidathion formulation 2E (25.2% a.i.). The 48-hour LC50 for 

methidathion (TGAI) is 7.2 μg a.i./L, and the LC50 for methidathion formulation 2E 

(25.2% a.i.) is 11.9 μg product/L or 3.0 μg a.i./L. The most sensitive endpoint, 48-hour 

LC50 of 3.0 μg a.i./L (MRID 42081704), will be used to calculate RQs for indirect effects 

to the aquatic-phase CRLF. The probit dose-response slope is the default of 4.5 (2–9) for 

this study because there are less than two concentrations at which the percent dead is 

between 0 and 100. 

4.1.2.2 Freshwater Invertebrates: Chronic Exposure Studies 

Chronic toxicity data are available for a common freshwater zooplankton, Daphnia 

magna (Appendix J). In a 21-day lifecycle study (MRID 42081707), survivorship was 

reduced by 97% at 1.1 μg a.i./L; the NOAEC (mortality) was 0.66 μg a.i./L. This 

endpoint will be used to calculate RQs in this assessment. There were no significant 

effects of methidathion on reproduction or growth in the study.

67
4.1.2.3 Freshwater Invertebrates: Open Literature Data 

There are no freshwater invertebrate toxicity studies identified in the open literature for 

methidathion (Appendix H). 

4.1.3 Toxicity to Aquatic Plants 

There are no aquatic plant toxicity data for methidathion with which to assess the 

potential for indirect effects to the CRLF via effects on habitat, cover, and/or primary 

productivity or effects to the primary constituent elements (PCEs) relevant to the aquaticphase CRLF. 

4.2 Toxicity of Methidathion to Terrestrial Organisms 

Table 4.4 summarizes the most sensitive terrestrial toxicity endpoints for the CRLF, 

based on an evaluation of both the submitted studies and the open literature. A brief 

summary of submitted and open literature data considered relevant to this ecological risk 

assessment for the CRLF is presented below. 

Table 4.4 Terrestrial Toxicity Profile for Methidathion 

Endpoint Species Toxicity Value Used in 

Risk Assessment 

MRID Study 

Classification 

Acute Direct Toxicity 

to Terrestrial-Phase 

CRLF (LD50) 

Mallard duck 

Anas platyrhynchos

LD50 = 6.7 mg/kg 00159201 Supplemental 

Acute Direct Toxicity 

to Terrestrial-Phase 

CRLF (LC50) 

Bobwhite quail 

Colinus virginianus

LC50 = 224 ppm 42081701 Acceptable 

Chronic Direct Toxicity 

to Terrestrial-Phase 

CRLF 

Mallard Duck 

Anas platyrhynchos

NOAEC = 10 ppm 

LOAEC = 30 ppm 

Number of normal 

hatchlings/live 3-week 

embryos 

44381602 Acceptable 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via acute toxicity to 

mammalian prey items)

Rat 

Rattus norvegicus LD50 = 12 mg/kg 

00012714 Acceptable 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via chronic toxicity to 

mammalian prey items)

Rat 

Rattus norvegicus

NOAEL = 5 ppm 

LOAEL = 25 ppm 

Decreased mating index, 

decreased pup weight, 

tremors, decreased food 

consumption during 

lactation, transient 

decrease in body weight 

40079812 

40079813

Acceptable 

68
Table 4.4 Terrestrial Toxicity Profile for Methidathion 

Endpoint Species Toxicity Value Used in MRID 

Risk Assessment 

Study 

Classification 

(males and females) 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via acute toxicity to 

terrestrial invertebrate 

prey items)

Honey Bee 

Apis mellifera LD50 = 0.236 μg a.i./bee 0036935 

Acceptable 

Seedling Emergence

Monocots 

Seedling Emergence

Dicots 

Vegetative Vigor

Monocots 

Indirect Toxicity to 

Terrestrial- and 

Aquatic-Phase CRLF 

(via toxicity to 

terrestrial plants) 

Vegetative Vigor

No data available 

Dicots 

Acute toxicity to terrestrial animals is categorized using the classification system shown 

in Table 4.5 (U.S. EPA, 2004). Toxicity categories for terrestrial plants have not been 

defined. 

Table 4.5 Categories of Acute Toxicity for Avian and Mammalian Studies 

Toxicity Category Oral LD50 Dietary LC50

Very highly toxic < 10 mg/kg < 50 ppm 

Highly toxic 10 - 50 mg/kg 50 - 500 ppm 

Moderately toxic 51 - 500 mg/kg 501 - 1000 ppm 

Slightly toxic 501 - 2000 mg/kg 1001 - 5000 ppm 

Practically non-toxic > 2000 mg/kg > 5000 ppm 

4.2.1 Toxicity to Birds 

As specified in the Overview Document, the Agency uses birds as a surrogate for 

terrestrial-phase amphibians when amphibian toxicity data are not available (U.S. EPA, 

2004). No terrestrial-phase amphibian data are available for methidathion; therefore, 

acute and chronic avian toxicity data are used to assess the potential direct effects of 

methidathion to terrestrial-phase CRLFs. 

4.2.1.1 Birds: Acute Exposure (Mortality) Studies 

Based on acute oral toxicity studies for several bird species, methidathion is categorized 

as moderately to very highly toxic to birds (Appendix J). The mallard duck was the most 

sensitive of the species tested, with an 8-day LD50 of 6.7 (5.4 – 8.4) mg/kg methidathion 

(MRID 00159201). This endpoint will be used for risk estimation in this assessment. 

These data do not fit the probit dose-response model; thus, a default slope of 4.5 (2-9) 

69
will be used to calculate the probability of individual effects to the terrestrial-phase 

CRLF. 

Avian subacute dietary studies are available for methidathion (TGAI) and a formulation 

(40% a.i.) (Appendix J). Based on the available information, methidathion and the tested 

formulation appear to be similarly toxic to birds on a subacute dietary basis. The 

bobwhite quail (Colinus virginianus) was the most sensitive of the species tested, with an 

8-day LC50 of 224 (177 – 281) ppm a.i. (MRID 42081701). This endpoint will be used 

for risk estimation in this assessment. The probit dose-response slope was 8.7 (3.5 – 

13.8). 

4.2.1.2 Birds: Chronic Exposure (Growth, Reproduction) Studies 

Several avian chronic toxicity studies are available for methidathion. The mallard duck 

study (MRID 44381602) reported the most sensitive chronic toxicity endpoint, which will 

be used for risk estimation. In this study, the one-generation reproductive toxicity of 

methidathion TGAI to 6-month-old mallard ducks was assessed over 140 days. 

Methidathion TGAI was administered to the birds in the diet at 0, 1, 10, and 30 ppm diet. 

The number of normal hatchlings/live 3-week embryos was significantly reduced (a ratio 

of 0.5 versus 0.61 or 18% less than the control ratio) at the highest treatment, 30 ppm 

(LOAEC); the NOAEC for this effect is 10 ppm. A significant increase in the number of 

eggs cracked was observed when compared to the control group at the 10 ppm dietary 

level. In addition, a decrease in the number of eggs not cracked/eggs laid was observed 

at the same dietary level (i.e., NOAEC = 1 ppm); however, this is not a relevant 

assessment endpoint for the CRLF because endpoints having to do with egg cracking are 

not relevant to frogs, whose eggs do not have shells. There were no apparent behavioral 

abnormalities or other treatment-related signs of toxicity on the parental generation. 

 4.2.1.3 Birds: Sublethal Effects and Additional Open Literature 

Information 

In the mallard duck acute oral toxicity study that was used to calculate RQs (MRID 

00159201), toxic symptoms included depression, reduced reaction to external stimuli, 

wing droop, convulsions, and salivation. In the bobwhite quail subacute dietary study 

used for risk estimation, toxic symptoms including depression, reduced reaction to 

external stimuli, wing droop, loss of coordination, lower limb weakness, ruffled 

appearance, prostrate posture, and loss of righting reflex were observed at and above 178 

ppm a.i., and body weight gain and food consumption were reduced at levels above 316 

ppm a.i. 

In the mallard duck chronic toxicity study (MRID 44381602) used for risk estimation, 

methidathion technical was administered to the birds in the diet at 0 (control), 1, 10, and 

30 ppm diet. None of the ducks showed symptoms of toxicity or behavioral abnormalities 

during the experiment. The number of eggs cracked was significantly increased in the 10 

ppm (150%) and 30 ppm (86%) exposure concentrations. The number of eggs not 

70
cracked/eggs laid was also significantly reduced in the 10 ppm and 30 ppm treatment 

groups. The number of normal hatchlings/live 3-week embryos was significantly reduced 

(5%) at the highest treatment, 30 ppm. Body weight of 14-day-old survivors was 

reduced (6%) at 10 ppm, but not at the higher treatment levels. Brain cholinesterase 

activity was significantly reduced in drakes at the 10 ppm treatment level during weeks 2, 

10, and 20, and at the 30 ppm level during weeks 10 and 20. During week 20, hens 

showed a trend toward reduced brain cholinesterase activity at the 10 ppm and 30 ppm 

treatment levels. 

Additional avian acute toxicity studies for methidathion were identified in the open 

literature (Appendices H and I). However, none of the open literature studies resulted in 

a more sensitive acute toxicity threshold than the 8-day mallard duck LD50 of 6.7 mg/kg 

(MRID 00159201). 

4.2.2 Toxicity to Mammals 

Mammalian toxicity data are used to assess potential indirect effects of methidathion to 

the terrestrial-phase CRLF. Effects to small mammals resulting from exposure to 

methidathion could also indirectly affect the CRLF via reduction in available food. As 

discussed in Section 2.5.3, over 50% of the prey mass of the CRLF may consist of 

vertebrates such as mice, frogs, and fish (Hayes and Tennant, 1985). 

The study used to expand the initial area of concern to define the action area examined 

the effects of subchronic methidathion administration on vascular wall damage (Yavuz et 

al., 2005; ECOTOX ref. 80451). Methidathion was administered by gavage for 5 days a 

week for 4 weeks at a dose level of 5 mg/kg/day. The levels of malondialdehyde (MDA), 

a biomarker for oxidative stress, were determined in the vascular tissue. 

Histopathological examination was performed in the thoracic aortic tissue. The level of 

MDA was significantly higher, and cholinesterase activity was significantly lower in the 

methidathion group (p < 0.01). Subchronic methidathion administration led to irregular, 

prominent breaks and fragmentation of the elastic fibers in the aortic wall. Aortic lesions 

such as these are similar to the precursor lesion of a human aortic aneurysm. Since only 

one treatment level was tested, the LOAEC is 5 mg/kg/day, and a NOAEC was not 

defined. Therefore, a threshold for this effect could not be defined, and thus, this study 

was used to define the action area as the entire state of California for this assessment. 

4.2.2.1 Mammals: Acute Exposure (Mortality) Studies 

Available acute toxicity information suggests that methidathion is very highly toxic 

(Category I) to small mammals on an acute oral basis (Appendices J and K). The most 

sensitive endpoint, the acute rat (weanling) LD50 of 12 mg/kg, will be used to estimate 

risk to the CRLF via indirect effects to mammals. The probit dose-response slope is 

assumed to be 4.5 (2–9) for this study because individual animal data are not readily 

available for this study. 

4.2.2.2 Mammals: Chronic Exposure (Growth, Reproduction) Studies 

71
Chronic mammalian reproduction toxicity studies are available for methidathion (Table 

9). In a 2-generation reproduction study in rats (MRID 40079812, -13), rats were treated 

with 0 (control), 5, 25, or 50 ppm methidathion in the diet. The parental systemic 

NOAEC was 5 ppm and the LOAEC was 25 ppm, based on tremors and decreased food 

consumption during lactation, and decreased ovarian weight. In addition, there was also a 

slight decrease in body weight early in the F1 growth phase at 50 ppm. The reproductive 

NOAEC was 5 ppm and the LOAEC was 25 ppm based on a decreased mating index and 

a generalized indication of pup unthriftyness while nursing (e.g., decreased pup weight 

and an increased incidence of hypothermia with an appearance of starvation). In addition, 

there was an increase in stillbirths and decreased pup survival at birth and during 

lactation at the 50 ppm treatment level. An NOAEL of 5 ppm will be used for risk 

estimation in this assessment. 

4.2.3 Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of 

methidathion to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting 

from exposure to methidathion could also indirectly affect the CRLF via reduction in 

available food. 

4.2.3.1 Terrestrial Invertebrates: Acute Exposure (Mortality) Studies 

Methidathion is classified as very highly toxic to bees, with an acute contact LD50 of 

0.236 μg/bee or 1.84 ppm (MRID 00036935). This endpoint will be used to 

quantitatively assess the risk to the CRLF via indirect effects to terrestrial invertebrates. 

The probit dose-response slope from this study was 9.06. 

In addition, a residual toxicity study indicates that the RT25 (i.e., the residual time to kill 

25% of the tested population) is greater than 3 days when methidathion (Supracide 2E, 

25.2% a.i.) is applied at a rate of 5 lbs a.i./A (MRID 42081708). That is, in addition to 

very high acute contact toxicity, field weathered spray residues of methidathion can result 

in significant honey bee mortality for more than several days. 

4.2.3.2 Terrestrial Invertebrates: Open Literature Studies 

There are several terrestrial invertebrate toxicity studies available in the open literature 

(Appendices H and I). However, none of these studies identified a more sensitive 

endpoint than the submitted honey bee toxicity study that determined an acute contact 

LD50 of 0.236 μg/bee or 1.84 ppm (MRID 00036935). 

4.2.4 Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for methidathion to affect 

riparian zone and upland vegetation within the action area for the CRLF. Impacts to 

riparian and upland (i.e., grassland, woodland) vegetation could result in indirect effects 

72
to both aquatic- and terrestrial-phase CRLFs, as well as modification to designated 

critical habitat PCEs via increased sedimentation, alteration in water quality, and 

reduction in of upland and riparian habitat that provides shelter, foraging, predator 

avoidance and dispersal for juvenile and adult CRLFs. 

There are no registrant-submitted terrestrial plant toxicity data for methidathion with 

which to assess the potential for indirect effects to the aquatic- and terrestrial-phase 

CRLF via effects to riparian vegetation or effects to the primary constituent elements 

(PCEs) relevant to the aquatic- and terrestrial-phase CRLF. However, there is limited 

evidence in the open literature that methidathion has the potential to elicit phototoxic 

effects: Godfrey and Holtzer (1992; Ecotox ref. 64451) reported that methidathion 

significantly affected corn photosynthesis when applied at 0.5 lbs a.i./A, a rate that is 

lower than many of the currently registered rates. In addition, potential phytotoxic effects 

have been highlighted on methidathion labels; e.g., EPA SLN No. TX-050003 warns that 

methidathion application may result in phytotoxic effects, such as spotting, reddening, or 

chlorosis of the leaves, in certain sorghum varieties. In the absence of vegetative vigor 

and seedling emergence toxicity data, the potential risk to the CRLF via indirect effects 

to terrestrial plants is described in a qualitative manner (Section 5.2). 

4.3 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 methidathion 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 

measures of effect for each taxonomic group that is relevant to this assessment. 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. 

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. 

4.4 Incident Database Review 

73
A review of the EIIS database for ecological incidents involving methidathion was 

completed on 08 September 2008. A total of two aquatic and six terrestrial incidents are 

reported, all of which occurred in California. The reported aquatic incidents occurred in 

1976 and 2002; all of the reported terrestrial incidents occurred in 1994 and 1995. A 

brief description of each of the reported incidents is provided below. A complete list of 

the incidents involving methidathion is included as Appendix L. 

4.4.1 Terrestrial Incidents 

On January 18, 1994, one red-tailed hawk was found dead in Colusa County, California 

following application of methidathion to an unknown treatment site. No residue analysis 

was completed. The certainty index for this incident (I003351-003) is probable. 

Following application of methidathion to an unknown treatment site, a red-tailed hawk 

was found dead on January 27, 1994, in Stanislaus County, California. Blood and 

footwash sample were collected and sent to the PIU (Department of Fish and Game 

Pesticides Investigation Unit (California)) for examination. Blood plasma cholinesterase 

and acetlycholinesterase levels were within the normal range. The results of the footwash 

showed detections of methidathion at 2.7 ppb. In addition, organophosphates including 

chlorpyrifos and diazinon were also detected in the footwash sample at respective 

concentrations of 0.2 ppb and 4.1 ppb. It is likely that the hawk was exposed by perching 

on substrates which had recently been coated with the detected pesticides. The certainty 

index for this incident (I005042-012) is probable. 

On February 3, 1994, a red-tailed hawk was found entangled in a fence near an almond 

orchard in Merced County. The hawk was handed over to the Stanislaus Wildlife Care 

Center. Upon admission to the care center, blood and footwash samples were collected. 

The hawk was released following recovery from the injuries. Samples were sent to the 

PIU for examination. Plasma cholinesterase and acetylcholinesterase levels were found 

significantly below the normal range. Footwash analysis results show that methidathion 

was detected at a concentration of 2.7 ppb. Chlorpyrifos and diazinon were also detected 

in the footwash sample at respective concentrations of 0.7 ppb and 0.4 ppb. It appears 

that the hawk was affected due to exposure via perching on substrates, such as tree 

branches, which had recently been treated with these organophosphate pesticides. The 

certainty index for this incident (I05042-010) is highly probable. 

A red-tailed hawk was recovered from an orchard on December 23, 1994 and taken to the 

Stanislaus Wildlife Care Center. The bird died one day later. A blood sample and the 

carcass were sent to the PIU for examination. The plasma cholinesterase level was low 

and the brain cholinesterase level was severely depressed. Feathers and footskin samples 

were analyzed. Methidathion was detected at a concentration of 0.7 ppm in the footskin. 

In addition, chloropyrifos and diazinon were detected in both the feathers and footskin at 

concentrations ranging from 0.02 to 0.08 ppb. It is likely that all three organophosphates 

contributed to the death of this bird. The certainty index for this incident (I005042-015) 

is possible.

 

74
Following spraying of plum trees in Madera County, California with methidathion on 

Feb. 4, 1995 and spraying of a nearby location on the 5th and 6th, pollination hives were 

placed in the orchard on those same dates. Heavy fog was present the 4th and 5th, which 

limited foraging activity of the bees. Dead bees (number not specified) were found 

beginning Feb. 8 and continuing until Feb. 13. Residue analysis of the dead bees was 

completed, showing the presence of 2.6 ppm methidathion and 0.02 ppm diazinon. The 

Madera County authorities did not indicate any fault or liability, and attributed the bee 

kill to poor weather and too early an introduction of the hives. The certainty index for 

this incident (I001920-001) is possible.

The most recent terrestrial incident involving methidathion was reported on March 15, 

1995. Following application of methidathion to an agricultural area, an unknown number 

of bees were killed in an unknown county in California. The incident was reported as 

accidental under the misuse category. No further information was provided in the 

incident report. No residue analysis was completed, and the certainty index for this 

incident (I005895-512) is probable.

4.4.2 Aquatic Incidents 

On May 25, 1976, approximately 3,000 fish of unknown species were killed following an 

aerial application of methidathion to an agricultural area in Sacramento County, 

California. Methidathion had been aerially sprayed on a nearby seed clover field, and 

irrigation water from the field had entered Laguna Creek in the area directly upstream of 

the fish kill. The incident was attributed to accidental misuse of methidathion. Samples 

of the water collected from the field drain six days after the material was applied to the 

field showed methidathion concentrations of 60 ppb. It is presumed that the 

concentration of methidathion would have been much higher if the samples had been 

collected immediately following aerial application of the field. No fish tissue analysis 

was completed. The certainty index for this incident (B0000-218-10) is highly probable. 

5. Risk Characterization 

Risk characterization is the integration of the exposure and effects characterizations. 

Risk characterization is used to determine the potential for direct and/or indirect effects to 

the CRLF or for modification to its designated critical habitat from the use of 

methidathion in CA. The risk characterization provides an estimation (Section 5.1) and a 

description (Section 5.2) of the likelihood of adverse effects; articulates risk assessment 

assumptions, limitations, and uncertainties; and synthesizes an overall conclusion 

regarding the likelihood of adverse effects to the CRLF or its designated critical habitat 

(i.e., “no effect,” “likely to adversely affect,” or “may affect, but not likely to adversely 

affect”). 

5.1 Risk Estimation 

Risk is estimated by calculating the ratio of exposure to toxicity. This ratio is the risk 

quotient (RQ), which is then compared to pre-established acute and chronic levels of 

75
concern (LOCs) for each category evaluated (Appendix E). For acute exposures to the 

CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC 

is 0.05. For acute exposures to the CRLF and mammals, the LOC is 0.1. The LOC for 

chronic exposures to CRLF and its prey, as well as acute exposures to plants is 1.0. 

Risk to the aquatic-phase CRLF is estimated by calculating the ratio of exposure to 

toxicity using 1-in-10 year EECs based on the label-recommended methidathion usage 

scenarios summarized in Table 3.2 and the appropriate aquatic toxicity endpoint from 

Table 4.1. Risks to the terrestrial-phase CRLF and its prey (e.g. terrestrial insects, small 

mammals and terrestrial-phase frogs) are estimated based on exposures resulting from 

applications of methidathion (Tables 3.5 through 3.6) and the appropriate toxicity 

endpoint from Table 4.3. Due to lack of toxicity data, exposures are not derived for 

terrestrial plants. 

The minor degradate methidathion oxon is of toxicological concern, but because there are 

no toxicity data, this degradate cannot be included directly in calculations of RQs. 

Potential risk from methidathion oxon is characterized in the risk description (Section 

5.2). 

5.1.1 Exposures in the Aquatic Habitat 

5.1.1.1 Direct Effects to Aquatic-Phase CRLF 

Direct effects acute RQs for the aquatic-phase CRLF are presented in Table 5.1. Based 

on 1-in-10 year peak aquatic EECs from the PRZM/EXAMS model and the lowest acute 

96-hour LC50 for freshwater fish (surrogate for the aquatic-phase CRLF), acute RQs for 

the aquatic-phase CRLF exceed the listed species LOC (0.05) for all of the assessed 

methidathion uses. Acute RQs for methidathion range from 2.1 for the alfalfa scenario to 

52.5 for the nursery scenario. As a note, for the spatial analyses sections, the lowest and 

highest EECs were estimated following a single application. The lowest acute RQ for a 

single application is calculated to be 0.20 for the lowest use scenario, on mangoes. 

Therefore, even after a single application, the acute listed species LOC would be 

exceeded for all of the methidathion use scenarios. 

76
Table 5.1 Summary of Acute Direct Effect RQs for the Aquatic-phase CRLFa

Scenario Crops Represented 

Peak 1-in-10 

Year EEC 

(μg/L)b

Acute 

RQc

Probability of 

Individual Effect at 

LOCd

Probability of 

Individual Effect 

at RQd

Alfalfa Alfalfa, Clover, 

Timothy 4.6 2.1 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1.2 

Almond Almond, Walnut 49.3 22.4 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Citrus 

Calemondin, Citron, 

Citrus, Grapefruit, 

Kumquat, Lemon, 

Lime, Mango, 

Pummelo, Tangelo, 

Tangerine 

18.3 8.3 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Corn Sunflower 20.7 9.4 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Cotton Cotton 13.0 5.9 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Grapes Kiwi 6.6 3.0 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1.1 

Tree Fruit 

Apple, Apricot, 

Cherry, Nectarine, 

Peach, Pear, Plum, 

Prune 

15.6 7.1 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Olive Olive 15.2 6.9 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Row Crop Artichoke 16.0 7.3 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Wheat Safflower 13.6 6.2 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

Nursery Nursery 115.6 52.5 

~1 in 12,400 

(~1 in 10,300,000 to 

~1 in 149) 

~1 in 1 

a

 Based on bluegill sunfish (Lepomis macrochirus) acute 96-hour LC50 = 2.2 μg/L b

 See Table 3.3. c

 Acute RQs that exceed the acute endangered species LOC of 0.05 are in bold. d

 The effect probability was calculated based on a probit dose-response slope of 2.9 (1.9 – 4.0) for the bluegill sunfish acute toxicity 

study. 

Direct chronic risk estimates for the aquatic-phase CRLF are based on the 60-day average 

EECs from the PRZM/EXAMS model and the lowest chronic NOAEC for freshwater 

fish (surrogate for the aquatic-phase CRLF). Using the estimated chronic NOAEC of 

0.07 ppb based on an ACR of 140 and the acute LC50 value of 10 ppb, the chronic RQs 

exceed the LOC (1.0) for all of the assessed methidathion uses (Table 5.2). Based on 

77
acute and chronic LOC exceedances, methidathion may affect the aquatic-phase of the 

CRLF. 

Table 5.2 Summary of Chronic Direct Effect RQs for the Aquatic-phase CRLFa

Scenario Crops Represented 60-day EEC (μg/L)b Chronic RQc

Alfalfa Alfalfa, Clover, Timothy 2.5 35.7 

Almond Almond, Walnut 26.8 383 

Citrus Calemondin, Citron, Citrus, Grapefruit, 

Kumquat, Lemon, Lime, Mango, Pummelo, 

Tangelo, Tangerine 

9.0 129 

Corn Sunflower 10.9 156 

Cotton Cotton 6.4 91.4 

Grapes Kiwi 3.2 45.7 

Tree Fruit Apple, Apricot, Cherry, Nectarine, Peach, Pear, 

Plum, Prune 5.0 71.4 

Olive Olive 7.6 109 

Row Crop Artichoke 8.7 124 

Wheat Safflower 6.6 94.3 

Nursery Nursery 54.3 776 a

 Based on estimated chronic NOAEC of 0.07 μg/L from ACR ratio (see Section 4.1.1.2).

b

 See Table 3.3. c

 Chronic RQs that exceed the chronic LOC of 1.0 are in bold. 

5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey 

(non-vascular aquatic plants, aquatic invertebrates, fish, and frogs) 

Non-vascular Aquatic Plants

Indirect effects of methidathion to the aquatic-phase CRLF (tadpoles) via reduction in 

non-vascular aquatic plants in its diet cannot be quantitatively estimated because there are 

no aquatic plant toxicity data available for methidathion. For a qualitative risk 

description, see Section 5.2.2.1. 

Aquatic Invertebrates

Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in 

aquatic habitats are based on peak 1-in-10 year EECs from the PRZM/EXAMS model 

and the lowest 48-hour LC50 for freshwater invertebrates. These acute RQs for aquatic 

invertebrates range from 1.5 to 38.5, and thus exceed the acute risk LOC for non-listed 

species (0.5) (Table 5.3). 

78
Table 5.3 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via 

Direct Effects on Aquatic Invertebrates as Dietary Food Items 

Scenario Crops Represented Peak 1-in-10 Year 

EEC (μg/L)b

Indirect Effects 

Acute RQc

% Expected Effect on 

Prey Population at RQd

Alfalfa Alfalfa, Clover, 

Timothy 4.6 1.5 79 

Almond Almond, Walnut 49.3 16.4 100 

Citrus 

Calemondin, Citron, 

Citrus, Grapefruit, 

Kumquat, Lemon, 

Lime, Mango, 

Pummelo, Tangelo, 

Tangerine 

18.3 6.1 100 

Corn Sunflower 20.7 6.9 100 

Cotton Cotton 13.0 4.3 100 

Grapes Kiwi 6.6 2.2 94 

Tree Fruit 

Apple, Apricot, Cherry, 

Nectarine, Peach, Pear, 

Plum, Prune 

15.6 5.2 100 

Olive Olive 15.2 5.1 100 

Row Crop Artichoke 16.0 5.3 100 

Wheat Safflower 13.6 4.5 100 

Nursery Nursery 115.6 38.5 100 a

 Based on water flea (Daphnia magna) acute 48-hour LC50 = 3.0 μg/L b

 See Table 3.3. c

 Acute RQs that exceed the acute LOC for nonlisted species of 0.5 are in bold. d

 The acute daphnid toxicity study data do not fit the probit model; therefore, the effect probability was calculated based on a default slope 

assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). 

Risk estimates for indirect effects to the CRLF via chronic effects to aquatic invertebrates 

are based on 21-day average EECs and the lowest chronic NOAEC for freshwater 

invertebrates. Chronic RQs for freshwater invertebrates range from 3.4 to 128.4, and thus 

exceed the LOC (1.0) for all of the assessed methidathion uses (Table 5.4). Based on 

acute and chronic LOC exceedences for all of the assessed uses, methidathion may 

indirectly affect the CRLF via reduction in freshwater invertebrates prey items. 

79
Table 5.4 Summary of Chronic RQs Used to Estimate Indirect Effects to the CRLF via 

Direct Effects on Aquatic Invertebrates as Dietary Food Items 

Scenario Crops Represented 21-day EEC (μg/L)b Indirect Effects 

Chronic RQc

Alfalfa Alfalfa, Clover, Timothy 3.4 5.2 

Almond Almond, Walnut 40.2 60.9 

Citrus Calemondin, Citron, Citrus, Grapefruit, 

Kumquat, Lemon, Lime, Mango, Pummelo, 

Tangelo, Tangerine 

13.6 20.6 

Corn Sunflower 16.2 24.5 

Cotton Cotton 9.3 14.1 

Grapes Kiwi 5.0 7.6 

Tree Fruit Apple, Apricot, Cherry, Nectarine, Peach, Pear, 

Plum, Prune 9.2 13.9 

Olive Olive 11.7 17.7 

Row Crop Artichoke 12.1 18.3 

Wheat Safflower 10.4 15.8 

Nursery Nursery 84.8 128.4 a

 Based on water flea (Daphnia magna) chronic NOAEC = 0.66 μg/L 

b

 See Table 3.3. c

 Chronic RQs that exceed the chronic LOC of 1.0 are in bold. 

Fish and Frogs

Fish and frogs also represent potential prey items of adult aquatic-phase CRLFs. RQs 

associated with acute and chronic direct toxicity to the CRLF (Tables 5.1 - 5.2) are used 

to assess potential indirect effects to the CRLF based on a reduction in freshwater fish 

and frogs as food items. Since the acute and chronic RQs exceed the LOC, methidathion 

may indirectly affect the CRLF via reduction in freshwater fish and frogs as food items. 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or 

Primary Productivity (Freshwater Aquatic Plants) 

Indirect effects to the CRLF via direct toxicity to aquatic plants cannot be quantitatively 

estimated because there are no aquatic plant toxicity data available for methidathion. For 

a qualitative risk description, see Section 5.2.3.1. 

5.1.2 Exposures in the Terrestrial Habitat 

5.1.2.1 Direct Effects to Terrestrial-phase CRLF 

As previously discussed in Section 3.3, potential direct effects to terrestrial-phase CRLFs 

are based on foliar applications of methidathion. Potential direct acute effects to the 

terrestrial-phase CRLF are derived by considering dose- and dietary-based EECs 

modeled in T-REX for a small bird (20 g) consuming small invertebrates (Table 3.5) and 

acute oral and subacute dietary toxicity endpoints for avian species. Potential direct 

chronic effects of methidathion to the terrestrial-phase CRLF are derived by considering 

80
dietary-based exposures modeled in T-REX for a small bird (20g) consuming small 

invertebrates. Chronic effects are estimated using the lowest available toxicity data for 

birds. EECs are divided by toxicity values to estimate chronic dietary-based RQs. Acute 

dose-based, acute dietary-based, and chronic RQs exceed the LOCs for all of the assessed 

methidathion uses (Table 5.5-5.7). As a result, methidathion may affect the terrestrialphase of the CRLF. 

Table 5.5. Summary of Direct Effect Acute Dose-Based RQs for the Terrestrialphase CRLF for Spray Applications of Methidathion

Broadleaf Plants/ 

Small Insects 

Use 

App. Rate (lb ai/A) 

Number of Apps. 

App. Interval (days) 

Probability of 

Individual 

Effect at LOCb

Acute DoseBased RQa

Probability of 

Individual 

Effect at RQb

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

57.39 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

222.94 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

65.39 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

94.95 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

88.39 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Mango 

0.25 lb a.i./acre 

5 apps. 

21 days 

12.42 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Nursery 

10 lb a.i./A 

1 app 

NA 

441.97 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Safflower, Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

37.77 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Tree Fruit, Olives 

3 lb a.i./acre 

1 app 

NA 

132.59 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

Walnut 

2 lb a.i./acre 

3 apps. 

2 days 

218.11 

~1 in 294,000 

(~1 in 8.86E+18 

to ~1 in 44) 

~1 in 1 

a

 Based on mallard duck acute oral LD50 = 6.7 mg/kg b The acute data do not fit the probit model; therefore, the effect probability was calculated based on a default 

slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). c

Acute RQs that exceed the acute endangered species LOC of 0.1 are in bold. 

81
Table 5.6. Summary of Direct Effect Acute Dietary-Based RQs for the 

Terrestrial-phase CRLF for Spray Applications of Methidathion

Broadleaf Plants/ 

Small Insects 

Use 

App. Rate (lb ai/A) 

Number of Apps. Acute 

Dietary-Based 

RQa

Probability of 

Individual 

Effect at LOCb

App. Interval (days) 

Probability of 

Individual 

Effect at RQb

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

0.78 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 5 

(~1 in 14 

to ~1 in 2) 

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

3.04 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

0.89 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 3 

(~1 in 4 

to ~1 in 2) 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

1.29 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

1.21 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

Mango 

0.25 lb a.i./acre 

5 apps. 

21 days 0.17 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 9.28E+10 

(~1 in 8.3E+25 

to ~1 in 

2.83E+2) 

Nursery 

10 lb a.i./A 

1 app 

NA 

6.03 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

Safflower, Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

0.52 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 148 

(~1 in 22,500 

to ~1 in 6) 

Tree Fruit, Olives 

3 lb a.i./acre 

1 app 

NA 

1.81 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

Walnut 

2 lb a.i./acre 

3 apps. 

2 days 

2.97 

~1 in 6.03E+17 

(~1 in 7.85E+42 

to ~1 in 4,300) 

~1 in 1 

a

 Based on bobwhite quail acute dietary LC50 = 224 ppm b The effect probability was calculated using a probit dose-response slope of 8.7 (3.5 – 13.8) from the bobwhite quail 

study. 

c

Acute RQs that exceed the acute endangered species LOC of 0.1 are in bold.

82
Table 5.7. Summary of Direct Effect Chronic RQs for the Terrestrial-phase 

CRLF for Spray Applications of Methidathion

Broadleaf Plants/ 

Use Small Insects App. Rate (lb ai/A) 

Number of Apps. 

App. Interval (days) Chronic RQa

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

17.53b

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

68.10 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

19.97 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

29.00 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

27.00 

Nursery 

10 lb a.i./A 

1 app 

NA 

135.00 

Mango 

0.25 lb a.i./acre 

5 apps. 

21 days 

3.79 

Safflower, Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

11.54 

Tree Fruit, Olives 

3 lb a.i./acre 

1 app 

NA 

40.50 

Walnut 

2 lb a.i./acre 

3 apps. 66.62 

2 days a

 Based on mallard duck chronic NOAEC = 10 ppm b

Chronic RQs that exceed the chronic LOC of 1.0 are in bold.

5.1.2.2 Indirect Effects to Terrestrial-Phase CRLF via Reduction in 

Prey (terrestrial invertebrates, mammals, and frogs) 

5.1.2.2.1 Terrestrial Invertebrates 

In order to assess the risks of methidathion to terrestrial invertebrates, which are 

considered prey of CRLF in terrestrial habitats, the honey bee is used as a surrogate for 

terrestrial invertebrates. The toxicity value for terrestrial invertebrates is calculated by 

multiplying the lowest available acute contact LD50 of 0.236 μg/bee by 1 bee/0.128g, 

which is based on the weight of an adult honey bee. EECs (ppm) calculated by T-REX 

for small and large insects are divided by the calculated toxicity value for terrestrial 

invertebrates, which is 1.84 ppm (Table 5.8). Based on acute LOC exceedances for all of 

83
the assessed uses, methidathion may affect the CRLF via reduction in terrestrial 

invertebrate prey items. 

Table 5.8 Summary of RQs For Indirect Effects to the Terrestrial-phase CRLF via 

Direct Effects on Terrestrial Invertebrates as Dietary Food Items

Use 

App. Rate (lb ai/A) 

Number of Apps. 

App. Interval (days) 

Small 

Insect 

RQ 

Large 

Insect 

RQ 

Small Insect % 

Expected Effect on 

Prey Population at 

RQb

Large Insect % 

Expected Effect on 

Prey Population at 

RQb

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

95 10 100 100 

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

370 41 100 100 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

109 12 100 100 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

158 17 100 100 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

147 16 100 100 

Mangos 

0.25 lb a.i./acre 

5 apps 

21 days 

21 2 100 100 

Nursery 

10 lb a.i./A 

1 app 

NA 

734 82 100 100 

Safflower, 

Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

63 7 100 100 

Tree Fruit, 

Olives 

3 lb a.i./acre 

1 app 

NA 

220 24 100 100 

Walnut 

2 lb a.i./acre 

3 apps. 

2 days 

362 40 100 100 

a Based on honey bee LD50 of 0.236 μg/bee or 1.84 ppm b The % population was calculated using a probit dose-response slope of 9.06 from the honey bee study. c

Acute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.

5.1.2.2.2 Mammals 

Risks associated with ingestion of contaminated small mammals by large terrestrial-phase 

CRLFs are derived for dietary-based and dose-based exposures modeled in T-REX for a 

small mammal (15g) consuming short grass. Acute and chronic effects are estimated 

using the most sensitive mammalian toxicity data. EECs are divided by the toxicity value 

to estimate acute dose-based RQs as well as chronic dietary- and dose-based RQs. As 

shown in Tables 5.9-5.10, the acute and chronic RQs exceed the LOC for all of the 

84
assessed methidathion uses. As a result, methidathion may affect the CRLF via reduction 

in small mammal prey items. 

Table 5.9. Summary of Acute RQs For Indirect Effects to the Terrestrial-phase 

CRLF via Direct Effects on Small Mammals as Dietary Food Items

Use 

App. Rate (lb ai/A) 

Number of Apps. 

App. Interval (days) 

Acute Dose-Based 

RQa % Expected Effect on 

Prey Population at RQb

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

11.27 100 

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

43.76 100 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

12.84 100 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

18.64 100 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

17.35 100 

Mango 

0.25 lb a.i./A 

5 apps 

21 days 

2.44 96 

Nursery 

10 lb a.i./A 

1 app 

NA 

86.76 100 

Safflower, Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

7.41 100 

Tree Fruit, Olives 

3 lb a.i./acre 

1 app 

NA 

26.03 100 

Walnut 

2 lb a.i./acre 

3 apps. 

2 days 

42.82 100 

a Based on rat acute oral LD50 = 12 mg/kg-bw. b The effect probability was calculated based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 

and 9 (Urban and Cook, 1986). 

c

Acute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.

85
Table 5.10. Summary of Chronic RQs For Indirect Effects to the Terrestrial-phase 

CRLF via Direct Effects on Small Mammals as Dietary Food Items

Use 

App. Rate (lb ai/A) 

Number of Apps. 

App. Interval (days) 

Chronic Dose-Based 

RQa Chronic Dietary-Based 

RQb

Artichoke 

1 lb a.i./acre 

8 apps 

14 days 

540.74 62.33 

Citrus 

5 lb a.i./acre 

2 apps 

45 days 

2100.70 242.13 

Clover 

1 lb a.i./acre 

2 apps. 

7 days 

616.11 71.01 

Cotton 

1 lb a.i./acre 

4 apps 

5 days 

894.65 103.12 

Kiwi 

2 lb a.i./acre 

1 app 

NA 

832.90 96.00 

Mango 

0.25 lb a.i./A 

5 apps 

21 days 

117.00 13.49 

Nursery 

10 lb a.i./A 

1 app 

NA 

4164.5 480.00 

Safflower, Sunflower 

0.5 lb a.i./acre 

3 apps 

7 days 

355.92 41.02 

Tree Fruit, Olives 

3 lb a.i./acre 

1 app 

NA 

1249.35 144.00 

Walnut 

2 lb a.i./acre 

3 apps. 

2 days 

2055.21 236.88 

a Based on rat NOAEL = 0.25 mg/kg-bw. 

b Based on rat NOAEC = 5 mg/kg-diet. 

c

 Chronic RQs that exceed the chronic LOC of 1 are in bold.

5.1.2.2.3 Frogs

An additional prey item of the adult terrestrial-phase CRLF is other species of frogs. In 

order to assess risks to these organisms, dietary-based and dose-based exposures modeled 

in T-REX for a small bird (20g) consuming small invertebrates are used. See Section 

5.1.2.1 and associated tables (Tables 5.3 – 5.5) for results. Acute and chronic RQs 

exceed the LOCs for all of the assessed methidathion uses. As a result, methidathion may 

affect the CRLF via reduction in frogs as prey items. 

 

86
5.1.2.3 Indirect Effects to CRLF via Reduction in Terrestrial Plant 

Community (Riparian and Upland Habitat) 

Indirect effects to the CRLF via reduction in terrestrial plant community cannot be 

quantitatively estimated because there are no vegetative vigor or seedling emergence 

plant toxicity data available for methidathion. For a qualitative risk description, see 

Section 5.2.3.2. 

5.1.3 Primary Constituent Elements of Designated Critical Habitat 

For methidathion use, the assessment endpoints for designated critical habitat PCEs 

involve a reduction and/or modification of food sources necessary for normal growth and 

viability of aquatic-phase CRLFs, and/or a reduction and/or modification of food sources 

for terrestrial-phase juveniles and adults. Because these endpoints are also being 

assessed relative to the potential for indirect effects to aquatic- and terrestrial-phase 

CRLF, the effects determinations for indirect effects from the potential loss of food items 

are used as the basis of the effects determination for potential modification to designated 

critical habitat. 

5.1.3.1 Aquatic-Phase (Aquatic Breeding Habitat and Aquatic NonBreeding Habitat) 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

• 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. 

• 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. 

• Reduction and/or modification of aquatic-based food sources for pre-metamorphs 

(e.g., algae). 

Risk estimations for potential effects to aquatic and/or terrestrial plants were not 

conducted because no toxicity data are available for either aquatic or terrestrial plants. 

Therefore, it cannot be estimated whether or not methidathion is likely to affect aquaticphase PCEs of designated habitat related to effects on aquatic and/or terrestrial plants. 

Risks to aquatic and terrestrial plants will be discussed qualitatively in sections 5.2.2.1, 

5.2.3.2 and 5.2.3.2. 

The remaining aquatic-phase PCE is “alteration of other chemical characteristics 

necessary for normal growth and viability of CRLFs and their food source.” To assess 

the impact of methidathion on this PCE, acute and chronic freshwater fish and 

87
invertebrate toxicity endpoints, as well endpoints for aquatic non-vascular plants, are 

used as measures of effects. RQs for these endpoints were calculated in Sections 5.1.1.1 

and 5.1.1.2. Based on exceedances of the listed and non-listed species LOCs following 

both acute and chronic exposure for freshwater fish and invertebrates, methidathion is 

likely to affect aquatic-phase PCEs of designated habitat related to effects of alteration of 

other chemical characteristics necessary for normal growth and viability of CRLFs and 

their food source. As stated previously, there are no aquatic plant toxicity data available 

for methidathion. Therefore, effects to this aquatic-phase PCE based on effects to aquatic 

non-vascular plants will be discussed qualitatively in Section 5.2.2.1. 

5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat) 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• 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 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 

The risk estimation for terrestrial-phase PCEs of designated habitat related to potential 

effects on terrestrial plants cannot be quantitatively addressed because there are no 

vegetative vigor or seedling emergence plant toxicity data available for methidathion. 

The risk will be discussed qualitatively in Section 5.2.3.2. 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of methidation on this PCE, 

acute and chronic toxicity endpoints for birds, mammals, and terrestrial invertebrates are 

used as measures of effects. RQs for these endpoints were calculated in Sections 5.1.2.1 

and 5.1.2.2. Acute and chronic LOCs for listed and non-listed species were exceeded for 

all uses for birds and mammals and acute LOCs for listed and non-listed terrestrial 

invertebrates were also exceeded for all uses. Therefore, methidiathion is likely to affect 

the third terrestrial-phase PCE. 

The fourth terrestrial-phase PC is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.1.2.1. Acute and/or chronic LOCs for listed and non-listed species were 

exceeded for freshwater fish and invertebrates, birds, mammals and terrestrial 

invertebrates. No data are available for either aquatic or terrestrial plants. However, 

based on the results with aquatic and terrestrial animals, methidathion is likely to affect 

the fourth terrestrial - phase PCE. 

88
5.1.4 Spatial Extent of Potential Effects 

Since this screening level risk assessment defines taxa that are predicted to be exposed 

through runoff and drift to methidathion at concentrations above the Agency’s Levels of 

Concern (LOC), analysis of the potential spatial extent of effects requires expansion of 

the area from the treated site to include all areas potentially impacted by this federal 

action. Two methods are used to define these areas: (1) the spray drift assessment for 

determining the extent of terrestrial and aquatic habitats potentially affected through drift 

alone; and (2) the down stream dilution assessment for determining the extent of the 

affected lotic aquatic habitats (flowing water). This analysis is described briefly below 

and in more detail in Appendix D. 

An LAA effects determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of methidathion’s use pattern is identified, using 

corresponding land cover data. The spatial extent of the effects determination also 

includes areas beyond the initial area of concern that may be impacted by runoff and/or 

spray drift. The identified direct and indirect effects and modification to critical habitat 

are anticipated to occur only for those currently occupied core habitat areas, CNDDB 

occurrence sections and designated critical habitat for the CRLF that overlap with the 

initial area of concern plus greater than 1000 feet from its boundary. It is assumed that 

non-flowing waterbodies (or potential CRLF habitat) are included within this area. 

In addition to the spray drift buffer, the results of the downstream dilution extent analysis 

result in a distance of 285 kilometers which represents the maximum continuous distance 

of downstream dilution from the edge of the initial area of concern. If any of these 

streams reaches flow into CRLF habitat, there is potential to affect either the CRLF or 

modify its habitat. These lotic aquatic habitats within the CRLF core areas and critical 

habitats potentially contain concentrations of methidathion sufficient to result in LAA 

determination or modification of critical habitat. 

The determination of the buffer distance and downstream dilution for spatial extent of the 

effects determination is described below. 

 

5.1.4.1 Spray Drift 

In order to determine terrestrial and aquatic habitats of concern due to methidathion 

exposures through spray drift, it is necessary to estimate the distance that spray 

applications can drift from the treated area and still be present at concentrations that 

exceed levels of concern. A quantitative analysis of spray drift distances was completed 

using AgDrift (v. 2.01). 

Spatial analysis of spray drift effects is limited to consideration of a single application 

because, due to variable wind conditions, multiple applications are not likely to impact 

the same location each time. Spray drift distances depend on both application rate and 

89
method, and so, in order to understand the range of possible impacts, modeling was done 

for the uses with the highest single application rates for each method, which include tree 

fruit and nuts treated by aerial spray, nursery stock treated by ground spray, and citrus 

treated by spray that is presumed to be airblast. Use on mangos was modeled as well to 

represent the lower boundary of potential effects. 

Methidathion labels have specific application requirements in order to reduce potential 

spray drift, including restrictions on wind speed, release height and droplet size. AgDrift 

inputs are based on these requirements, as presented in Table 5.11. For inputs not 

specified on the labels, default values are used. When Tier I models were used, it is 

because no higher tier models are available. 

Table 5.11. Input parameters for simulation of methidathion in spray drift using 

AgDrift (v. 2.01) 

Parameter Description Tree Fruit / 

Nuts 

Nursery Stock Citrus Mangos 

Single Application Rate 3 

lb a.i./A 

10 

lb a.i./A 

5 

lb a.i./A 

0.5 

lb a.i./A 

Application method Aerial Ground Airblast Airblast 

Droplet Size 

Distribution (DSD) 

Medium to 

Coarse 

Fine to 

Medium 1 NA NA 

Release height NA High Boom 2 NA NA 

Orchard type NA NA Dense Orchard Dense Orchard 

1

 Labels require a medium to coarse DSD, but AgDrift does not include this as an option for Tier I Ground mode. 2

 Labels require a release height for ground spray applications of less than 4 feet above the crop canopy. AgDrift does 

not use specific release heights, so modeling for this use is based high boom spray, the option with the highest release 

height, 4 ft (1.27 m). 

The terrestrial analysis is based on the honey bee acute contact LD50 of 0.236 μg/bee, or 

1.84 ppm, which is the most sensitive terrestrial endpoint and is used as a surrogate for 

terrestrial invertebrates. Based on this endpoint, the initial average deposition level that 

would lead to exceedance of the terrestrial invertebrate LOC (0.05) is 0.00068 lb a.i./A, 

where the deposition is calculated as the RQ divided by the LOC and multiplied by the 

application rate. The most sensitive endpoint upon which the aquatic analysis is based is 

the bluegill sunfish 96-hour acute LC50 of 2.2 μg a.i./L. With this toxicity endpoint, the 

endangered species LOC of 0.05 will be exceeded at any concentration greater than 0.11 

μg a.i./L. 

Table 5.12 includes uses with the maximum single application rates for each application 

method and presents a summary of the buffer distances at which spray drift deposition 

from these uses drop below levels of concern (e.g., RQs will be below LOCs). These 

distances represent the maximum extent where effects are possible using the most 

sensitive data and the listed species LOC of 0.05 for either terrestrial invertebrates or 

freshwater fish. For each application method, lower application rates will yield a smaller 

buffer distance than reported here. 

90
Table 5.12 Summary of maximum predicted distances for potential spray drift 

effects. 

Application 

Method 

Application Rate 

(lb a.i/A) Uses Represented Terrestrial 

LD50 Distance (ft) 

Aquatic 

LC50 Distance (ft)

Aerial 3 Almond & 

Tree Fruit > 1000 >1000 

Ground 10 Nursery > 1000 > 1000 

Airblast 5 Citrus > 1000 > 1000 

Airblast 0.25 Mango 197 23 

The use of methidathion on almonds and tree fruit is predicted to have effects for 

terrestrial invertebrates at distances greater than 1,000 ft (0.3 km), the limit of AgDrift 

modeling. Given the uncertainty in AgDrift and the assumptions used in the modeling, 

the potential for effects from drift cannot be quantified beyond 1000 ft and so this value 

is used to represent the potential for effects to terrestrial invertebrates and to define the 

spatial extent of the effects determination (i.e., this buffer distance is added to the initial 

area of concern). Given the toxicity of methidathion and the calculated RQs, the extent 

of spray drift effects may be at least an order of magnitude higher than this. 

5.1.4.2 Downstream Dilution Analysis 

The maximum downstream extent of methidathion exposure in streams and rivers where 

the EEC can potentially be above levels that would exceed the most sensitive LOC may 

be estimated by utilizing the greatest ratio of aquatic RQ to LOC. This is based on the 

assumption that there is uniform runoff across the landscape in treated areas with streams 

flowing through them (i.e. the initial area of concern), represented by the modeled EECs, 

and that as those waters move downstream, the influx of non-impacted water will dilute 

the concentrations of methidathion present. 

Using an LC50 value of 2.2 ug/L for freshwater fish (the most sensitive species) and a 

maximum peak EEC of 114.7 μg/L for applications to nursery stock yields an RQ/LOC 

ratio of 1042 (52.2/0.05). Using the downstream dilution approach (described in more 

detail in Appendix D) yields a target percent crop area (PCA) of 27.8%. This value has 

been input into the downstream dilution approach and results in a distance of 285 

kilometers, which represents the maximum continuous distance of downstream dilution 

from the edge of the initial area of concern. Similar to the spray drift buffer described 

above, the LAA/NLAA determination is based on the area defined by the point where 

concentrations exceed the LC50 value. 

5.1.4.3 Overlap between CRLF habitat and Spatial Extent of 

Potential Effects 

An LAA effects determination is made for those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat 

and the area overlaps with the core areas, critical habitat and available occurrence data 

for CRLF. 

91
Figure 5.1 shows that there is some overlap between the initial area of concern mapped 

according to methidathion’s use pattern and the CRLF habitat, including currently 

occupied core areas, CNDDB occurrence sections, and designated critical habitat. This 

map does not include areas beyond the initial area of concern that may be impacted by 

runoff and/or spray drift. It is expected that any additional areas of CRLF habitat that are 

located >1000 ft (to account for offsite migration via spray drift) and 285 kilometers of 

stream reach (to account for downstream dilution) outside the initial area of concern may 

also be impacted and are part of the full spatial extent of the LAA/modification of critical 

habitat effects determination. 

92
Figure 5.1. Overlap Map: CRLF Habitat and Methidathion Initial Area of Concern 

5.2 Risk Description 

93
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 CRLF and its designated 

critical habitat. 

If the RQs presented in the Risk Estimation (Section 5.1) show no direct or indirect 

effects for the CRLF, and no modification to PCEs of the CRLF’s designated critical 

habitat, a “no effect” determination is made, based on methidathion’s use within the 

action area. However, if direct or indirect effect LOCs are exceeded or effects may 

modify the PCEs of the CRLF’s critical habitat, the Agency concludes a preliminary 

“may affect” determination for the FIFRA regulatory action regarding methidathion. A 

summary of the results of the risk estimation results are provided in Table 5.13 for 

direct and indirect effects to the CRLF and in Table 5.14 for the PCEs of designated 

critical habitat for the CRLF. 

These results represent risk from exposure to applied methidathion and do not include 

the additional risk posed by any degradates of toxicological concern. Given that acute 

and chronic risk quotients are exceeded for the parent compound alone, any contribution 

in toxicity from any of the major degradates or from methidathion oxon or other OP 

degradates would increase the risk estimates. Potential risk from methidathion oxon is 

characterized in Section 5.2.1.3.

Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to 

CRLF 

Assessment Endpoint 

LOC 

Exceedances 

(Y/N) 

Description of Results of Risk Estimation 

Aquatic Phase 

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

Direct Effects 

Survival, growth, and reproduction 

of CRLF individuals via direct 

effects on aquatic phases 

Y 

There are LOC exceedances for listed species 

following both acute and chronic exposure using 

freshwater fish as the surrogate for aquatic-phase 

amphibians. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects to 

food supply (i.e., fish, freshwater 

invertebrates, non-vascular plants) Y 

The acute LOC for non-listed species (all uses) and 

the chronic LOC (all uses except alfalfa, clover, 

timothy and kiwi) are exceeded for freshwater fish. 

There are LOC exceedances for non-listed species 

following both acute and chronic exposure to 

freshwater invertebrates. No aquatic plant data are 

available. A qualitative discussion of risk will be 

provided. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

habitat, cover, and/or primary 

productivity (i.e., aquatic plant 

community) 

Unknown No aquatic plant data are available. A qualitative 

discussion of risk to aquatic plants will be provided. 

94
Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to 

CRLF 

LOC 

Exceedances 

(Y/N) 

Assessment Endpoint Description of Results of Risk Estimation 

Indirect Effects 

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. 

No terrestrial plant data are available. A qualitative 

discussion of risk will be provided. Unknown 

Terrestrial Phase 

(Juveniles and adults)

Direct Effects 

Survival, growth, and reproduction 

of CRLF individuals via direct 

effects on terrestrial phase adults and 

juveniles 

Y 

There are LOC exceedances for listed species 

following both acute and chronic exposure using birds 

as the surrogate for terrestrial-phase amphibians. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

prey (i.e., terrestrial invertebrates, 

small terrestrial mammals and 

terrestrial phase amphibians) 

Y 

There are LOC exceedances for non-listed species 

following both acute and/or chronic exposure to 

terrestrial invertebrates, mammals and birds (surrogate 

to terrestrial-phase amphibians). 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

habitat (i.e., riparian vegetation) 

No terrestrial plant data are available. A qualitative 

discussion of risk will be provided. Unknown 

Table 5.14 Risk Estimation Summary for Methidathion – PCEs of Designated Critical Habitat 

for the CRLF 

Assessment Endpoint Habitat Modification 

(Y/N) Description of Results of Risk Estimation 

Aquatic Phase PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)

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. 

Unknown 

No aquatic and terrestrial plant data are available. 

A qualitative discussion of risk will be provided. 

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. 

Unknown 

No aquatic and terrestrial plant data are available. 

A qualitative discussion of risk will be provided. 

95
Table 5.14 Risk Estimation Summary for Methidathion – PCEs of Designated Critical Habitat 

for the CRLF 

Habitat Modification 

(Y/N) Assessment Endpoint Description of Results of Risk Estimation 

Alteration of other chemical characteristics 

necessary for normal growth and viability of 

CRLFs and their food source. 

Y 

LOC exceedances for both listed and non-listed 

species following both acute and chronic exposure 

for freshwater fish and invertebrates. No aquatic 

plant toxicity data are available. 

Reduction and/or modification of aquaticbased food sources for pre-metamorphs 

(e.g., algae) 

No aquatic plant data are available. A qualitative 

Unknown discussion of risk will be provided. 

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 

habitat that are comprised of grasslands, 

woodlands, and/or wetland/riparian plant 

species that provides the CRLF shelter, 

forage, and predator avoidance 

Unknown No terrestrial plant data are available. A qualitative 

discussion of risk will be provided. 

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 

Unknown No terrestrial plant data are available. A qualitative 

discussion of risk will be provided. 

Reduction and/or modification of food 

sources for terrestrial phase juveniles and 

adults Y 

Acute and chronic LOCs for listed and non-listed 

species were exceeded for all uses for birds and 

mammals and acute LOCs for listed and non-listed 

terrestrial invertebrates were also exceeded for all 

uses. 

Alteration of chemical characteristics 

necessary for normal growth and viability of 

juvenile and adult CRLFs and their food 

source. 

Acute and/or chronic LOCs for listed and non-listed 

species were exceeded for freshwater fish and 

invertebrates, birds, mammals and terrestrial 

invertebrates. No data are available for either 

aquatic or terrestrial plants. 

Y 

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. 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 and its designated critical habitat. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the CRLF and its designated critical habitat include the following: 

96
• 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. 

• Likelihood of the Effect Occurring: Discountable effects are those that are 

extremely unlikely to occur. 

• 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 and its designated critical habitat is provided in Sections 5.2.1 

through 5.2.3. 

5.2.1 Direct Effects 

5.2.1.1 Aquatic-Phase CRLF 

The aquatic-phase considers life stages of the frog that are obligatory aquatic organisms, 

including eggs and larvae. It also considers submerged terrestrial-phase juveniles and 

adults, which spend a portion of their time in water bodies that may receive runoff and 

spray drift containing methidathion. 

As stated in the risk estimation section, acute RQs for the aquatic-phase CRLF exceed the 

listed species LOC (0.05) for all of the assessed methidathion uses. The acute RQs range 

from 2.1 for the alfalfa scenario to 52.5 for the nursery scenario. In addition, the lowest 

acute RQ following a single application is estimated to be 0.20 for the lowest mango 

scenario, which still exceeds the acute LOC for listed species. 

Limited acute toxicity data are available to assess sensitivity across freshwater fish 

species. The lowest and highest acute 96-hour LC50s values from the submitted studies 

on bluegill sunfish, rainbow trout and goldfish are 2.2 (bluegill sunfish) and 14 μg/L 

(rainbow trout). Estimated acute RQs from these studies indicate that they would exceed 

the listed species acute LOC for all of the assessed methidathion uses for any one of these 

three surrogate freshwater fish species. It is noted that one study on the common eel 

(Anguilla anguilla) was available in the open literature with a 96-hr acute LC50 that is 

close to three orders of magnitude higher (1510 μg/L) than the most sensitive submitted 

freshwater fish species. If acute RQs were estimated based on that study, they would 

range from 0.003 for the alfalfa scenario to 0.08 for the nursery scenario, the only 

97
scenario that exceeds the acute LOC for listed species. Therefore, there will be some fish 

species that may not be at risk following acute exposure to some of the methidathion use 

scenarios, and the relative sensitivity of aquatic amphibians to fish is unknown. 

Based on a probit dose-response slope of 2.9 (1.9 – 4.0) from the bluegill sunfish acute 

toxicity study with an acute LC50 of 2.2 μg/L, the probability of an individual effect at the 

LOC is estimated to be 1 in 12,400 and the probability of an individual effect at the RQ is 

close to 1 in 1 for all scenarios. Sublethal effects were reported in the bluegill sunfish 

acute toxicity study at concentration levels higher than the endpoints utilized in this risk 

assessment (MRID 00011841). In this study, it was reported that test organisms in the 

highest exposure group (1000 μg/L) exhibited slight spastic motions and swam on their 

sides. 

As stated previously, the chronic NOAEC of 6.3 μg/L for the fathead minnow is similar 

to the acute 96-hour LC50 of several other test species, ranging from 2.2 to 14 μg/L. A 

chronic NOAEC for any of these other species would likely be lower than the available 

fathead minnow NOAEC. Therefore, this assessment estimated a chronic NOAEC for 

rainbow trout using an acute to chronic ratio (ACR) calculation from data from other 

organophosphates. The most conservative acute to chronic ratio from the other 

organophosphate data is 140. Using that ACR value with an acute LC50 value of 10 ppb 

for rainbow trout, the chronic NOAEC used in this assessment is estimated to be 0.07 

ppb. With this value, the chronic RQs exceed the LOC (1.0) for all of the assessed 

methidathion uses (Table 5.2). If the chronic NOAEC of 6.3 ppb from the fathead 

minnow study is used, then the chronic RQs would exceed the LOC (1.0) for all of the 

assessed methidathion uses except for the alfalfa scenario (RQ = 0.40: alfalfa, clover and 

timothy); the grape scenario (RQ = 0.51: kiwi) and the tree fruit scenario (RQ = 0.79: 

apple, apricot, cherry, nectarine, peach, pear, plum and prune). Therefore, even with the 

endpoint from the chronic study in fathead minnows, the chronic LOC is still exceeded 

with many methidathion uses. 

Sublethal effects in the chronic study include effects on total length and wet weight at the 

highest measured treatment level (12 μg ai/L) relative to the pooled control. 

Surface water monitoring data support the conclusion of risk determined based on 

calculated RQs. Monitoring in the Sacramento Valley found 46 detections out of 472 

samples with a maximum of 15.1 ug/L and an average across the detections of 1.2 ug/L. 

In the San Joaquin Valley, the maximum of 30 detections out of 738 samples was 2.4 

ug/L and the average was 0.3 ug/L. Atmospheric monitoring detected methidathion 

concentrations in fog of 15.5 ug/L. Compared to 96-hr acute LC50 values of 2.2 ug/L to 

14 ug/L, these data demonstrate the potential for aquatic exposures that could lead to 

direct effects, particularly because monitoring is generally not expected to capture peak 

values. 

In 1976, approximately 3,000 fish of unknown species in a nearby creek were killed 

following an aerial application of methidathion to an agricultural area in Sacramento 

County, California. The incident was attributed to accidental misuse of methidathion. 

98
Samples of the water collected from the field drain six days after the material was applied 

to the field showed methidathion concentrations of 60 ppb. The certainty index for this 

incident is highly probable. 

In summary, methidathion is very highly toxic to freshwater fish, the surrogate for the 

aquatic-phase CRLF. The acute LOC (listed species) and the chronic LOC are exceeded 

for all of the methidathion uses. There is one study in the open literature that indicates 

that there will be some fish species that may not be at risk following acute exposure from 

some of the methidathion use scenarios. This is an uncertainty. The probability of an 

individual effect on an acute basis approaches 100% at the RQ levels. Incident data 

indicate that freshwater fish are vulnerable, particularly if there are misuses. Spatial 

analyses indicate that a significant overlap will exist between the use sites for 

methidathion and the CRLF habitat, particularly when the spraydrift and downstream 

dilution buffers are applied to the “footprint” of the methidathion use area. Therefore, 

based on the weight-of-evidence, there is a potential for direct impact to the aquaticphase CRLF based on the endpoints generated from the freshwater fish data and the 

effects determination is Likely to adversely affect (LAA). 

As stated previously, the only detected degradate of toxicological concern is the minor 

degradate methidathion oxon, which may be more toxic than the parent. However, no 

data are available on the environmental fate, toxicity, and occurrence of methidathion 

oxon in the environment and so it is not considered directly in calculated RQs. Given 

that acute and chronic risk quotients are exceeded for the parent compound alone, any 

contribution in toxicity from any of the major degradates or from methidathion oxon or 

other OP degradates would increase the risk estimates. Further characterization of risk 

from methidathion oxon is provided in Section 5.2.1.3. 

5.2.1.2 Terrestrial-Phase CRLF 

As stated in the risk estimation section, acute dose-based and acute dietary-based RQs for 

the terrestrial-phase CRLF exceed the listed species LOC (0.1) for all of the assessed 

methidathion uses. The acute RQs range from 0.17 for mangos on a dietary basis to 442.0 

for nursery uses on a dose basis. Limited acute toxicity data are available to assess 

sensitivity across avian species on a dose-basis. The acute oral LD50’s range from 6.7 

mg/kg for mallard ducks (Anas platyrhynchos) (MRID 000159201) to 225 mg/kg for 

Chukar (Alectoris chukar) (MRID 00060823). Other species fall within that range. Even 

with the highest LD50 for the Chukar, the dose-based acute RQ for avian species is still 

over 150 times higher than the acute LOC of 0.1 for listed terrestrial animals with the 

nursery uses. With the lowest application rate of 0.25 lb a.i./A for mangos and the 

highest acute LD50 value for the Chukar, the acute dose-based RQ for small birds eating 

small insects (0.29) is still greater than the acute LOC of 0.1 for endangered birds. On a 

dietary basis, there are only two studies with the technical material, one with bobwhite 

quail and one with mallard ducks. The acute dietary LC50s are 224 and 543 ppm for 

bobwhite quail and mallard ducks, respectively. Again, with the lowest application rate 

of 0.25 lb a.i./A for mangos and using the bobwhite quail LC50 value of 224 ppm, the 

acute RQ on a dietary basis for small birds eating small insects is 0.17, which exceeds the 

99
LOC for listed species. Using the mallard duck LC50 value of 543 ppm, the acute RQ on 

a dietary basis for small birds eating small insects is 0.07, which is just under the LOC 

for endangered species. However, with the next highest application rate of 0.5 lbs a.i./A 

for safflower, even when applied only once per year, the RQ using the highest LC50 of 

543 ppm for mallard ducks, the RQ would be 0.12, which is greater than the acute LOC 

for endangered birds. Therefore, even within the range of dose- and dietary-based acute 

toxicity values for avian species, there is potential acute risk to endangered birds for all 

uses with the possible exception of mangos with some of the less sensitive birds. 

Chronic RQs also exceed the chronic LOC for all of the assessed methidathion uses 

(Table 5.7). The chronic endpoint used in this assessment (10 ppm) is based on a study 

with mallard ducks (MRID 44381602). In bobwhite quail, no effects were observed in 

two studies at the highest concentrations tested (30 and 35 ppm; MRIDs 44381601 and 

44381602, respectively). Using the highest bobwhite quail NOAEC value of 35 ppm, the 

chronic RQ with the lowest application rate (mangos) is 1.08, which is still greater than 

the chronic LOC of 1. Therefore, within the available range of dietary-based chronic 

toxicity values for avian species (two), there is potential risk following chronic exposure 

for all of the assessed methidathion uses. 

In an effort to refine the acute dose-based risk estimates, the T-REX model was modified 

to account for the lower metabolic rate and lower caloric requirement of amphibians 

(compared to birds). Acute dose-based RQs were recalculated using the T-HERPS 

(Version 1.0) model for small (1 g), medium (37 g), and large (238 g) frogs (Table 5.15, 

Appendix M). Using this refinement, the acute dose-based RQs still exceed the LOC 

(0.1) for nearly all of the modeled scenarios for methidathion. The T-HERPS model can 

only be used to refine dose-based risk estimates at this time; thus, further refinement of 

the acute or chronic dietary-based RQs is not possible. 

Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients 

EECs and RQs 

Broadleaf 

Plants/ 

Small 

Insects 

Fruits/Pods/

Seeds/ 

Large Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

Size 

Class 

(grams) 

Adjusted 

LD50

EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1

ARTICHOKE 

1.4 6.70 6.81 1.02 0.76 0.11 N/A N/A N/A N/A N/A N/A 

37 6.70 6.69 1.00 0.74 0.11 194.24 28.99 12.14 1.81 0.23 0.03 

238 6.70 4.39 0.65 0.49 0.07 30.20 4.51 1.89 0.28 0.15 0.02 

CITRUS 

1.4 6.70 26.46 3.95 2.94 0.44 N/A N/A N/A N/A N/A N/A 

37 6.70 26.00 3.88 2.89 0.43 754.62 112.63 47.16 7.04 0.90 0.13 

238 6.70 17.04 2.54 1.89 0.28 117.31 17.51 7.33 1.09 0.59 0.09 

CLOVER 

1.4 6.70 7.76 1.16 0.86 0.13 N/A N/A N/A N/A N/A N/A 

37 6.70 7.63 1.14 0.85 0.13 221.32 33.03 13.83 2.06 0.26 0.04 

100
Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients 

EECs and RQs 

Broadleaf 

Plants/ 

Small 

Insects 

Fruits/Pods/

Seeds/ 

Large Insects 

Small 

Herbivore 

Mammals 

Small 

Amphibians 

Small 

Insectivore 

Mammal 

Size 

Class 

(grams) 

EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1

Adjusted 

LD50 

EEC RQ1

238 6.70 5.00 0.75 0.56 0.08 34.41 5.14 2.15 0.32 0.17 0.03 

COTTON 

1.4 6.70 11.27 1.68 1.25 0.19 N/A N/A N/A N/A N/A N/A 

37 6.70 11.07 1.65 1.23 0.18 321.38 47.97 20.09 3.00 0.38 0.06 

238 6.70 7.26 1.08 0.81 0.12 49.96 7.46 3.12 0.47 0.25 0.04 

KIWI 

1.4 6.70 10.49 1.57 1.17 0.17 N/A N/A N/A N/A N/A N/A 

37 6.70 10.31 1.54 1.15 0.17 299.20 44.66 18.70 2.79 0.36 0.05 

238 6.70 6.76 1.01 0.75 0.11 46.51 6.94 2.91 0.43 0.23 0.04 

MANGO 

1.4 6.70 1.47 0.22 0.16 0.02 N/A N/A N/A N/A N/A N/A 

37 6.70 1.45 0.22 0.16 0.02 42.03 6.27 2.63 0.39 0.05 0.01 

238 6.70 0.95 0.14 0.11 0.02 6.53 0.98 0.41 0.06 0.03 0.00 

NURSERY 

1.4 6.70 2.62 0.39 0.29 0.04 N/A N/A N/A N/A N/A N/A 

37 6.70 2.58 0.38 0.29 0.04 74.80 11.16 4.67 0.70 0.09 0.01 

238 6.70 1.69 0.25 0.19 0.03 11.63 1.74 0.73 0.11 0.06 0.01 

SAFFLOWER, SUNFLOWER 

1.4 6.70 4.48 0.67 0.50 0.07 N/A N/A N/A N/A N/A N/A 

37 6.70 4.41 0.66 0.49 0.07 127.85 19.08 7.99 1.19 0.15 0.02 

238 6.70 2.89 0.43 0.32 0.05 19.88 2.97 1.24 0.19 0.10 0.01 

TREE FRUIT, OLIVES 

1.4 6.70 15.73 2.35 1.75 0.26 N/A N/A N/A N/A N/A N/A 

37 6.70 15.46 2.31 1.72 0.26 448.79 66.98 28.05 4.19 0.54 0.08 

238 6.70 10.14 1.51 1.13 0.17 69.77 10.41 4.36 0.65 0.35 0.05 

WALNUT

1.4 6.70 25.88 3.86 2.88 0.43 N/A N/A N/A N/A N/A N/A 

37 6.70 25.44 3.80 2.83 0.42 738.28 110.19 46.14 6.89 0.88 0.13 

238 6.70 16.67 2.49 1.85 0.28 114.77 17.13 7.17 1.07 0.58 0.09 aAcute RQs that exceed the acute endangered species LOC of 0.1 are in bold.

On a dose basis, based on a default probit dose-response slope of 4.5 (2 – 9) from the 

mallard duck acute oral toxicity study with an acute LD50 of 6.7 mg/kg bw, the 

probability of an individual effect at the RQ is approximately 1 in 1 for all scenarios. On 

a dietary basis, the probit dose-response slope was 8.7 (3.5-13.8) from the bobwhite quail 

dietary study with an LC50 of 224 ppm. The probability of an individual effect at the RQ 

ranged from 1 in 1 for 5 use scenarios to 1 in 9.28E+10 for use on mangos (see Tables 

5.5 and 5.6). 

101
Sublethal effects were reported in the mallard acute oral toxicity study (MRID 

00159201). Toxic symptoms included depression, reduced reaction to external stimuli, 

wing droop, convulsions, and salivation. In ducks, pheasants and/or partridges (MRID 

00060823), toxic symptoms included goose-stepping, ataxia, dyspnea, lacrimation, 

salivation, ataxia, wing-spread seizures, and terminal opisthotonos. 

Symptoms appeared as soon as 20 minutes after dosage; recovery of 

survivors took 1-2 days. The NOAELs and LOAELs for these sublethal effects were not 

reported. 

There are no other acute or chronic toxicity data with birds that have lower endpoints 

than the studies used in this assessment. 

Four incidents were reported in which red-tailed hawks were found either dead or injured 

after potential exposure to methidathion. One incident was classified as possible, two as 

probable and the fourth as highly probable. The incidents were all reported in 1994 in 

several counties in California (Colusa, Stanislaus and Merced). In one case, blood 

plasma cholinesterase and acetlycholinesterase levels were within the normal range; 

however, the results of the footwash showed detections of methidathion at 2.7 ppb (other 

organophosphates, including chlorpyrifos and diazinon were also detected in the 

footwash sample at respective concentrations of 0.2 ppb and 4.1 ppb). In a second case, 

the hawk recovered. Plasma cholinesterase and acetylcholinesterase levels were found 

significantly below the normal range. Footwash analysis results show that methidathion 

was detected at a concentration of 2.7 ppb. Again, chlorpyrifos and diazinon were also 

detected in the footwash sample at respective concentrations of 0.7 ppb and 0.4 ppb. In 

the third case where the hawk died, the plasma cholinesterase level was low and the brain 

cholinesterase level was severely depressed. Methidathion was detected at a 

concentration of 0.7 ppm in the footskin. Chloropyrifos and diazinon were detected in 

both the feathers and footskin at concentrations ranging from 0.02 to 0.08 ppb. It is 

likely that all three organophosphates contributed to the death of this bird.

In summary, the acute dose-based, acute dietary-based and/or chronic dietary-based RQs 

for the terrestrial-phase CRLF, using avian data as a surrogate, exceed the acute listed 

species LOC (0.1) and/or the chronic non-listed species LOC (1) for all of the assessed 

methidathion uses. In addition, the probability of an individual effect on an acute basis is 

high at the RQ levels. Incident data indicate that birds are vulnerable. Spatial analyses 

indicate that a significant overlap will exist between the use sites for methidathion and 

the CRLF habitat, particularly when the spraydrift buffers are applied to the “footprint” 

of the methidathion use area. Therefore, based on the weight-of-evidence, there is a 

potential for direct impact to the terrestrial-phase CRLF using the endpoints generated 

from the avian data and the effects determination is LAA. 

As stated previously, the only detected degradate of toxicological concern is the minor 

degradate methidathion oxon, which may be more toxic than the parent. However, no 

data are available on the environmental fate, toxicity, and occurrence of methidathion 

oxon in the environment and so it is not considered directly in calculated RQs. Given 

that acute and chronic risk quotients are exceeded for the parent compound alone, any 

102
contribution in toxicity from any of the major degradates or from methidathion oxon or 

other OP degradates would increase the risk estimates. Further characterization of risk 

from methidathion oxon is provided in Section 5.2.1.3. 

5.2.1.3 Direct Effects from Methidathion Oxon to Aquatic-Phase and 

Terrestrial-Phase CRLF 

As described in Section 2.1.4, methidathion oxon formation in soil was demonstrated in 

soil photolysis, aerobic soil metabolism, and terrestrial field dissipation studies, and 

formation on foliage was demonstrated in dislodgeable residue studies. Long range 

transport has also been shown to occur, with methidathion oxon detected in air and on 

pine needles at more than 20 km from the nearest use site. Although formed at less than 

10%, methidation oxon is a degradate of concern based on comparison with other OP 

pesticides (e.g., diazinon, chlorpyrifos), for which oxon degradates have been shown in 

many cases to be of equal or greater toxicity than the parent. This analysis does not 

include consideration of other oxon degradates which, based on the degradation pathway 

for methidathion, may have been formed but were not detected in fate studies due to the 

lack of appropriate labeling. 

No environmental fate or ecological effects data are available for methidathion oxon. 

Therefore, it is difficult to quantify the increased risk which may result from potential 

exposure to this degradate; however, based on conservative assumptions, some broad 

conclusions can be reached. In laboratory studies, transformation to the oxon occurred at 

maximum amounts under soil photolysis conditions, where 5% of the applied parent was 

present as methidathion oxon at study termination. For quantitative modeling purposes, 

then, the assumption can be made that methidathion oxon is initially present on soil and 

on foliage at 5% of the initially applied parent compound. 

For both aquatic modeling using PRZM/EXAMS and terrestrial modeling using T-REX, 

EECs are linear with the application rate. Therefore, based on an application rate of 5% 

of the applied methidathion for the methidathion oxon, and assuming that its fate 

properties are similar to the parent, aquatic and terrestrial EECs for the oxon alone would 

be approximately 5% of those estimated for the parent7. If the oxon were of equal 

toxicity to the parent, then the acute aquatic and terrestrial RQs for the oxon alone would 

be 5% of those calculated for the parent. For acute aquatic risk, the acute RQs would 

range from 0.01 for the use on mangos to 1.12 for almonds and 2.62 for the nursery use, 

as shown in Table 5.16. For terrestrial risk, acute dose-based RQs would range from 

0.62 for mangos to 11.1 for citrus and 22.1 for the nursery use, as shown in Table 5.17. 

Therefore, at equal toxicity to the parent, risk from the oxon degradate alone would 

exceed the endangered species LOCs for acute risk to both the aquatic phase CRLF (LOC 

= 0.05) and the terrestrial phase CRLF (LOC = 0.1) for all except the lowest application 

rates. 

 

7

 Modeled aquatic EECs for the oxon would not be exactly 5% because spray drift would not be a factor 

and so model inputs would change to 100% application efficiency and 0% drift, rather than 95% and 2%, 

respectively. This, along with a correction for molecular weight, would be likely to change the percentage 

slightly but not enough to affect conclusions. 

103
However, oxons can be considerably more toxic than the parent and risk from 

methidathion oxon alone could be even greater than that from the parent. In the 

cumulative assessment for OP insecticides8

, if there were no toxicity data available for a 

given OP-oxon, high-end adjustment factors of 10X to 100X were applied to account for 

the presumed increased toxicity of the oxon relative to the parent chemical. To explore 

the potential risk if methidathion oxon were more toxic than the parent, RQs based on 

these factors are reported as well in Tables 5.16 and 5.17, for direct effects to the aquatic 

and terrestrial phase CRLF, respectively. 

Table 5.16 Summary of Direct Effect Acute RQs for the Aquatic-phase CRLF, 

based on exposure to methidathion oxon degradate

Oxon Only (no parent) b

Use Parent Only a

Equal Toxicity 10X Toxicity 100X Toxicity 

Mango 0.20 0.01 0.1 1.0 

Alfalfa 2.1 0.10 1.0 10 

Almond 22.4 1.12 11.2 112 

Nursery 52.5 2.62 26.2 262 a

 Previously calculated in Table 5.1 using bluegill sunfish (Lepomis macrochirus) acute 96-hour LC50 = 2.2 μg/L b

 Assumes EECs to be 5% of those for the parent, based on the maximum percent formation observed in laboratory 

studies. 

Table 5.17 Summary of Direct Effect Acute Dose-based RQs for the 

Terrestrial-phase CRLF, based on exposure to methidathion oxon degradate

Oxon Only (no parent) b

Use Parent Only a

Equal Toxicity 10X Toxicity 100X Toxicity 

Mango 12.4 0.62 6.2 60 

Clover 65.4 3.3 33 330 

Citrus 222.9 11.1 111 1110 

Nursery 442.0 22.1 221 2210 a

 Previously calculated in Table 5.5 based on mallard duck acute oral LD50 = 6.7 mg/kg. b

 Assumes EECs to be 5% of those for the parent, based on the maximum percent formation observed in laboratory 

studies. 

There is a great deal of uncertainty in these estimates because of the assumptions on 

which they are based, including those for formation (e.g., fate properties) and toxicity of 

the methidathion oxon. For the aquatic-phase CRLF, exposure estimates are based on the 

assumptions that all of the applied methidathion is available on the soil surface for 

photolysis and that all of the transformation product is available for runoff at one time. 

For the terrestrial-phase CRLF, exposure estimates assume that laboratory soil photolysis 

rates are also reflective of the photolysis rates that may occur on foliage. Additionally, 

the fate properties of methidathion oxon are unknown, and assuming the same fate 

behavior of the parent may lead to overestimation because OP oxons tend to be more 

 

8 More detail on the OP cumulative assessment and the characterization of additional risk due to oxon 

occurrence may be found at http://www.epa.gov/pesticides/cumulative/2006-op/op_cra_appendices_part2.pdf

104
transient than the parent. Methidathion oxon has not been detected in surface water 

monitoring done in two studies, even in samples in which the parent methidathion was 

detected. Despite the uncertainty, the potential for acute direct effects to aquatic-phase 

and terrestrial-phase CRLF from exposure to methidathion oxon alone cannot be 

precluded, even if the toxicity is no greater than that of the parent. Exposure from the 

parent methidathion already leads to conclusions of risk; thus, consideration of the oxon 

degradate as well increases the likelihood of direct effects. 

5.2.2 Indirect Effects (via Reductions in Prey Base) 

5.2.2.1 Algae (non-vascular plants) 

 

As discussed in Section 2.5.3, the diet of CRLF tadpoles is composed primarily of 

unicellular aquatic plants (i.e., algae and diatoms) and detritus. Due to a lack of aquatic 

plant toxicity data indirect effects to the CRLF via direct toxicity to aquatic plants cannot 

be quantitatively estimated for methidathion. There are no incident data for aquatic 

plants and no relevant data in the open literature. An examination of the completed 

CRLF assessments for nine other organophosphates indicates that nearly all of the effects 

determinations for aquatic non-vascular plants were either “no effect” or “not likely to 

adversely affect”. In addition, the mode of action for methidathion as an 

organophosphate insecticide is by disrupting nervous system function of exposed animals 

via acetylcholinesterase inhibition. Unless there is a separate herbicidal mode of action 

as with bensulide, a pre-emergent organophosphate herbicide which inhibits cell division 

in root tips and inhibits seedling growth by conjugation of acetyl co-enzyme A, 

this mode of action on animals is not expected to affect plants. Methidathion also has a 

history of being applied to a myriad of agricultural crops (as per the label), with no 

known incidents of adverse phytotoxic effects to aquatic plants. Therefore, based on the 

weight-of-evidence, the potential indirect impact to the CRLF is expected to be minimal. 

The effects determination is may affect, not likely to adversely affect (NLAA). 

5.2.2.2 Aquatic Invertebrates 

The potential for methidathion to elicit indirect effects to the CRLF via effects on 

freshwater invertebrate food items is dependent on several factors including: (1) the 

potential magnitude of effect on freshwater invertebrate individuals and populations; and 

(2) the number of prey species potentially affected relative to the expected number of 

species needed to maintain the dietary needs of the CRLF. Together, these data provide a 

basis to evaluate whether the number of individuals within a prey species is likely to be 

reduced such that it may indirectly affect the CRLF. 

The acute RQs for aquatic invertebrates range from 1.5 to 38.5, thus exceeding the acute 

risk LOC for non-listed species for all of the assessed uses (LOC = 0.5; see Table 5.3). 

Chronic RQs for freshwater invertebrates range from 8.9 to145.0, thus exceeding the 

LOC (1.0) for all of the assessed methidathion uses (see Table 5.4). Only one other 

acute toxicity study with the same species (daphnia magna, MRID 00011350) provides 

any other data relevant to this assessment, either from submitted studies or from the open 

105
literature. With the endpoint from that study, the lowest acute RQ would be 1.0, which 

still exceeds the acute LOC for non-listed species. At the lowest application rate with the 

default slope of 4.5 and the lowest RQ of 1.5, the probability of an individual effect is 

approximately 1 in 1.3 and the percentage effect to the aquatic invertebrate prey base is 

79% (e.g., the percentage of the aquatic invertebrate population that is expected to be 

affected following exposure to methidathion). Based on the weight-of-evidence, there is 

a potential indirect impact to the CRLF via effects on freshwater invertebrate food items. 

The effects determination is LAA. 

 

5.2.2.3 Fish and Aquatic-phase Frogs 

Methidathion is very highly toxic to freshwater fish, the surrogate for the aquatic-phase 

CRLF. The acute LOC for non-listed species (all uses) and the chronic LOC (all uses) 

are exceeded; however, one study in the open literature indicates that there will be some 

fish species that may not be at risk following acute exposure from some of the 

methidathion use scenarios. At the lowest application rate, the probability of an 

individual effect on an acute basis at the RQ is approximately 1 in 1 and the percentage 

effect to the fish and aquatic-phase frog prey base is 93.4%. In addition, the incident data 

indicate that freshwater fish are vulnerable. Therefore, based on the weight-of-evidence, 

there is a potential indirect impact to the aquatic-phase CRLF based on the endpoints 

generated from the freshwater fish data. The effects determination is LAA. 

5.2.2.4 Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. The RQs for both small and large insects exceed 

the acute LOC for non-listed species for all methidathion uses, even with only a single 

use. The acute RQs for small insects range from 21 (mangos) to 734 (nursery) and the 

acute RQs for large insects range from 2.3 (mangos) to 82 (nursery). Following a single 

use, the lowest RQ is 2.0 for large insects (mangos). There are insufficient studies to 

conduct a sensitivity analysis; however, the highest terrestrial invertebrate endpoint is 

from the open literature with an acute toxicity value of 0.416 μg/bee (ECOTOX reference 

70351). The lowest RQ, using the endpoint from this study is 1.1 (mangos), which still 

exceeds the acute list species LOC of 0.05. The probability of an individual effect at the 

LOC of 0.5 is 1 in 4.41E+31 and the probability of an individual effect at the RQ for all 

uses is 1 in 1. In addition, the percentage effect to the terrestrial invertebrate prey base is 

99.9 - 100%. 

Two incidents were reported following application of methidathion in California. The 

first involved spraying of plum trees near some pollination hives. Dead bees (number not 

specified) were found along with residues of methidathion and diazinon. This incident 

was rated as possible. The second incident reported that an unknown number of bees 

were killed in an unknown county. The incident was attributed to accidental misuse. No 

residue analysis was completed; however, the certainty index for this incident is probable.

106
Based on the weight-of-evidence, there is a potential indirect impact to the CRLF based 

on this endpoint. The effects determination is LAA. 

5.2.2.5 Mammals 

Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume 

terrestrial vertebrates, including mice. The acute and chronic RQs exceed the listed 

species acute LOC and the chronic LOC for all uses. The acute RQs range from 2.4 

(mangos) to 86.8 (nursery). The chronic dietary RQs range from 117 (mangos) to 4164.5 

(nursery, dose-based) and from 13.5 (mangos) to 480.0 (nursery, dietary-based). There is 

one study from the open literature conducted with the mouse (mus musculus) that appears 

to have a lower endpoint (E85173); however, the study is not available for review at this 

time. The acute LD50 for this study is 10.5 mg a.i./kg. Using the endpoint from this 

study and assuming that a mouse weighs 20 g, the lowest RQ (mangos) would be 5.70, 

which exceeds the non-listed species LOC by 10 times. No other relevant studies are 

available in the open literature. At the lowest application rate (0.25 lb a.i./A for mangos), 

with the default slope of 4.5 and the lowest RQ of 2.4, the probability of an individual 

effect is approximately 1 in 1 and the percentage effect to the mammalian prey base is 

95.6%. Based on the weight-of-evidence, the labeled uses for methidathion may 

indirectly impact the CRLF through effects to the mammalian prey base. The effects 

determination is LAA. 

5.2.2.6 Terrestrial-phase Amphibians 

Terrestrial-phase adult CRLFs also consume frogs. RQ values representing direct 

exposures of methidathion to terrestrial-phase CRLFs are used to represent exposures of 

methidathion to frogs in terrestrial habitats. With the exception of mangos on an acute 

dietary basis, the acute dose-based, acute dietary-based and/or chronic dietary-based RQs 

for terrestrial-phase amphibians, using avian data as a surrogate, exceed the acute nonlisted species LOC and the chronic LOC for all of the assessed methidathion uses. 

Refining the RQ values with T-HERPS, the acute dose-based RQs still exceed the nonlisted LOC (0.5) for at least one food category for all of the modeled scenarios for 

methidathion. 

Table 5.18. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients 

EECs and RQs 

Broadleaf 

Plants/ 

Small 

Insects 

Fruits/Pods/

Seeds/ 

Large Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

Size 

Class 

(grams) 

Adjusted 

LD50

EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1

ARTICHOKE 

1.4 6.70 6.81 1.02 0.76 0.11 N/A N/A N/A N/A N/A N/A 

37 6.70 6.69 1.00 0.74 0.11 194.24 28.99 12.14 1.81 0.23 0.03 

238 6.70 4.39 0.65 0.49 0.07 30.20 4.51 1.89 0.28 0.15 0.02 

CITRUS 

107
Table 5.18. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients 

EECs and RQs 

Broadleaf 

Plants/ 

Small 

Insects 

Fruits/Pods/

Seeds/ 

Large Insects 

Small 

Herbivore 

Mammals 

Small 

Amphibians 

Small 

Insectivore 

Mammal 

Size 

Class 

(grams) 

EEC RQ1 EEC RQ1 EEC RQ1 EEC RQ1

Adjusted 

LD50 

EEC RQ1

1.4 6.70 26.46 3.95 2.94 0.44 N/A N/A N/A N/A N/A N/A 

37 6.70 26.00 3.88 2.89 0.43 754.62 112.63 47.16 7.04 0.90 0.13 

238 6.70 17.04 2.54 1.89 0.28 117.31 17.51 7.33 1.09 0.59 0.09 

CLOVER 

1.4 6.70 7.76 1.16 0.86 0.13 N/A N/A N/A N/A N/A N/A 

37 6.70 7.63 1.14 0.85 0.13 221.32 33.03 13.83 2.06 0.26 0.04 

238 6.70 5.00 0.75 0.56 0.08 34.41 5.14 2.15 0.32 0.17 0.03 

COTTON 

1.4 6.70 11.27 1.68 1.25 0.19 N/A N/A N/A N/A N/A N/A 

37 6.70 11.07 1.65 1.23 0.18 321.38 47.97 20.09 3.00 0.38 0.06 

238 6.70 7.26 1.08 0.81 0.12 49.96 7.46 3.12 0.47 0.25 0.04 

KIWI 

1.4 6.70 10.49 1.57 1.17 0.17 N/A N/A N/A N/A N/A N/A 

37 6.70 10.31 1.54 1.15 0.17 299.20 44.66 18.70 2.79 0.36 0.05 

238 6.70 6.76 1.01 0.75 0.11 46.51 6.94 2.91 0.43 0.23 0.04 

MANGO 

1.4 6.70 1.47 0.22 0.16 0.02 N/A N/A N/A N/A N/A N/A 

37 6.70 1.45 0.22 0.16 0.02 42.03 6.27 2.63 0.39 0.05 0.01 

238 6.70 0.95 0.14 0.11 0.02 6.53 0.98 0.41 0.06 0.03 0.00 

NURSERY 

1.4 6.70 2.62 0.39 0.29 0.04 N/A N/A N/A N/A N/A N/A 

37 6.70 2.58 0.38 0.29 0.04 74.80 11.16 4.67 0.70 0.09 0.01 

238 6.70 1.69 0.25 0.19 0.03 11.63 1.74 0.73 0.11 0.06 0.01 

SAFFLOWER, SUNFLOWER 

1.4 6.70 4.48 0.67 0.50 0.07 N/A N/A N/A N/A N/A N/A 

37 6.70 4.41 0.66 0.49 0.07 127.85 19.08 7.99 1.19 0.15 0.02 

238 6.70 2.89 0.43 0.32 0.05 19.88 2.97 1.24 0.19 0.10 0.01 

TREE FRUIT, OLIVES 

1.4 6.70 15.73 2.35 1.75 0.26 N/A N/A N/A N/A N/A N/A 

37 6.70 15.46 2.31 1.72 0.26 448.79 66.98 28.05 4.19 0.54 0.08 

238 6.70 10.14 1.51 1.13 0.17 69.77 10.41 4.36 0.65 0.35 0.05 

WALNUT

1.4 6.70 25.88 3.86 2.88 0.43 N/A N/A N/A N/A N/A N/A 

37 6.70 25.44 3.80 2.83 0.42 738.28 110.19 46.14 6.89 0.88 0.13 

238 6.70 16.67 2.49 1.85 0.28 114.77 17.13 7.17 1.07 0.58 0.09 aAcute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.

In addition, on an acute dose-basis, the probability of an individual effect is 1 in 1 at the 

lowest RQ level and the percentage effect to the avian/terrestrial-phase amphibian prey 

base is 100%. Incident data indicate that birds are vulnerable. Therefore, based on the 

108
weight-of-evidence, there is a potential indirect impact to the terrestrial-phase CRLF 

based on these endpoints. The effects determination is LAA. 

5.2.3 Indirect Effects (via Habitat Effects) 

5.2.3.1 Aquatic Plants (Vascular and Non-vascular) 

Aquatic plants serve several important functions in aquatic ecosystems. Non-vascular 

aquatic plants are primary producers and provide the autochthonous energy base for 

aquatic ecosystems. Vascular plants provide structure as attachment sites and refugia for 

many aquatic invertebrates, fish, and juvenile organisms, such as fish and frogs. In 

addition, vascular plants also provide primary productivity and oxygen to the aquatic 

ecosystem. Rooted plants help reduce sediment loading and provide stability to 

nearshore areas and lower streambanks. In addition, vascular aquatic plants are important 

as attachment sites for egg masses of CRLFs. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production would normally be assessed using RQs from freshwater aquatic vascular and 

non-vascular plant data; however, there are no aquatic plant data for methidathion and 

quantitative risk estimations cannot be conducted. There are no incident data for aquatic 

plants and no relevant data in the open literature. An examination of the completed 

CRLF assessments for nine other organophosphates indicates that nearly all of the effects 

determinations for aquatic vascular and non-vascular plants were either no effect or not 

likely to adversely affect. Not all of the other CRLF organophosphate chemicals have 

aquatic vascular plant data. In those cases, determinations were based on the nonvascular plant data and any available terrestrial plant data. As stated previously, the 

mechanism of action for methidathion as an organophosphate insecticide is by disrupting 

nervous system function of exposed animals via acetylcholinesterase inhibition, which is 

not a mode of action that is expected to affect plants. Methidathion also has a history of 

being applied to a myriad of agricultural crops (as per the label), with no known incidents 

of adverse phytotoxic effects to aquatic plants. Therefore, based on the weight-ofevidence, the potential indirect impact to the CRLF is minimal. The effects 

determination is may affect, NLAA. 

5.2.3.2 Terrestrial Plants 

Terrestrial plants serve several important habitat-related functions for the CRLF. In 

addition to providing habitat and cover for invertebrate and vertebrate prey items of the 

CRLF, terrestrial vegetation also provides shelter for the CRLF and cover from predators 

while foraging. Terrestrial plants also provide energy to the terrestrial ecosystem through 

primary production. Upland vegetation including grassland and woodlands provides 

cover during dispersal. Riparian vegetation helps to maintain the integrity of aquatic 

systems by providing bank and thermal stability, serving as a buffer to filter out sediment, 

nutrients, and contaminants before they reach the watershed, and serving as an energy 

source. 

109
Again, as with aquatic plants, there are no registrant-submitted terrestrial plant toxicity 

data for methidathion for assessment of the potential for indirect effects to the aquaticand terrestrial-phase CRLF via effects to riparian vegetation or effects to the primary 

constituent elements (PCEs) relevant to the aquatic- and terrestrial-phase CRLF. 

However, there is limited evidence in the open literature that methidathion has the 

potential to elicit phototoxic effects (Ecotox ref. 64451). This paper reported that 

methidathion significantly affected corn photosynthesis when applied at 0.5 lbs a.i./A, a 

rate that is lower than many of the currently registered rates. In addition, potential 

phytotoxic effects have been highlighted on methidathion labels (i.e., spotting, reddening, 

or chlorosis of the leaves) in certain sorghum varieties. An examination of the completed 

CRLF assessments for nine other organophosphates indicates that the majority of the 

effects determinations for terrestrial plants were either “no effect” or “not likely to 

adversely affect”. Two were determined to be “LAA”; however, one had herbicidal 

activity from a known mechanism and the other used surrogate data from another 

pesticide. For some of these organophosphates, as with the study mentioned above, there 

is the potential for some damage to plants. Nevertheless, the conclusions in those cases 

were generally that while effects to terrestrial plants may affect the CRLF via habitat 

modification, they are not likely to adversely affect the CRLF based on the type and 

extent of damage as observed. The mode of action for methidathion as an 

organophosphate insecticide is not one known to affect plants. Therefore, based on the 

weight-of-evidence, methidathion is not likely to impact plants to an extent that is 

expected to adversely affect the CRLF at the labeled application rates. Although some 

phytotoxicity could occur, these effects are considered to be insignificant in the context 

of a “take”. The effects determination is may affect, NLAA. 

5.2.4 Modification to Designated Critical Habitat 

The risk conclusions for the designated critical habitat are based on conclusions described 

for indirect effects previously described. Potential habitat modification is described 

below. 

5.2.4.1 Aquatic-Phase PCEs 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

• 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. 

• 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. 

• Reduction and/or modification of aquatic-based food sources for pre-metamorphs 

(e.g., algae). 

110
Conclusions for potential indirect effects to the CRLF via direct effects to aquatic and 

terrestrial plants are used to determine whether modification to critical habitat may occur. 

The potential for habitat medication via impacts to aquatic plants (Sections 5.2.2.1 and 

5.2.3.1) and terrestrial plants (5.2.3.2) is not considered to be significant. No habitat 

modification is expected. 

The remaining aquatic-phase PCE is “alteration of other chemical characteristics 

necessary for normal growth and viability of CRLFs and their food source.” Other than 

impacts to algae as food items for tadpoles (discussed above), this PCE is assessed by 

considering direct and indirect effects to the aquatic-phase CRLF via acute and chronic 

freshwater fish and invertebrate toxicity endpoints as measures of effects. Based on the 

potential direct impact to the aquatic-phase CRLF (Section 5.2.1.1) and impacts to 

freshwater invertebrates and fish as food items (Sections 5.2.2.2 and 5.2.2.3), there is a 

potential for habitat modification. 

 5.2.4.2 Terrestrial-Phase PCEs 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• 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 drip line surrounding aquatic and riparian 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. 

No habitat modification is expected through impacts to terrestrial plants (5.2.3.2). 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of methidathion on this PCE, 

acute and chronic toxicity endpoints for terrestrial invertebrates, mammals, and 

terrestrial-phase frogs are used as measures of effects. There is a potential for habitat 

modification based on potential reductions in prey base (Section 5.2.2.4 for terrestrial 

invertebrates, Section 5.2.2.5 for mammals, and 5.2.2.6 for frogs). 

The fourth terrestrial-phase PCE is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. There is a potential for habitat modification based on potential direct (Section 

5.2.1.2) and indirect effects (Sections 5.2.2.4, 5.2.2.5, and 5.2.2.6) to terrestrial-phase 

CRLFs. 

111
6. Uncertainties 

6.1 Exposure Assessment 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 pest resistance, timing of applications, cultural practices, 

and market forces. In particular, the application rate assumed for use on nursery stock is 

conservative. The label does not provide explicit application rates for this use and so it is 

assessed based on an assumption of application at the general maximum application rate 

required by the labels. 

6.1.2 Aquatic Exposure Modeling of Methidathion

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 

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. CRLFs prefer habitat with 

perennial (present year-round) or near-perennial water and do not frequently inhabit 

112
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 aquatic-phase 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). 

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 

an agricultural 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, 

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 buffers 

on runoff and loadings. The effect of buffers on runoff is highly dependent on the 

condition of the vegetative strip. For example, a well-established, 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. 

In order to account for uncertainties associated with modeling, available monitoring data 

were compared to PRZM/EXAMS estimates of peak EECs. The surface water dataset for 

methidathion has several thousand samples collected from 135 sites primarily in 

agricultural areas, and includes several monitoring studies targeted to water bodies 

113
receiving runoff from high OP use areas and with a high frequency of sampling, factors 

which reduce the uncertainty inherent in monitoring data. This monitoring detected 

methidathion in surface waters at concentrations of up to 15.1 ug/L. Peak modeling 

EECs range from 7.3 ug/L for alfalfa to 52.4 ug/L for the almond use. (The maximum 

modeled EEC is 114.7 ug/L for the use on nursery stock, but this result is based on 

conservative assumptions about application rates.) The highest monitoring detection of 

15.1 ug/L is within this range, below the peak EECs for some uses but higher than EECs 

for others. Although the use in the location with the highest detection is unknown, the 

majority of the sampling was conducted in areas with high acreage of fruit and nut 

orchards, which had modeled EECs of 17.8 ug/L for fruit trees, 20.0 ug/L for olives, and 

52.4 ug/L, suggesting that the detection is not in excess of relevant modeled EECs. 

Additionally, the next highest monitored detection from this relatively large dataset is 2.4 

ug/L, suggesting that the modeled EECs are reasonably conservative representations of 1-

in-10 year concentrations. 

6.1.3 Potential Groundwater Contributions to Surface Water 

Chemical Concentrations 

Although the potential impact of discharging ground water on CRLF populations is not 

explicitly delineated, it should be noted that ground water could provide a source of 

pesticide to surface water bodies – especially low-order streams, headwaters, and ground 

water-fed pools. Terrestrial field dissipation studies did not find leaching of 

methidathion below 12 inches, but given the fact that methidathion is soluble and 

classified as moderatley mobile, and that its abiotic and biotic degradation in anaerobic 

aquatic environments may be slow, the possibility of methidathion reaching ground water 

cannot be precluded. Much of available ground water will eventually be discharged to 

the surface – often supporting stream flow in the absence of rainfall. Continuously 

flowing low-order streams in particular are sustained by ground water discharge, which 

can constitute 100% of stream flow during baseflow (no runoff) conditions. Thus, it is 

important to keep in mind that pesticides in groundwater may have a major (detrimental) 

impact on surface water quality, and on CRLF habitats. 

Concentrations in a receiving water body resulting from groundwater discharge cannot be 

explicitly quantified, but it can be assumed that significant attenuation and retardation of 

the chemical will have occurred prior to discharge. Nevertheless, groundwater could still 

be a significant consistent source of chronic background concentrations in surface water, 

and may also add to surface runoff during storm events (as a result of enhanced 

groundwater discharge typically characterized by the ‘tailing limb’ of a storm 

hydrograph). In this assessment, long-term chronic concentrations derived from the 

PRZM-EXAMS model are assumed to reflect background concentrations that might be 

found in discharged groundwater/stream baseflow, but this factor remains an uncertainty. 

6.1.4 Action Area Uncertainties

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 

114
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 

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. 

6.1.5 Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Eight years of data (1999 – 

2006) were included in this analysis. 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 

115
not likely to be reported. As with all pesticide usage 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. 

6.1.6 Terrestrial Exposure Modeling of Methidathion

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. 

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. 

For the terrestrial exposure analysis of this 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. 

116
6.1.7 Spray Drift Modeling 

Although there may be multiple methidathion applications at a single site, it is unlikely 

that the same organism would be exposed to the maximum amount of spray drift from 

every application made. In order for an organism to receive the maximum concentration 

of methidathion from multiple applications, each application of methidathion would have 

to occur under identical atmospheric conditions (e.g., same wind speed and – for plants – 

same wind direction) and (if it is an animal) the animal being exposed would have to be 

present directly downwind at the same distance after each application. Although there 

may be sites where the dominant wind direction is fairly consistent (at least during the 

relatively quiescent conditions that are most favorable for aerial spray applications), it is 

nevertheless highly unlikely that plants in any specific area would receive the maximum 

amount of spray drift repeatedly. It appears that in most areas (based upon available 

meteorological data) wind direction is temporally very changeable, even within the same 

day. Additionally, other factors, including variations in topography, cover, and 

meteorological conditions over the transport distance are not accounted for by the 

AgDRIFT/AGDISP model (i.e., it models spray drift from aerial and ground applications 

in a flat area with little to no ground cover and a steady, constant wind speed and 

direction). Therefore, in most cases, the drift estimates from AgDRIFT/AGDISP may 

overestimate exposure even from single applications, especially as the distance increases 

from the site of application, since the model does not account for potential obstructions 

(e.g., large hills, berms, buildings, trees, etc.). Furthermore, conservative assumptions 

are often made regarding the application method (e.g., aerial), release heights and wind 

speeds. Alterations in any of these inputs would change the area of potential effect. 

6.2 Effects Assessment Uncertainties 

6.2.1 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 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 CRLF. 

117
6.2.2 Use of Surrogate Species Effects Data

Guideline toxicity tests and open literature data on methidathion are not available for 

frogs or any other aquatic-phase amphibian; therefore, freshwater fish are used as 

surrogate species for aquatic-phase amphibians. Efforts are made to select the organisms 

most likely to be affected by the type of compound and usage pattern; however, there is 

an inherent uncertainty in extrapolating across phyla. In addition, 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.

6.2.3 Sublethal Effects 

When assessing acute risk, 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 effects determination 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. However, the full suite of sublethal effects from valid 

open literature studies is considered for the purposes of defining the action area. 

When assessing acute risk, 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 effects determination 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. However, the full suite of sublethal effects from valid 

open literature studies is considered for the purposes of defining the action area. 

In fish, slight spastic motions and swimming on their sides were observed following 

acute exposure at 1000 μg/L (MRID 00011841). Following chronic exposure, total length 

and wet weight are affected (MRID 00015730 or 45822701) with a NOAEC/LOAEC of 

6.3/12 μg ai/L respectively. No sublethal effects were reported in the freshwater 

invertebrate studies. 

In birds, depression, reduced reaction to external stimuli, wing droop, convulsions, and 

salivation were observed following acute exposure via gavage. In subacute dietary 

studies, toxic symptoms including depression, reduced reaction to external stimuli, wing 

droop, loss of coordination, lower limb weakness, ruffled appearance, prostrate posture, 

and loss of righting reflex were observed at 178 ppm a.i. and above. Reductions on body 

weight gain and food consumption were reduced at levels above 316 ppm a.i.. Following 

chronic exposure (MRID 44381602), none of the ducks showed symptoms of toxicity or 

behavioral abnormalities during the experiment. An increase in the number of eggs 

118
cracked and a decrease in the number of eggs not cracked/eggs laid were observed at 10 

ppm. Brain cholinesterase was inconsistently affected at 30 ppm. 

The study used to define the action area examined the effects of subchronic methidathion 

administration on vascular wall damage in rats (ECOTOX ref. 80451). At the lowest dose 

tested (5 mg/kg/day), the levels of malondialdehyde (MDA), a biomarker for oxidative 

stress was significantly higher and cholinesterase activity was significantly lower. There 

were irregular, prominent breaks and fragmentation of the elastic fibers in the aortic wall. 

In a 2-generation reproduction study in rats (MRID 40079812, -13), the parental systemic 

NOAEC was 5 ppm and the LOAEC was 25 ppm, based on tremors and decreased food 

consumption during lactation, and decreased ovarian weight. In addition, there was also a 

slight decrease in body weight early in the F1 growth phase at 50 ppm. The reproductive 

NOAEC was 5 ppm and the LOAEC was 25 ppm based on a decreased mating index and 

a generalized indication of pup unthriftyness while nursing. In addition, there was an 

increase in stillbirths and decreased pup survival at birth and during lactation at the 50 

ppm treatment level. 

To the extent to which sublethal effects are not considered in this assessment, the 

potential direct and indirect effects of methidathion on CRLF may be underestimated. 

6.2.4 Location of Wildlife Species 

For the terrestrial exposure analysis of this 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. 

7. Risk Conclusions 

In fulfilling its obligations under Section 7(a)(2) of the Endangered Species Act, the 

information presented in this endangered species risk assessment represents the best data 

currently available to assess the potential risks of methidathion to the CRLF and its 

designated critical habitat. 

Based on the best available information, the Agency makes a Likely to Adversely Affect 

determination for the CRLF from the use of methidathion. Additionally, the Agency has 

determined that there is the potential for modification of CRLF designated critical habitat 

from the use of the chemical. 

There is potential for direct risk to the aquatic- and terrestrial-phase CRLF following 

either acute or chronic exposure to methidathion with its current use patterns in 

California. The acute listed species LOC for freshwater fish (surrogate to the aquatic119
phase CRLF) is exceeded for all uses; however, there are limited data which indicate that 

there will be some fish species that may not be at risk following acute exposure from 

some of the methidathion use scenarios. The chronic LOC for freshwater fish is also 

exceeded for all uses. For the terrestrial-phase CRLF, using avian data as a surrogate, 

with the exception of mangos on a dietary basis for non-listed species, the acute dosebased, acute dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF, 

exceed both the acute listed and non-listed species LOC and chronic LOC for all of the 

assessed uses. 

There is a potential for indirect effects to both the aquatic- and terrestrial-phase CRLF. 

For aquatic invertebrates (food source), the acute and chronic RQs exceed the acute nonlisted LOC for aquatic animals and chronic LOC for aquatic animals respectively, for all 

assessed uses. For fish and aquatic-phase amphibians (food source), the acute non-listed 

species LOC is exceeded for all uses. The chronic LOC is also exceeded for all uses. No 

studies are available for aquatic vascular and non-vascular plants (food source and 

habitat); however, the weight of the evidence indicates an effect determination of may 

affect, not likely to adversely affect (NLAA). For terrestrial invertebrates (food source), 

the RQs for both small and large insects exceed acute LOC for non-listed species for all 

methidathion uses, even with only a single use. For terrestrial-phase amphibians (food 

source), see the direct effects paragraph. For mammals (food source), the acute and 

chronic RQs exceed the non-listed species acute LOC and the chronic LOC, respectively 

for all uses. No studies are available for terrestrial plants (habitat). There is some 

evidence for potential for terrestrial plant damage; however, the weight of the evidence 

indicates that the effect to terrestrial plants is may affect, not likely to adversely affect 

(NLAA). Habitat modification is expected for both the aquatic- and terrestrial-phase 

CRLF based on indirect effects from the reduction in prey base. A significant overlap is 

expected to exist between the use sites and CRLF habitat, particularly when the spraydrift 

and downstream dilution buffers are added to the methidathion use area. Spraydrift 

distances range from 23 to greater than 1000 feet and the downstream dilution buffer is 

285 km. Figure 5.1 references the overlap of frog habitat with NLCD land cover data for 

use patterns that result in LAA determinations. Given the LAA determination for the 

CRLF and potential modification of designated critical habitat, a description of the 

baseline status and cumulative effects for the CRLF is provided in Attachment 2. 

A summary of the risk conclusions and effects determinations for the CRLF and its 

critical habitat, given the uncertainties discussed in Section 6, is presented in Tables 7.1 

and 7.2.

120
Table 7.1 Effects Determination Summary for Methidathion Use and the CRLF

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

Aquatic-phase (Eggs, Larvae, and Adults): 

Highly toxic to freshwater fish. Acute listed species LOC exceeded for all uses 

for three tested surrogate species. Chronic effects based on survival and growth. 

Chronic LOC exceeded for all uses. Probability of an individual effect on an 

acute basis is high, both at the acute LOC and at the RQ levels. Incident data 

also indicate freshwater fish vulnerability, and monitoring has detected surface 

water concentrations above the acute endpoint. Significant overlap expected to 

exist between use sites and CRLF habitat, particularly when the spraydrift and 

downstream dilution buffers are added to the methidathion use area. Spraydrift 

distances range from 23 to over 1000 feet and the downstream dilution buffer is 

285 km. 

Terrestrial-phase (Juveniles and Adults): 

Moderately toxic to very highly toxic to avian species. Acute dose-based, acute 

dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF, 

using avian data as a surrogate, exceed acute listed species LOC and chronic 

LOC for all of the assessed uses. Chronic effects based on reduction in number 

of normal hatchlings/live 3-week embryos. Limited data indicate that on a dose 

basis, the acute LOC is exceeded for all uses, even with the least sensitive 

species at the lowest application rate (mangos). On a dietary basis, the same is 

true for all uses except mangos. Probability of an individual effect on an acute 

basis is high at the RQ levels. Incident data indicate that birds are vulnerable. 

Significant overlap exists between use sites and CRLF habitat, particularly when 

the spraydrift buffers are applied to the methidathion use area. Spraydrift 

distances range from 194 to over 1000 feet. 

Potential for Indirect Effects

Aquatic prey items, aquatic habitat, cover and/or primary productivity 

Very highly toxic to freshwater invertebrates. Acute and chronic RQs exceed 

acute LOC for non-listed species and chronic LOC, respectively for all assessed 

uses. The percentage effect to the aquatic invertebrate prey base for all uses is 

very high. For fish (aquatic-phase amphibians), acute non-listed species LOC 

exceeded for all uses. Chronic LOC exceeded for all uses. Percentage effect to 

the freshwater fish/aquatic-phase amphibian prey base is very high. No studies 

are available for aquatic vascular and non-vascular plants. 

Survival, growth, 

and/or reproduction 

of CRLF 

individuals 

LAA1

Terrestrial prey items, riparian habitat 

RQs for both small and large insects exceed acute LOC for non-listed species for 

all methidathion uses, even with only a single use. Insufficient studies to 

conduct a sensitivity analysis; however, lowest RQ, using highest terrestrial 

invertebrate endpoint still exceeds the acute list species LOC. Percentage effect 

to the terrestrial invertebrate prey base for all uses is very high. For terrestrialphase amphibians using avian data as a surrogate, with the exception of mangos 

on an acute dietary basis, the acute dose-based, acute dietary-based and chronic 

dietary-based RQs exceed the acute non-listed species LOC and the chronic LOC 

for all of the assessed methidathion uses, respectively. Percentage effect to the 

avian/terrestrial-phase amphibian prey base is very high. For mammals, the 

acute and chronic RQs exceed the non-listed species acute LOC and the chronic 

LOC, respectively for all uses. At the lowest RQ, the percentage effect to the 

mammalian prey base is very high. No studies are available for terrestrial plants.

121
1

 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely 

affect (LAA)

Table 7.2 Effects Determination Summary for Methidathion Use and CRLF Critical Habitat 

Impact Analysis

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase PCE No studies available for aquatic vascular and non-vascular plants and 

terrestrial plants. 

For the aquatic-phase CRLF, acute non-listed species LOC for freshwater 

fish (aquatic-phase amphibians) exceeded for all uses. Chronic LOC for 

freshwater fish exceeded for all uses. Probability of an individual effect and 

percentage effect to the freshwater fish/aquatic-phase amphibian prey base 

on an acute basis at the RQ are high. Incident data support freshwater fish 

vulnerability. For freshwater invertebrates, acute and chronic RQs exceed 

acute LOC (non-listed species) and chronic LOC, respectively for all 

assessed uses. Again, the percentage effect to the aquatic invertebrate prey 

base is very high. Significant overlap expected between use sites and CRLF 

habitat, particularly when the spraydrift and downstream dilution buffers are 

added to the use area. Spraydrift distances range from 23 (mangos) to over 

1000 feet and the downstream dilution buffer is 285 km. 

Modification of 

terrestrial-phase 

PCE 

Habitat 

Modification1

No studies are available for terrestrial plants. 

For the terrestrial-phase CRLF, using avian data as a surrogate, with the 

exception of mangos on a dietary basis for non-listed species, the acute dosebased, acute dietary-based and chronic dietary-based RQs for the terrestrialphase CRLF, exceed both the acute listed and non-listed species LOC and 

chronic LOC for all of the assessed uses. Probability of an individual effect 

on an acute basis at the lowest RQ level and percentage effect to the 

avian/terrestrial-phase amphibian prey base are very high. Incident data 

indicate that birds are vulnerable. RQs for both small and large insects 

exceed acute LOC for non-listed species for all methidathion uses, even with 

only a single use. Percentage effect to terrestrial invertebrate prey base for 

all uses are very high. For mammals, the acute and chronic RQs exceed the 

non-listed species acute LOC and the chronic LOC, respectively for all uses. 

The percentage effect to the mammalian prey base is very high for all uses. 

Significant overlap exists between use sites and CRLF habitat, particularly 

when the spraydrift buffers are applied to the methidathion use area. 

Spraydrift distances range from 194 (mangos) to over 1000 feet. 1

 Habitat Modification or No effect (NE)

Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

and Wildlife Service under Section 7 of the Endangered Species Act should be initiated 

to seek concurrence with the LAA determinations and to determine whether there are 

reasonable and prudent alternatives and/or measures to reduce and/or eliminate potential 

incidental take. 

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 

122
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 

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 modification to critical habitat.

8. References 

Altig, R. and R.W. McDiarmid. 1999. Body Plan: Development and Morphology. In 

R.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. 

123
Aston, L.S. and J. N. Seiber. 1997. Fate of summertime airborne organophosphate 

pesticide residues in the Sierra Nevada Mountains. J. Environ. Qual. 26:1483-

1492. 

Burns, L.A. 1997. Exposure Analysis Modeling System (EXAMSII) Users Guide for 

Version 2.97.5, Environmental Research Laboratory, Office of Research and 

Development, U.S. Environmental Protection Agency, Athens, GA. 

Carsel, R.F. , J.C. Imhoff, P.R. Hummel, J.M. Cheplick and J.S. Donigian, Jr. 1997. 

PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in Crop Root and 

Unsaturated Soil Zones: Users Manual for Release 3.0; Environmental Research 

Laboratory, Office of Research and Development, U.S. Environmental Protection 

Agency, Athens, GA. 

Fellers, G.M, L.L. McConnell, D. Pratt, S. Datta. 2004. Pesticides in Mountain YellowLegged Frogs (Rana Mucosa) from the Sierra Nevada Mountains of California, 

USA. Environmental Toxicology & Chemistry 23 (9):2170-2177. 

Fellers, Gary M. 2005a. Rana draytonii Baird and Girard 1852. California Red-legged 

Frog. Pages 552-554. In: M. Lannoo (ed.) Amphibian Declines: The Conservation 

Status of United States Species, Vol. 2: Species Accounts. University of 

California Press, Berkeley, California. xxi+1094 pp. 

(http://www.werc.usgs.gov/pt-reyes/pdfs/Rana%20draytonii.PDF) 

Fellers, Gary M. 2005b. California red-legged frog, Rana draytonii Baird and Girard. 

Pages 198-201. In: L.L.C. Jones, et al (eds.) Amphibians of the Pacific Northwest. 

xxi+227. 

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

of the EPA food-chain (Kenaga) nomogram, and instrument for estimating 

pesticide residues on plants. Environmental Toxicology and Chemistry 13 

(9):1383-1391. 

Ganapathy, C., C. Nordmark, K. Bennet, and A. Bradley. 1997. Temporal distribution 

of insecticide residues in four California Rivers, California Department of 

Pesticide Regulation, report EH 97-06. 

http://www.cdpr.ca.gov/docs/sw/swemreps.htm . 

Hayes, M.P. and M.R. Jennings. 1988. Habitat correlates of distribution of the 

California red-legged frog (Rana aurora draytonii) and the foothill yellow-legged 

frog (Rana boylii): Implications for management. Pp. 144-158. In Proceedings 

of the symposium on the management of amphibians, reptiles, and small 

mammals in North America. R. Sarzo, K.E. Severson, and D.R. Patton (technical 

coordinators). USDA Forest Service General Technical Report RM-166. 

124
Hayes, M.P. and M.M. Miyamoto. 1984. Biochemical, behavioral and body size 

differences between Rana aurora aurora and R. a. draytonii. Copeia 1984(4): 

1018-22. 

Hayes and Tennant. 1985. Diet and feeding behavior of the California red-legged frog. 

The Southwestern Naturalist 30(4): 601-605. 

Hoerger, F., and E.E. Kenaga. 1972. Pesticide residues on plants: Correlation of 

representative data as a basis for estimation of their magnitude in the 

environment. In F. Coulston and F. Korte, eds., Environmental Quality and 

Safety: Chemistry, Toxicology, and Technology, Georg Thieme Publ, Stuttgart, 

West Germany, pp. 9-28. 

Jennings, M.R. and M.P. Hayes. 1985. Pre-1900 overharvest of California red-legged 

frogs (Rana aurora draytonii): The inducement for bullfrog (Rana catesbeiana) 

introduction. Herpetological Review 31(1): 94-103. 

Jennings, M.R. and M.P. Hayes. 1994. Amphibian and reptile species of special concern 

in California. Report prepared for the California Department of Fish and Game, 

Inland Fisheries Division, Rancho Cordova, California. 255 pp. 

Jennings, M.R., S. Townsend, and R.R. Duke. 1997. Santa Clara Valley Water District 

California red-legged frog distribution and status – 1997. Final Report prepared 

by H.T. Harvey & Associates, Alviso, California. 22 pp. 

Karvonen, T., Koivusalo, H., Jauhiainen, M., Palko, J. and Weppling, K. 1999. A 

hydrological model for predicting runoff from different land use areas, Journal of 

Hydrology, 217(3-4): 253-265. 

Kupferberg, S. 1997. Facilitation of periphyton production by tadpole grazing: 

Functional differences between species. Freshwater Biology 37:427-439. 

Kupferberg, S.J., J.C. Marks and M.E. Power. 1994. Effects of variation in natural 

algal and detrital diets on larval anuran (Hyla regilla) life-history traits. Copeia 

1994:446-457.

LeNoir, J.S., L.L. McConnell, G.M. Fellers, T.M. Cahill, J.N. Seiber. 1999. 

Summertime Transport of Current-use pesticides from California’s Central Valley 

to the Sierra Nevada Mountain Range,USA. Environmental Toxicology & 

Chemistry 18(12): 2715-2722. 

MacCoy, D., K.L. Crepeau, and K.M. Kuivila. (1995) Dissolved pesticide data for the 

San Joaquin River at Vernalis and the Sacramento River at Sacramento, 

California, 1991-94. U.S. Geological Survey open-file report 95-110. 

125
Majewski , M.S. and P.D. Capel. 1995. Pesticides in the atmosphere: distribution, trends, 

and governing factors. Ann Arbor Press, Inc. Chelsea, MI. 

McConnell, L.L., J.S. LeNoir, S. Datta, J.N. Seiber. 1998. Wet deposition of current-use 

pesticides in the Sierra Nevada mountain range, California, USA. Environmental 

Toxicology & Chemistry 17(10):1908-1916. 

McDonald M.A.1; Healey J.R.; Stevens P.A. 2002. The effects of secondary forest 

clearance and subsequent land-use on erosion losses and soil properties in the 

Blue Mountains of Jamaica. Agriculture, Ecosystems & Environment, Volume 

92, Number 1: 1-19. 

Okisaka S.; Murakami A.; Mizukawa A.; Ito J.; Vakulenko S.A.; Molotkov I.A.; Corbett 

C.W.; Wahl M.; Porter D.E.; Edwards D.; Moise C. 1997. Nonpoint source runoff 

modeling: A comparison of a forested watershed and an urban watershed on the 

South Carolina coast. Journal of Experimental Marine Biology and Ecology, 

Volume 213, Number 1: 133-149. 

Phuong V.T. and van Dam J. Linkages between forests and water: A review of research 

evidence in Vietnam. In: Forests, Water and Livelihoods European Tropical 

Forest Research Network. ETFRN NEWS (3pp). 

Poletika, N.N and C.K. Robb. (1998) A monitoring study to characterize chlorpyrifos 

concentration patterns and ecological risk in an agriculturally dominated tributary 

of the San Joaquin River. Dow AgroSciences LLC Study ENV96055. 

Rathburn, G.B. 1998. Rana aurora draytonii egg predation. Herpetological Review, 

29(3): 165. 

Reis, D.K. Habitat characteristics of California red-legged frogs (Rana aurora draytonii): 

Ecological differences between eggs, tadpoles, and adults in a coastal brackish 

and freshwater system. M.S. Thesis. San Jose State University. 58 pp. 

Seale, D.B. and N. Beckvar. 1980. The comparative ability of anuran larvae (genera: 

Hyla, Bufo and Rana) to ingest suspended blue-green algae. Copeia 1980:495-

503.

Sparling, D.W., G.M. Fellers, L.L. McConnell. 2001. Pesticides and amphibian 

population declines in California, USA. Environmental Toxicology & Chemistry 

20(7): 1591-1595. 

Teske, Milton E., and Thomas B. Curbishley. 2003. AgDisp ver 8.07 Users Manual. 

USDA Forest Service, Morgantown, WV. 

U.S. Environmental Protection Agency (U.S. EPA). 1998. Guidance for Ecological Risk 

Assessment. Risk Assessment Forum. EPA/630/R-95/002F, April 1998. 

126
U.S. EPA. 1999. Methidathion – Environmental Fate and Effects Reregistration 

Eligibility Decision Chapter. Office of Pesticide Programs, Environmental Fate 

and Effects Division. Novermber 30, 1999. Addended, April 4, 2001. 

U.S. EPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of 

Pesticide Programs. Office of Prevention, Pesticides, and Toxic Substances. 

Office of Pesticide Programs. Washington, D.C. January 23, 2004. 

U.S. Fish and Wildlife Service (USFWS). 1996. Endangered and threatened wildlife and 

plants: determination of threatened status for the California red-legged frog. 

Federal Register 61(101):25813-25833. 

USFWS. 2002. Recovery Plan for the California Red-legged Frog (Rana aurora 

draytonii). Region 1, USFWS, Portland, Oregon. 

(http://ecos.fws.gov/doc/recovery_plans/2002/020528.pdf) 

USFWS. 2006. Endangered and threatened wildlife and plants: determination of critical 

habitat for the California red-legged frog. 71 FR 19244-19346. 

USFWS. Website accessed: 30 December 2006. 

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

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

(NMFS). 1998. Endangered Species Consultation Handbook: Procedures for 

Conducting Consultation and Conference Activities Under Section 7 of the 

Endangered Species Act. Final Draft. March 1998. 

USFWS/NMFS. 2004. 50 CFR Part 402. Joint Counterpart Endangered Species Act 

Section 7 Consultation Regulations; Final Rule. FR 47732-47762. 

Vogel, J.R., M.S. Majewski, and P.D. Capel. 2008. Pesticides in rain in four agricultural 

watersheds in the United States. J. Environ. Qual. 37:1101-1115. 

Washburn, A. 2003. The Environmental Fate of Methidathion. California Department 

of Pesticide Regulation, Envrionmental Monitoring Branch, Fate Reviews. 

http://www.cdpr.ca.gov/docs/emon/pubs/fatememo/methidathion.pdf

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

Environmental Contamination and Toxicology. 100:23-73. 

Wassersug, R. 1984. Why tadpoles love fast food. Natural History 4/84. 

127