Risks of Oxamyl 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 

2/12/2009 
Primary Authors: 

Alicia Korol 

Greg Orrick 

Kristina Garber 

Secondary Reviewers: 

Marietta Echeverria 

Anita Pease 

Branch Chief, Environmental Risk Assessment Branch 4: 

Elizabeth Behl 

2 
Table of Contents 

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

2. Problem Formulation .................................................................................................................13 

2.1. Purpose................................................................................................................................13 

2.2. Scope...................................................................................................................................15 

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

2.4. Stressor Source and Distribution ........................................................................................16 

2.4.1. Environmental Fate Properties.....................................................................................16 

2.4.2. Mechanism of Action...................................................................................................19 

2.4.3. Use Characterization....................................................................................................20 

2.5. Assessed Species.................................................................................................................26 

2.5.1. Distribution ..................................................................................................................26 

2.5.2. Reproduction................................................................................................................29 

2.5.3. Diet ..............................................................................................29 

2.6. Designated Critical Habitat.................................................................................................31 

2.7. Action Area.........................................................................................................................33 

2.8. Assessment Endpoints and Measures of Ecological Effect ................................................35 

2.8.1. Assessment Endpoints for the CRLF...........................................................................35 

2.8.2. Assessment Endpoints for Designated Critical Habitat...............................................36 

2.9. Conceptual Model...............................................................................................................39 

2.9.1. Risk Hypotheses...........................................................................................................39 

2.9.2. Diagram........................................................................................................................39 

2.10. Analysis Plan ....................................................................................................................42 

2.10.1. Measures to Evaluate the Risk Hypothesis and Conceptual Model ..........................43 

2.10.2. Data Gaps...................................................................................................................47 

3. Exposure Assessment................................................................................................................47 

3.1. Label Application Rates and Intervals................................................................................47 

3.2. Aquatic Exposure Assessment............................................................................................49 

3.2.1. Modeling Approach .....................................................................................................49 

3.2.2. Model Inputs ................................................................................................................50 

3.2.3. Results..........................................................................................................................52 

3.2.4. Existing Monitoring Data ............................................................................................53 

3.3. Terrestrial Animal Exposure Assessment...........................................................................54 

3.4. Terrestrial Plant Exposure Assessment...............................................................................57 

4. Effects Assessment ....................................................................................................................58 

4.1. Toxicity of Oxamyl to Aquatic Organisms.........................................................................60 

4.1.1. Toxicity to Freshwater Fish .........................................................................................61 

4.1.2. Toxicity to Freshwater Invertebrates ...........................................................................61 

4.1.3. Toxicity to Aquatic Plants ...........................................................................................62 

4.2. Toxicity of Oxamyl to Terrestrial Organisms.....................................................................62 

4.2.1. Toxicity to Birds ..........................................................................................................64 

4.2.2. Toxicity to Mammals...................................................................................................65 

4.2.3. Toxicity to Terrestrial Invertebrates ............................................................................65 

4.2.4. Toxicity to Terrestrial Plants .......................................................................................66 

4.3. Incident Database Review ..................................................................................................66 

3
 
5. Risk Characterization................................................................................................................68 

5.1. Risk Estimation...................................................................................................................68 

5.1.1. Exposures in the Aquatic Habitat ................................................................................68 

5.1.2. Exposures in the Terrestrial Habitat ............................................................................71 

5.1.3. Primary Constituent Elements of Designated Critical Habitat ....................................75 

5.2. Risk Description .................................................................................................................76 

5.2.1. Direct Effects ...............................................................................................................79 

5.2.2. Indirect Effects (via Reductions in Prey Base)............................................................89 

5.2.3. Indirect Effects (via Habitat Effects) ...........................................................................96 

5.2.4. Modification to Designated Critical Habitat................................................................97 

5.3. Uncertainties ......................................................................................................................99 

5.3.1. Exposure Assessment Uncertainties ............................................................................99 

5.3.2. Effects Assessment Uncertainties..............................................................................104 

5.4. Addressing the risk hypotheses.........................................................................................106 

6. Risk Conclusions ....................................................................................................................107 

7. References
 

..................................................................................................110 

Appendices 

Appendix A. Intersection of Oxamyl Use Area and California Red-legged frog Habitat 

Appendix B. The Risk Quotient Method and Levels of Concern 

Appendix C. PRZM/EXAMS Input Files and Output File Data 

Appendix D. Example output from T-REX v.1.3.1 

Appendix E. Example output from Terrplant v.1.2.2 

Appendix F. List of citations accepted and rejected by ECOTOX criteria 

Appendix G. Detailed spreadsheet of available ECOTOX open literature for oxamyl 

Appendix H. Summary of human health effects data for oxamyl 

Appendix I. Example output from analysis of likelihood of individual mortality 

Appendix J. Example output from T-HERPS v.1.0 

Appendix K. Use of fugacity approach to estimate exposures to small mammals 

consuming earthworms contaminated with oxamyl from treatment sites 

Appendix L. Degradate summary 

Attachments 

Attachment 1: Status and Life History of California Red-legged Frog 

Attachment 2: Baseline Status and Cumulative Effects for the California Red-legged Frog 

4
 
List of Tables 

Table 1. Description of evidence supporting effects determination for oxamyl use in 

California. Assessment endpoint is survival, growth and reproduction of CRLF 

individuals......................................................................................................................... 11 

Table 2. Summary of effects determination for CRLF critical habitat based on uses of
 

Table 3. Summary of Chemical Properties and Environmental Fate Parameters of 

Table 4. Oxamyl uses involving foliara

 applications. These uses are registered for use in 

CA and considered part of the FIFRA regulatory action relevant to this endangered 

Table 5. Uses of oxamyl involving soil applications. These uses are registered for use in 

CA and considered part of the FIFRA regulatory action relevant to this endangered 

Table 6. Summary of California Department of Pesticide Registration (CDPR) Pesticide 

Table 7. Summary of California Department of Pesticide Registration (CDPR) Pesticide 

Table 9. Summary of Assessment Endpoints and Measures of Ecological Effect for 

Table 12. PRZM Scenarios and Input Parameters Describing Oxamyl Uses in California.
 

Table 15. Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for 

Table 16. Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Table 17. EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to 

Table 18. TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and SemiTable 23. Pesticides and their associated certainty classification associated with incident 

oxamyl in California. ........................................................................................................ 12 

Oxamyl.............................................................................................................................. 17 

species assessment. ........................................................................................................... 21 

species assessment. ........................................................................................................... 22 

Use Reporting (PUR) Data from 1999 to 2006 by County............................................... 24 

Use Reporting (PUR) Data from 1999 to 2006 for Currently Registered Oxamyl Uses.. 25 

Table 8. Assessment Endpoints and Measures of Ecological Effects. ............................. 36 

Primary Constituent Elements of Designated Critical Habitat. ........................................ 38 

Table 10. Oxamyl Uses by Foliar1

 Application Assessed for the CRLF.2 ....................... 48 

Table 11. Oxamyl Uses by Soil1 Application Assessed for the CRLF.2........................... 49 

........................................................................................................................................... 51 

Table 13. PRZM and EXAMS Chemical Input Parameters for Oxamyl.......................... 52 

Table 14. Aquatic 1-in-10-year EECs (μg/L) for Oxamyl Uses in California.................. 53 

oxamyl with T-REX.......................................................................................................... 55 

Exposures of the CRLF and its Prey to Oxamyl............................................................... 56 

Terrestrial Invertebrate Prey Items. .................................................................................. 57 

aquatic Areas Exposed to Oxamyl via Runoff and Drift. ................................................. 58 

Table 19. Freshwater Toxicity Profile for Oxamyl........................................................... 60 

Table 20. Categories of Acute Toxicity for Aquatic Organisms. ..................................... 61 

Table 21. Terrestrial Toxicity Profile for Oxamyl............................................................ 63 

Table 22. Categories of Acute Toxicity for Avian and Mammalian Studies.................... 64 

#I000799-003.................................................................................................................... 67 

Table 24. Summary of Direct Effect RQs for the Aquatic-phase CRLF. ......................... 69 

Table 25. Acute and Chronic RQs for aquatic invertebrates exposed to oxamyl............. 70 

5
 
Table 26. RQs for the terrestrial-phase CRLF based on acute and chronic exposures from
 

foliar applications of oxamyl at maximum use rates. These RQs were generated using TREX................................................................................................................................... 72 

Table 27. RQs for acute exposures to small and large terrestrial insects from foliar 

Table 28. RQs for acute and chronic exposures of small mammals consuming short grass 

Table 29. RQs for monocots and dicots inhabiting dry and semi-aquatic areas exposed to 

Table 30. Risk Estimation Summary for Oxamyl - Direct and Indirect Effects to CRLF.77 

Table 31. Risk Estimation Summary for Oxamyl – PCEs of Designated Critical Habitat 

Table 32. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for Dose-based 

Table 33. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for DietaryTable 34. Dose-based RQs for 1.4g CRLF consuming different food items (calculated 

Table 35. Refined Dose-based RQs for 37g CRLF consuming different food items 

Table 36. Refined Dose-based RQs for 238g CRLF consuming different food items 

Table 37. Refined Acute Dietary-based RQs for CRLF consuming different food items 

Table 38. Revised Chronic Dietary-based RQs for CRLF consuming different food items 

Table 39. Acute RQs and corresponding likelihood of individual effects and chronic RQs 

Table 40. Acute, dose-based RQ values for small mammals (15 g) exposed to oxamyl 

through consumption of food items on the treated field. RQs correspond to single 

Table 41. Concentration where LOC is not exceeded for small mammals (15 g) receiving 

acute, dose-based exposures of oxamyl through consumption of contaminated food items 

Table 42. Chronic, dose-based RQ values for small mammals (15 g) exposed to oxamyl 

through consumption of food items on the treated field. RQs correspond to single 

Table 43. Chronic, dietary-based RQ values for small mammals (15 g) exposed to 

oxamyl through consumption of food items on the treated field. RQs correspond to single 

Table 44. Acute dose-based RQs and associated likelihood of individual effects to 

Table 45. Description of evidence supporting effects determination for oxamyl use in 

Table 46. Summary of effects determination for CRLF critical habitat based on uses of 

applications of oxamyl at maximum use rates.................................................................. 73 

to oxamyl from foliar applications at maximum use rates................................................ 74 

oxamyl via spray drift and runoff. .................................................................................... 75 

for the CRLF..................................................................................................................... 78 

Exposures of the CRLF and its Prey to Oxamyl............................................................... 81 

based Exposures of the CRLF and its Prey to Oxamyl..................................................... 82 

using T-HERPS). .............................................................................................................. 83 

(calculated using T-HERPS)............................................................................................. 83 

(calculated using T-HERPS)............................................................................................. 84 

(calculated using T-HERPS)............................................................................................. 85 

(RQs calculated using T-HERPS)..................................................................................... 88 

for aquatic invertebrates exposed to oxamyl. ................................................................... 90 

applications of oxamyl at 1 lb a.i./A and 2 lb a.i./A. ........................................................ 93 

and distance from edge of treatment field where LOC is not exceeded. .......................... 94 

applications of oxamyl at 1 lb a.i./A and 2 lb a.i./A. ........................................................ 95 

applications of oxamyl at 1 lb a.i./A and 2 lb a.i./A. ........................................................ 95 

terrestrial-phase frogs (prey) due to oxamyl exposures.................................................... 96 

California. ....................................................................................................................... 108 

oxamyl in California. ...................................................................................................... 109 

6
 
List of Figures 

Figure 1. Estimated Annual Oxamyl Usage in the U.S. ................................................... 23 

Figure 2. Recovery Unit, Core Area, Critical Habitat, and Occurrence Designations for 

Figure 5. Conceptual Model for Pesticide effects on Aquatic-Phase of the CRLF and its 

Figure 6. Conceptual Model for Pesticide Effects on Terrestrial-Phase of the CRLF and 

CRLF................................................................................................................................. 28 

Figure 3. CRLF Reproductive Events by Month.............................................................. 29 

Figure 4. Initial area of concern for oxamyl use in California.......................................... 34 

Critical Habitat.................................................................................................................. 41 

its Critical Habitat............................................................................................................. 42 

 Figure 7. Chance of individual mortality to terrestrial-phase CRLF when considering
 

dose-based RQs................................................................................................................. 86 

Figure 8. Chance of individual mortality to terrestrial-phase CRLF when considering 

Figure 9. Likelihood of individual mortality to terrestrial invertebrates at different 

distances from the edge of a field treated with a single aerial application of oxamyl at 1 lb 

Figure 10. Likelihood of individual mortality to terrestrial invertebrates at different 

acute dietary-base RQs. .................................................................................................... 87 

a.i./A.................................................................................................................................. 91 

distances from the edge of a field treated with a single ground application of oxamyl at 2 

lb a.i./A.............................................................................................................................. 92 

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 oxamyl on 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. 

Oxamyl [(EZ)-N,N-dimethyl-2-methylcarbamoyloxyimino-2-(methylthio)acetamide; 

CAS# 23135-22-0; PC Code 103801] is an N-methyl carbamate. The mode of action of 

these pesticides is inhibition of the enzyme acetylcholinesterase. Oxamyl is currently 

registered as a restricted use plant growth regulator, acaricide, insecticide, miticide, and 

nematicide for the control of a broad spectrum of insects, mites, ticks, and nematodes on 

various field crops, vegetables, fruits, and non-bearing trees. The active ingredient is 

applied to treatment sites in liquid formulations by aerial, ground or chemigation 

application equipment or by soil injection. The following uses of oxamyl are registered 

in California and, therefore, considered part of the federal action evaluated in this 

assessment: apples, celery, cherries, citrus, clover, cotton, cucumber, eggplant, garlic, 

melons, onions, peaches, pears, peppers, potatoes, pumpkins, squash, tobacco, and 

tomatoes. Oxamyl has additional uses that are not registered for use in California (i.e., 

bananas, carrots, ginger, mint, peanuts, pineapple, plantains, sweet potatoes, and yams). 

Because oxamyl is not expected to be applied for these uses within California, which is 

the action area for oxamyl use that is relevant to the CRLF, these uses are not considered 

part of the federal action evaluated in this assessment. 

Oxamyl is hydrophilic, highly mobile, and relatively nonvolatile. The compound 

dissipates in the environment by abiotic and microbially-influenced degradation and by 

leaching, with estimated half-lives on the order of several hours to several weeks. 

Because oxamyl is relatively nonvolatile and does not bind well to soil, the major routes 

of off-site transport for the compound are expected to include surface water runoff and 

spray drift. 

Major degradates of oxamyl include oxime [2-hydroxyamino-N,N-dimethyl-2­

(methylthio)acetamide], DMOA [N,N-dimethyl-oxalamic acid], DMCF [cyanomethanoic acid dimethylamide ], DMEA [N,N-dimethyl-oxalamide], and carbon dioxide. 

Similar to oxamyl parent, oxime is highly mobile; DMOA, DMCF, and DMEA are 

expected to be highly mobile as well. Furthermore, oxime and DMOA are more 

8
 
persistent than oxamyl in certain conditions. Degradates of oxamyl are not considered to 

be of toxicological concern, based on limited mammalian toxicity data considered in the 

Reregistration Eligibility Decision (U.S. EPA 2000). Therefore, exposure to degradates 

of oxamyl was not considered in this assessment. 

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

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

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

resulting from runoff and spray drift from different uses. Peak model-estimated aquatic 

exposure estimates resulting from different oxamyl uses range from 3.2 to 45 μg/L. 

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

(insects, mice and terrestrial-phase frogs), the T-REX model is used. The T-HERPS 

model is used to allow for further characterization of amphibian-specific dietary 

exposures used to represent oxamyl exposures to terrestrial-phase CRLFs. T-REX and THERPS are used for oxamyl uses involving applications to foliar surfaces only, not uses 

involving applications to soil. Exposures of small mammals consuming earthworms 

contaminated with oxamyl applied directly to the soil are characterized using a fugacitybased approach. AgDRIFT is used to estimate deposition of oxamyl on terrestrial habitats 

from spray drift. The TerrPlant model is used to estimate oxamyl exposures to terrestrialphase CRLF habitat. 

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

on the survival, reproduction, and growth of the CRLF itself. 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. The acute toxicity endpoint 

for the aquatic-phase CRLF is a 96-h LC50 of 4.2 mg a.i./L. The chronic toxicity 

endpoint for the aquatic-phase CRLF is a NOAEC of 0.77 mg a.i./L, with a LOAEC of 

1.5 mg a.i./L based on embryo hatching and larval swim-up. In the terrestrial habitat, 

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

for terrestrial-phase amphibians. The acute and sub-acute endpoints for the terrestrialphase CRLF are LD50 = 3.16 mg/kg-bw and LC50 = 340 mg/kg-diet, respectively. The 

chronic endpoint for the terrestrial-phase CRLF is a NOAEC of 10 mg/kg-diet, which is 

based on a LOAEC of 50 mg/kg-diet, where effects to egg production and fertility were 

observed (in mallard ducks). 

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. The 

toxicity endpoint for algae, representing prey of tadpole-stage CRLF is 0.12 mg a.i./L 

(120-h EC50). The acute and chronic endpoints for aquatic invertebrates are 0.018 mg 

a.i./L (48-h EC50) and 0.012 mg a.i./L (NOAEC derived from an acute-to-chronic ratio), 

respectively. The endpoints for aquatic plants are 0.12 mg a.i./L (120-h EC50) and 30 mg 

a.i./L (14-d EC50) for non-vascular and vascular plants, respectively. 

9
 
In the terrestrial habitat, indirect effects due to depletion of prey are assessed by 

considering effects to terrestrial insects, small terrestrial mammals, and frogs. For 

terrestrial insects, the LD50 is 0.31 μg/bee. For terrestrial mammals, the acute and 

chronic endpoints are a 2.5 mg/kg-bw LD50 and a 25 mg/kg-diet NOAEC, with a LOAEC 

of 75 mg/kg-diet, based on decreased food consumption and altered body weight in first 

generation rats. The endpoints used to represent acute, sub-acute and chronic effects to 

the terrestrial-phase CRLF are also used to represent effects to terrestrial-phase frogs 

serving as prey to the CRLF (e.g., Pacific tree frogs). Indirect effects due to modification 

of the terrestrial habitat are characterized by available data for terrestrial monocots and 

dicots. The endpoint for terrestrial plants is an EC25 > 2.1 lbs a.i./A. 

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 oxamyl 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 oxamyl 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. 

Based on the best available information, the Agency makes a May Affect, and Likely to 

Adversely Affect (LAA) determination for the CRLF from the use of oxamyl. 

Additionally, the Agency has determined that there is the potential for modification of 

CRLF designated critical habitat from the use of the chemical. Summaries of the risk 

conclusions and effects determinations for the CRLF and its critical habitat are presented 

in Table 1 and in Table 2, respectively. These conclusions are based on potential effects 

of oxamyl to the terrestrial-phase CRLF and its prey, including terrestrial insects, small 

mammals and terrestrial-phase frogs. 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. 

10
 
Table 1. Description of evidence supporting effects determination for oxamyl use in California. 

Assessment endpoint is survival, growth and reproduction of CRLF individuals. 

Effects Habitat Basis for LAA Determination 

Direct Aquatic Direct effects to aquatic-phase CRLF resulting from acute and chronic exposures of oxamyl are not 

expected. 

- RQs for acute and chronic exposures resulting from all uses of oxamyl are below the levels of concern. 

Direct Terrestrial Oxamyl has the potential to directly affect the terrestrial-phase CRLF. 

-Acute and chronic RQs for all uses of oxamyl exceed the Agency’s LOCs for the terrestrial-phase 

CRLF. 

- The likelihoods of individual mortality range from 0.00034% to 100% for terrestrial-phase CRLF on a 

field treated with uses of oxamyl that have LOC exceedances (relevant to California). 

- Comparison of the LOAEC directly to chronic dietary-based EECs for CRLF consuming small insects 

and small herbivore mammals indicate that EECs for all uses are sufficient to exceed the concentration 

where reproductive effects were observed in the laboratory. 

Indirect Aquatic Indirect effects to the CRLF through effects to its prey in the aquatic habitat are discountable. 

- prey - RQs representing exposures to non-vascular aquatic plants (which serve as prey to tadpole stage 

CRLF) are below LOCs for all uses of oxamyl. 

- RQs representing acute and chronic exposures to fish (which serve as prey to adult CRLF) are below 

LOCs for all uses of oxamyl. 

- Although some RQs representing acute exposures to aquatic invertebrates are above the LOC, the 

likelihood of individual effects chance indicates that all uses of oxamyl are expected to result in ≤0.34% 

chance of effects to an individual aquatic invertebrate. Therefore, potential indirect effects to the CRLF 

due to effects of oxamyl to aquatic invertebrates resulting from acute exposures are discountable. 

- RQs representing chronic exposures to aquatic invertebrates (serving as prey to juvenile and adult 

CRLF) are below LOCs, with the exception of 2 uses (garlic and celery). EECs for garlic and celery do 

not reach the NOAEC for less sensitive species of aquatic invertebrates (i.e., Daphnia magna), 

suggesting that even though sensitive species of aquatic invertebrates may be affected by chronic 

exposures of oxamyl, less sensitive species, such as daphnids, may not be affected. Because the CRLF is 

an opportunistic feeder, the impact of effects to sensitive aquatic invertebrates from chronic exposures 

on the CRLF is discountable. 

Indirect Aquatic Indirect effects to the CRLF through modification of its aquatic habitat are not expected. 

- habitat -RQs representing exposures to non-vascular and vascular aquatic plants are below LOCs. 

-Estimated exposures of oxamyl to riparian vegetation are above the EC25 values for monocots and 

dicots. 

Indirect Terrestrial Oxamyl has the potential to indirectly affect the terrestrial-phase CRLF through effects to its prey. 

- prey -RQs representing exposures to terrestrial insects are above the LOC for all uses of oxamyl. 

- The likelihood of individual mortality is >99.9% for a terrestrial invertebrate on a field treated with any 

use of oxamyl (relevant to California). 

- >22% chance of mortality to individual small insects can occur within 950 feet from the edge of a field 

treated with 1 lb a.i./A oxamyl by aerial methods. 

-RQs representing acute and chronic exposures to small terrestrial mammals are above LOCs for all uses 

of oxamyl. 

- The likelihood of individual mortality is >99.9% for a small mammal consuming short grass on a field 

treated with any use of oxamyl (relevant to California). 

-Dose-based and dietary-based EECs are above levels where subacute effects to mammals were 

observed in chronic toxicity studies. 

-RQs representing exposures to terrestrial-phase frogs are above the LOC for all uses of oxamyl. 

-There is a >10% chance of effects to an individual terrestrial phase frog (serving as prey to CRLF) 

consuming small insects for all oxamyl uses, and a >10% chance of effects to an individual terrestrial 

phase frog consuming large insects for a majority of oxamyl uses (relevant to California). 

-Dose-based and dietary-based EECs are above the levels where subacute effects to birds were observed 

in chronic toxicity studies. 

Indirect 

- habitat 

Terrestrial Indirect effects to the CRLF through modification of its terrestrial habitat are not expected. 

-Estimated exposures of oxamyl to terrestrial vegetation are above the EC25 values for monocots and 

dicots. 

11
 
Table 2. Summary of effects determination for CRLF critical habitat based on uses of oxamyl in 

California. 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase 

primary constituent 

elements 

Habitat 

Modification 

Modification of habitat based on the aquatic-phase PCEs is discountable. 

- RQs for acute and chronic exposures directly to the CRLF resulting from all 

uses of oxamyl are below the levels of concern. 

- Indirect effects to the CRLF through effects to its prey in the aquatic habitat are 

discountable (see Table 1). 

-RQs representing exposures to non-vascular and vascular aquatic plants are 

below LOCs. 

-Estimated exposures of oxamyl to riparian vegetation are above the EC25 values 

for monocots and dicots. 

Modification of 

terrestrial-phase 

primary constituent 

elements 

Oxamyl has the potential to modify habitat based on the terrestrial-phase PCEs. 

- Oxamyl has the potential to directly affect the terrestrial-phase CRLF (See 

Table 1). 

Oxamyl has the potential to indirectly affect the terrestrial-phase CRLF through 

effects to its prey (see Table 1). 

-Estimated exposures of oxamyl to terrestrial vegetation are below the EC25 

values for monocots and dicots. 

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 

12
 
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. 

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 

oxamyl on agricultural crops. In addition, this assessment evaluates whether use on 

agricultural 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)) settlement 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, TerrPlant, and AgDRIFT, all of 

which are described at length in the Overview Document. Additional refinements 

13
 
include use of the T-HERPS model to refine estimates of exposure to terrestrial-phase 

CRLF as well as a fugacity based approach for estimating oxamyl concentrations in 

earthworms and resulting exposures to small mammals consuming earthworms. 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 oxamyl is based on an action area. The action area is the area directly or 

indirectly affected by the federal action, as indicated by the exceedance 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 oxamyl 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 oxamyl in accordance with current labels: 

• “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 oxamyl 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 oxamyl. 

If a determination is made that use of oxamyl 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 

14
 
habitat and oxamyl use sites) and further evaluation of the potential impact of oxamyl 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 oxamyl is expected to directly impact living organisms within the action area 

(defined in Section 2.7), critical habitat analysis for oxamyl 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 oxamyl 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. 

2.2. Scope 

The 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 oxamyl in accordance with the approved product labels for California is 

“the action” relevant to this ecological risk assessment. 

Oxamyl is an N-methyl carbamate currently registered for the control of a broad 

spectrum of insects, mites, ticks, and nematodes on various field crops, vegetables, fruits, 

and non-bearing trees. Current registrations of oxamyl allow for use nationwide with 

many use patterns limited to specific states including or excluding California. This 

ecological risk assessment and effects determination addresses currently registered uses 

of oxamyl 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. 

Major degradates of oxamyl include oxime [2-hydroxyamino-N,N-dimethyl-2­

(methylthio)acetamide], DMOA [N,N-dimethyl-oxalamic acid], DMCF [cyanomethanoic acid dimethylamide ], DMEA [N,N-dimethyl-oxalamide], and carbon dioxide. 

Similar to oxamyl parent, oxime is highly mobile; DMOA, DMCF, and DMEA are 

expected to be highly mobile as well. Furthermore, oxime and DMOA are more 

15
 
persistent than oxamyl in certain conditions. Degradates of oxamyl are not considered to 

be of toxicological concern, based on limited mammalian toxicity data on oxime and 

DMCF that were considered in the RED (U.S. EPA 2000). Therefore, exposure to 

degradates of oxamyl was not considered in this assessment. 

The Agency does not routinely include, in its risk assessments, an evaluation of mixtures 

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

formulations or those in the applicator’s tank. In the case of the product formulations of 

active ingredients (that is, a registered product containing more than one active 

ingredient), each active ingredient is subject to an individual risk assessment for 

regulatory decision regarding the active ingredient on a particular use site. If effects data 

are available for a formulated product containing more than one active ingredient, they 

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

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

2004). Nevertheless, oxamyl does not have any registered products that contain multiple 

active ingredients. No data were identified in the scientific literature to describe the 

effects of mixtures containing oxamyl on non-target organisms. 

2.3. Previous Assessments 

Oxamyl was first registered as an insecticide in the U.S. in 1974. The Agency issued a 

Registration Standard for oxamyl in 1987, which was updated in 1991. In 2000, the 

Agency completed an Interim Reregistration Eligibility Decision (IRED) for oxamyl (U.S 

EPA 2000), which was finalized in 2007. 

Since then, an ecological risk assessment for the new use of oxamyl on sugar beets has 

been conducted (U.S. EPA 2007). 

2.4. Stressor Source and Distribution 

2.4.1. Environmental Fate Properties 

Oxamyl [(EZ)-N,N-dimethyl-2-methylcarbamoyloxyimino-2-(methylthio)acetamide; 

CAS# 23135-22-0; PC Code 103801] is hydrophilic, highly mobile, and relatively 

nonvolatile. The compound dissipates in the environment by chemical and microbiallyinfluenced degradation and by leaching, with estimated half-lives on the order of several 

hours to several weeks. Table 3 summarizes the physiochemical and environmental fate 

properties of oxamyl. 

16
 
Table 3. Summary of Chemical Properties and Environmental Fate Parameters of Oxamyl. 

Parameter Value Reference 

Selected Physical/Chemical Parameters 

Molecular mass 219.3 g/mol MRID 40499702 

Vapor pressure (25°C) 3.84 x 10-7 torr MRID 42526101 

Water solubility (20°C) 2.82 x 105

 mg/L MRID 40499702 

n-Octanol-water partition coeff. (KOW) 0.36 MRID 40499702 

Persistence 

Hydrolysis half-life pH 5: >31 d 

pH 7: 8 d 

pH 9: 0.125 d 

MRID 40606516 

Aqueous photolysis half-life 14.2 d (pH 5) MRID 40606515; 

41058801 

Soil photolysis half-life No evidence of degradation ACC.# 147704 

Aerobic soil metabolism half-life 11 d (silt loam, pH 6.4, OM 2.8%) 

17 d (silt loam, pH 6.4, OM 2.8%) 

ACC.# 63012 

11 d (sandy clay loam, pH 7.7, OM 1.5%) MRID 42820001 

2.9 d (silt loam, pH 7.0, OM 0.4%) 

4.6 d (silt loam, pH 7.8, OM 2.1%) 

112 d (silty clay loam, pH 4.8, OM 4.4%) 

MRID 45176602 

Anaerobic soil metabolism half-life 5.2 d (silt loam, pH 4.6, OM 3.7%) 

5.8 d (sandy clay loam, pH 7.7, OM 1.5%) 

MRID 41346201 

MRID 42820001 

Aerobic aquatic metabolism half-life 

(half-life corrected for hydrolysis at 

pH 7) 

3.4 d (6.1 d) (sandy loam, pH 6.6-7.8) 

3.5 d (6.3 d) (sandy loam, pH 6.9-8.3) 

MRID 45045305 

Soil Mobility 

Organic carbon partition coefficient 

(KOC) 

10-60 L/kgOC (5 soils) 

6-10 L/kgOC (3 soils) 

2.5-8.7 L/kgOC (6 soils) 

MRID 46237301 

Bilkert and Rao, 1985 

Bromilow et al., 1980 

Column leaching (% parent in leachate; 

% identified residues in leachate) 

<0.2-83%; 89-100% (6 unaged soils) 

21-50%; 37-67% (3 aged soils) 

ACC.# 141395/ 

MRID 40606514 

Field Dissipation 

Terrestrial field dissipation half-life Not determined (NY) 

Not determined (CA) 

4 d (DE) 

3 d (FL), 4 d (CA), 19 d (WA) 

8.6 d (MS) 

(Oxamyl 

detected at 

deepest 

sample 

depths of 

each study.) 

ACC.# 145302 

ACC.# 149231 

ACC.# 40494 

MRID 41573201; 

41963901 

MRID 45045304 

17
 
2.4.1.1. Degradation 

Hydrolysis of oxamyl is pH-dependent, as oxamyl degrades rapidly in neutral to alkaline 

environments (half-life of 8 days and 3 hours at pH 7 and 9, respectively) and persists in 

acidic conditions (relatively stable at pH 5). Photolysis appears to be significant in 

acidic, clear, near-surface water (half-life of 14 days at pH 5), but not on soil (relatively 

stable). The major hydrolysis (pH 7 and 9) and aqueous photolysis transformation 

product is oxime, which comprised 83-93% of the applied radioactivity by the end of the 

hydrolysis studies (pH 7 and 9) and up to 75% of the applied by the end of the photolysis 

study. Although these studies were not conducted long enough to track a pattern of 

decline, they suggest oxime may be more persistent to abiotic degradation (i.e., 

hydrolysis and photolysis) than oxamyl. 

In aerobic aquatic systems, oxamyl degrades with a half-life of 3.4-3.5 days at pH 6.6­

8.3. The biodegradation half-life corrected for hydrolysis at pH 7 is 6.1-6.3 days (i.e., the 

pH 7 hydrolysis rate constant was subtracted from the degradation rate constants in 

aerobic aquatic systems in order to yield these rate constants for biodegradation alone). 

The major transformation products are oxime, DMOA, DMCF, DMEA, and carbon 

dioxide. In one study system, oxime reached 59% of the applied radioactivity after 1 day 

and DMOA totaled 79% of the applied after 30 days. In another study system, DMCF 

and DMEA were up to 55% and 14% of the applied, respectively, after 2 days. Carbon 

dioxide in these systems totaled 31-75% of the applied. 

In aerobic soil, oxamyl degrades with a half life ranging from 2.9 to 112 days (6 soils; pH 

4.8-7.8). The wide range in half-lives is likely due to variation in pH, as degradation may 

reflect hydrolysis as well as microbial metabolism. The major transformation products 

are oxime, DMOA, and carbon dioxide. In one aerobic metabolism study, oxime peaked 

at 24% of the applied radioactivity after 10 days, DMOA reached 20% of the applied 

after 21 days, and carbon dioxide comprised 45% of the applied after 51 days (MRID 

42820001). In another study, oxime comprised up to 51% of the applied after 7 days; 

DMOA was a maximum of 35% of the applied in a separate soil after 10 days; in both 

soils, carbon dioxide totaled 73-76% of the applied at study termination (MRID 

45176602). 

In anaerobic soil, oxamyl degrades with a half-life of 5 to 6 days (2 soils; pH 4.6-7.7). In 

a 32-day anaerobic study, oxime peaked at 70% at 20 days of flooding, declining to 22% 

at the end of the study; DMOA peaked at 23% at 32 days (MRID 42820001). In another 

anaerobic study, oxime only formed a maximum of 2% of the applied, while DMOA 

peaked at 86% of the applied after 30 days of flooding, remaining 74% of the applied at 

study termination (MRID 41346201). 

Major degradates of oxamyl include oxime [2-hydroxyamino-N,N-dimethyl-2­

(methylthio)acetamide], DMOA [N,N-dimethyl-oxalamic acid], DMCF [cyanomethanoic acid dimethylamide ], DMEA [N,N-dimethyl-oxalamide], and carbon dioxide. 

Similar to oxamyl parent, oxime is highly mobile; DMOA, DMCF, and DMEA are 

expected to be highly mobile as well. Furthermore, oxime and DMOA are more 

18
 
persistent than oxamyl in certain conditions, such as abiotic conditions in the case of 

oxime. 

2.4.1.2. Transport 

Oxamyl has little affinity for adsorption on a variety of soils and is mobile to highly 

mobile according to the FAO soil mobility classification scheme (U.S. EPA 2006). In a 

submitted batch equilibrium study (5 soils), average soil-water partition coefficients (Kd) 

ranged from 0.12 to 0.80 L/kg and organic carbon partitioning coefficients (KOC) ranged 

from 10 to 60 L/KgOC (adsorption to one soil was too low to calculate a Freundlich 

isotherm). Adsorption was correlated to organic carbon content, demonstrated by less 

variability in KOC values compared to that in Kd values. Batch equilibrium studies in the 

open literature reported lower organic carbon partition coefficients (range of 2.5 to 10 

L/KgOC for 9 soils; Bilkert and Rao, 1985; Bromilow et al., 1980). Oxime has similar 

mobility to oxamyl parent, with Kd values ranging from 0.33 to 0.67 L/kg (5 soils) and 

KOC values ranging from 18 to 66 L/KgOC. 

Soil column leaching studies confirm the mobility of oxamyl. In a study using 2 soils, 

83-100% of the unaged parent was collected in the leachate. In a second study with 4 

soils, <0.2-83% of the unaged parent and 89-95% of unaged residues were collected in 

the leachate. While aging reduces the mobility of oxamyl residues, significant amounts 

were still detected; 67% of 7-day aged residues, and 37% of 18-day aged residues, 

compared to 95% in unaged residues (12-inch long column). Oxime and DMOA were 

found in both the unaged and aged residue leachate. In an 18-inch long column study, 

61-63% of the applied radioactivity of oxamyl residues aged 30 days were recovered in 

the leachate. 

Oxamyl has a relatively low partial vapor pressure (3.8 x 10-7 torr at 25°C) and is soluble 

in water up to 2.8 x 105 mg/L at 20°C. This indicates that the compound will not readily 

volatilize from soil or water or precipitate from water. Oxamyl has a low n-octanol-water 

partition coefficient (KOW = 0.36) and, therefore, is not expected to bioaccumulate. 

2.4.1.3. Field Dissipation 

In the field, half of the applied oxamyl dissipated from the surface in less than 3 weeks 

(DT50 range of 3 to 19 days) in studies from Florida, California and Washington. When 

both oxamyl and oxime residues are considered, the combined DT50 values range from 4 

to 39 days. Field dissipation studies show that both oxamyl and oxime leach through the 

soil, confirming that these residues have a low affinity for adsorption and are mobile in 

soil. Oxamyl residues reached the lowest sampled soil depth within several weeks of 

application in a variety of crops and sites. 

2.4.2. Mechanism of Action 

Oxamyl is an N-methyl carbamate insecticide, acaricide, miticide, nematocide, and plant 

growth regulator. Carbamate toxicity (to target pests) is based on the inhibition of the 

19
 
enzyme acetylcholine esterase, which cleaves the neurotransmitter acetylcholine. 

Inhibition of acetylcholine esterase by carbamate insecticides, such as oxamyl, interferes 

with proper neurotransmission in cholinergic synapses and neuromuscular junctions. 

In rodents and humans, which are non-target organisms, N-methyl carbamates cause 

neurotoxicity via the inhibition of cholinesterase by carbamylation of the serine hydroxyl 

group located in the active site of the enzyme, which leads to accumulation of 

acetylcholine and ultimately clinical signs (U.S. EPA 2007). 

2.4.3. Use Characterization 

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

action. The current labels for oxamyl represent 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. 

Oxamyl is registered for 29 agricultural uses in the United States; however, not all of 

these uses are federally labeled for use in California. Section 3 uses of oxamyl that are 

federally labeled for use in California include: tree fruit (apples, cherries, citrus, peaches, 

pears), fruits and vegetables (celery, cucumber, eggplant, garlic, melons, onions, peppers, 

potatoes, pumpkins, squash and tomatoes), cotton, and tobacco. In addition, one special 

local need registration exists for use of oxamyl on clover grown for seed in California 

(SLN # CA-06-0028). Although current labels do not exclude the use of oxamyl on 

tobacco in California, tobacco is not grown in the State. U.S. Department of Agriculture 

census data confirm that tobacco has not been grown in California from at least the 1992 

report, forward (USDA 2008). It is unlikely that tobacco agriculture will be initiated in 

California in the future. Therefore, use of oxamyl on tobacco was not quantitatively 

evaluated for exposure in this assessment. The 18 agricultural uses allowed in California 

for existing crops are evaluated for exposure in this assessment. 

Oxamyl has some additional uses that are registered in other states, but prohibited (by the 

federal label) in California (i.e., bananas, carrots, ginger, mint, peanuts, pineapple, 

plantains, sweet potatoes, yams). Because oxamyl is not expected to be applied to these 

crops within California, which is the action area for oxamyl use that is relevant to the 

CLRF, these additional uses are not considered part of the federal action evaluated in this 

assessment. 

Applications of oxamyl are made to soil, foliar surfaces of crops and to transplants. 

Application methods for soil uses include injection, band treatment, incorporation, infurrow, and broadcast. Foliar applications include the use of chemigation, ground and 

aerial equipment. Target pests of oxamyl include nematodes, mites and insects. Single 

maximum application rates of oxamyl range from 1-4 lb a.i./A, depending upon the 

specific use and application method. At this time, there are 2 approved formulated 

products containing oxamyl that are used in the United States (registrations 352-532 and 

352-372). Both products are water soluble liquids. Table 4 and Table 5 present oxamyl 

20
 
uses and corresponding maximum application rates and application methods considered 

in this assessment. 

Table 4. Oxamyl uses involving foliara applications. These uses are registered for use in CA and 

considered part of the FIFRA regulatory action relevant to this endangered species assessment. 

Use Application Method 

Maximum Single 

Application Rate 

(lb a.i./A) 

Maximum # 

Applications per 

Season 

Apple (bearing fruit) Ground 2.0 1 

Celery Ground and Aerial 1.0 6 

Citrus (bearing fruit) Ground and Aerial 1.0 6 

Chemigation 2.0 3 

Clover (for seed)b Ground and Aerial 1.0 2 

Cotton Ground and Aerial 1.0 3 

Cucumber Ground, Aerial, Chemigation 1.0 6 

Eggplant Ground 1.0 6 

Garlic Ground and Chemigation 2.0 2.25d 

Aerial 1.0 4.5 

Melonsc Ground, Aerial, Chemigation 1.0 6 

Onion (dry bulb) Ground and Chemigation 2.0 2.25d 

Aerial 1.0 4.5 

Non-bearing fruite Ground 2.0 4 

Aerial 1.0 8 

Pepper Ground, Aerial, Chemigation 1.0 6 

Potato Ground, Aerial, Chemigation 1.0 8 

Pumpkin Ground, Aerial, Chemigation 1.0 6 

Squash Ground, Aerial, Chemigation 1.0 6 

Tomato Ground and Aerial 1.0 8 

Chemigation 2.0 4 

a Foliar application includes ground, aerial, and chemigation. b Use on clover is registered under FIFRA Section 24(c). 

c Melons includes cantaloupe, honeydew, and watermelon.
 d Oxamyl product labels allow for a maximum of 2 applications for use on garlic and onion per season at
 

2.0 lb a.i./A with one additional application of 0.5 lb a.i./A. e Non-bearing fruit includes apple, cherry, citrus, peach, and pear. 

21
 
Table 5. Uses of oxamyl involving soil applications. These uses are registered for use in CA and 

considered part of the FIFRA regulatory action relevant to this endangered species assessment. 

Use Maximum Single 

Application Rate 

(lb a.i./A) 

Maximum # 

applications per 

season 

Apples (bearing fruit) NA NA 

Celery 1.0 6 

Citrus (bearing fruit) NA NA 

Clover (for seed)b NA NA 

Cotton NA NA 

Cucumber 4.0 1d 

Eggplant NA NA 

Garlic 2.0 2.25 

Melonsc 4.0 1d 

Onion (dry bulb) 2.0 2.25 

Non-bearing fruite 4.0 1d 

Pepper 1.0 6 

Potatof 4.0 1d 

Pumpkin 4.0 1d 

Squash 4.0 1d 

Tomato 0.75-1.25g 3 

NA = not applicable
 a Soil applications include injection, incorporation, or in-furrow. b

 Use on clover is registered under FIFRA Section 24(c). 

c Melons includes cantaloupe, honeydew, and watermelon.
 d This application is intended to be made preplant soil; therefore, it is assumed that 

there is one application per season. 

e Non-bearing fruit includes apple, cherry, citrus, peach, and pear uses. 

f Label allows for a preplant soil application of 4 lb a.i./A. 

g For this use, maximum application rates should be as follows: the first application
 

should be 0.75 lb a.i./A, the second should be 1.25 lb a.i./A, and the third should be 1.0 

lb a.i./A. 

In order to understand the extent of the use of oxamyl, national level data are considered. 

Figure 1 depicts estimated annual oxamyl use in the continental United States in 2002. 

As of 2002, approximately 700,000 pounds of oxamyl were applied annually. The highest 

uses of oxamyl included cotton and potatoes (76% of total use). 

22
 
Figure 1. Estimated Annual Oxamyl Usage in the U.S. 

(from http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m6045) 

[The pesticide use maps available from this site show the average annual pesticide use intensity expressed 

as average weight (in pounds) of a pesticide applied to each square mile of agricultural land in a county. 

The area of each map is based on state-level estimates of pesticide use rates for individual crops that were 

compiled by the CropLife Foundation, Crop Protection Research Institute during based on information 

collected during 1999 through 2004 and on 2002 Census of Agriculture county crop acreage. The maps do 

not represent a specific year, but rather show typical use patterns over the five year period 1999 through 

2004.] 

Data describing documented uses of oxamyl in the state of California are also considered. 

These data were obtained form the California’s Department of Pesticide Regulation 

Pesticide Use Reporting (CDPR PUR) database1

. According to CDPR PUR data, the 

greatest quantity of oxamyl was applied in Fresno County with a yearly average 

application of 40,801 lbs a.i./year, followed by Kings County, Kern County and Ventura 

County with 22,642 lbs a.i./year, 11,738 lbs a.i./year, and 10,914 lbs a.i./year, 

respectively (Table 6). 

1

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

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

23
 
Table 6. Summary of California Department of Pesticide Registration (CDPR) Pesticide Use 

Reporting (PUR) Data from 1999 to 2006 by County. 

County* 

Average Annual 

Application 

(lbs a.i./year) 

% of total average 

annual use in CA 

Fresno 40,801 36.4% 

Kings 22,642 20.2% 

Kern 11,738 10.5% 

Ventura 10,914 9.7% 

Monterey 6,393 5.7% 

Tulare 3,470 3.1% 

Merced 3,264 2.9% 

Imperial 2,326 2.1% 

Riverside 2,202 2.0% 

San Diego 2,066 1.8% 

Santa Barbara 1,389 1.2% 

San Benito 1,159 1.0% 

* Orange , Santa Clara, San Luis Obispo, Madera, San Joaquin, Modoc, Stanlius, 

Colusa, Yolo, Santa Cruz, Los Angeles, Sutter, Siskiyou, contra Costa, Glenn, Solano, 

Yuba, Sacramento, Butte and Tehama Counties have <1000 pounds applied annually, 

which is <1% of the total average annual use in the state of California. 

According to data from the PUR database, from 1999 to 2006, oxamyl was used in 

greatest quantity on cotton with an average yearly application of 78,118 lbs a.i./year. 

Celery, tomatoes (produce), onions (dry), and peppers (fruiting) followed with 17,125 lbs 

a.i./year, 4,890 lbs a.i./year, 2,854 lbs a.i./year, and 2,484 lbs a.i./year, respectively 

(Table 7). 

24
 
Table 7. Summary of California Department of Pesticide Registration (CDPR) Pesticide Use 

Reporting (PUR) Data from 1999 to 2006 for Currently Registered Oxamyl Uses. 

Crop 

Average Annual StateWide Application 

(lbs a.i./ year) 

Application Rate (lbs a.i./acre) 

Mean Maximum 

Cotton 78,118 0.74 8.46 

Celery 17,125 0.67 6.81 

Tomato, produce 4,890 0.68 7.96 

Onion, dry 2,854 0.91 9.37 

Pepper, fruiting 2,484 0.69 7.47 

Tomato, processing 1,672 0.73 4.98 

Cantalope 1,505 0.87 3.93 

Melon 1,397 0.88 14.93 

Garlic 577 1.34 1.99 

Apple 314 0.95 3.88 

Eggplant 209 0.73 10.45 

Watermelon 194 0.63 5.12 

Potato 141 0.65 0.95 

Cucumber 139 0.72 2.21 

Squash 114 0.61 1.19 

Orange 98 0.81 1.98 

Squash, summer 77 0.79 1.99 

Pumpkin 75 0.72 1.00 

Pepper, spice 40 0.72 1.00 

Pepper, pimento 19 0.43 0.50 

Peach 13 10.16 10.35 

Nectarine 7.5 5.04 9.96 

Squash, zucchini 4.6 2.07 2.99 

Lemon 3.1 0.91 1.00 

Tangelo 2.5 0.66 0.66 

Onion, green 2.0 0.50 0.50 

Tangerine 0.62 0.50 0.50 

Squash, winter 0.34 0.55 0.55 

A number of maximum application rates for different uses exceeded the label-permitted 

rate. For example, the maximum rate given for use on eggplant was 10.45 lbs a.i./acre, 

an exceedance from the maximum label rate by about 1000%. However, a review of the 

CDPR PUR individual recorded uses for the years 2005 and 2006 demonstrate that these 

extremely high values are either just rare incidences of misuse, or perhaps misreported 

values. For 2005, the average application rate was 0.79 lbs a.i./year, the 75th %ile 

application rate was 1.00 lbs a.i./year, and the 95th %ile application rate was 1.00 lbs 

a.i./year. Similarly in 2006, the average application rate was 0.82 lbs a.i./year, the 75th 

%ile was 1.00 lbs a.i./year, and the 95th percentile rate was 1.01 lbs a.i./year. These 

25
 
values are below or just at the maximum label-permitted application rates; thus, the usage 

data confirm that assessing exposure of oxamyl to the CRLF based on maximum label 

rates is reflective of actual use rates for a majority of use sites (Table 7). 

It should be noted that the uses considered in this risk assessment represent all currently 

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

this assessment. Any reported use, such as may be seen in the CDPR PUR database, 

represent either historic uses that have been cancelled, misreported uses, or misuse. 

Historical uses, misreported uses, and misuse are not considered part of the federal action 

and, therefore, are not considered in this assessment. 

2.5. Assessed Species 

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

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

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

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

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

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

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

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

for the CRLF is provided in Section 2.6. 

2.5.1. Distribution 

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

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

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

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

an 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). 

26
 
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). Recovery units, core areas, and other known occurrences of the CRLF from 

the CNDDB are described in further detail in Attachment I, 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. 

27
 
 and Western 

o Bay 

es and Tehachapi 

nsular Ranges 

Recovery Units 

1. Sierra Nevada Foothills and C entral Valley

2. North Coast Range Foothills 

Sacramento River Valley 

3. North Coast and North San Fr ancisco Bayancisco Bay

4. South and East San Francisc 

5. Central Coast 

6. Diablo Range and Salinas Val ley

7. Northern Transverse Rang 

Mountains 

8. Southern Transverse and Peni 

Core Ar eas

1. Feather River 	 Feather River

2. Yuba River- S. Fork Feather River 	 Yuba River- S. Fork Feather River

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

4. Cosumnes River 	 Cosumnes River

5. South Fork Calaveras River* 	 South Fork Calaveras River*

6. Tuolumne River* 	 Tuolumne River*

7. Piney Creek* 	 Piney Creek*

8. Cottonwood Creek 	 Cottonwood Creek

9. Putah Creek – Cache Creek* 	 Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries 10. Lake Berryessa Tributaries

11. Upper Sonoma Creek 11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek 12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula 13. Pt. Reyes Peninsula

14. Belvedere Lagoon 	 Belvedere Lagoon

15. Jameson Canyon – Lower Napa River 	 Jameson Canyon – Lower Napa River

16. East San Francisco Bay 	 East San Francisco Bay

17. Santa Clara Valley 	 Santa Clara Valley

18. South San Francisco Bay 	 South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough 19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia 20. Carmel River – Santa Lucia

21. Gablan Range 21. Gablan Range

22. Estero Bay 22. Estero Bay

23. Arroyo Grange River 23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River 24. Santa Maria River – Santa Ynez River

25. Sisquoc River 25. Sisquoc River

26. Ventura River – Santa Clara River 26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams 27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River 28. Estrella River

29. San Gabriel Mountain* 29. San Gabriel Mountain*

30. Forks of the Mojave* 30. Forks of the Mojave*

31. Santa Ana Mountain* 31. Santa Ana Mountain*

32. Santa Rosa Plateau 32. Santa Rosa Plateau

33. San Luis Ray* 33. San Luis Ray*

34. Sweetwater* 34. Sweetwater*

35. Laguna Mountain* 35. Laguna Mountain*

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

Figure 2. Recovery Unit, Core Area, Critical Habitat, and Occurrence Designations for CRLF 

28
 
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 3 depicts CRLF annual reproductive timing. 

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

Figure 3. CRLF Reproductive Events by Month 

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 

29
 
many as 42 different invertebrate taxa, including Arachnida, Amphipoda, Isopoda, 

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

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

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

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

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

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

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

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

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

2.5.4. Habitat 

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

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

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

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

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

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

1997). Dense vegetation 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 

30
 
Jennings and Hayes (1994), CRLFs also use small mammal burrows and moist leaf litter 

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

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

exposure (Alvarez 2000). 

2.6. Designated Critical Habitat 

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

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

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

areas (previously discussed in Section 2.5.1) is provided in Attachment I. 

‘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 (Section 7) 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 I. 

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 

31
 
when a designation limited to its present range would be inadequate to ensure the 

conservation of the species. For the CRLF, all designated critical habitat units contain all 

four of the PCEs, and were occupied by the CRLF at the time of FR listing notice in 

April 2006. The FR notice designating critical habitat for the CRLF includes a special 

rule exempting routine ranching activities associated with livestock ranching from 

incidental take prohibitions. The purpose of this exemption is to promote the 

conservation of rangelands, which could be beneficial to the CRLF, and to reduce the rate 

of conversion to other land uses that are incompatible with CRLF conservation. Please 

see Attachment I for a full explanation on this special rule. 

USFWS has established modification standards for designated critical habitat (USFWS 

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

the PCEs and jeopardize the continued existence of the species. Evaluation of actions 

related to use of oxamyl 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 oxamyl is expected to directly impact living 

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

32
 
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 oxamyl 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 (U.S. EPA 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 oxamyl may be expected to have on the 

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

best available information concerning the use of oxamyl 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 

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 oxamyl. An analysis of labeled uses and review of available product labels was 

completed (See use characterization in section 2.4.3). For those uses relevant to the 

CRLF, the analysis indicates that, for oxamyl, only agricultural uses are considered as 

part of the federal action evaluated in this assessment: apple, celery, cherry, citrus, clover, 

cotton, cucumber, eggplant, garlic, melons (cantaloupe, honeydew, and watermelon), 

onion, peach, pear, pepper, potato, pumpkin, squash, and tomato. 

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

of oxamyl 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 oxamyl is presented in Figure 4. 

33
 
Figure 4. Initial area of concern for oxamyl use in California. 

34
 
Once the initial area of concern is defined, the next step is to compare the extent of that 

area with the results of the screening level risk assessment. In this assessment, transport 

of oxamyl through runoff and spray drift is considered in deriving quantitative estimates 

of oxamyl exposure to CRLF, its prey and its habitats. Since this screening level risk 

assessment defines taxa that are predicted to be exposed through runoff and drift to 

oxamyl at concentrations above the Agency’s Levels of Concern (LOC), there is need to 

expand the action area to include areas that are affected indirectly by this federal action. 

Because of the lack of a NOAEC in the dietary-based acute study for the bobwhite quail 

(MRID: 406065-11), described later in Section 4.2.1, the action area for oxamyl is 

established as the entire state of California. Additional analysis related to the intersection 

of the oxamyl action area and CRLF habitat used in determining the final action area is 

described in Appendix A. 

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.”1

 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 oxamyl 

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

to oxamyl (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 (U.S. EPA 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. 

1

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

35
 
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 oxamyl is provided in Table 8. 

Table 8. Assessment Endpoints and Measures of Ecological Effects. 

Assessment Endpoint Measures of Ecological Effects3 

Aquatic-Phase CRLF (Eggs, larvae, juveniles, and adults)a 

Direct Effects 

1. Survival, growth, and reproduction of CRLF 1a. Rainbow trout (Oncorhynchus mykiss) LC50 

1b. Rainbow trout (Oncorhynchus mykiss) NOEC 

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) 

2a. Algae (Navicula pelliculosa) EC50 

2b. Rainbow trout (Oncorhynchus mykiss) LC50 

2c. Midge (Chironomus plumosus) EC50 

2d. Rainbow trout (Oncorhynchus mykiss) NOEC 

2e. Midge (Chironomus plumosus) NOEC 

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. Algae (Navicula pelliculosa) EC50 

3b. Duckweed (Lemna Gibba) EC50 

4. Survival, growth, and reproduction of CRLF 

individuals via effects to riparian vegetation 

4a. Monocots: Seedling Emergence, Vegetative 

Vigor EC25 

4b. Dicots: Seedling Emergence, Vegetative 

Vigor EC25 

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. Northern Bobwhite Quail (Colinus virginianus) LC50 

5b. Mallard duck (Anas platyrhynchos) NOEAC 

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. Laboratory rat (Rattus norvegicus) 

Female LD50 and Male LD50 

6b. Laboratory rat (Rattus norvegicus) NOAEC 

6c. Mallard duck (Anas platyrhynchos) LD50 

6d. Honey bee (Apis mellifera) LD50 

6e. Northern Bobwhite Quail (Colinus virginianus) LC50 

6f. Mallard duck (Anas platyrhynchos) NOEAC 

7. Survival, growth, and reproduction of CRLF 

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

riparian and upland vegetation) 

7a. Monocots: Seedling Emergence, Vegetative Vigor EC25 

7b. Dicots: Seedling Emergence, Vegetative Vigor EC25 

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 oxamyl 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 

3

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

Appendix A. 

36
 
are those that alter the PCEs and jeopardize the continued existence of the CRLF. 

Therefore, these actions are identified as assessment endpoints. It should be noted that 

evaluation of PCEs as assessment endpoints is limited to those of a biological nature (i.e., 

the biological resource requirements for the listed species associated with the critical 

habitat) and those for which oxamyl effects data are available (see Table 9). 

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

in Section 2.6. 

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

CRLF are described in section 2.6. 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). 

37
 
Table 9. Summary of Assessment Endpoints and Measures of Ecological Effect for Primary 

Constituent Elements of Designated Critical Habitat. 

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 1a. Algae (Navicula pelliculosa) EC50 

stream channel or pond: aquatic habitat (including 1b. Duckweed (Lemna Gibba) EC50 

riparian vegetation) provides for shelter, foraging, 1c. Monocots: Seedling Emergence, Vegetative Vigor EC50 

predator avoidance, and aquatic dispersal for juvenile 1d. Dicots: Seedling Emergence, Vegetative Vigor EC25 

and adult CRLFs. 

Alteration in water chemistry/quality including 2a. Algae (Navicula pelliculosa) EC50 

temperature, turbidity, and oxygen content necessary 2b. Duckweed (Lemna Gibba) EC50 

for normal growth and viability of juvenile and adult 2c. Monocots: Seedling Emergence, Vegetative Vigor EC25 

CRLFs and their food source. 2d. Dicots: Seedling Emergence, Vegetative Vigor EC25 

3a. Rainbow trout (Oncorhynchus mykiss) LC50 

Alteration of other chemical characteristics necessary 3b. Midge (Chironomus plumosus) EC50 

for normal growth and viability of CRLFs and their 3c. Rainbow trout (Oncorhynchus mykiss) NOEC 

food source. 3d. Midge (Chironomus plumosus) NOEC 

3e. Algae (Navicula pelliculosa) EC50 

Reduction and/or modification of aquatic-based food 

sources for pre-metamorphs (e.g., algae) 4. Algae (Navicula pelliculosa) 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 

5a. Monocots: Seedling Emergence, Vegetative Vigor EC25 

5b. Dicots: Seedling Emergence, Vegetative Vigor EC25 

5c. Laboratory rat (Rattus norvegicus) Female LD50 and

 Male LD50 

5d. Laboratory rat (Rattus norvegicus) NOAEC 

5e. Mallard duck (Anas platyrhynchos) LD50 

5f. Honey bee (Apis mellifera) LD50 

5g. Northern Bobwhite Quail (Colinus virginianus) LC50 

5h. Mallard duck (Anas platyrhynchos) NOEAC 

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 

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. 

38
 
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 oxamyl to the environment. 

The following risk hypotheses are presumed for this endangered species assessment: 

The labeled use of oxamyl 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 oxamyl release mechanisms, biological receptor types, and effects 

endpoints of potential concern. The conceptual models for terrestrial and aquatic 

39
 
exposures are shown in Figure 5 and in Figure 6, respectively, which include the 

conceptual models for the aquatic and terrestrial PCE components of critical habitat. 

Based on available fate and transport data for oxamyl (described in Section 2.4.1), 

potential transport mechanisms of this pesticide include surface water runoff, ground 

water leachate, spray drift, and secondary drift of volatilized or soil-bound residues 

leading to deposition onto nearby or more distant ecosystems. Because oxamyl is 

relatively nonvolatile and does not bind well to soil, secondary drift is not expected to 

appreciably occur. Also, because oxamyl degrades in aquatic environments by 

metabolism and hydrolysis in non-acidic environments, and ground water flow tends to 

be attenuated relative to surface water flow, exfiltrated ground water is expected to 

contribute attenuated amounts of oxamyl to surface water bodies after residues in runoff 

have largely dissipated. Therefore, ground water discharge is not expected to contribute 

greater exposure to surface water bodies than surface water runoff. And, thus, the major 

routes of exposure for oxamyl are expected only to include surface water runoff and 

spray drift. 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 appreciably less than that 

of potential exposure routes shown in solid lines. 

40
 
Stressor 

Source 

Exposure 

Media 

Receptors 

Attribute 

Oxamyl applied to agriculture use site 

Spray drift 

Surface water/ 

Sediment 

Runoff Long range 

atmospheric 

transport 

Wet/dry deposition 

Soil Groundwater 

Uptake/gills Uptake/cell, 

Aquatic Animals 

Invertebrates 

Vertebrates 

or integument roots, leaves 

Uptake/gills 

or integument 

Ingestion Ingestion 

Aquatic Plants 

Non-vascular 

Vascular 

Riparian plant 

terrestrial 

exposure 

pathways 

Fish/aquaticphase amphibians 

**Piscivorous 

mammals and 

birds 

Habitat integrity 

Change Individual organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain 

Reduction in algae 

Reduction in prey 

Modification of PCEs 

related to prey 

availability 

Reduction in primary productivity 

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Figure 5. Conceptual Model for Pesticide effects on Aquatic-Phase of the CRLF and its Critical 

Habitat. 

41
 
Stressor 

Source 

Receptors 

Attribute 

Oxamyl applied to agriculture use site 

Direct 

application Spray drift 

Birds/terrestrialphase 

amphibians/ 

reptiles/mammals 

Terrestrial 

insects 

Individual organisms Habitat integrity 

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 

Terrestrialphase 

amphibians 

Ingestion 

Reduced survival Food chain Change Reduced growth	 Reduction in prey 

Reduced reproduction	 Modification of 

PCEs related to 

prey availability
 

Reduction in primary productivity 

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Figure 6. Conceptual Model for Pesticide Effects on Terrestrial-Phase of the CRLF and its Critical 

Habitat. 

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 oxamyl 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 oxamyl is estimated 

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

actual calculated risk quotient value. 

42
 
2.10.1. Measures to Evaluate the Risk Hypothesis and Conceptual 

Model 

2.10.1.1. Measures of Exposure 

The environmental fate properties of oxamyl along with available monitoring data 

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

oxamyl to the aquatic and terrestrial habitats of the CRLF. In this assessment, transport 

of oxamyl through runoff and spray drift is considered in deriving quantitative estimates 

of oxamyl exposure to CRLF, its prey and its habitats. 

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

environmental concentrations (EECs) of oxamyl 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; Carousel et al., undated) and EXAMS (v2.98.4.6, April 

2005; Burns, 2004) are screening simulation models coupled with the graphical user 

interface PE (v5.0, Aug 2007) to generate daily exposures and 1-in-10 year EECs of 

oxamyl that may occur in surface water bodies adjacent to application sites receiving 

oxamyl 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 oxamyl. 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 (v1.3.1, 12/07/2006; U.S. 

EPA. 2006c). 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 

43
 
field measurements (Hoerger and Kenega, 1972). For modeling purposes, direct 

exposures of the CRLF to oxamyl through contaminated food are estimated using the 

EECs for the small bird (20 g) which consumes small insects. Dietary-based and dosebased 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 oxamyl 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 (v.1.3.1) has been refined to the T-HERPS model (v. 1.0, 5/15/2007; U.S. EPA. 

2007a), 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; (U.S. EPA. 2006b). 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. 

In order to explore the potential exposures of mammals to oxamyl following applications 

involving ground application, a simple fugacity approach was employed to estimate 

oxamyl concentrations in earthworms and subsequent exposures to mammals consuming 

earthworms. 

The spray drift model, AgDRIFT, is used to assess exposures of terrestrial phase CRLF 

and its prey to oxamyl deposited on terrestrial habitats by spray drift. 

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 U.S. EPA, 

44
 
Office of Research and Development, and the National Health and Environmental Effects 

Research Laboratory's Mid-Continent Ecology Division. 

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

assumption that toxicity of oxamyl 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 terrestrial-phase 

amphibians represent potential prey of the CRLF in the terrestrial habitat. Aquatic, semiaquatic, 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). 

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 oxamyl 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 

45
 
estimate) as the slope parameter for the spreadsheet. In addition, the acute RQ is entered 

as the desired threshold. 

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 (U.S. EPA 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 

oxamyl, 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 oxamyl 

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) (U.S. EPA, 2004) (see 

Appendix B). 

For this endangered species assessment, listed species LOCs are used for comparing RQ 

values for acute and chronic exposures of oxamyl directly to the CRLF. If estimated 

exposures directly to the CRLF of oxamyl 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 oxamyl 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 nonlisted 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 B. 

46
 
2.10.2. Data Gaps 

Acceptable data regarding the anaerobic aquatic metabolism of oxamyl are not available. 

These data are relevant to the environmental fate of residues in anaerobic aquatic 

environmental compartments including saturated subsurface soils. However, submitted 

hydrolysis and anaerobic soil metabolism data indicate that oxamyl will not persist under 

anaerobic, aquatic environments, especially in neutral to alkaline conditions, due to 

hydrolysis. 

3. Exposure Assessment 

Oxamyl is formulated as a water-soluble liquid. Application methods include aerial, 

air-blast, and ground foliar broadcasts, chemigation, soil injection, band treatment, 

row treatment, incorporation, in-furrow spray or drench, and transplant water 

treatment. Application equipment includes various aircraft, ground booms, sprayers 

and irrigation equipment. 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. Therefore, risks from ground boom and aerial applications are expected 

to result in the highest off-target concentrations of oxamyl due to generally higher 

spray drift levels. 

3.1. Label Application Rates and Intervals 

Oxamyl labels may be categorized into two types: labels for manufacturing uses 

(including technical grade oxamyl and its formulated products) and end-use products. 

While technical products, which contain oxamyl 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 nematodes, mites, and insects. The formulated product 

labels legally limit oxamyl’s potential use to only those sites that are specified on the 

labels. 

Currently registered agricultural uses of oxamyl within California include apple, 

celery, cherry, citrus, clover, cotton, cucumber, eggplant, garlic, melons (cantaloupe, 

honeydew, and watermelon), onion, peach, pear, pepper, potato, pumpkin, squash, 

tobacco, and tomato. Although use of oxamyl is not prohibited on tobacco in 

California, this crop is not grown in the State, and is therefore not considered in this 

endangered species assessment. The maximum use patterns being assessed for uses 

of oxamyl are summarized in Table 10 and in Table 11. Current labels limit 

applications per season rather than per year. However, the only assessed use for 

which multiple seasons may occur per year is celery. Therefore, labeled limits for 

number of applications per season can be interpreted as annual limits for all uses 

except celery. In California, 2.5 crops of celery may be grown per year (USDA, 

2000). Therefore, for this assessment, three seasons of celery were assumed possible 

per year. 

47
 
Table 10. Oxamyl Uses by Foliar1 Application Assessed for the CRLF.2 

Use Application Method 

Maximum Single 

Application Rate 

(lbs a.i./A) 

Maximum # 

Applications 

per Season 

Application 

Interval (days) 

Apple (bearing fruit) Ground 2.0 1 ND 

Celery [1 season]3 Ground, Aerial 1.0 6 5 

Celery [3 seasons]3 Ground, Aerial 1.0 18 5 

Citrus (bearing fruit) 

Ground 1.0 6 14 

Aerial 1.0 6 ND 

Chemigation 2.0 3 30 

Clover (for seed) Ground, Aerial 1.0 2 14 

Cotton Ground, Aerial 1.0 3 6 

Cucumber Ground, Aerial 1.0 6 7 

Chemigation 1.0 6 14 

Eggplant Ground 1.0 6 7 

Garlic Ground, Chemigation 2.0 2.25 14 

Aerial 1.0 4.5 7 

Melons4 Ground, Aerial 1.0 6 7 

Chemigation 1.0 6 14 

Onion (dry bulb) Ground, Chemigation 2.0 2.25 14 

Aerial 1.0 4.5 7 

Non-bearing fruit5 Ground 2.0 4 ND 

Aerial 1.0 8 ND 

Pepper Ground, Aerial, 

Chemigation 1.0 6 7 

Potato Ground, Aerial, 

Chemigation 1.0 8 5 

Pumpkin Ground, Aerial 1.0 6 7 

Chemigation 1.0 6 14 

Squash Ground, Aerial 1.0 6 7 

Chemigation 1.0 6 14 

Tomato Ground and Aerial 1.0 8 5 

Chemigation 2.0 4 7 

ND = not defined 

1 Foliar application includes ground, aerial, and chemigation. 2 Uses assessed based on spreadsheet from BEAD dated June 10, 2008, confirmed by email from SRRD
 

dated June 30, 2008. 

3

 If one season of celery is assumed per year, then 6 applications are modeled per year with an interval of 5 

days. If three seasons of celery are assumed per year, then 18 applications are modeled per year with an
 

interval of 5 days between within-season applications and between the last application in one season and the 

first application in the following season. 

4 Melons include cantaloupe, honeydew, and watermelon. 

5 Non-bearing fruit includes apple, cherry, citrus, peach, and pear uses. 

48
 
Table 11. Oxamyl Uses by Soil1

 Application Assessed for the CRLF.2 

Use Maximum Single 

Application Rate (lbs a.i./A) 

Maximum # 

Applications per 

Season 

Application Interval 

(days) 

Apples (bearing fruit) NA NA NA 

Celery [1 season] 1.0 6 21 

Celery [3 seasons] 1.0 18 21 

Citrus (bearing fruit) NA NA NA 

Clover (for seed) NA NA NA 

Cotton NA NA NA 

Cucumber 4.0 1 NA 

Eggplant NA NA NA 

Garlic 2.0 2.25 14 

Melons3 4.0 1 NA 

Onion (dry bulb) 2.0 2.25 14 

Non-bearing fruit4 4.0 1 NA 

Pepper 1.0 6 7 

Potato 4.0 1 NA 

Pumpkin 4.0 1 NA 

Squash 4.0 1 NA 

Tomato 1.3 3 21 

NA = not applicable

1 Soil applications include injection, incorporation, or in-furrow. 

2 Uses assessed based on a spreadsheet from BEAD dated June 10, 2008 that was confirmed via an email 

from SRRD dated June 30, 2008.
 3 Melons include cantaloupe, honeydew, and watermelon. 

4 Non-bearing fruit includes apple, cherry, citrus, peach, and pear uses. 

3.2. Aquatic Exposure Assessment 

3.2.1. Modeling Approach 

Aquatic exposures are quantitatively estimated for all of the assessed uses using scenarios 

that represent high exposure sites. Each of these sites represents a 10 hectare field that 

drains into a 1-hectare pond that is 2 meters deep and has no outlet. Exposure estimates 

generated using the standard 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 first-order 

streams. As a group, there are factors that make these water bodies more or less 

vulnerable than the standard surrogate pond. Static water bodies that have larger ratios of 

drainage area to water body volume would be expected to have higher peak EECs than 

the standard pond. These water bodies will be either shallower or have large drainage 

areas (or both). Shallow water bodies tend to have limited additional storage capacity, 

49
 
and thus, tend to overflow and carry pesticide in the discharge whereas the standard pond 

has no discharge. As watershed size increases beyond 10 hectares, at some point, it 

becomes unlikely that the entire watershed is planted to a single crop, which is all treated 

with the pesticide. Headwater streams can also have peak concentrations higher than the 

standard pond, but they tend to persist for only short periods of time and are then carried 

downstream. 

Crop-specific management practices for all of the assessed uses of oxamyl were used for 

modeling, including application rates, number of applications per year, application 

intervals, and the first application date for each crop. The date of first application was 

developed within the constraints of the modeled scenarios based on several sources of 

information including data provided by BEAD, a summary of individual applications 

from the CDPR PUR data, and Crop Profiles maintained by the USDA. More detail on 

the crop profiles and the previous assessments may be found at: 

http://www.ipmcenters.org/cropprofiles/index.cfm. 

3.2.2. Model Inputs 

The model input parameters used in PRZM to simulate oxamyl applications are provided 

in Table 12. These use patterns are those listed in Table 10 and in Table 11 that produce 

the maximum estimated aquatic exposure for each use. Because oxamyl use is not 

allowed on fruit-bearing cherries, pears, or peaches, a use pattern for non-bearing fruit 

was assessed that represents use on non-bearing apples, cherries, citrus, pears, and 

peaches. Uses on fruit-bearing apples and citrus were assessed separately. 

Applications were modeled as applied by aircraft for all uses other than fruit-bearing 

apples and citrus, eggplant, and garlic for the following reasons. The use patterns for 

fruit-bearing apples and eggplant do not include aerial application. Estimated exposure 

for ground applications to fruit-bearing citrus and garlic are higher than that for aerial 

applications. Furthermore, the application rate labeled for in-furrow applications to 

garlic is higher than that for other ground applications. Therefore, garlic is the only use 

for which exposure was estimated assuming direct application to the ground surface 

[chemical application method (CAM) = 1] rather than assuming application to the foliage 

(CAM = 2). 

Although uses of oxamyl are seasonally limited, whereas model inputs must be annually 

limited, all uses of oxamyl have only one season per year with the exception of celery. 

Therefore, for uses other than celery, seasonal use patterns were modeled as annual use 

patterns. Use on celery was modeled assuming a range of one to three seasons per year in 

order to assess the potential exposure from different annual cropping practices. However, 

the PRZM scenario used to model celery (CA row crop RLF) is parameterized for only 

one crop per year, which adds uncertainty to exposure estimates reflecting three seasons 

of use per year. Because the second and later seasons of use per year occur in the model 

on a post-harvest field with no foliar interception, exposure estimates for three seasons of 

50
 
oxamyl use per year are expected to be more conservative than those that would result 

with a scenario parameterized for three crops per year. 

Table 12. PRZM Scenarios and Input Parameters Describing Oxamyl Uses in California. 

Uses Scenario1 

App. Rate 

(lbs 

a.i./A) 

App. 

per 

Year 

App. 

Interval 

(days) 

Date of 

Initial 

App. 

CAM 

Input 

IPSCND 

Input 

Application 

Efficiency/ 

Spray Drift 

Apple (bearing fruit) CA fruit STD 2.0 1 N/A May 1 2 3 0.99/0.01 

Celery [1 season]2

CA row crop RLF 1.0 6 5 Jan 15 2 1 0.95/0.05 

Celery [3 seasons]2 18 5 & 120 

Citrus (bearing fruit) CA citrus STD 1.0 6 15 Mar 1 2 3 0.99/0.01 

Clover CA alfalfa OP 1.0 2 14 Feb 1 2 1 0.95/0.05 

Cotton CA cotton STD 1.0 3 6 Sep 19 2 1 0.95/0.05 

Cucumber, melons, 

pumpkin, squash CA melons RLF 1.0 6 7 May 30 2 1 0.95/0.05 

Eggplant CA melons RLF 1.0 6 7 May 30 2 1 0.99/0.01 

Garlic CA garlic RLF 2.0 2.253 14 Mar 1 1 1 0.99/0.01 

Non-bearing fruit CA fruit STD 1.0 8 7 Mar 1 2 3 0.95/0.05 

Onion CA onion STD 1.0 4.53 7 Feb 1 2 1 0.95/0.05 

Pepper CA row crop RLF 1.0 6 7 Feb 15 2 1 0.95/0.05 

Potato CA potato RLF 1.0 8 5 Mar 1 2 1 0.95/0.05 

Tomato CA tomato STD 1.0 8 5 Apr 15 2 1 0.95/0.05 1

 All modeled scenarios were parameterized for irrigation. 

2

 If one season of celery is assumed per year, then 6 applications are modeled per year with an interval of 5 

days. If three seasons of celery are assumed per year, then 18 applications are modeled per year with an 

interval of 5 days between within-season applications and an interval of 120 days between the last 

application in one season and the first application in the following season. 3

 Fractions of applications per year indicate that a final application was modeled at that fraction of the 

maximum application rate. 

 The general chemical and environmental fate data for oxamyl listed in Table 3 were used 

for generating model input parameters for PRZM and EXAMS (listed in Table 13). 

These inputs were determined in accordance with the Division’s “Guidance for 

Chemistry and Management Practice Input Parameters for Use in Modeling the 

Environmental Fate and Transport of Pesticides” dated February 28, 2002. This guidance 

indicates that the hydrolysis rate at pH 7 (half-life of 8.0 days for oxamyl) should be 

modeled, which was done for risk estimation. However, oxamyl is relatively stable to 

hydrolysis in acidic water bodies. Therefore, stability to hydrolysis was also modeled in 

order to characterize potential exposure in acidic water bodies. Input/output files for 

PRZM/EXAMS modeling used to support risk estimation are available in Appendix C. 

51
 
Table 13. PRZM and EXAMS Chemical Input Parameters for Oxamyl. 

Input Parameter Value Comment Source (MRID) 

Molecular Mass (g/mol) 219 Product chemistry data 40499702 

Vapor Pressure (torr) 3.8 x 10-7 Product chemistry data 42526101 

Solubility in Water (mg/L) 2.8 x105 Product chemistry data 40499702 

Organic Carbon Partition 

Coefficient (KOC) (L/kgOC) 

35 Represents the average KOC. 46237301 

Aerobic Soil Metabolism 

Half-life (days) 

52 Represents the upper 90% confidence bound on 

the mean of six half-lives. 

ACC.# 63012 

42820001 

45176602 

Aerobic Aquatic Metabolism 

Half-life (days) 

6.6 Represents the upper 90% confidence bound on 

the mean of two half-lives adjusted for hydrolysis 

at pH 7. 

45045305 

Anaerobic Aquatic 

Metabolism Half-life (days) 

0 No data; assumed stable. Aqueous dissipation 

will be dominated by hydrolysis. 

Not applicable 

Hydrolysis Half-life (days) 8.0 Half-life at pH 7 40606516 

Aqueous Photolysis 

Half-life (days) 

14 Represents the maximum environmental 

phototransformation half-life. 

40606515; 

41058801 

3.2.3. Results 

The aquatic 1-in-10-year EECs for the various scenarios and application practices are 

listed in Table 14. The use pattern resulting in the highest potential aquatic exposure is 

use on celery, assuming multiple seasons occur per year. If use on celery is limited to 

one season per year, the use patterns of maximum aquatic exposure are garlic and celery, 

followed closely by tomato. The 1-in-10-year exposure estimates from all use patterns 

range across approximately one order of magnitude. Variability between these EECs is 

mainly due to variations in modeled application rates, dates of application, 

meteorological conditions, and soil conditions. Because California has a Mediterranean 

climate and receives less precipitation in the summer than in other seasons, use patterns 

modeled with initial application dates in May produced the lowest exposure estimates. 

If oxamyl is applied adjacent to acidic water bodies, where it is expected to be relatively 

stable to hydrolysis, peak exposure estimates slightly increase and time-averaged 

exposure estimates increase by two- to three-fold. For example, 1-in-10-year peak, 21­

day mean, and 60-day mean EECs increase to 53 ppb, 41 ppb, and 24 ppb, respectively, 

for use on celery with three seasons per year and assuming stability to hydrolysis. 

Respective EECs for one season of celery per year and assuming stability to hydrolysis 

are 37 ppb, 26 ppb, and 19 ppb. 

52
 
Table 14. Aquatic 1-in-10-year EECs (μg/L) for Oxamyl Uses in California. 

Uses Scenario 

Seasonal 

App. Rate 

(lbs a.i./A) 

Date of 

Initial 

App. Peak EEC 

21-day 

average 

EEC 

60-day 

average 

EEC 

Celery (3 seasons/yr) CA row crop RLF 18 15-Jan 45 18 9.0 

Celery (1 season/yr) CA row crop RLF 6 15-Jan 31 13 6.8 

Garlic CA garlic RLF 4.5 1-Mar 34 13 5.6 

Tomato CA tomato STD 8 15-Apr 28 8.1 4.4 

Non-bearing fruit CA fruit STD 8 1-Mar 21 8.2 4.6 

Cotton CA cotton STD 3 19-Sep 19 7.2 3.1 

Pepper CA row crop RLF 6 15-Feb 18 7.5 4.5 

Citrus (bearing fruit) CA citrus STD 6 1-Mar 12 3.4 1.2 

Clover CA alfalfa OP 2 1-Feb 7.8 3.7 1.7 

Onion CA onion STD 4.5 1-Feb 7.6 4.2 2.1 

Potato CA potato RLF 8 1-Mar 7.3 4.8 3.3 

Cucumber, melons, 

pumpkin, squash CA melons RLF 6 30-May 6.7 1.9 1.2 

Eggplant CA melons RLF 6 30-May 3.3 1 0.52 

Apple (bearing fruit) CA fruit STD 2 1-May 3.2 1.2 0.51 

3.2.4. Existing Monitoring Data 

A critical step in the process of characterizing EECs is comparing the modeled estimates 

with available surface water monitoring data. Included in this assessment are available 

data on oxamyl and its degradates from monitoring conducted in California, as reported 

in the U. S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) 

program (http://water.usgs.gov/nawqa) and in the California Department of Pesticide 

Regulation (CDPR) Surface Water Database 

(http://www.cdpr.ca.gov/docs/emon/surfwtr/surfdata.htm). CDPR does not provide 

ground water monitoring data for oxamyl or its degradates because they are not currently 

listed on the Ground Water Protection List 

(http://www.cdpr.ca.gov/docs/emon/grndwtr/). The only analysis of a degradate of 

oxamyl in these data was an analysis of the oxime degradate of oxamyl in ground water 

conducted for the NAWQA program. 

3.2.4.1. USGS NAWQA Surface Water Data 

NAWQA data indicate that 33 surface water sites in California were monitored for 

oxamyl at various times from March 1993 to September 2005. Degradates of oxamyl 

were not analyzed for. Oxamyl was detected once at 0.025 μg/L in Orestimba Creek in 

53
 
Stanislaus Country in April, 2002. All other results are reported as less than the limit of 

quantitation (LOQ = 0.012-0.4 μg/L). Oxamyl was not detected in the ephemeral 

Orestimba Creek two weeks before or after the detection in April, 2002 (U.S. Geological 

Survey 2007). 

3.2.4.2. USGS NAWQA Ground Water Data 

NAWQA data indicate that oxamyl was analyzed in ground water at 318 sites in 

California at various times from August 1993 to September 2006. Oxamyl was not 

detected in ground water above the level of quantitation (0.012-0.54 μg/L) at any site. 

The oxime degradate of oxamyl was analyzed in ground water once in April 2001, 

yielding no detection (LOQ=0.064 μg/L). 

3.2.4.3. California Department of Pesticide Regulation (CDPR) 

Surface Water Database 

The CDPR Surface Water Database indicates that oxamyl was analyzed at 183 surface 

water sites in California at various times from February 1991 to October 2006. 

Degradates of oxamyl were not analyzed for. Oxamyl was detected at 11 of those sites at 

concentrations ranging from less than the level of quantitation (0.1-0.5 μg/L) to 2.8 μg/L. 

Detections occurred in the San Jose River and its tributaries (Stanislaus and Merced 

counties) in April of 1991, 1992, and 2002 and in the Pajaro River and its tributaries 

(Santa Cruz and Monterey counties) on December 13, 1994. The highest detection of 2.8 

μg/L occurred in a drainage ditch connected to the Pajaro River. Sites with detections 

were often reanalyzed for oxamyl within a few weeks to a few months, resulting in no 

detections, which suggests that the presence of oxamyl likely correlates with upslope 

usage and that residues dissipate in flowing water bodies. 

The NAWQA and CDPR monitoring data are not targeted to observe peak runoff from 

local uses of oxamyl. Therefore, the highest monitored detections of the compound are 

expected to reflect time-averaged concentrations, such as 21-day mean concentrations, 

rather than peak concentrations in the environment. Also, monitored surface water sites 

are generally located in flowing water bodies; whereas modeled exposure estimates are 

based on a static water body adjacent to the use site where dilution and flow do not occur. 

Therefore, the highest monitored concentrations are not expected to often exceed 

modeled time-average exposure estimates. 

The highest detection of oxamyl (2.8 μg/L) in the NAWQA and CDPR monitoring data is 

consistent with or within an order of magnitude of 1-in-10-year 21-day mean EECs (1-18 

μg/L) of oxamyl for uses on individual crops. 

3.3. Terrestrial Animal Exposure Assessment 

T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of oxamyl for 

the CRLF and its potential prey (e.g. small mammals and terrestrial insects) inhabiting 

54
 
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, foliar spray applications of oxamyl are considered using TREX. Applications directly to the soil are not modeled using T-REX. 

Terrestrial EECs for foliar formulations of oxamyl were derived for the uses summarized 

in Table 15. Given that no suitable data on interception and subsequent dissipation from 

foliar surfaces is available for oxamyl, the EFED default foliar dissipation half-life of 35 

days is used based on the work of Willis and McDowell (1987). Two values specific to 

oxamyl are available in Willis and McDowell; however, these half-lives represent data 

collected from dislodgable residues, and are, therefore, not representative of the foliar 

dissipation half-life. Use-specific input values, including number of applications, 

application rate and application interval are provided in Table 15. The maximum 

application rate for each use is modeled in T-REX and upper bound EECs are used for 

RQ calculations. An example output from T-REX is available in Appendix D. 

Table 15. Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for oxamyl with 

T-REX. 

Use (Application Method) Application Rate 

(lbs a.i./A) 

Number of 

Applications 

Application Interval 

(days) 

Apple (bearing fruit) 

(ground) 2.0 1 NA 

Celery (ground) [1season] 1.0 6 5 

Celery (ground) [3 seasons] 1.0 18 5 

Citrus (bearing fruit) 

(ground) 1.0 6 5* 

Clover (for seed) (ground) 1.0 2 14 

Cotton (ground) 1.0 3 6 

Cucumber (ground) 1.0 6 7 

Eggplant (ground) 1.0 6 7 

Garlic (ground) 2.0 2.25 14 

Melons (ground) 1.0 6 7 

Onion (ground) 2.0 2.25 14 

Non-bearing (ground) 2.0 4 5* 

Pepper (ground) 1.0 6 7 

Potato (ground) 1.0 8 5 

Pumpkin (ground) 1.0 6 7 

Squash (ground) 1.0 6 7 

Tomato (chemigation) 2.0 4 7 

NA = not applicable 

*because application interval is undefined on product labels, a 5 day interval value is assumed for modeling 

purposes based on the lowest interval value listed for any use 

55
 
For modeling purposes, exposures of the CRLF, as well as other frog species serving as 

prey to the CRLF, to oxamyl 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 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 16). 

As indicated above, T-REX is also used to calculate EECs for terrestrial insects exposed 

to oxamyl. Dietary-based EECs calculated by T-REX for small and large insects (units of 

μg a.i./g) are used to bound an estimate of exposure to terrestrial insects. Available acute 

contact toxicity data for bees exposed to oxamyl (in units of μg a.i./bee), are converted to 

μg a.i./g (of bee) by multiplying by 1 bee/0.128 g. Dietary-based EECs for terrestrial 

insects are later compared to the adjusted acute contact toxicity data for bees in order to 

derive RQs. Dietary-based EECs for small and large insects reported by T-REX as well 

as the resulting adjusted EECs are available in Table 17. 

Table 16. Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based Exposures of the 

CRLF and its Prey to Oxamyl. 

Use 

EECs for CRLF EECs for Prey 

(small mammals) 

Dietary-based 

EEC (ppm) 

Dose-based EEC 

(mg/kg-bw) 

Dietary-based 

EEC (ppm) 

Dose-based EEC 

(mg/kg-bw) 

Apple (bearing fruit) 270 308 480 458 

Celery (1 season) 641 730 1140 1087 

Celery (3 seasons) 1191 1356 2117 2018 

Citrus (bearing fruit) 641 730 1140 1087 

Clover (for seed) 237 270 421 402 

Cotton 361 411 642 612 

Cucumber 588 670 1047 998 

Eggplant 588 670 1047 998 

Garlic 629 717 1119 1274 

Melons 588 670 1047 998 

Onion 629 717 1119 1274 

Non-bearing 936 1066 1665 1587 

Pepper 588 670 1047 998 

Potato 783 892 1392 1327 

Pumpkin 588 670 1047 998 

Squash 588 670 1047 998 

Tomato 887 1011 1578 1504 

56
 
Table 17. EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to Terrestrial 

Invertebrate Prey Items. 

Use Small Insect EEC Large Insect EEC 

Apple (bearing fruit) 270 30.0 

Celery (1 season) 641 71.3 

Celery (3 seasons) 1191 132 

Citrus (bearing fruit) 641 71.3 

Clover (for seed) 237 26.4 

Cotton 361 40.2 

Cucumber 588 65.4 

Eggplant 588 65.4 

Garlic 629 70.0 

Melons 588 65.4 

Onion 629 70.0 

Non-bearing fruit 936 104 

Pepper 588 65.4 

Potato 783 87.1 

Pumpkin 588 65.4 

Squash 588 65.4 

Tomato 887 98.6 

3.4. Terrestrial Plant Exposure Assessment 

TerrPlant (version 1.2.2) is used to calculate EECs for non-target plants inhabiting dry 

and semi-aquatic areas. Parameter values for application rate, drift assumption and 

incorporation depth are based upon the use and related application method A runoff 

value of 0.05 is utilized based on oxamyl’s solubility, which is classified by TerrPlant as 

>100 mg/L. For aerial and ground application methods, drift is assumed to be 5% and 

1%, respectively. 

Terrplant was used to model EECs from soil and foliar applications relevant to terrestrial 

plants. These EECs are calculated in the apple scenario in Table 18. The non-bearing 

fruit scenario reflects the highest applications rates, and thus the highest EECs, for soil 

and foliar applications. All foliar applications rates (≤ 2 lbs a.i./acre) are below the 

exposure level at which no effects were observed (2.1 lbs a.i./acre) in the available tier 1 

study; however, soil application rates at 4 lbs a.i./acre are above this exposure level. 

Thus, modeling for soil applications generated EECs with the potential to adversely 

affect terrestrial plants. Due to oxamyl’s solubility, all pesticide concentrations were 

considered in runoff where highest exposure was expected to occur. An example output 

from TerrPlant v.1.2.2 is available in Appendix E. 

57
 
Table 18. TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and Semi-aquatic Areas 

Exposed to Oxamyl via Runoff and Drift. 

Use 

Application 

rate 

(lbs a.i./A) 

Application method 

Drift 

Value 

(%) 

Spray drift 

EEC 

(lbs a.i./A) 

Dry area 

EEC 

(lbs a.i./A) 

Semiaquatic 

area EEC 

(lbs a.i./A) 

Nonbearing 

fruit 

4.0 Soil – incorporation, 

injection, infurrow 0 0 0.2 2.0 

2.0 Foliar - ground 0.01 0.02 0.12 1.02 

1.0 Foliar – aerial 0.05 0.01 0.06 0.51 

4. Effects Assessment 

This assessment evaluates the potential for oxamyl to directly or indirectly affect the 

CRLF or modify its designated critical habitat. As 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 (short-term) and chronic 

(long-term) toxicity information is characterized based on registrant-submitted studies 

and a comprehensive review of the open literature on oxamyl. 

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). In order to be included in the ECOTOX 

database, papers must meet the following minimum criteria: 

o the toxic effects are related to single chemical exposure; 

58
 
o	 the toxic effects are on an aquatic or terrestrial plant or animal species; 

o	 there is a biological effect on live, whole organisms; 

o	 a concurrent environmental chemical concentration/dose or application rate is 

reported; and 

o	 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 

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 oxamyl. 

Toxicity data for oxamyl available in the ECOTOX database on 7/31/2008 were 

considered for this assessment. 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 F. 

Appendix F 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 accepted ECOTOX open literature 

data, including the full suite of lethal and sublethal endpoints is presented in Appendix G. 

Appendix H includes a summary of the human health effects data for oxamyl. 

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 oxamyl. A summary of the available incident 

information for oxamyl is provided below. 

It should be noted that where data were provided in study reports, sublethal effects 

observed during acute toxicity studies are described in this effects characterization. 

These sublethal effects raise concern about the effects of oxamyl; however, it is not 

possible to quantitatively link these effects to the selected assessment endpoints for the 

listed CRLF (i.e., survival, growth, and reproduction of individuals and maintenance of 

critical habitat PCEs). Therefore, potential sublethal effects on specific taxa are 

59
 
evaluated qualitatively. Definitive endpoints, such as LC50s are used for quantifying RQs 

in this assessment. 

4.1. Toxicity of Oxamyl to Aquatic Organisms 

Table 19 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of the submitted studies as previously discussed. Open literature data 

for aquatic organisms was not considered relevant to this ecological risk assessment 

because none of the toxicity endpoints were more sensitive that those from submitted 

studies. Toxicity to aquatic fish and invertebrates is categorized using the system shown 

in (Table 20). Toxicity categories for aquatic plants have not been defined. 

Table 19. Freshwater Toxicity Profile for Oxamyl. 

Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Citation 

MRID # 

Comment 

Acute Direct Toxicity 

to Aquatic-Phase 

CRLF 

Oncorhynchus 

mykiss 

(Rainbow 

Trout) 

96-hr LC50 = 

4.2 mg a.i./L 

Slope = NA 

400980-01 

Supplemental – per EFED 

policy (USEPA 2006); 

some deviations from 

guideline requirements 

Chronic Direct Oncorhynchus NOAEC = Acceptable – NOEC is 

Toxicity to Aquatic­ mykiss 0.77 mg a.i./L 409011-01 based on embryo hatching 

Phase CRLF LOAEC = 

1.5 mg a.i./L 

Slope = NA 

and larval swim-up 

Indirect Toxicity to 

Aquatic-Phase CRLF 

via Acute Toxicity to 

Freshwater 

Invertebrates (i.e. 

prey items) 

Chironomus 

plumosus 

(midge) 

48-hr EC50 = 

 0.18 mg a.i./L 

Slope = NA 

400980-01 

Acceptable – NOAEC 

endpoint based on egg 

production and egg 

fertility 

Indirect Toxicity to 

Aquatic-Phase CRLF 

via Chronic Toxicity 

to Freshwater 

Invertebrates (i.e. 

prey items) 

Chironomus 

plumosus 

(midge) 

NOAEC = 

0.012 mg a.i./L 

Slope = NA 

N/A 

Based on an acute to 

chronic ratio using 

daphnid (Daphnia magna) 

endpoints [4.2/0.027 = 

15.5 (ACR)] 

Indirect Toxicity to Navicula 120-hourEC50 = Acceptable – NOEC and 

Aquatic-Phase CRLF 

via Toxicity to Nonpelliculosa 

(algae) 

0.12 mg a.i./L 455461-04 EC50 values are based on 

cell density. 

vascular Aquatic Slope = 0.872 (±) 

Plants 0.168 

Indirect Toxicity to Lemna gibba 14-day EC50= Supplemental – due to 

Aquatic-Phase CRLF 

via Toxicity to 

(duckweed) 30 mg a.i./L 455461-03 lack of constant exposure 

of test organisms to the 

Vascular Aquatic Slope = 4.56 (±) test material 

Plants 0.441 

60
 
Table 20. Categories of Acute Toxicity for Aquatic Organisms. 

LC50 (mg/L) 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 oxamyl toxicity data are not available for aquatic-phase amphibians, 

freshwater fish data are used as a surrogate to estimate direct acute and chronic risks to 

the CRLF. Freshwater fish toxicity data are also used to assess potential indirect effects 

of oxamyl to the CRLF. Effects to freshwater fish resulting from exposure to oxamyl 

could 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). 

Oxamyl is moderately toxic to freshwater fish on an acute exposure basis. The most 

sensitive freshwater species tested is rainbow trout, with a 96-hour LC50 value of 4.2 mg 

a.i./L (MRID: 400980-01). Two acute studies available for bluegill sunfish (Lepomis 

macrochirus) and rainbow trout (Salmo gairderii) reported 96-hour LC50 values of 5.6 

mg a.i./L (MRID: 66914) and 4.2 mg a.i./L (MRID: 66916), respectively. 

Chronic toxicity data are available for the fathead minnow and the rainbow trout exposed 

to oxamyl. The chronic exposure involving the fathead minnow resulted in effects 

observed at all test concentrations and a LOAEC <1.0 mg/L (effects to larval survival 

were observed) (MRID: 94663). The chronic exposure involving the rainbow trout 

resulted in a discrete NOAEC of 0.77 mg/L and a LOAEC of 1.5 mg/L, with effects to 

embryo hatch and larval swim-up observed at the LOAEC (MRID 409011-01). The 

toxicity data from the rainbow trout study is used to represent the chronic toxicity 

endpoint for the aquatic-phase CRLF because both the NOAEC and the LOAEC are 

discrete values. By using the rainbow trout study, however, chronic risk is potentially 

underestimated due to lack of a NOAEC in the fathead minnow study. 

4.1.2. Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data are used to assess potential indirect effects 

of oxamyl to the CRLF. Effects to freshwater invertebrates resulting from exposure to 

oxamyl 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. 

61
 
Oxamyl is highly toxic (defined as: LC50 >0.1-1 mg a.i./L) to freshwater invertebrates on 

an acute exposure basis. The most sensitive species tested is Chironomus plumosus 

(midge), with a 48-hour EC50 value of 0.18 mg a.i./L (MRID: 400980-01). Acute toxicity 

data are also available for Daphnia magna (daphnid). Two studies on oxamyl exposure 

to daphnids resulted in a 48-hour LC50 of 0.42 mg a.i./L (MRID: 400980-01) and 5.7 mg 

a.i./L (MRID: 94664). 

The most sensitive chronic toxicity data available indicated a NOAEC of 0.027 mg a.i./L 

and a LOAEC of 0.050 mg a.i./L based on effects to growth and reproduction of Daphnia 

magna (MRID: 450678-01). The 48-hour LC50 (0.42 mg a.i./L) from the study on 

daphnids is higher than the lowest acute exposure level tested in the aforementioned 

study on midges (MRID: 400980-01). Therefore, the chronic endpoint used in this 

assessment was derived using the acute-to-chronic ratio (ACR) for daphnids applied to 

the more sensitive acute midge endpoint (0.18 mg a.i./L). The calculated NOAEC value 

for midge is 0.012 mg a.i./L based on the daphnid-derived ACR of 15.5. 

4.1.3. Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether oxamyl could affect primary production and the availability of aquatic plants as 

food for CRLF tadpoles. Primary productivity is essential for indirectly supporting the 

growth and abundance of the CRLF. 

A submitted study on algae indicated oxamyl has an EC50 of 120-hour EC50 value of for 

decreased cell density (MRID: 455461-04). A submitted study on aquatic vascular plants 

(duckweed, Lemna gibba) reported a 14-day EC50 value of 30 mg a.i./L based on 

decreased frond count (MRID: 455461-03). 

4.2. Toxicity of Oxamyl to Terrestrial Organisms 

Table 21 summarizes the most sensitive terrestrial toxicity endpoints for the CRLF, based 

on an evaluation of submitted studies. No additional toxicity data for terrestrial 

organisms exposed to oxamyl were identified in the open literature. Acute toxicity to 

terrestrial animals is categorized using the classification system shown in Table 22 (U.S. 

EPA, 2004). Toxicity categories for terrestrial plants have not been defined. 

62
 
Table 21. Terrestrial Toxicity Profile for Oxamyl. 

Endpoint Species Toxicity Value 

Used in Risk 

Assessment 

Citation 

MRID# 

Comment 

Acute Direct Toxicity 

to Terrestrial-Phase 

CRLF (LD50) 

Anas platyrhynchos 

(Mallard Duck) 

LD50 = 

 3.16 mg/kg-bw 

Slope = NA 

94660 Acceptable 

Acute Direct Toxicity 

to Terrestrial-Phase 

CRLF (LC50) 

(Colinus 

virginianus) 

(Northern 

Bobwhite Quail) 

LC50 = 

 340 mg/kg-diet 

Slope = NA 

406065-11 Acceptable – NOAEC < 39 

mg/kg-diet based on signs of 

toxicity and reduction in body 

weight gain and feed 

consumption 

Chronic Direct Toxicity 

to Terrestrial-Phase 

CRLF 

Anas platyrhynchos 

(Mallard Duck) 

NOAEC = 

 10 mg/kg-diet 

Slope = NA 

00116609 Acceptable – NOAEC 

endpoint based on egg 

production and egg fertility 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via acute toxicity to 

mammalian prey items) 

Rattus norvegicus 

(laboratory rat) 

Male LD50 = 

 3.1 mg/kg-bw 

Female LD50 = 

 2.5 mg/kg-bw 

63011 Acceptable 

-Slope for male LD50=9.3 ± 

4.6 

-Slope for female LD50=18.0 ± 

8.6 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via chronic toxicity to 

mammalian prey items) 

Rattus norvegicus 

(laboratory rat) 

NOAEC = 

 25 mg/kg-diet 

Slope = NA 

416608-01 Acceptable – Decreased body 

weight and live pups/litter 

during lactation observed at 

LOAEC (75 mg/kg-diet) 

Indirect Toxicity to 

Terrestrial-Phase CRLF 

(via acute toxicity to 

terrestrial invertebrate 

prey items) 

Apis mellifera 

(Honey bee) 

LD50 = 

 0.31 μg/bee 

Slope = NA 

05001991 Acceptable 

Indirect Toxicity to 

Terrestrial- and 

Aquatic-Phase CRLF 

(via toxicity to 

terrestrial plants) 

Seedling 

Emergence 

Monocots 

21-day EC25 

 >2.1 lbs a.i./A 

Slope = NA 

455461-01 Acceptable –Tier I Terrestrial 

Plant Seedling Emergence 

toxicity study 

Seedling 

Emergence 

Dicots 

21-day EC25

 >2.1 lbs a.i./A 

Slope = NA 

455461-01 Acceptable –Tier I Terrestrial 

Plant Seedling Emergence 

toxicity study 

Vegetative Vigor 

Monocots 

21-day EC25 

 >2.1 lbs a.i./A 

Slope = NA 

455461-02 Supplemental—per EFED 

policy (USEPA 2006); 

insufficient numbers of plants 

per treatment group 

Vegetative Vigor 

Dicots 

21-day EC25

 >2.1 lbs a.i./A 

Slope = NA 

455461-02 Supplemental—per EFED 

policy (USEPA 2006); 

insufficient numbers of plants 

per treatment group 

63
 
Table 22. Categories of Acute Toxicity for Avian and Mammalian Studies. 

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

Very highly toxic < 10 < 50 

Highly toxic 10 - 50 50 - 500 

Moderately toxic 51 - 500 501 - 1000 

Slightly toxic 501 - 2000 1001 - 5000 

Practically non-toxic > 2000 > 5000 

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 for this taxa 

(U.S. EPA, 2004). No terrestrial-phase amphibian data are available for oxamyl; 

therefore, acute and chronic avian toxicity data are used to assess the potential direct 

effects of oxamyl to terrestrial-phase CRLFs. 

Oxamyl is considered very highly toxic to birds on an acute oral exposure basis and 

highly toxic to birds on a dietary exposure basis (Table 22). The oral acute LD50 value 

for mallard duck (Anas platyrhynchos), the most sensitive species tested, is 3.16 mg/kgbw (MRID: 94660). 

Sublethal effects were noted in the acute oral exposure studies on mallard ducks (MRID: 

94660) and bobwhite quail (MRID: 406065-11). Clinical observations of oxamyl-treated 

mallard ducks reported the following effects: loss of coordination at 1.00 and 3.16 

mg/kg-diet; ruffled feathers at 1.00 mg/kg-diet; weakness and slow movement at 3.16 

mg/kg-diet; discharge from eyes and nose at 3.16 and 10.0 mg/kg-diet; and apparent leg 

paralysis at 3.16, 10.0, 31.6, and 100 mg/kg-diet. Reported signs of overt toxicity in 

bobwhite quail occurred at test concentrations of 78 ppm and higher, and included 

lethargy, ruffled appearance, depression, wing droop, loss of coordination, reduced 

reaction to external stimuli, shallow and rapid respiration, lower limb weakness, head 

tremors, and prostrate posture. 

The dietary-based acute LC50 value for the bobwhite quail is 340 mg/kg-diet (MRID: 

406065-11). In the acute dietary-based study, the NOAEC was < 39 mg/kg-diet, the 

lowest test concentration, based on signs of toxicity and reduction in body weight gain 

and feed consumption. 

Reproduction toxicity studies are used to assess chronic avian exposure to oxamyl. A 

study submitted to the Agency involving mallard ducks exposed to oxamyl (MRID: 

00116609) reported a NOAEC of 10 mg/kg-diet and a LOAEC of 50 mg/kg-diet. The 

LOAEC determination was based on effects to egg production and egg fertility. 

64
 
4.2.2. Toxicity to Mammals 

Mammalian toxicity data are used to assess potential indirect effects of oxamyl to the 

terrestrial-phase CRLF. Effects to small mammals resulting from exposure to oxamyl 

could 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). 

Available data indicate that oxamyl is very highly toxic to mammals (LD50 <10 mg/kg) 

on an acute oral exposure basis. The most sensitive endpoint based on submitted studies 

on laboratory rats (Rattus norvegicus) is an LD50 of 2.5 mg/kg-bw for females and 3.1 

mg/kg-bw for males (MRID: 63011). Sublethal effects from acute exposure were also 

observed, although dosage concentration was not reported: tremors, fasciculations 

(muscle twitches), exophthalmos (eye bulging), salivation, chromodacryorrhea (bloody 

tears), stained faces and perineal areas, and slight weight loss. 

A two-generation reproduction study on laboratory rats was conducted to determine 

effects of oxamyl on mammalian reproduction. Reproductive toxicity was observed in 

both generations as a significantly decreased viability index and significantly decreased 

body weight and live pups/litter during lactation. The study reported a NOAEC of 25 

mg/kg-diet (approximately 1.7 and 2.0 mg/kg/day for males and females, respectively) 

and a LOAEC of 75 mg/kg-diet (approximately 5.2 and 6.6 mg/kg/day for males and 

females, respectively) (MRID: 416608-01). 

4.2.3. Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of oxamyl 

to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting from exposure 

to oxamyl could indirectly affect the CRLF via reduction in available food. 

Oxamyl is classified as very highly toxic to beneficial insects (LD50 = 0.31 μg/bee) on an 

acute exposure basis. 

A 1986 study conducted by Atkins, E.L. and D. Kellum (ECOTOX reference #70351) 

examined the toxicological and morphogenic effects to honeybee brood from the 

formulated product, Vydate® L, with a purity of 24% active ingredient oxamyl. Larvae 

inside of the hive of 1-2, 3-4, and 5-6 days old were exposed to varying levels of oxamyl 

in their food to determine LD50 values for each group. According to the study, oxamyl 

was generally much more toxic to larvae than to adults, particularly to those of 1-2 days. 

For larvae of 1-2 days old, LD50 values were reported at 0.00013 μg/individual. For 

larvae 3-4 and 5-6 days old, LD50 values were reported at 0.026 μg/individual and 0.057 

μg/individual, respectively. The study resulted in an overall LD50 value of 0.367 

μg/individual for all larvae tested. It should be noted, however, that there are several 

limitations associated with this study. First, no raw mortality data were provided to allow 

65
 
the reviewer to recalculate the reported LD50 values. Second, the formulation Vydate® L 

was used instead of the technical product. Third, dosage number and dosage levels were 

not reported directly. 

4.2.4. Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for oxamyl 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 

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 of upland and riparian habitat that provides shelter, foraging, predator 

avoidance and dispersal for juvenile and adult CRLFs. 

In tests involving exposures of oxamyl to terrestrial plants, effects were only observed in 

dicots. In a Tier I seedling emergence study (MRID: 455461-01) involving 4 monocot 

species and 6 dicot species, significant effects were observed in tomatoes (significant 

effects to height) and percent inhibition ranged from -14.3% to 11.5% compared to the 

control; however no effect was ≥25% relative to the control. In a Tier 1 vegetative vigor 

study (MRID: 455461-02) involving 4 monocot species and 6 dicot species, significant 

effects to height and weight were observed in soybeans with a percent inhibition of 

-13.1%; however, no effect was ≥25% relative to the control. This study was classified 

as supplemental due to insufficient numbers of plants per treatment group. 

4.3. Incident Database Review 

A search of the EIIS (Environmental Incident Information System) database for 

ecological incidents (run on December 2, 2008) identified 3 incidents involving 

exposures of oxamyl. 

The first incident involved the death of hundreds of fish and over 100 birds in Alamance 

County, North Carolina, in May of 1991. Fish kills were observed in several ponds (≥1 

Acre in size) near farms following high runoff events. Several pesticides (See Table 23) 

had recently been used in the area, and motor oil ran into the ponds. More than 100 

ducks, 1 turkey, 5 cardinals and 7 blackbirds were observed dead. The certainty that the 

deaths of the animals were associated with exposure to oxamyl was considered probable 

but the legality of use was indicated as “intentional misuse” (Incident # I000799-003). 

66
 
Table 23. Pesticides and their associated certainty classification associated with incident #I000799­

003. 

Pesticide Use Site Legality of Use 

Certainty that incident was 

associated with pesticide 

exposure* 

2,4-D Home/lawn Undetermined Probable 

Carbaryl Shrubbery Undetermined Possible 

Diazinon Residential turf Undetermined Possible 

Dicamba NR Undetermined Unlikely 

Mecoprop NR Undetermined Unlikely 

Oxamyl NR Intentional misuse** Probable 

Pentachlorophenol NR Undetermined Possible 

* Incidents included in EIIS are defined by a certainty index associated with the likelihood that the 

pesticide application described resulted in the observed incident. The certainty index defines incidents as 

unrelated, unlikely, possible, probable and highly probable. 

**Corn was laced with oxamyl as bait for ducks. 

NR = not reported 

The second incident, with a certainty index of ‘possible’ and legality of use indicated as 

undetermined, involved the death of a red-cockaded woodpecker in Sussex County, 

Virginia, in February of 2003. The immediate cause of death was identified as head 

trauma. Tissue analysis identified the presence of oxamyl in the liver and gastrointestinal 

track at 7.3 and 1.2 ppm, respectively. Brain acetylcholine esterase levels were depressed 

(83% of normal) (Incident #I014344-001). 

The third incident involved the deaths of 13 laughing gulls and 1 cattle egret in the 

British Virgin Islands in 2004. The cause of mortality was defined as “toxicosis caused 

by both methomyl and oxamyl.” The certainty that the deaths of the birds were associated 

with exposure to either pesticide was considered possible and the legality of use was 

indicated as undetermined (Incident #I018980-010). 

One source of uncertainty associated with this database is the nature of reporting of 

incidents. Many more incidents may have occurred due to oxamyl exposures but may not 

have been reported due to various factors, such as a lack of reporting, or a lack of 

witnessing of effects. Therefore, the lack of an incident report does not necessarily 

indicate a lack of an incident. 

67
 
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 oxamyl 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”). 

The risk characterization describes some of the uncertainties associated with this 

endangered species assessment (Section 5.3). The risk characterization is concluded by 

revisiting the risk hypotheses (Section 5.4) defined in the problem formulation. 

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 

concern (LOCs) for each category evaluated (Appendix B). RQs representing acute and 

chronic exposures to the CLRF, its prey and its habitat are defined below. 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 terrestrial-phase 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. 

5.1.1. Exposures in the Aquatic Habitat 

5.1.1.1. Direct Effects to Aquatic-Phase CRLF 

Direct acute effects to the aquatic-phase CRLF are based on peak 1-in-10-year EECs in 

the standard pond and the lowest acute toxicity value for freshwater fish. In order to 

assess direct chronic risks to the CRLF, 1-in-10-year 60-day-mean EECs and the lowest 

chronic toxicity value for freshwater fish are used. These EECs are divided by acute and 

chronic toxicity data for the rainbow trout (Table 19), a surrogate species for the aquaticphase CRLF. 

RQs are derived based on the highest aquatic EEC, which corresponds to 3 seasons of 

oxamyl applications to celery per year (Table 14). The resulting RQs representing acute 

and chronic exposures to the aquatic-phase CRLF (both 0.01; Table 24) are less than the 

acute and chronic LOCs (0.05 and 1.0, respectively). All other EECs would result in 

lower RQs. Therefore, all uses of oxamyl relevant to California result in RQ values that 

are below LOCs for acute and chronic exposures of the aquatic-phase CRLF to oxamyl. 

68
 
Table 24. Summary of Direct Effect RQs for the Aquatic-phase CRLF. 

Exposure 1-in-10-year EEC (μg/L)a Toxicity Value (μg/L)b RQ 

Acute 45 (Peak) LC50 = 4,200 0.01 

Chronic 9 (60-d mean) NOAEC = 770 0.01 a

 The highest 1-in-10-year EECs based on 3 applications per year of oxamyl to celery, (Table 14). b

 Based on data for the rainbow trout, a surrogate species for the aquatic-phase CRLF. 

5.1.1.2. Indirect Effects to Aquatic-Phase CRLF via Reduction 

in Prey 

Non-vascular Aquatic Plants 

Indirect effects of oxamyl to the aquatic-phase CRLF (tadpole stage) via reduction in 

non-vascular aquatic plants in its diet are based on 1-in-10-year peak EECs from the 

standard pond and the lowest toxicity value (EC50) for aquatic non-vascular plants. The 

highest RQ for non-vascular aquatic plants is 0.38, calculated by dividing the highest 

peak EEC (45 μg/L; Table 14) by the lowest toxicity value for non-vascular aquatic 

plants (120 μg/L; Table 19). RQs generated for all other uses of oxamyl are <0.38. 

Therefore, RQs representing exposures of non-vascular aquatic plants to oxamyl applied 

at maximum label rates to all uses relevant to California are less than the LOC for aquatic 

plants (1.0). 

Aquatic Invertebrates 

Indirect acute effects to the aquatic-phase CRLF via effects to aquatic invertebrates (prey 

of juvenile and adult aquatic-phase CRLF) are based on 1-in-10-year peak EECs in the 

standard pond and the lowest acute toxicity value for freshwater invertebrates. For 

chronic risks, 1-in-10-year 21-day-mean EECs and the lowest chronic toxicity value for 

invertebrates are used to derive RQs. 

RQ values representing acute oxamyl exposures to aquatic invertebrates indicate that the 

majority of oxamyl uses result in RQs (Table 25) that are sufficient to exceed the listed 

species LOC (0.05). Celery, modeled with three seasons, has the highest acute RQ with a 

value of 0.25. 

RQ values representing chronic oxamyl exposures to aquatic invertebrates indicate that 

two uses of oxamyl (specifically celery and garlic) result in RQs (Table 25) that are 

sufficient to exceed the listed species LOC (1.0). The highest RQ for chronic exposures 

of aquatic invertebrates to oxamyl is 1.5, calculated by dividing the highest 1-in-10 year 

21-day mean (18 μg/L; Table 14) by the lowest chronic toxicity value for aquatic 

invertebrates (12 μg/L; Table 19). 

69
 
Table 25. Acute and Chronic RQs for aquatic invertebrates exposed to oxamyl. 

Use 

1-in-10­

year Peak 

EEC (μg/L) 

Indirect 

Effects 

Acute RQ1 

Individual 

Chance of 

Effects3 

1-in-10-year 

21-day-mean 

(μg/L) 

Indirect Effects 

Chronic RQ4 

Celery (3 seasons) 45 0.252 0.34% 18 1.55 

Celery (1 season) 31 0.172 0.03% 13 1.15 

Garlic 34 0.192 0.06% 13 1.15 

Tomato 28 0.162 0.02% 8.1 0.7 

Non-bearing fruit 21 0.122 <0.01% 8.2 0.7 

Pepper 18 0.102 <0.01% 7.5 0.6 

Cotton 19 0.112 <0.01% 7.2 0.6 

Citrus (bearing 

fruit) 12 0.072 <0.01% 3.4 0.3 

Potato 7.3 0.04 <0.01% 4.8 0.4 

Clover 7.8 0.04 <0.01% 3.7 0.3 

Onion 7.6 0.04 <0.01% 4.2 0.4 

Cucumber, 

melons, pumpkin, 

squash 

6.7 

0.04 

<0.01% 1.9 

0.2 

Apple (bearing 

fruit) 3.2 0.02 

<0.01% 1.2 0.1 

Eggplant 3.3 0.02 <0.01% 1.0 0.1 1

 Acute RQ = use-specific 1-in-10-year peak EEC / acute freshwater invertebrate endpoint (EC50 = 180 

μg/L).

2

Exceeds listed species LOC for acute exposures (0.05). 3

Calculated using default slope = 4.5. 

4

 Chronic RQ = use-specific 1-in-10-year 21-day-mean EEC / chronic freshwater ACR endpoint 

(NOAEC = 12 μg/L). 

5

Exceeds listed species LOC for chronic exposures (1.0). 

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 are used to assess potential 

indirect effects to the CRLF based on a reduction in freshwater fish and frogs as food 

items. Acute and chronic RQs (≤0.01) are less than the acute and chronic LOCs for fish 

and frogs representing potential prey of CRLF (0.05 and 1.0, respectively). 

5.1.1.3. Indirect Effects to CRLF via Reduction in Habitat 

and/or Primary Productivity (Freshwater Aquatic Plants) 

Indirect effects to the aquatic-phase CRLF via reduction in habitat resulting from oxamyl 

exposures are based on 1-in-10-year peak EECs from the standard pond and the lowest 

toxicity value (EC50) for aquatic non-vascular and vascular plants. Because there are no 

obligate relationships between the CRLF and any aquatic plant species, the most sensitive 

EC50 values, rather than NOAEC values, are used to derive RQs 

70
 
The highest RQ for non-vascular aquatic plants is 0.38, calculated by dividing the highest 

peak EEC (45 μg/L; Table 14) by the lowest toxicity value for non-vascular aquatic 

plants (120 μg/L; Table 19). Non-vascular plant RQs generated for all other uses of 

oxamyl relevant to California are <0.38. Therefore, RQs for representing exposures of 

non-vascular aquatic plants to oxamyl applied to all labeled uses relevant to California 

are less than the LOC for aquatic plants (1.0). 

The highest RQ for vascular aquatic plants is <0.01, calculated by dividing the highest 

peak EEC (45 μg/L; Table 14) by the lowest toxicity value for vascular aquatic plants 

(30,000 μg/L; Table 19). Vascular plant RQs generated for all other uses of oxamyl are 

<0.01. Therefore, RQs for representing exposures of vascular aquatic plants to oxamyl 

applied to all labeled uses relevant to California are less than the LOC for aquatic plants 

(1.0). 

5.1.2. Exposures in the Terrestrial Habitat 

5.1.2.1. Direct Effects to Terrestrial-phase CRLF 

As discussed in Section 3.3, potential direct effects to terrestrial-phase CRLFs are based 

on foliar (ground, aerial, chemigation) applications of oxamyl. Only foliar applications 

are modeled, because T-REX is not appropriate for modeling soil applications with 

incorporation. 

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 16) and acute oral and subacute dietary toxicity endpoints for 

avian species (Table 21). Available exposure and effects data for birds exposed to 

oxamyl are used as surrogates for the terrestrial-phase CRLF. Resulting acute dietaryand dose-based RQs for all assessed uses of oxamyl exceed the Agency’s acute 

endangered species LOC of 0.1 for the CRLF (Table 26). 

Potential direct chronic effects of oxamyl to the terrestrial-phase CRLF are derived by 

considering dietary-based exposures modeled in T-REX for a small bird (20g) consuming 

small invertebrates (Table 16). Chronic effects are estimated using the lowest available 

toxicity data for birds. EECs are divided by toxicity values to estimate chronic dietarybased RQs. The chronic dietary-based RQs for the terrestrial-phase CRLF exceed the 

Agency’s chronic LOC of 1.0 (Table 26) for all assessed uses of oxamyl. 

71
 
Table 26. RQs for the terrestrial-phase CRLF based on acute and chronic exposures from foliar 

applications of oxamyl at maximum use rates. These RQs were generated using T-REX. 

Use Dosed-based 

Acute RQ 

Dietary-based 

Acute RQ 

Dietary-based 

Chronic RQ 

Apple (bearing fruit) 187 0.79 27 

Celery (1 season) 445 1.9 64 

Celery (3 seasons) 827 3.5 119 

Citrus (bearing fruit) 445 1.9 64 

Clover (for seed) 164 0.70 24 

Cotton 250 1.1 36 

Cucumber 408 1.7 59 

Eggplant 408 1.7 59 

Garlic 437 1.9 63 

Melons 408 1.7 59 

Onion 437 1.9 63 

Non-bearing fruit 650 2.8 94 

Pepper 408 1.7 59 

Potato 643 2.3 78 

Pumpkin 408 1.7 59 

Squash 408 1.7 59 

Tomato 616 2.6 89 

5.1.2.2. Indirect Effects to Terrestrial-Phase CRLF via 

Reduction in Prey 

Terrestrial Invertebrates 

In order to assess the risks of oxamyl exposures to terrestrial invertebrates, which are 

prey of CRLF in terrestrial habitats, the honey bee is used to represent terrestrial 

invertebrates. The toxicity value for terrestrial invertebrates is based on the weight of an 

adult honey bee and is calculated by multiplying the lowest available acute contact LD50 

of 0.31 μg a.i./bee by 1 bee/0.128g, which equals 2.4 ppm (μg a.i./g) of bee. EECs (μg 

a.i./g of bee) calculated by T-REX for small and large insects (Table 17) are divided by 

the toxicity value for terrestrial invertebrates to generate RQs for terrestrial invertebrates. 

For all uses of oxamyl, RQ values exceed the LOC (0.05) for acute exposures to 

terrestrial invertebrates by several orders of magnitude (Table 27). 

72
 
Table 27. RQs for acute exposures to small and large terrestrial insects from foliar applications of 

oxamyl at maximum use rates. 

Use Small Insect RQ Large Insect RQ 

Apple (bearing fruit) 113 13 

Celery (1 season) 267 30 

Celery (3 seasons) 496 55 

Citrus (bearing fruit) 267 30 

Clover (for seed) 98.7 11 

Cotton 150 9.6 

Cucumber 245 27 

Eggplant 245 27 

Garlic 262 29 

Melons 245 27 

Onion 262 29 

Non-bearing fruit 390 43 

Pepper 245 27 

Potato 326 36 

Pumpkin 245 27 

Squash 245 27 

Tomato 369 41 

Mammals 

In order to assess the risks of oxamyl exposures to small mammals, which are prey of 

CRLF in terrestrial habitats, dietary-based and dose-based exposures are modeled in TREX for a small mammal (15g) consuming short grass (Table 16). Acute and chronic 

effects are estimated using the most sensitive mammalian toxicity data (Table 21). EECs 

are divided by the toxicity value to estimate acute and chronic dose-based RQs as well as 

chronic dietary-based RQs (Table 28). RQs representing acute and chronic exposures of 

small mammals consuming short grass on the treated field contaminated with oxamyl 

exceed the LOCs for all uses of oxamyl relevant to California. 

73
 
Table 28. RQs for acute and chronic exposures of small mammals consuming short grass to oxamyl 

from foliar applications at maximum use rates. 

Use Chronic RQ Acute RQ 

Dose-based1 Dietary-based 2 Dose-based3 

Apple (bearing fruit) 167 19 83 

Celery (1 season) 395 46 197 

Celery (3 seasons) 735 85 367 

Citrus (bearing fruit) 395 46 197 

Clover (for seed) 146 17 73 

Cotton 222 26 111 

Cucumber 363 42 181 

Eggplant 363 42 181 

Garlic 388 45 194 

Melons 363 42 181 

Onion 388 45 194 

Non-bearing fruit 577 67 288 

Pepper 363 42 181 

Potato 483 56 241 

Pumpkin 363 42 181 

Squash 363 42 181 

Tomato 547 63 273 1

 Based on dose-based EEC and oxamyl rat NOAEL = 1.25 mg/kg-bw. 

2 Based on dietary-based EEC and oxamyl rat NOAEC = 25 mg/kg-diet. 

3

 Based on dose-based EEC and oxamyl female rat acute oral LD50 = 2.5 mg/kg-bw. 

Frogs 

An additional prey item of the adult terrestrial-phase CRLF is other species of frogs (e.g., 

the Pacific tree frog). In order to assess risks to these prey organisms, dietary-based and 

dose-based exposures modeled in T-REX for a small bird (20g) consuming small 

invertebrates are used (Table 26). As noted in Section 5.1.2.1, acute and chronic RQs 

representing exposures from all uses of oxamyl that are relevant to California exceed 

LOCs. 

5.1.2.3. Indirect Effects to CRLF via Reduction in Terrestrial 

Plant Community (Riparian and Upland Habitat) 

Potential indirect effects to the CRLF resulting from effects on riparian and upland 

vegetation are assessed using RQs from terrestrial plant seedling emergence and 

vegetative vigor studies. Based on all RQs from soil and foliar applications (represented 

by the non-bearing fruit scenario) being below the LOC of 1.0 (Table 29), oxamyl will 

have no effect on the CRLF via reduction in terrestrial plants. Example output from 

TerrPlant v.1.2.2 is provided in Appendix E. 

74
 
Table 29. RQs for monocots and dicots inhabiting dry and semi-aquatic areas exposed to oxamyl via 

spray drift and runoff. 

Plant type 

Applicatio 

n rate 

(lbs a.i./A) 

Application method 

Drift 

Value 

(%) 

Spray drift 

RQ 

Dry area 

RQ 

Semiaquatic 

area RQ 

Monocot 

4.0 Soil – incorporation, 

infurrow, injection 0 < 0.1 < 0.1 <0.95 

2.0 Foliar – ground 0.05 < 0.1 < 0.1 <0.52 

1.0 Foliar – aerial 0.01 < 0.1 < 0.1 <0.24 

Dicot 

4.0 Soil – incorporation, 

infurrow, injection 0 < 0.1 < 0.1 <0.95 

2.0 Foliar – ground 0.05 < 0.1 < 0.1 <0.52 

1.0 Foliar – aerial 0.01 < 0.1 < 0.1 <0.24 

5.1.3. Primary Constituent Elements of Designated Critical Habitat 

5.1.3.1. Aquatic-Phase (Aquatic Breeding Habitat and Aquatic 

Non-Breeding 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). 

RQs used to determine potential effects to these assessment endpoints are provided in 

Sections 5.1.1.2, 5.1.1.3, and 5.1.2.3. As described in these sections, none of the RQs for 

aquatic or riparian plants exceed the LOC. 

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 oxamyl on this PCE, acute and chronic freshwater fish and 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. As discussed in these sections, RQs for direct effects to CRLF are below LOCs 

for acute and chronic exposures. RQs for non-vascular plants, representing dietary items 

of the tadpole stage of the CRLF are below the LOC. Some RQs for acute exposures of 

aquatic invertebrates to oxamyl exceed the LOC (Table 25). 

75
 
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 

RQs used to determine potential effects to these assessment endpoints are provided in 

Section 5.1.2.3. These RQs do not exceed the LOC for plants (Table 29). 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of oxamyl on this PCE, acute 

and chronic toxicity endpoints for frogs, mammals, and terrestrial invertebrates are used 

as measures of effects. RQs for these endpoints were calculated in Section 5.1.2.2. RQs 

for these endpoints exceed the LOCs for all uses of oxamyl (Table 26, Table 27 and 

Table 28). 

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. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.2.1.2. RQs for these endpoints exceed the LOCs for all uses of oxamyl (Table 

26). 

5.2. Risk Description 

The risk description synthesizes an overall conclusion regarding the likelihood of adverse 

impacts leading to an effects determination (i.e., “no effect,” “may affect, but not likely 

to adversely affect,” or “likely to adversely affect”) for the 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 (i.e., there are no LOC exceedances), and no modification to PCEs 

of the CRLF’s designated critical habitat, a “no effect” determination is made, based on 

oxamyl’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 oxamyl. 

76
 
Because some RQs exceeded their respective LOCs, the preliminary effects 

determination for exposures of oxamyl to the CLRF is “May Affect” based on direct and 

indirect effects and effects to the CRLF’s critical habitat. A summary of the results of 

the risk estimation results are provided in Table 30 for direct and indirect effects to the 

CRLF and in Table 31 for the PCEs of designated critical habitat for the CRLF. 

Table 30. Risk Estimation Summary for Oxamyl - 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 

No RQs for acute and chronic exposures to fish resulting 

from all uses of oxamyl are below LOCs. 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects to food 

supply (i.e., freshwater invertebrates, 

non-vascular plants) 

Yes RQs are above the LOC for acute and chronic 

exposures to aquatic invertebrates 

RQs are below the LOC for exposures to nonvascular aquatic plants, acute and chronic exposures 

to fish 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects on habitat, 

cover, and/or primary productivity (i.e., 

aquatic plant community) 

No RQs for exposures of non-vascular and vascular 

aquatic plants are below aquatic plant LOC. 

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 EECs are below level where no effects to plants 

were observed in Tier 1 test. 

Terrestrial Phase (Juveniles and adults) 

Direct Effects 

Survival, growth, and reproduction of 

CRLF individuals via direct effects on 

terrestrial phase adults and juveniles 

Yes 

Acute and chronic RQs for all uses exceed LOCs for 

the CRLF. 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects on prey 

(i.e., terrestrial invertebrates, small 

terrestrial mammals and terrestrial 

phase amphibians) 

Yes 

Acute and chronic RQs for terrestrial invertebrates, 

small terrestrial mammals, and terrestrial phase 

amphibians, exceed LOCs for all uses. 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects on habitat 

(i.e., riparian vegetation) 

No EECs are below level where no effects to terrestrial 

plants were observed in Tier 1 test. 

77
 
Table 31. Risk Estimation Summary for Oxamyl – PCEs of Designated Critical Habitat for the 

CRLF. 

Assessment Endpoint 

LOC 

Exceedances 

(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. 

No RQs for non-vascular and vascular aquatic 

plants are below aquatic plant LOC. 

EECs are below level where no effects to 

terrestrial plants were observed in Tier 1 test. 

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. 

No RQs for non-vascular and vascular aquatic 

plants are below aquatic plant LOC. 

EECs are below level where no effects to 

terrestrial plants were observed in Tier 1 test. 

Alteration of other chemical characteristics 

necessary for normal growth and viability of 

CRLFs and their food source. 

Yes RQs for acute and chronic exposures to fish 

resulting from all uses of oxamyl are below 

LOCs. 

Acute RQs for some uses of oxamyl exceed the 

LOC for aquatic invertebrates. 

RQs for non-vascular plants are below aquatic 

plant LOC. 

Reduction and/or modification of aquatic-based 

food sources for pre-metamorphs (e.g., algae) 

No RQs for non-vascular plants are below aquatic 

plant LOC. 

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 

No EECs are below level where no effects to 

plants were observed in Tier 1 test. 

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 EECs are below level where no effects to 

plants were observed in Tier 1 test. 

Reduction and/or modification of food sources 

for terrestrial phase juveniles and adults 

Yes Acute and chronic RQs for terrestrial 

invertebrates, small terrestrial mammals, and 

terrestrial phase amphibians, exceed the LOCs 

for all uses. 

Alteration of chemical characteristics necessary 

for normal growth and viability of juvenile and 

adult CRLFs and their food source. 

Yes Avian acute and chronic RQs exceed the LOCs 

for all uses. 

78
 
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 

(e.g., habitat range, feeding preferences) 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: 

•	 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 5.2.4. 

5.2.1. Direct Effects 

5.2.1.1. Aquatic-Phase CRLF 

The aquatic-phase of the CRLF’s life cycle is relevant to stages that require that CRLF 

individuals inhabit aquatic areas. Relevant aquatic-phase life stages include eggs and 

larvae, as well as submerged juveniles and adults, which spend a portion of their time in 

water bodies that may receive runoff and spray drift containing oxamyl. 

For all uses of oxamyl that are relevant to California, acute and chronic exposures to 

CRLF in the aquatic habitat are insufficient to result in RQs exceeding the LOC. For 

acute exposures, the probability of individual mortality to an aquatic phase CLRF is 1 in 

≥8.9x1018 (assuming default slope of 4.5). While the available fathead minnow chronic 

toxicity endpoint was not used definitively to measure direct effects to the CRLF, the 

reported LOAEC value of < 1000 μg a.i./L is a source of uncertainty in making a direct 

79
 
effects determination. However, this LOAEC value would have to be reduced by a factor 

of 100 to result in an LOC exceedance for chronic effects (1000 / 100 = 10 μg /L). 

Therefore, the determination for direct effects to the CRLF in aquatic habitats is “No 

Effect” for all uses of oxamyl that are relevant to California. 

5.2.1.2. Terrestrial-Phase CRLF 

RQs representing acute dose-based, acute dietary-based and chronic dietary-based 

exposures to the terrestrial-phase CRLF exceeded their respective LOCs, resulting in a 

“may affect” determination for all uses of oxamyl (Table 26). These RQs were derived 

using the T-REX model, which estimates exposures that are specific to food intake 

equations for birds. RQs generated for birds are used as surrogates to represent RQs for 

the terrestrial-phase CRLF. The RQs generated by T-REX are based on the maximum 

application rates and exceeded the LOC of 0.1 by 7 to 6500 times. 

In order to explore influences of amphibian-specific food intake equations on potential 

dose-based and dietary-based exposures of the terrestrial phase CRLF to oxamyl, THERPS is used. Modeling with T-HERPS incorporates the same application rates, 

intervals and number of applications for each use as defined for modeling using T-REX 

(Table 16). The dose-based and dietary-based EECs generated by T-HERPS are 

provided in Table 32 and in Table 33, respectively. An example output from T-HERPS 

is available in Appendix I. 

80
 
Table 32. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-bw) for Dose-based Exposures 

of the CRLF and its Prey to Oxamyl. 

Scenario CRLF 

Size (g) 

Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals1 

Small 

Insectivore 

Mammals1 

Small 

TerrestrialPhase 

Amphibians 

1.4 10.5 1.17 NA NA NA 

Apple (bearing fruit) 37 10.3 1.15 299 18.7 0.358 

238 6.76 0.751 46.5 2.91 0.235 

Celery [1 season], 

Citrus (bearing fruit) 

1.4 24.9 2.77 NA NA NA 

37 24.5 2.72 711 44.4 0.850 

238 16.1 1.78 111 6.91 0.557 

1.4 46.3 5.14 NA NA NA 

Celery [3 seasons] 37 45.5 5.05 1320 82.5 1.58 

238 29.8 3.31 205 12.8 1.04 

1.4 9.22 1.02 NA NA NA 

Clover 37 9.06 1.01 263 16.4 2.55 

238 5.94 0.660 40.9 0.315 0.206 

1.4 14.0 1.56 NA NA NA 

Cotton 37 13.8 1.53 400 25.0 0.479 

238 9.04 1.01 62.2 3.89 0.314 

Cucumber, Melons, 1.4 22.9 2.54 NA NA NA 

Pumpkin, Squash, 

Eggplant, Pepper 

37 22.5 2.50 653 40.8 0.781 

238 14.7 1.64 101 6.31 0.512 

1.4 24.5 2.72 NA NA NA 

Garlic, Onion 37 24.0 2.67 698 43.6 0.83 

238 15.8 1.75 108 6.78 0.55 

1.4 36.4 4.04 NA NA NA 

Non-bearing fruit 37 35.8 3.97 1037 64.9 1.24 

238 23.4 2.60 161 10.1 0.814 

1.4 27.7 3.07 NA NA NA 

Potato 37 16.6 1.85 483 30.2 0.577 

238 8.24 0.916 56.7 3.55 0.286 

1.4 34.5 3.83 NA NA NA 

Tomato 37 33.9 3.77 984 61.5 1.18 

238 22.2 2.47 153 9.56 0.77 

1

weight of small herbivore and insectivore mammals is 35g 

81
 
Table 33. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for Dietary-based 

Exposures of the CRLF and its Prey to Oxamyl. 

Scenario Small Insects Large Insects 

Small 

Herbivore 

Mammals1 

Small 

Insectivore 

Mammals1 

Small 

TerrestrialPhase 

Amphibians 

Apple (bearing fruit) 270 30.0 316 19.8 9.37 

Celery [1 season], 

Citrus (bearing fruit) 641 71.3 751 47.0 22.3 

Celery [3 seasons] 1191 132 1395 87.2 41.3 

Clover 237 26.4 278 17.4 8.24 

Cotton 361 40.1 423 26.5 12.5 

Cucumber, Melons, 

Pumpkin, Squash, 

Eggplant, Pepper 

589 65.4 690 43.1 20.4 

Eggplant, Pepper 589 65.4 690 43.1 20.4 

Garlic, Onion 630 70.0 738 46.1 21.9 

Non-bearing fruit 937 104 1097 68.6 32.5 

Potato 785 87.1 918 57.4 27.2 

Tomato 888 98.6 1040 65 30.8 

1

weights of small herbivore and insectivore mammals are 35g 

Acute RQs 

RQs are calculated for the terrestrial-phase CRLF on the basis of dose and diet. It should 

be noted that although dietary-based RQ values are considerably lower than dose-based 

RQ values, the former do not take into account that different-sized animals consume 

differing amounts of food and that depending on the forage item, an animal has to 

consume varying amounts due to differing nutrition levels in the food item. If dietarybased RQ values are adjusted to account for differential food consumption, the adjusted 

RQ value would likely approximate the dose-based RQ value. 

Refined dose-based RQs for small sized (1.4g) CRLF consuming small and large insects 

exceed the acute listed species LOC (0.1) for all uses of oxamyl (Table 34), with larger 

exceedances for CRLF consuming small insects. RQs range from 0.39 to 42, suggesting 

that small CRLF consuming small and large insects are potentially affected by acute 

exposures to oxamyl. 

82
 
Table 34. Dose-based RQs for 1.4g CRLF consuming different food items (calculated using THERPS). 

Scenario Small Insects Large Insects 

Apple (bearing fruit) 9.5 1.1 

Celery [1 season], Citrus (bearing fruit) 9.4 1.1 

Celery [3 seasons] 42 4.7 

Clover 3.5 0.39 

Cotton 5.3 0.59 

Cucumber, Melons, 

Pumpkin, Squash, Eggplant, Pepper 8.7 0.96 

Garlic, Onion 22 2.5 

Non-bearing fruit 33 3.7 

Potato 28 3.1 

Tomato 31 3.5 

Refined dose-based RQs for medium sized (37g) CRLF consuming small insects, large 

insects, small herbivore mammals, small insectivore mammals, and small terrestrialphase amphibians exceed the acute listed species LOC (0.1) for all uses of oxamyl (Table 

35). LOC exceedances for all uses of oxamyl relevant to California indicate that the 

medium sized CRLF could potentially be affected by acute exposures to oxamyl. 

Table 35. Refined Dose-based RQs for 37g CRLF consuming different food items (calculated using THERPS). 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammals 

Small 

TerrestrialPhase 

Amphibians 

Apple (bearing fruit) 5.7 0.64 166 10 0.20 

Celery [1 season], Citrus 

(bearing fruit) 14 1.5 395 25 0.47 

Celery [3 seasons] 25 2.8 733 46 0.88 

Clover 5.0 0.56 146 9.1 0.18 

Cotton 7.7 0.85 222 14 0.27 

Cucumber, Melons, 

Pumpkin, Squash, 

Eggplant, Pepper 

13 1.4 362 23 0.43 

Garlic, Onion 13. 1.5 388 24 0.46 

Non-bearing fruit 20 2.2 58 36 0.69 

Potato 17 1.9 483 30 0.58 

Tomato 19 2.1 547 34 0.65 

Refined dose-based RQs for large sized (238g) CRLF consuming small insects, large 

insects, small herbivore mammals, small insectivore mammals, and small terrestrialphase amphibians all exceed the acute listed species LOC (0.1) for all uses of oxamyl, 

except for clover (Table 36). Only for the application rate specified for clover, the 

scenario of CRLF (238g) consuming small terrestrial-phased amphibians did not exceed 

83
 
the LOC (0.087). Due to exceedances for almost all uses, acute exposures to oxamyl 

have the potential to adversely affect the medium sized CRLF. 

Table 36. Refined Dose-based RQs for 238g CRLF consuming different food items (calculated using 

T-HERPS). 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammals 

Small 

Terrestrial-Phase 

Amphibians 

Apple (bearing fruit) 2.8 0.32 20 1.2 0.10 

Celery [1 season], Citrus 

(bearing fruit) 6.8 0.75 47 2.9 0.23 

Celery [3 seasons] 13 1.4 86 5.4 0.43 

Clover 2.5 0.28 17 1.1 0.09 

Cotton 3.8 0.42 26 1.6 0.13 

Cucumber, Melons, 

Pumpkin, Squash, 

Eggplant, Pepper 

6.2 0.69 43 2.7 0.22 

Garlic, Onion 6.6 0.74 46 2.9 0.23 

Non-bearing fruit 9.9 1.1 68 4.2 0.34 

Potato 8.2 0.92 57 3.6 0.29 

Tomato 9.3 1.0 64 4.0 0.32 

Although dietary-based RQs are generally lower than dose-based RQs, they follow a 

similar trend when compared to dose-based RQs. Refined acute dietary-based RQs for 

CRLFs consuming small insects and small herbivorous mammals meet or exceed the 

acute listed species LOC (0.1) for all uses of oxamyl (Table 37). For CLRF consuming 

large insects in the apple (bearing) and clover scenario, and for CRLF consuming small 

insectivore mammals in the scenarios of apple (bearing fruit), celery [1 season], citrus, 

clover, cotton, cucumber, melons, pumpkin, squash, eggplant, or pepper, RQs do not 

exceed the listed species acute dietary-based LOC. 

While the CRLF consuming small terrestrial-amphibians may only be affected by acute 

dietary-based exposures from oxamyl uses on apple, cherry, citrus, peach, and pear, the 

range of RQs from the other food items indicates that CRLF consuming small and large 

insects, and small herbivore and insectivore mammals could potentially be affected by 

acute dietary-based exposures to oxamyl at maximum application rates. 

84
 
Table 37. Refined Acute Dietary-based RQs for CRLF consuming different food items (calculated 

using T-HERPS). 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammals 

Small 

Terrestrial-Phase 

Amphibians 

Apple (bearing fruit) 0.79 0.09 0.93 0.06 0.03 

Celery [1 season], Citrus 

(bearing fruit) 1.9 0.21 2.2 0.14 0.07 

Celery [3 seasons] 3.5 0.39 4.1 0.26 0.12 

Clover 0.70 0.08 0.82 0.05 0.02 

Cotton 1.1 0.12 1.3 0.08 0.04 

Cucumber, Melons, 

Pumpkin, Squash, 

Eggplant, Pepper 

1.7 0.19 2.0 0.13 0.06 

Garlic, Onion 1.9 0.21 2.1 0.14 0.06 

Non-bearing fruit 2.8 0.31 3.2 0.20 0.10 

Potato 2.3 0.26 2.7 0.17 0.08 

Tomato 2.6 0.29 3.1 0.19 0.09 

Based on an analysis of the likelihood of individual mortality considering the range of 

refined acute dose-based RQs for terrestrial-phase CRLFs (Table 34 - Table 36) and a 

probit dose-response of 4.5 (slope), the chance of mortality for RQs which meet or 

exceed the LOC (0.1) range from 1 in 294,000 individuals to 1 in 1 individual. 

Considering alternate probit dose-responses of 9 and 2, the chance of mortality for RQs 

which exceed the LOC range from 1 in 8 E+18 to 1 in 1 individual, and 1 in 44 to 1 in 1 

individual, respectively (Figure 7). This range of RQs is relevant to all sizes of CRLF 

consuming small and large insects, small herbivore and insectivore mammals, and small 

terrestrial-phase amphibians. This range is not relevant to large CRLF consuming small 

terrestrial-phase amphibians modeled by the clover scenario, for which there was no LOC 

exceedance. 

The graph in Figure 7 shows the chance of individual mortality for the terrestrial-phase 

CRLF considering three probit dose-responses of 4.5, 9, and 2. These dose-responses 

converge at an RQ of 1, equivalent to a chance of mortality of 1 in 2 individuals. All 

small sized CRLFs consuming small insects exceed this RQ of 1, and thus exceed an 

individual chance of mortality of 50%, as do all medium and large sized CRLF 

consuming small insects, small herbivore mammals, and small insectivore mammals. 

For medium and large sized frogs consuming small herbivorous mammals, the chance of 

individual mortality is approximately 1 in 1 individual for all uses of oxamyl. The range 

of acute dose-based RQs indicates that most applications at maximum labeled rates pose 

risks of mortality to the CRLF. In exploring lower applications rates that result in 

reduced risks to the CRLF and do not cause LOC exceedances, a single application rate 

of 0.01 lbs a.i./acre was found to be the highest application rate that would not incur 

acute dosed-based LOC exceedances for any sized frog consuming all food items. 

85
 
Acute Dosed Based 

Chance of individual effect (1 in ...)

1.00E+19 

1.00E+18 

1.00E+17 

1.00E+16 

1.00E+15 

1.00E+14 

1.00E+13 

1.00E+12 

1.00E+11 

1.00E+10 

1.00E+09 

1.00E+08 

1.00E+07 

1.00E+06 

1.00E+05 

1.00E+04 

1.00E+03 

1.00E+02 

1.00E+01 

1.00E+00 

Series1 

Series2 

Series3

0.1 1 10 100 1000 

RQ 

 Figure 7. Chance of individual mortality to terrestrial-phase CRLF when considering dose-based 

RQs. 

Considering acute dietary-based RQs for the terrestrial phase CRLF (Table 37) and a 

probit dose-response of 4.5 (slope), the chance for mortality of RQs exceeding the LOC 

(0.1) range from 1 in 294,000 individuals to 1 in 1 individual (Figure 8). A probit doseresponse of 9 indicates that the chance for mortality ranges from 1 in 8 E+

18 individuals 

to 1 in 1 individual; considering a probit dose-response of 2, the chance for mortality 

ranges from 1 in 44 individuals to 1 in 1.12 individuals (Figure 8). 

The graph in Figure 8 shows the chance of individual mortality for the terrestrial-phase 

CRLF considering three probit dose-responses of 4.5, 9, and 2. These dose-responses 

converge at an RQ of 1, equivalent to a chance of mortality of 1 in 2 individuals. For the 

CRLF consuming small terrestrial-phase amphibians for all uses of oxamyl, all RQs are 

below the convergence point of an RQ of 1, and thus the chance of individual mortality is 

less than 50%. In exploring lower applications rates that present reduced risks to the 

CRLF and do not cause LOC exceedances, a single application rate of 0.21 lbs a.i./acre 

was found to be the highest application rate that would not cause acute dietary-based 

LOC exceedances for any exposure based on any food item. 

86
 
Acute Dietary Based 

Chance of individual effect (1 in ...)

1.00E+19 

1.00E+18 

1.00E+17 

1.00E+16 

1.00E+15 

1.00E+14 

1.00E+13 

1.00E+12 

1.00E+11 

1.00E+10 

1.00E+09 

1.00E+08 

1.00E+07 

1.00E+06 

1.00E+05 

1.00E+04 

1.00E+03 

1.00E+02 

1.00E+01 

1.00E+00 

Series1 

Series2 

Series3 

0.1 1 10 

RQ 

Figure 8. Chance of individual mortality to terrestrial-phase CRLF when considering acute dietarybase RQs. 

Sublethal effects are an indicator of direct effects from oxamyl exposure to the terrestrialphase CRLF. Two oral acute exposure studies on mallard ducks (MRID: 94660) and 

bobwhite quail (MRID: 406065-11) recorded sublethal effects. Clinical observations of 

oxamyl-treated mallard ducks reported the following effects: loss of coordination at 1.00 

and 3.16 mg/kg-diet; ruffled feathers at 1.00 mg/kg-diet; weakness and slow movement at 

3.16 mg/kg-diet; discharge from eyes and nose at 3.16 and 10.0 mg/kg-diet; and apparent 

leg paralysis at 3.16, 10.0, 31.6, and 100 mg/kg-diet. Reported signs of overt toxicity in 

bobwhite quail occurred at test concentrations of 78 mg/kg-diet and higher, and included 

lethargy, ruffled appearance, depression, wing droop, loss of coordination, reduced 

reaction to external stimuli, shallow and rapid respiration, lower limb weakness, head 

tremors, and prostrate posture. The dose of 31.6 mg/kg-diet corresponds to the EEC 

value of 30.0 ppm in T-HERPS, and causes an LOC exceedance with an RQ value of 3.0. 

This indicates that oxamyl concentrations at which some of these sublethal effects occur 

at (>30 mg/kg-diet), have the potential to cause effects to survival, growth and 

reproduction of the CRLF. 

87
 
Based on the information in this section, for all uses of oxamyl, the effects determination 

for acute effects to the terrestrial phase CRLF is “likely to adversely affect (LAA)” based 

on potential mortality through consumption of oxamyl-contaminated food items. 

Chronic RQs 

Refined chronic dietary-based RQs for CRLFs consuming insects and mammals using the 

T-HERPS model exceed the chronic listed species LOC (1.0) for all uses of oxamyl. The 

chronic listed species LOC is exceeded for CRLF consuming terrestrial-phase 

amphibians for every use except clover (Table 38). 

Table 38. Revised Chronic Dietary-based RQs for CRLF consuming different food items (RQs 

calculated using T-HERPS). 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammals 

Small 

Terrestrial-Phase 

Amphibians 

Apple (bearing fruit) 27 3.0 32 2.0 0.94 

Celery [1 season], Citrus 

(bearing fruit) 64.1 7.13 75.1 4.70 2.23 

Celery [3 seasons] 119 13 140 8.7 4.13 

Clover 23.7 2.64 27.8 1.74 0.824 

Cotton 36.1 4.02 42.3 2.65 1.25 

Cucumber, Melons, 

Pumpkin, Squash, 

Eggplant, Pepper 

58.9 6.54 69.0 4.31 2.04 

Garlic, Onion 63.0 7.00 73.8 4.61 2.19 

Non-bearing fruit 93.7 10.4 110 6.86 3.25 

Potato 78.3 8.71 91.8 5.74 2.72 

Tomato 88.8 9.86 104 6.50 3.08 

In the available chronic study where Northern bobwhite quail were exposed to oxamyl, 

the NOAEC was 10 mg/kg-diet, and the LOAEC was 50 mg/kg-diet, based on effects to 

egg production and egg fertility. Comparison of the LOAEC directly to chronic dietarybased EECs for CRLF consuming small insects and small herbivore mammals indicate 

that EECs for all uses are sufficient to exceed the concentration where reproductive 

effects were observed in the laboratory. For CRLF consuming large insects, the majority 

of the oxamyl uses have EECs which are insufficient to exceed the LOAEC. Therefore, 

for some CRLF feeding categories, oxamyl EECs are at levels where reproductive effects 

were observed in birds, which serve as surrogates for the CRLF. 

In exploring lower applications rates that present reduced risks to the CRLF and do not 

cause LOC exceedances, a single application rate of 2.13 lbs a.i./acre was found to be the 

highest application rate that would not cause chronic dietary-based LOC exceedances for 

any exposure based on any food item. 

88
 
Based on this information, the effects determination for chronic effects from all oxamyl 

uses to the terrestrial-phase CRLF is “LAA” based on potential for effects to 

reproduction. 

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 nonvascular aquatic plants (i.e., algae and diatoms) and detritus. For all uses of oxamyl 

relevant to California, exposures in the aquatic habitat to non-vascular aquatic plants are 

insufficient to result in RQs exceeding the LOC. Therefore, there is no effect to tadpolestage CRLFs via effects on aquatic non-vascular plants from exposures resulting from 

any use of oxamyl. 

5.2.2.2. Aquatic Invertebrates 

RQ values representing acute exposures to aquatic invertebrates indicate that several uses 

of oxamyl can potentially result in effects to invertebrates. Therefore, indirect effects are 

may affect CRLF juveniles and adults, through decreases in prey base. The results of an 

analysis of the likelihood of individual mortality using acute RQs for aquatic 

invertebrates and a probit dose-response of 4.5 (based on default) are displayed in Table 

39. Based on this analysis, all uses of oxamyl result in ≤0.34% chance of effects to an 

individual aquatic invertebrate. Therefore, although acute oxamyl exposures may affect 

individual CRLF through indirect effects caused by effects to aquatic invertebrates, the 

impact of these effects on the CRLF is discountable for all uses. Therefore, effects to 

aquatic invertebrates resulting from acute exposures are not likely to adversely affect the 

CRLF. 

For the majority of uses of oxamyl relevant to California, chronic exposures in the 

aquatic habitat to aquatic invertebrates are insufficient to result in RQs exceeding the 

LOC. Chronic exposures for the uses of garlic and celery resulted in RQs exceeding the 

LOC. The chronic RQs are based on a derived chronic NOAEC for aquatic invertebrate 

(0.012 mg a.i./L) from an ACR. If the ACR method was used to derive an adjusted 

LOAEC for chironomids, that value would be 0.027 mg a.i./L. This adjusted LOAEC is 

above the chronic EECs for aquatic invertebrates (max EEC = 0.018 mg a.i./L). In 

addition, EECs are below the available NOAEC value for daphnids (NOAEC=0.027 mg 

a.i./L and LOAEC=0.050 mg a.i./L, MRID: 450678-01), suggesting that even though 

sensitive species of aquatic invertebrates may be affected by chronic exposures of 

oxamyl, less sensitive species, such as daphnids are unlikely to be affected. Given that 

the CRLF is an opportunistic feeder, the impact of effects to sensitive aquatic 

invertebrates from chronic exposures on the CRLF is discountable. Therefore, although 

chronic oxamyl exposures may affect individual CRLF through indirect effects caused by 

effects to aquatic invertebrates, the impact of these effects on the CRLF is discountable 

for all uses. Therefore, effects to aquatic invertebrates resulting from chronic exposures 

are not likely to adversely affect the CRLF. 

89
 
Table 39. Acute RQs and corresponding likelihood of individual effects and chronic RQs for aquatic 

invertebrates exposed to oxamyl. 

Use 

Indirect 

Effects 

Acute RQ1 

Individual 

Chance of 

Effects3 

Indirect Effects 

Chronic RQ4 

Celery (3 seasons) 0.252 0.34% 1.55 

Celery (1 season) 0.172* 0.03% 1.15 

Garlic 0.192* 0.06% 1.15 

Tomato 0.162 0.02% 0.7 

Non-bearing fruit 0.122 <0.01% 0.7 

Pepper 0.102 <0.01% 0.6 

Cotton 0.112 <0.01% 0.6 

Citrus (bearing fruit) 0.072 <0.01% 0.3 

Potato 0.04 <0.01% 0.4 

Clover 0.04 <0.01% 0.3 

Onion 0.04 <0.01% 0.4 

Cucumber, melons, 

pumpkin, squash 0.04 <0.01% 0.2 

Apple (bearing fruit) 0.02 <0.01% 0.1 

Eggplant 0.02 <0.01% 0.1 1

 Acute RQ = use-specific 1-in-10-year peak EEC / acute freshwater 

invertebrate endpoint (EC50 = 180 μg/L). 2

Exceeds listed species LOC for acute exposures (0.05). 3

Calculated using default slope = 4.5. 

4

 Chronic RQ = use-specific 1-in-10-year 21-day-mean EEC / chronic 

freshwater ACR endpoint (NOAEC = 12 μg/L). 

5

 Exceeds listed species LOC for chronic exposures (1.0). 

5.2.2.3. Fish and Aquatic-phase Frogs 

For all uses of oxamyl acute and chronic exposures in the aquatic habitat to fish and 

aquatic-phase frogs (serving as prey to adult CRLF) are insufficient to result in RQs 

exceeding the LOC. Therefore, indirect effects of oxamyl by reductions in this food 

source of the adult CRLF are not expected based on the animal’s diet during this life 

stage for all uses of oxamyl. Therefore, there is no effect to aquatic-phase CRLFs via 

effects on fish and aquatic-phase amphibians from aquatic exposures arising from any use 

of oxamyl that is relevant to California. 

5.2.2.4. Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. RQs representing acute exposures of terrestrial 

invertebrates are represented by oxamyl exposures to small and large terrestrial insects 

located on the treatment field. Resulting RQ values exceed the acute LOC (0.05) by 

several orders of magnitude, with values ranging 9.57-390 (Table 27). 

90
 
An analysis of the likelihood of individual mortality indicates that an RQ of 5 or greater 

results in a >99.9% probability of mortality to an individual terrestrial invertebrate 

located on a field treated with oxamyl (assuming a default probit dose-response value of 

4.5) Table 25. Since all RQs for terrestrial invertebrates are >5, the likelihood of 

individual mortality to a terrestrial invertebrate on a field treated with any use of oxamyl 

(relevant to California) is >99.9%. 

Based on the lowest acute toxicity value for terrestrial invertebrates (LD50 = 2.4 ppm), 

exposure concentrations <0.12 ppm would be below levels of concern for this taxa. For 

large and small insects, this exposure would be expected at single applications of <0.008 

and <0.0009 lbs a.i./A, respectively. These exposure concentrations are well below 

maximum single application rates allowed on product labels, as well as several orders of 

magnitude below estimated average single application rates of oxamyl in California, 

which range 0.43-10 lbs a.i./A (Table 7). 

A spray drift exposure analysis (using AgDRIFT) indicates that oxamyl concentrations 

resulting from a single aerial application of 1 lb a.i./A would be at levels sufficient to 

exceed the LOC for large and small terrestrial insects at <990 feet from the edge of the 

treated field (assuming the default ASAE fine to medium droplet size distribution). A 

likelihood of individual mortality analysis (using IEC v. 1.1) indicates that >10% chance 

of mortality to individual large insects can occur <120 ft from the edge of the field and 

>22% chance of mortality to individual small insects can occur <950 feet from the edge 

of the field (Figure 9). 

100 Large insect 

Small insect 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

0 100 200 300 400 500 600 700 800 900 1000 

Distance from edge of field (ft) 

Figure 9. Likelihood of individual mortality to terrestrial invertebrates at different distances from 

the edge of a field treated with a single aerial application of oxamyl at 1 lb a.i./A. Liklihood of individual mortality (%) 

91
 
A spray drift exposure analysis (using AgDRIFT) indicates that oxamyl concentrations 

resulting from a single ground application of 2 lb a.i./A would be at levels sufficient to 

exceed the LOC for large and small terrestrial insects at 209 and <990 feet from the edge 

of the treated field (assuming low boom, ASAE very fine to fine droplet size 

distribution). A likelihood of individual mortality analysis (using IEC v. 1.1) indicates 

that >10% chance of mortality to individual large and small insects can occur within 

approximately 20 ft and 250 feet, respectively, from the edge of the field (Figure 10). 

100.00 Large insect 

Small insect 

90.00 

80.00 

70.00 

60.00 

50.00 

40.00 

30.00 

20.00 

10.00 

0.00 

0 100 200 300 400 500 600 700 800 900 1000 

Distance from edge of field (ft) 

Figure 10. Likelihood of individual mortality to terrestrial invertebrates at different distances from 

the edge of a field treated with a single ground application of oxamyl at 2 lb a.i./A. 

Therefore, it is possible that there will be indirect effects to terrestrial-phase CRLFs via 

effects on terrestrial insects from oxamyl exposures arising from any use of oxamyl that 

is relevant to California. The extent of effects is unknown because a range of sensitivities 

of invertebrates is expected to exist. 

5.2.2.5. Mammals 

Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume 

terrestrial mammals, including mice. RQs representing acute and chronic exposures of 

terrestrial mammals are represented by oxamyl exposures to animals located on the 

treatment field. 

Acute Exposures Liklihood of individual mortality (%) 

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Acute dose-based RQ for small mammals consuming short grass exceed the acute LOC 

(0.1) by several orders of magnitude for all uses of oxamyl, with values ranging 73.2-288 

(Table 28). Single applications of 1 or 2 lbs a.i./A of oxamyl result in acute, dose-based 

RQs which are sufficient to exceed the LOC for small mammals consuming other food 

items, including tall grass, broadleaf plants, small insects, fruits, pods, large insects and 

seeds (Table 40). 

Table 40. Acute, dose-based RQ values for small mammals (15 g) exposed to oxamyl through 

consumption of food items on the treated field. RQs correspond to single applications of oxamyl at 1 

lb a.i./A and 2 lb a.i./A. 

Diet 1 lb a.i./A 2 lb a.i./A 

Short Grass 41.64 83.29 

Tall Grass 19.09 38.17 

Broadleaf plants/small insects 23.43 46.85 

Fruits/pods/large insects 2.60 5.21 

Seeds 0.58 1.16 

An analysis of the likelihood of individual mortality indicates that an RQ of 3 or greater 

results in a >99.9% probability of mortality to an individual mammal located on a field 

treated with oxamyl (assuming a probit dose-response value of 9.3 and 18.0, MRID 

00063011) Table 25. Since all acute, dose-based RQs for small mammals consuming 

short grass are >3, the likelihood of individual mortality to a terrestrial mammal on a field 

treated with any use of oxamyl (relevant to California) is >99.9%. In addition, the 

likelihood of individual mortality to mammals consuming other food items, besides short 

grass, is also >99.9%, based on RQs>3 (Table 40). 

Based on the lowest acute toxicity value for mammals (LD50 = 2.5 mg/kg-bw), exposure 

concentrations <0.0025 lbs a.i./A would be below levels of concern for small mammals. 

This exposure concentration is below maximum single application rates allowed on 

product labels, as well as several orders of magnitude below estimated average single 

application rates of oxamyl in California, which range 0.43-1.34 lb a.i./A (Table 7). A 

spray drift exposure analysis (using AgDRIFT) indicates that oxamyl concentrations 

resulting from a single aerial application of 1 lb a.i./A would be at levels sufficient to 

exceed the LOC for small mammals at >990 feet from the edge of the treated field. 

Exposure concentrations up to 801 feet from the edge of a field treated by ground 

methods with 2 lb a.i./A oxamyl would be at levels sufficient to exceed the LOC for 

small mammals (Table 41). 

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Table 41. Concentration where LOC is not exceeded for small mammals (15 g) receiving acute, dosebased exposures of oxamyl through consumption of contaminated food items and distance from edge 

of treatment field where LOC is not exceeded. 

Diet 

Concentration (lb a.i./A) 

where acute LOC is not 

exceeded* 

Distance (ft) from edge of field 

where LOC is not exceeded 

Single aerial 

application of 1 

lb a.i./A 

Single ground 

application of 2 

lb a.i./A 

Short Grass 0.0025 >990** 801 

Tall Grass 0.0050 >990** 423 

Broadleaf plants/small insects 0.0044 >990** 479 

Fruits/pods/large insects 0.0400 236 46 

Seeds 0.1700 52 13 

* Based on the lowest acute toxicity value for mammals (LD50 = 2.5 mg/kg-bw) and LOC of 0.1 

**The limit of AgDRIFT’s predictability is 990 ft from the edge of the treated field. 

The T-REX model is useful for assessing exposures of terrestrial animals to pesticides 

applied to foliar surfaces of crops. The model cannot be used to assess pesticide 

exposures to terrestrial animals resulting from applications involving soil incorporation. 

In order to explore the potential exposures of mammals to oxamyl following applications 

involving ground application, a simple fugacity approach was employed to estimate 

oxamyl concentrations in earthworms and subsequent exposures to mammals consuming 

earthworms. This approach is explained in detail in Appendix K. If it is assumed that the 

small mammal being assessed eats only earthworms on the day of treatment (meaning 

that there would be no degradation of oxamyl), the acute dose of oxamyl to a 20 g 

mammal would be 0.024 to 0.078 mg/kg-bw. This range of exposure values is 2 orders of 

magnitude below the LD50 values for rats (2.5-3.1 mg/kg-bw; MRID 00063011) exposed 

to oxamyl. The likelihood of individual mortality to a small mammal consuming only 

earthworms from a treated field would be <0.01%. 

Chronic Exposures 

Dose-based chronic RQ values for small mammals (15 g) consuming short grass exceed 

the chronic LOC (1.0) by several orders of magnitude, with values ranging 146-577. 

Dietary-based chronic RQ values also exceed the chronic LOC (1.0) by several orders of 

magnitude, with values ranging 16.8-66.6 (Table 28). Single applications of 1 or 2 lbs 

a.i./A of oxamyl result in chronic, dose-based and dietary-based RQs which are sufficient 

to exceed the LOC for small mammals consuming other food items, including tall grass, 

broadleaf plants, small insects, fruits, pods, large insects and seeds (Table 42 and Table 

43). 

Chronic RQ values are derived with available NOAEC data. If chronic, dose-based and 

dietary-based EECs (Table 16) were compared to the LOAEC and LOAEL values for 

mammals exposed to oxamyl, the EECs exceed these levels (75 mg/kg-diet) where 

effects to mammals were observed in the laboratory. Observed effects include decreased 

body weight during lactation (MRID 416608-01). 

94
 
Table 42. Chronic, dose-based RQ values for small mammals (15 g) exposed to oxamyl through 

consumption of food items on the treated field. RQs correspond to single applications of oxamyl at 1 

lb a.i./A and 2 lb a.i./A. 

Diet 1 lb a.i./A 2 lb a.i./A 

Short Grass 83.29 166.58 

Tall Grass 38.17 76.35 

Broadleaf plants/small insects 46.85 93.70 

Fruits/pods/large insects 5.21 10.41 

Seeds 1.16 2.31 

Table 43. Chronic, dietary-based RQ values for small mammals (15 g) exposed to oxamyl through 

consumption of food items on the treated field. RQs correspond to single applications of oxamyl at 1 

lb a.i./A and 2 lb a.i./A. 

Diet 1 lb a.i./A 2 lb a.i./A 

Short Grass 9.60 19.20 

Tall Grass 4.40 8.80 

Broadleaf plants/small insects 5.40 10.80 

Fruits/pods/large insects 0.60 1.20 

Seeds 0.60 1.20 

Summary 

Based on this information, there are potential indirect effects to terrestrial-phase CRLFs 

via effects on small mammals from oxamyl exposures arising from any use of oxamyl 

that is relevant to California. 

5.2.2.6. Terrestrial-phase Amphibians 

In order to explore influences of amphibian-specific food intake equations on potential 

dose-based and dietary-based exposures of amphibians (prey of CRLF) to oxamyl, the THERPS model is used. The Pacific tree frog is used to represent the amphibian prey 

species. The weight of the animal is assumed to be 2.3 g, and its diet is assumed to be 

composed of small and large insects. For frogs consuming small and large insects, the 

acute LOC (0.1) is exceeded for all uses of oxamyl (Table 44). 

Based on an analysis of the likelihood of individual mortality using acute dose-based 

RQs for terrestrial phase frogs and a default probit dose-response of 4.5, the likelihood of 

individual mortality for each use is available in Table 44. Based on this analysis all of 

uses of oxamyl result in >10% chance of effects to an individual terrestrial phase frog 

consuming small insects. The majority of uses of oxamyl also result in >10% chance of 

effects to an individual terrestrial phase frog consuming large insects. Based on acute 

dose-based RQs and individual effects chance for terrestrial-phase frogs (prey of CRLF) 

which consume small insects, there is potential for effects to this taxa due to oxamyl 

exposures from all uses. 

95
 
Table 44. Acute dose-based RQs and associated likelihood of individual effects to terrestrial-phase 

frogs (prey) due to oxamyl exposures. 

Frogs consuming 

Small Insects 

Frogs consuming 

Large Insects 

Use RQ Likelihood of 

individual 

acute effect (%) 

RQ Likelihood of 

individual acute 

effect (%) 

Apple (bearing fruit) 2.97 98 0.33 2 

Celery [1 season], Citrus (bearing fruit) 7.05 ~100 0.78 31 

Celery [3 seasons] 13.1 ~100 1.45 77 

Clover 2.61 97 0.29 1 

Cotton 3.97 ~100 0.44 5 

Cucumber, Melons, 

Pumpkin, Squash, Eggplant, Pepper 

6.47 ~100 0.72 26 

Garlic, Onion 6.92 ~100 0.77 30 

Non-bearing fruit 10.3 ~100 1.14 60 

Potato 8.61 ~100 0.96 47 

Tomato 9.75 ~100 1.08 56 

Acute dietary-based RQs for the CRLF, which do not account for the weight of the 

animal being assessed, can also be used to assess risks to the terrestrial frog prey (Table 

37). For frogs which consume small insects, RQs exceed the acute LOC (0.1) for all 

oxamyl uses. For frogs which consume large insects, the acute LOC is exceeded for all 

uses of oxamyl, with the exception of oxamyl use on clover. 

Chronic dietary-based RQs for the CRLF, which do not account for the weight of the 

animal being assessed, can also be used to assess risks to the terrestrial frog prey (Table 

38). Refined dietary-based RQs indicate that, for all oxamyl uses, there is potential for 

chronic effects to terrestrial frogs feeding on small and large insects. Chronic RQs are 

exceeded by factors ranging 2.6X to 94X. 

Summary 

Based on this information, there are potential indirect effects to terrestrial-phase CRLFs 

via effects on small terrestrial phase frogs from oxamyl exposures arising from any use of 

oxamyl that is relevant to California. 

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 

96
 
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 were assessed using RQs from freshwater aquatic vascular and non-vascular 

plant data. RQs for both aquatic vascular and non-vascular plants are below the LOC, 

indicating that exposures of oxamyl to aquatic plants will not result in effects to plants. In 

addition exposures of oxamyl to riparian vegetation are below levels where no effects to 

terrestrial plants were observed in greenhouse studies. Therefore, the determination for 

indirect effects to the CRLF caused by effects to aquatic plants resulting from use of 

oxamyl is “No Effect.” 

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. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production were assessed using RQs from monocot and dicot plant data. RQs for both 

monocots and dicots were below the Agency’s LOC, indicating that exposures of oxamyl 

to terrestrial plants will not result in effects to plants. While effects to height and weight 

were observed in dicots in the vegetative vigor and seedling emergence studies, these 

effects were ≤25% relative to the control. Based on the supplemental nature of the 

vegetative vigor study, the ≤25% effects relative to the controls, and no RQ exceedances, 

the determination for indirect effects to the CRLF caused by effects to terrestrial plants 

resulting from use of oxamyl is “No Effect.” 

5.2.4. Modification to Designated Critical Habitat 

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 

97
 
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). 

Based on the risk estimation for potential effects to aquatic and terrestrial plants provided 

in Sections 5.1.1.2, 5.1.1.3, and 5.1.2.3 (i.e., a lack of LOC exceedances by RQs) oxamyl 

is not likely to affect aquatic-phase PCEs of designated habitat related to effects on 

aquatic and/or terrestrial plants. 

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. RQs for these 

endpoints were calculated in Sections 5.1.1.1 and 5.1.1.2. As discussed in these sections, 

RQs for direct effects to CRLF are below LOCs for acute and chronic exposures. RQs 

for non-vascular plants, representing dietary items of the tadpole stage of the CRLF are 

below the LOC. Although some RQs for acute exposures of aquatic invertebrates to 

oxamyl exceed the LOC, these effects are considered discountable. Therefore, oxamyl 

may affect, but is not likely to adversely 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. 

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 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 

Based on the risk estimation for terrestrial-phase PCEs of designated habitat related to 

potential effects on terrestrial plants is provided in Section 5.1.2.3, oxamyl is not likely 

to affect terrestrial-phase PCEs of designated habitat related to effects on terrestrial 

plants. 

98
 
The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of oxamyl 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 Section 5.1.2.2. 

Based on RQ exceedances for acute and chronic exposure to frogs, mammals, and 

terrestrial invertebrates, oxamyl is likely to result in habitat modification based on 

potential impacts to 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 5.2.1.2. Based on RQ exceedances for acute and chronic exposure to birds, 

mammals, and terrestrial invertebrates, oxamyl is likely to result in habitat 

modification based on potential impacts to the fourth terrestrial-phase PCEs. 

5.3. Uncertainties 

5.3.1. Exposure Assessment Uncertainties 

From 1994 to 2006 oxamyl was not used on cherries, based on the CDPR PUR data. 

While the lack of use indicates that oxamyl most likely was not used in the past decade 

on cherries, it is still a registered crop and has the potential to be used in the future. Thus, 

risk to CRLF from use on cherries is considered in this assessment. 

In contrast, U.S. Department of Agriculture census data confirm that tobacco has not 

been grown in California from at least the 1992 report, forward (USDA 2008) and it is 

unlikely that tobacco agriculture will be initiated in California in the future. Therefore, 

use of oxamyl on tobacco was not quantitatively evaluated for exposure in this 

assessment. 

5.3.1.1. 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. 

5.3.1.2. Aquatic Exposure Modeling of Oxamyl 

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 

99
 
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 

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). 

Furthermore, if aquatic-phase CRLFs inhabit acidic water bodies that are exposed to 

oxamyl, the duration of exposure may be longer than was estimated using the standard 

aquatic modeling guidance, as oxamyl is relatively stable to hydrolysis at pH 5. In this 

case, aquatic invertebrates used as prey may be at higher risk to chronic exposure and at 

slightly higher risk to acute exposure than was assessed. Although this is an uncertainty 

in the assessment, the increase in risk is relatively small and potential indirect effects to 

the aquatic-phase CRLF due to effects of oxamyl to aquatic invertebrates are not 

expected. 

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 

100
 
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. Also, 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. However, the model input for the aerobic soil metabolism half-life remains 

uncertain due to an uncharacteristically high value in the data. 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 a 

vegetative setback on runoff and loadings. The effectiveness of vegetative setbacks is 

highly dependent on the condition of the vegetative strip. For example, a wellestablished, healthy vegetative setback can be a very effective means of reducing runoff 

and erosion from agricultural fields. Alternatively, a setback of poor vegetative quality 

or a setback that is channelized can be ineffective at reducing loadings. Until such time 

as a quantitative method to estimate the effect of vegetative setbacks on various 

conditions on pesticide loadings becomes available, the aquatic exposure predictions are 

likely to overestimate exposure where healthy vegetative setbacks exist and 

underestimate exposure where poorly developed, channelized, or bare setbacks exist. 

In order to account for uncertainties associated with modeling, available monitoring data 

were compared to PRZM/EXAMS estimates of peak EECs for the different uses. As 

discussed above, several data values were available from NAWQA for oxamyl 

concentrations measured in surface waters receiving runoff from agricultural areas. The 

specific use patterns (e.g. application rates and timing, crops) associated with the 

agricultural areas are unknown, however, they are assumed to be representative of 

potential oxamyl use areas. 

5.3.1.3. 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 

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 

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characteristics of: a) undeveloped (especially forested) areas, which exhibit the least 

amount of surface runoff but the greatest amount of ground water 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. 

5.3.1.4. Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Four years of data (2002 – 

2005) were included in this analysis because statistical methodology for identifying 

outliers, in terms of area treated and pounds applied, was provided by CDPR for these 

years only. No methodology for removing outliers was provided by CDPR for 2001 and 

earlier pesticide data; therefore, this information was not included in the analysis because 

it may misrepresent actual usage patterns. CDPR PUR documentation indicates that 

errors in the data may include the following: a misplaced decimal; incorrect measures, 

area treated, or units; and reports of diluted pesticide concentrations. In addition, it is 

possible that the data may contain reports for pesticide uses that have been cancelled. 

The CPDR PUR data does not include home owner applied pesticides; therefore, 

residential uses are not likely to be reported. As with all pesticide usage data, there may 

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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. 

5.3.1.5. Terrestrial Exposure Modeling of Oxamyl 

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. 

5.3.1.6. Spray Drift Modeling 

Although there may be multiple oxamyl applications at a single site, it is unlikely that the 

same organism would be exposed to the maximum amount of spray drift from every 

103
 
application made. In order for an organism to receive the maximum concentration of 

oxamyl from multiple applications, each application of oxamyl 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 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, the drift estimates from AgDRIFT 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 droplet 

size distributions being modeled (‘ASAE Very Fine to Fine’ for orchard uses and ‘ASAE 

Very Fine’ for agricultural uses), the application method (e.g., aerial), release heights and 

wind speeds. Alterations in any of these inputs would change the area of potential effect. 

5.3.1.7. 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. 

5.3.2. Effects Assessment Uncertainties 

5.3.2.1. 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 

104
 
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. 

5.3.2.2. Use of Surrogate Species Effects Data 

CRLF 

Guideline toxicity tests and open literature data on oxamyl are not available for frogs or 

any other aquatic-phase amphibian; therefore, freshwater fish are used as surrogate 

species for aquatic-phase amphibians. Endpoints based on freshwater fish ecotoxicity 

data are assumed to be protective of potential direct effects to aquatic-phase amphibians 

including the CRLF, and extrapolation of the risk conclusions from the most sensitive 

tested species to the aquatic-phase CRLF is likely to overestimate the potential risks to 

those species. 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. 

Terrestrial Plants 

Plant toxicity data from both registrant-submitted studies and studies in the scientific 

literature were reviewed for this assessment. Registrant-submitted studies are conducted 

under conditions and with species defined in EPA toxicity test guidelines. Sub-lethal 

endpoints such as plant growth, dry weight, and biomass are evaluated for both monocots 

and dicots, and effects are evaluated at both seedling emergence and vegetative life 

stages. Guideline studies generally evaluate toxicity to ten crop species. A drawback to 

these tests is that they are conducted on herbaceous crop species only, and extrapolation 

of effects to other species, such as the woody shrubs and trees and wild herbaceous 

species, contributes uncertainty to risk conclusions. 

Commercial crop species have been selectively bred, and may be more or less resistant to 

particular stressors than wild herbs and forbs. The direction of this uncertainty for 

specific plants and stressors, including oxamyl, is largely unknown. Homogenous test 

plant seed lots also lack the genetic variation that occurs in natural populations, so the 

range of effects seen from tests is likely to be smaller than would be expected from wild 

populations. 

5.3.2.3. 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 t is 

exercised on a case-by-case basis and only after careful consideration of the nature of the 

105
 
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. To the 

extent to which sublethal effects are not considered in this assessment, the potential direct 

and indirect effects of oxamyl on CRLF may be underestimated. 

5.3.2.4. Reproductive effects to plants 

Potential reproductive effects to plants are not considered as endpoints in this assessment. 

The toxicity data used in this assessment indicate that 2.1 lb a.i./A of oxamyl causes less 

than 25% effects to the vegetative vigor and seedling emergence of tested (crop) species. 

However, oxamyl is used to thin apples at single application rates of 0.5-1.0 lb a.i./A 

(registration # 352-372). Based on this information, it is possible that oxamyl applications 

could result in reproductive effects to non-target plants. 

5.4. Addressing the risk hypotheses 

In order to conclude this risk assessment, it is necessary to address the risk hypotheses 

defined in Section 2.9.1. Based on the results of this assessment, several hypotheses can 

be rejected, meaning that they are not of concern for the CRLF. However, several of the 

original hypotheses cannot be rejected, meaning that the statements represent concerns in 

terms of effects of oxamyl on the CRLF. 

Based on the results of this assessment, the following hypotheses can be rejected: 

•	 Labeled uses of oxamyl within the action area may indirectly affect the CRLF 

and/or adversely modify designated critical habitat by reducing or changing the 

composition of the aquatic plant community in the ponds and streams comprising 

the species’ current range and designated critical habitat, thus affecting primary 

productivity and/or cover; 

•	 Labeled uses of oxamyl within the action area may indirectly affect the CRLF 

and/or adversely modify designated critical habitat by reducing or changing the 

composition of the terrestrial plant community (i.e., riparian habitat) required to 

maintain acceptable water quality and habitat in the ponds and streams 

comprising the species’ current range and designated critical habitat; 

•	 Labeled uses of oxamyl within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing upland habitat 

within 200 ft of the edge of the riparian vegetation necessary for shelter, foraging, 

and predator avoidance. 

•	 Labeled uses of oxamyl within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing dispersal habitat 

within designated units and between occupied locations within 0.7 mi of each 

106
 
other that allow for movement between sites including both natural and altered 

sites which do not contain barriers to dispersal. 

•	 Labeled uses of oxamyl within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing or changing breeding and 

non-breeding aquatic habitat (via modification of water quality parameters, 

habitat morphology, and/or sedimentation). 

Based on the results of this assessment, the following hypotheses can not be rejected. 

•	 Labeled uses of oxamyl within the action area may directly affect the CRLF by 

causing mortality or by adversely affecting growth or fecundity; 

•	 Labeled uses of oxamyl within the action area may indirectly affect the CRLF by 

reducing or changing the composition of food supply; 

•	 Labeled uses of oxamyl within the action area may adversely modify the 

designated critical habitat of the CRLF by reducing the food supply required for 

normal growth and viability of juvenile and adult CRLFs; 

•	 Labeled uses of oxamyl within the action area may adversely modify the 

designated critical habitat of the CRLF by altering chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs. 

6. 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 oxamyl to the CRLF and its designated 

critical habitat. 

Based on the best available information, the Agency makes a May Affect, and Likely 

to Adversely Affect (LAA) determination for the CRLF from the use of oxamyl. 

Additionally, the Agency has determined that there is the potential for modification 

of CRLF designated critical habitat from the use of the chemical. These conclusions 

are based on potential effects of oxamyl to the terrestrial-phase CRLF and its prey, 

including terrestrial insects, small mammals and terrestrial-phase frogs. Summaries of 

the risk conclusions and effects determinations for the CRLF and its critical habitat are 

presented in Table 45 and Table 46, respectively. Analysis related to the intersection of 

the oxamyl action area and CRLF habitat used in determining use patterns that result in 

LAA determinations are described in Appendix A. The LAA determination for the 

CRLF and potential modification of designated critical habitat is described in the baseline 

status and cumulative effects for the CRLF and is provided in Attachment 2. 

107
 
Table 45. Description of evidence supporting effects determination for oxamyl use in California. 

Effects Habitat Basis for LAA Determination 

Direct Aquatic Direct effects to aquatic-phase CRLF resulting from acute and chronic exposures of oxamyl are not 

expected. 

- RQs for acute and chronic exposures resulting from all uses of oxamyl are below the levels of concern. 

Direct Terrestrial Oxamyl has the potential to directly affect the terrestrial-phase CRLF. 

-Acute and chronic RQs for all uses of oxamyl exceed the Agency’s LOCs for the terrestrial-phase 

CRLF. 

- The likelihoods of individual mortality range from 0.00034% to 100% for terrestrial-phase CRLF on a 

field treated with uses of oxamyl that have LOC exceedances (relevant to California). 

- Comparison of the LOAEC directly to chronic dietary-based EECs for CRLF consuming small insects 

and small herbivore mammals indicate that EECs for all uses are sufficient to exceed the concentration 

where reproductive effects were observed in the laboratory. 

Indirect Aquatic Indirect effects to the CRLF through effects to its prey in the aquatic habitat are not expected. 

- prey - RQs representing exposures to non-vascular aquatic plants (which serve as prey to tadpole stage 

CRLF) are below LOCs for all uses of oxamyl. 

- RQs representing acute and chronic exposures to fish (which serve as prey to adult CRLF) are below 

LOCs for all uses of oxamyl. 

- Although some RQs representing acute exposures to aquatic invertebrates are above the LOC, the 

likelihood of individual effects chance indicates that all uses of oxamyl are expected to result in ≤0.34% 

chance of effects to an individual aquatic invertebrate. Therefore, potential indirect effects to the CRLF 

due to effects of oxamyl to aquatic invertebrates resulting from acute exposures are discountable. 

- RQs representing chronic exposures to aquatic invertebrates (serving as prey to juvenile and adult 

CRLF) are below LOCs, with the exception of 2 uses (garlic and celery). EECs for garlic and celery do 

not reach the NOAEC for less sensitive species of aquatic invertebrates (i.e., Daphnia magna), 

suggesting that even though sensitive species of aquatic invertebrates may be affected by chronic 

exposures of oxamyl, less sensitive species, such as daphnids, may not be affected. Because the CRLF is 

an opportunistic feeder, the impact of effects to sensitive aquatic invertebrates from chronic exposures 

on the CRLF is discountable. 

Indirect Aquatic Indirect effects to the CRLF through modification of its aquatic habitat are not expected. 

- habitat -RQs representing exposures to non-vascular and vascular aquatic plants are below LOCs. 

-Estimated exposures of oxamyl to riparian vegetation are above the EC25 values for monocots and 

dicots. 

Indirect Terrestrial Oxamyl has the potential to indirectly affect the terrestrial-phase CRLF through effects to its prey. 

- prey -RQs representing exposures to terrestrial insects are above the LOC for all uses of oxamyl. 

- The likelihood of individual mortality is >99.9% for a terrestrial invertebrate on a field treated with any 

use of oxamyl (relevant to California). 

- >22% chance of mortality to individual small insects can occur within 950 feet from the edge of a field 

treated with 1 lb a.i./A oxamyl by aerial methods. 

-RQs representing acute and chronic exposures to small terrestrial mammals are above LOCs for all uses 

of oxamyl. 

- The likelihood of individual mortality is >99.9% for a small mammal consuming short grass on a field 

treated with any use of oxamyl (relevant to California). 

-Dose-based and dietary-based EECs are above levels where subacute effects to mammals were 

observed in chronic toxicity studies. 

-RQs representing exposures to terrestrial-phase frogs are above the LOC for all uses of oxamyl. 

-There is a >10% chance of effects to an individual terrestrial phase frog (serving as prey to CRLF) 

consuming small insects for all oxamyl uses, and a >10% chance of effects to an individual terrestrial 

phase frog consuming large insects for a majority of oxamyl uses (relevant to California). 

-Dose-based and dietary-based EECs are above the levels where subacute effects to birds were observed 

in chronic toxicity studies. 

Indirect 

- habitat 

Terrestrial Indirect effects to the CRLF through modification of its terrestrial habitat are not expected. 

-Estimated exposures of oxamyl to terrestrial vegetation are above the EC25 values for monocots and 

dicots. 

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Table 46. Summary of effects determination for CRLF critical habitat based on uses of oxamyl in 

California. 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase 

primary constituent 

elements 

Habitat 

Modification 

Modification of the aquatic-phase PCE is not expected. 

- RQs for acute and chronic exposures directly to the CRLF resulting from all 

uses of oxamyl are below the levels of concern. 

- Indirect effects to the CRLF through effects to its prey in the aquatic habitat are 

not expected (see Table 1). 

-RQs representing exposures to non-vascular and vascular aquatic plants are 

below LOCs. 

-Estimated exposures of oxamyl to riparian vegetation are above the EC25 values 

for monocots and dicots. 

Modification of 

terrestrial-phase 

primary constituent 

elements 

Oxamyl has the potential to modify the terrestrial-phase PCE. 

- Oxamyl has the potential to directly affect the terrestrial-phase CRLF (See 

Table 1). 

Oxamyl has the potential to indirectly affect the terrestrial-phase CRLF through 

effects to its prey (see Table 1). 

-Estimated exposures of oxamyl to terrestrial vegetation are below the EC25 

values for monocots and dicots. 

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 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 aquatic- and 

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 

109
 
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. 

7. References 

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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. 

Atkins. E.L., E.A. Greywood, and R.L. MacDonald. 1975. Toxicity of pesticides and 

other agricultural chemicals to honey bees. Laboratory studies. Univ. of Calif., Div. 

Agric. Sci. Leaflet 2287. 38 pp. (MRID# 000369-35). 

Burns, L.A. 2004. Exposure Analysis Modeling System (EXAMS): User Manual and 

System Documentation. Revision G. EPA/600/R-00/081. National Exposure Research 

Laboratory, Office of Research and Development, U.S. Environmental Protection 

Agency, Research Triangle Park, NC. May, 2004. 

Carousel, R.F., J.C. Imhoff, P.R. Hummel, J.M. Cheplick, A.S. Donigian, Jr., and L.A. 

Suarez. Undated. PRZM-3, A Model for Predicting Pesticide and Nitrogen Fate in the 

Crop Root and Unsaturated Soil Zones: Users Manual for Release 3.12.2. National 

Exposure Research Laboratory, Office of Research and Development, U.S. 

Environmental Protection Agency, Athens, GA; AQUA TERRA Consultants, Mountain 

View, CA; Waterborne Environmental, Inc., Leesburg, VA. 

Crawshaw, G.J. 2000. Diseases and Pathology of Amphibians and Reptiles in: 

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Bishop. SETAC Publication Series, Columbia, MO. 

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Fellers, G. M., et al. 2001. Overwintering tadpoles in the California red-legged frog 

(Rana aurora draytonii). Herpetological Review, 32(3): 156-157. 

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/ptreyes/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. 

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