Risks of Pendimethalin Use to Federally 

Threatened California Red-legged Frog 

(Rana aurora draytonii) 

CH3 

CH3 

H3C NH

H3C N 

O 

O 

N O 

O 

Pesticide Effects Determination 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

Washington, D.C. 20460 

June 16, 2009 

1
Primary Authors: 

Michael Davy, Agronomist 

William P. Eckel, Ph.D., Physical Scientist 

Secondary Review: 

Jean Holmes, DVM,MPH 

Risk Assessment Process Leader 

Final Review: 

Tom Bailey, Ph.D., Branch Chief 

Environmental Risk Assessment Branch 2 

2
Table of Contents 

1.0 Executive Summary.......................................................................................... 10 

2.0 Problem Formulation ........................................................................................ 15 

2.1 Purpose.............................................................................................................. 15 

2.2 Scope................................................................................................................. 17 

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

2.4 Stressor Source and Distribution ...................................................................... 19 

2.4.1 Environmental Fate Assessment ................................................................... 19 

2.4.2 Mechanism of Action.................................................................................... 21 

2.4.3 Use Characterization..................................................................................... 22 

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

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

2.5.2 Reproduction................................................................................................. 31 

2.5.3 Diet................................................................................................................ 31 

2.5.4 Habitat........................................................................................................... 32 

2.6 Designated Critical Habitat............................................................................... 33 

2.7 Action Area....................................................................................................... 35 

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

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

2.8.2 Assessment Endpoints for Designated Critical Habitat ................................ 41 

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

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

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

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

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

2.10.1.1 Measures of Exposure....................................................................... 46 

2.10.1.2 Measures of Effect ............................................................................ 48 

2.10.1.3 Integration of Exposure and Effects ................................................. 49 

3.0 Exposure Assessment ....................................................................................... 49 

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

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

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

3.2.2 Model Inputs ................................................................................................. 52 

3.2.3 Results........................................................................................................... 53 

3.2.4 Existing Monitoring Data ............................................................................. 54 

3.2.4.1 USGS NAWQA Surface Water Data ....................................................... 54 

3.2.4.2 USGS NAWQA Groundwater Data ......................................................... 54 

3.2.4.3 California Department of Pesticide Regulation (CPR) Data .................... 55 

3.2.4.4 Atmospheric Monitoring Data .................................................................. 56 

3.2.4.5 Spray Drift Buffer and Downstream Dilution Analysis for Action Area . 56 

3.3 Terrestrial Animal Exposure Assessment......................................................... 57 

3.4 Terrestrial Plant Exposure Assessment............................................................. 59 

4.0 Effects Assessment ........................................................................................... 60 

4.1 Evaluation of Aquatic Ecotoxicity Studies....................................................... 62 

4.1.1 Toxicity to Freshwater Fish .......................................................................... 63 

3
4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies............................. 63 

4.1.1.2 Freshwater Fish: Chronic Exposure (Early Life Stage and Reproduction) 

Studies....................................................................................................... 64 

4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open Literature 

Information ............................................................................................... 64 

4.1.1.4 Aquatic-phase Amphibian: Acute and Chronic Studies .......................... 64 

4.1.2 Toxicity to Freshwater Invertebrates ............................................................ 65 

4.1.2.1 Freshwater Invertebrates: Acute Exposure (Mortality) Studies................ 65 

4.1.2.2 Freshwater Invertebrates: Chronic Exposure (Reproduction) Studies...... 65 

4.1.2.3 Freshwater Invertebrates: Sublethal Effects and Open Literature Data.... 66 

4.1.3 Toxicity to Aquatic Plants ............................................................................ 66 

4.1.3.1 Aquatic Plants: Laboratory Data............................................................... 66 

4.1.3.2 Freshwater Field Studies........................................................................... 67 

4.2 Toxicity of Pendimethalin to Terrestrial Organisms ........................................ 67 

4.2.1 Toxicity to Birds ........................................................................................... 68 

4.2.1.1 Birds: Acute Exposure (Mortality) Studies............................................... 69 

4.2.1.2 Birds: Chronic Exposure (Growth, Reproduction) Studies ...................... 69 

4.2.1.3 Birds: Open Literature Studies.................................................................. 69 

4.2.2 Toxicity to Mammals.................................................................................... 69 

4.2.2.1 Mammals: Acute Exposure (Mortality) Studies ....................................... 70 

4.2.2.2 Mammals: Chronic Exposure (Growth, Reproduction) Studies............... 70 

4.2.2.3 Mammals: Open Literature Studies .......................................................... 70 

4.2.3 Toxicity to Terrestrial Invertebrates ............................................................. 70 

4.2.3.1 Terrestrial Invertebrates: Acute Exposure (Mortality) Studies................. 70 

4.2.3.2 Terrestrial Invertebrates: Open Literature Studies.................................... 71 

4.2.4 Toxicity to Terrestrial Plants ........................................................................ 71 

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

Endangered Species Levels of Concern............................................................ 72 

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

4.4.1 Terrestrial Incidents ...................................................................................... 73 

4.4.2 Plant Incidents............................................................................................... 74 

4.4.3 Aquatic Incidents .......................................................................................... 74 

5.0 Risk Characterization........................................................................................ 76 

5.1 Risk Estimation................................................................................................. 76 

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

5.1.1.1 Direct Effects to Aquatic-Phase CRLF..................................................... 76 

5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey (nonvascular aquatic plants, aquatic invertebrates, fish, and frogs)................. 78 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or Primary 

Productivity (Freshwater Aquatic Plants)................................................. 80 

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

5.1.2.1 Direct Effects to Terrestrial-phase CRLF................................................. 80 

5.1.2.2 Indirect Effects to Terrestrial-Phase CRLF via Reduction in Prey 

(terrestrial invertebrates, mammals, and frogs) ........................................ 83 

5.1.2.3 Indirect Effects to CRLF via Reduction in Terrestrial Plant Community 

(Riparian and Upland Habitat).................................................................. 86 

4
5.1.3 Primary Constituent Elements of Designated Critical Habitat ..................... 88 

5.1.3.1 Aquatic-Phase (Aquatic Breeding Habitat and Aquatic Non-Breeding 

Habitat) ..................................................................................................... 88 

5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat) ....................... 89 

5.2 Risk Description ............................................................................................... 90 

5.2.1 Direct Effects ................................................................................................ 93 

5.2.1.1 Aquatic-Phase CRLF ................................................................................ 93 

5.2.1.2 Terrestrial-Phase CRLF ............................................................................ 94 

5.2.2 Indirect Effects (via Reductions in Prey Base) ............................................. 98 

5.2.2.1 Algae (non-vascular plants) ...................................................................... 98 

5.2.2.2 Aquatic Invertebrates................................................................................ 99 

5.2.2.3 Fish and Aquatic-phase Frogs................................................................. 100 

5.2.2.4 Terrestrial Invertebrates.......................................................................... 100 

5.2.2.5 Mammals................................................................................................. 101 

5.2.2.6 Terrestrial-phase Amphibians................................................................. 102 

5.2.3 Indirect Effects (via Habitat Effects) .......................................................... 102 

5.2.3.1 Aquatic Plants (Vascular and Non-vascular).......................................... 102 

5.2.3.2 Terrestrial Plants ..................................................................................... 103 

5.2.4 Effects to Designated Critical Habitat ........................................................ 104 

5.2.4.1 Aquatic-Phase PCEs ............................................................................... 104 

5.2.4.2 Terrestrial-Phase PCEs ........................................................................... 105 

5.2.5 Spatial Extent of Potential Effects .............................................................. 106 

5.2.5.1 Spray Drift............................................................................................. 106 

5.2.5.2 Downstream Dilution Analysis.............................................................. 108 

5.2.5.3 Overlap between CRLF habitat and Spatial Extent of Potential Effects 108 

6.0 Uncertainties.................................................................................................. 111 

6.1 Exposure Assessment Uncertainties ............................................................... 111 

6.1.1 Maximum Use Scenario.............................................................................. 111 

6.1.2 Aquatic Exposure Modeling of Pendimethalin........................................... 111 

6.1.3 Usage Uncertainties .................................................................................... 113 

6.1.4 Terrestrial Exposure Modeling of Pendimethalin....................................... 113 

6.1.5 Spray Drift Modeling.................................................................................. 114 

6.1.6 Volatility..................................................................................................... 115 

6.1.7 Bioaccumulation........................................................................................ 115 

6.2 Effects Assessment Uncertainties................................................................... 116 

6.2.1 Age Class and Sensitivity of Effects Thresholds........................................ 116 

6.2.2 Use of Surrogate Species Effects Data ....................................................... 116 

6.2.3 Sublethal Effects ......................................................................................... 116 

6.2.4 Location of Wildlife Species....................................................................... 117 

7.0 Risk Conclusions ............................................................................................ 117 

8.0 References....................................................................................................... 121 

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List of Tables 

Table 1-1 Effects Determination Summary for Pendimethalin Use and the CRLF ......... 13 

Table 1-2 Effects Determination Summary for Pendimethalin Use and CRLF Critical 

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

Table 2-1 Summary of Pendimethalin Environmental Fate Properties ............................ 20 

Table 2-2 Pendimethalin Uses Assessed for the CRLF.................................................... 22 

Table 2-3 Summary of California Department of Pesticide Registration (CDPR) 

Pesticide Use Reporting (PUR) Data from 1994 to 2006 for Currently 

Registered Pendimethalin Uses: By County................................................... 26 

Table 2-5 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Table 2-4 Assessment Endpoints and Measures of Ecological Effects....................... 40 

Primary Constituent Elements of Designated Critical Habitata

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

Table 3-1 Pendimethalin Uses, Scenarios, and Application Information for the CRLF 

risk assessment1 .............................................................................................. 50 

Table 3-2 Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Pendimethalin Endangered Species Assessment for the 

CRLF 1............................................................................................................. 52 

Table 3-3 Aquatic EECs (μg/L) for Pendimethalin Uses in California ............................ 53 

Table 3-4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for 

Table 3-5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Table 3-6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to 

Table 3-7 TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and SemiPendimethalin with T-REX............................................................................. 57 

Exposures of the CRLF and its Prey to Pendimethalin................................... 58 

Terrestrial Invertebrate Prey Items ................................................................. 58 

aquatic Areas Exposed to Pendimethalin via Runoff and Drift...................... 59 

Table 4-1 Freshwater Aquatic Toxicity Profile for Pendimethalin .................................. 62 

Table 4-2 Categories of Acute Toxicity for Fish and Aquatic Invertebrates.................... 63 

Table 4-3. Terrestrial Toxicity Profile for Pendimethalin ............................................... 67 

Table 4-4 Categories of Acute Toxicity for Avian and Mammalian Studies ................... 68 

Table 4-5 Non-target Terrestrial Plant Seedling Emergence and Vegetative Vigor 

Toxicity (Tier II) Data .................................................................................... 71 

Table 5-1 Summary of Direct Effect RQsa

 for the Aquatic-phase CRLF ........................ 77 

6
Table 5-2 Summary of RQs Used to Estimate Indirect Effects to the CRLF via Effects to 

Non-Vascular Aquatic Plants a (diet of CRLF in tadpole life stage and habitat 

of aquatic-phase CRLF).................................................................................. 78 

Table 5-3 Summary of Acute and Chronic RQsa

 Used to Estimate Indirect Effects to 

the CRLF via Direct Effects on Aquatic Invertebrates as Dietary Food 

Items (prey of CRLF juveniles and adults in aquatic habitats)................ 79 

Table 5-4 Summary of RQs Used to Estimate Indirect Effects to the CRLF via Effects to 

Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a

............................ 80 

Table 5-5 Summary of Acute RQs* Used to Estimate Direct Effects to the Terrestrialphase CRLF .................................................................................................... 81 

Table 5-6 Summary of Chronic RQs* Used to Estimate Direct Effects to the Terrestrialphase CRLF .................................................................................................... 82 

Table 5-7 Summary of RQs Used to Estimate Indirect Effects to the Terrestrial-phase

CRLF via Direct Effects on Terrestrial Invertebrates as Dietary Food Items 83 

Table 5-8 Summary of Acute and Chronic RQs* Used to Estimate Indirect Effects to the 

Terrestrial-phase CRLF via Direct Effects on Small Mammals as Dietary 

Food Items ...................................................................................................... 85 

Table 5-9 RQs* for Monocots Inhabiting Dry and Semi-Aquatic Areas Exposed to 

Pendimethalin via Runoff and Drift................................................................ 86 

Table 5-10 RQs* for Dicots Inhabiting Dry and Semi-Aquatic Areas Exposed to 

Pendimethalin via Runoff and Drift................................................................ 87 

Table 5-11 Risk Estimation Summary for Pendimethalin - Direct and Indirect Effects to 

CRLF............................................................................................................... 90 

Table 5-12 Risk Estimation Summary for Pendimethalin – PCEs of Designated Critical 

Habitat for the CRLF ...................................................................................... 91 

Table 5-13 Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients (1 lbs a.i./acre, 1 application) ......................................................... 96 

Table 5-14 Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients (1.24 lbs a.i./acre, 1 application) .................................................... 96 

Table 5-15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients (1.425 lbs a.i./acre, 1 application) .................................................. 96 

Table 5-16. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk 

Quotients (6 lbs a.i./Acre, 1 application) ........................................................ 96 

Table 5-17 RQ Comparisons among Weight Class and Applications Rates for CRLF 

Consuming Small Herbivore Mammal That Recently Consumed 

Contaminated Short Grass .............................................................................. 97 

Table 5-18. Range of Risk Quotients for Non-Target Terrestrial Plants from 

Pendimethalin Usage .................................................................................... 103 

Table 5-19 Summary of AgDrift Predicted Terrestrial Spray Drift Distances ............... 107 

7
Table 5-20 Summary of AgDrift Predicted Aquatic Spray Drift Distances ................. 107 

Table 7-1 Effects Determination Summary for Pendimethalin Use and the CRLF ...... 118 

Table 7-2 Effects Determination Summary for Pendimethalin Use and CRLF Critical 

Habitat Impact Analysis................................................................................ 119 

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List of Figures 

Figure 2.1 Pendimethalin Use in Total Pounds per County.............................................. 24 

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

CRLF............................................................................................................... 30 

Figure 2.3 CRLF Reproductive Events by Month........................................................... 31 

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

Figure 2.5 Conceptual Model for Pesticide Effects on Terrestrial Phase of the CRLF... 44 

Figure 2.6 Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF....... 45 

Appendices 

Appendix A Ecological Effects Data 

Appendix B Multi-ai Product Analysis Pendimethalin 

Appendix C RQ Method and LOCs 

Appendix D CRLF Spatial Summary Pendimethalin 

Appendix E T-REX Output 

Appendix F TerrPlant Output 

Appendix G ECOTOX Bibliography 

Appendix H Accepted ECOTOX Data Table 

Appendix I HED Effects Pendimethalin 

Appendix J PRZM-EXAMS Output 

Appendix K KABAM Model Output 

Attachment I. Status and Life History of the California Red-legged Frog 

Attachment II. Baseline Status and Cumulative Effects for the California Red-legged 

Frog 

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1.0 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 pendimethalin on agricultural and non-agricultural 

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

result in effects to 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. 

The following uses are considered as part of the federal action evaluated in this 

assessment: alfalfa, lupine for seed, almond, beech nut, brazil nut, butternut, cashew, 

chestnut, chinquapin, filbert (hazelnut), hickory nut, macadamia nut (bushnut), mayhaw 

(hawthorn), pecan, pistachio, walnut (English/black), calamondin, citron (citrus), citrus, 

citrus hybrids other than tangelo, grapefruit, kumquat, lemon, lime, orange, pummelo 

(shaddock), tangelo, tangerines, Brassica (head and stem) vegetables, broccoli, broccoli 

(Chinese), brussels sprouts, cabbage, cabbage (Chinese), cauliflower, kohlrabi, mustard 

cabbage (gai choy - pak-choi), corn (field, pop, sweet), sunflower, cotton, forest trees (all 

species), Christmas tree plantations, apple, apricot, cherry, crabapple, fig, loquat, 

nectarine, peach, pear, pepino (melon pear), plum, plum (Japanese), pomegranate, prune, 

quince, small fruits, garlic, leek, grapes, eggplant, olive, onion (green, scallions, spring), 

shallot, potato (white/Irish), nonagricultural rights-of-way/fencerows/hedgerows, 

artichoke, asparagus, beans (dry, succulent/lima/snap), carrot (including tops), garbanzos 

(including chick peas), legume vegetables, lentils, peanuts, peas (dry, early dwarf, 

pigeon, southern/cowpea), pepper, strawberry, tomato, groundcherry (strawberry tomato, 

tomatillo), airports/landing fields, commercial/industrial lawns, golf course turf, 

industrial areas (outdoor), ornamental and/or shade trees, ornamental ground cover, 

ornamental herbaceous plants, ornamental lawns and turf, ornamental nonflowering 

plants, ornamental sod farm (turf), ornamental woody shrubs and vines, recreation area 

lawns, residential lawns, shelterbelt plantings, sorghum, wheat, and rice. 

Ground and aerial spray applications of pendimethalin (including some forms of 

chemigation) will potentially result in spray drift onto non-target plants, soil, and water 

adjacent to a treated field. Pendimethalin is expected to be persistent in the terrestrial 

environment and under the right conditions, may accumulate in the soil with repeated 

applications resulting in the potential for pendimethalin to reach the aquatic environment 

on suspended soil in runoff water. Laboratory dissipation studies in soil under aerobic 

10
conditions, demonstrate a half-life of approximately 72-172 days, and field dissipation 

studies in the Midwest similarly had half-lives that ranged from 84-147 days. However, 

field dissipation studies in Louisiana and Mississippi had half-lives of <20 days, 

demonstrating that pendimethalin can be relatively short-lived under some environmental 

conditions. Field half-lives less than 20 days may be due to volatilization, which has a 

measured half-life of 12.5 days in moist soil. In laboratory studies, pendimethalin can 

photodegrade in water with a half-life of approximately 42 days. However, in the 

environment, pendimethalin will not be available for photodegradation because the 

pendimethalin in water will tend to sorb to soil or sediment particles as indicated by Koc's 

which range from 13000-29400 ml/g. Thus, pendimethalin is considered hardly mobile 

according to FAO classification. 

Pendimethalin may accumulate in fish based on submitted data. Pendimethalin residues 

accumulated in bluegill sunfish exposed to 3 ppb of pendimethalin, with BCFs of 1400X 

for edible, 5800X for nonedible, and 5100X for whole fish. Depuration was rapid, with 

87- 91% of the 14C-residues eliminated from the fish tissues by 14 days of depuration. 

The major terminal degradates in terrestrial environments include several compounds 

formed at insignificant levels (<10 % of applied). Therefore, no degradates are included 

in the definition of the chemical stressor. 

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

prey to pendimethalin are assessed separately for the two habitats. Tier-II aquatic 

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

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

environmental concentrations resulting from different pendimethalin uses range from 

1.74 ppb (rights-of-way/impervious) to 16.6 μg/L (forestry) and 48 ppb (rice paddies). 

These estimates are supplemented with analysis of available California surface water 

monitoring data from U. S. Geological Survey’s National Water Quality Assessment 

(NAWQA) program and the California Department of Pesticide Regulation. The 

maximum concentration of pendimethalin reported by NAWQA for California surface 

waters with agricultural watersheds is 0.68 μg/L. This value is approximately 71 times 

less than the maximum model-estimated environmental concentration. The maximum 

concentration of pendimethalin reported by the California Department of Pesticide 

Regulation surface water database (3.5 μg/L) is roughly 14 times lower than the highest 

peak model-estimated environmental concentration. 

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

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

foliar uses. AgDRIFT and AGDISP models are also used to estimate deposition of 

pendimethalin on terrestrial and aquatic habitats from aerial and ground spray drift. The 

TerrPlant model is used to estimate pendimethalin exposures to terrestrial-phase CRLF 

habitat, including plants inhabiting semi-aquatic and dry areas, resulting from uses 

involving foliar pendimethalin applications. The T-HERPS model is used to allow for 

further characterization of dietary exposures of terrestrial-phase CRLFs relative to birds. 

11
The effects determination assessment endpoints for the CRLF include direct toxic effects 

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

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

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

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

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

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

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

freshwater aquatic invertebrates and aquatic plants, toxicity information for these 

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

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

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

characterized by available data for terrestrial monocots and dicots. 

Pendimethalin is stable to most of the environmental degradation processes measured, 

meaning there is little production of degradates. Thus, no degradates were considered in 

the assessment, which was based on parent pendimethalin alone. There was monitoring 

data for one possible degradate, 4-hydroxypendamethalin, in the NAWQA data. It was 

not detected (less than 0.143 ppb) in 61 ground water samples, and 8 surface water 

samples, all taken from the San Joaquin-Tulare Basin. 

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 pendimethalin use within the action area has the potential to 

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

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

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

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

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

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

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

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

Adversely Affect determination for the CRLF from the use of pendimethalin. 

Additionally, the Agency has determined that there is the potential for effects to CRLF 

designated critical habitat from the use of the chemical. There is potential for direct and 

indirect effects to the aquatic-phase and terrestrial-phase CRLF from the use of 

pendimethalin. Potential effects to the habitat of the aquatic and terrestrial phase CRLF 

has been determined due to effects to terrestrial and aquatic non-target plants. A summary 

of the risk conclusions and effects determinations for the CRLF and its critical habitat is 

presented in Table 1-1 and Table 1-2. Further information on the results of the effects 

12
determination is included as part of the Risk Description in Section 5.2. Given the LAA 

determination for the CRLF and potential effects to designated critical habitat, a 

description of the baseline status and cumulative effects for the CRLF is provided in 

Attachment 2. 

Table 1-1 Effects Determination Summary for Pendimethalin Use and the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1 

Basis for Determination 

Survival, growth, Potential for Direct Effects 

and/or reproduction 

of CRLF 

individuals 

LAA1 Aquatic-phase (Eggs, Larvae, and Adults): 

Acute LOCs were exceeded for fish or aquatic-phase amphibians 

Terrestrial-phase (Juveniles and Adults): 

Acute and chronic LOCs were exceeded for birds. The available toxicity 

data suggest that amphibians are less sensitive than birds to pendimethalin, 

and considering factors such as lower food intake of terrestrial phase 

amphibians relative to birds reduces EECs and RQs, but does not reduce 

RQs to levels that are below LOCs. Likely to adversely affect CRLF. 

Potential for Indirect Effects 

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

LOC is exceeded only for non-vascular aquatic plants. RQs are below LOC 

for freshwater invertebrates (chronic and acute effects) and fish or frogs 

(chronic and acute effects). Pendimethalin could potentially impact 

terrestrial and aquatic plants to an extent that could result in indirect effects 

to the CRLF or effects to critical habitat. 

Terrestrial prey items, riparian habitat: 

Chronic LOC is exceeded for mammal and birds (surrogate for frog). 

Pendimethalin is practically non-toxic to honeybees with no mortalities observed 

in testing. LOC is exceeded for terrestrial invertebrates. Uncertainty in toxicity 

to insects from EEC that is above highest bee concentration tested does not 

preclude potential risk. Pendimethalin may adversely affect insects. Acute 

LOCs were not exceeded for insectivorous amphibians using the T-HERPS 

model at the highest rate of application. 

LOC is exceeded for non-target terrestrial plants and thereby critical habitat 

could be affected as a result of these potential impacts. 1

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

affect (LAA) 

Table 1-2 Effects Determination Summary for Pendimethalin Use and CRLF 

Critical Habitat Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase PCE 

Habitat effects 

Effects to riparian vegetation (terrestrial plants) and aquatic non-vascular and 

vascular plants result in LOC exceedances. These effects may indirectly affect 

the CRLF via reduction in food supply, changes in available cover, physical 

parameters of the waterbody (e.g. increase temperature or turbidity) 

LOC is exceeded for effects to non-vascular aquatic plants, freshwater 

invertebrates (chronic and acute effects) and fish or frogs (chronic and acute 

effects). 

13
Modification of 

terrestrial-phase 

PCE 

Effects to riparian vegetation (terrestrial plants) result in LOC exceedances. 

Effects may result in changes in community composition or relative abundance 

of riparian plant species, possibly altering terrestrial – phase CRLF habitat. 

Chronic LOC is exceeded for mammal. Although there are NLAA 

determinations for other prey items, small mammals constitute up to half of the 

food intake for CRLF and reduction in small mammalian populations may be 

significant. 

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 effects determinations, it is important to note that pesticide exposures and 

predicted risks to the species and its resources (i.e., food and habitat) are not expected to 

be uniform across the action area. In fact, given the assumptions of drift and downstream 

transport (i.e., attenuation with distance), pesticide exposure and associated risks to the 

species and its resources are expected to decrease with increasing distance away from the 

treated field or site of application. Evaluation of the implication of this non-uniform 

distribution of risk to the species would require information and assessment techniques 

that are not currently available. Examples of such information and methodology required 

for this type of analysis would include the following: 

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

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

action area. This information would allow for quantitative extrapolation 

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

proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

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

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

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

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

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

repeated exposure events and the inherent demographic characteristics of 

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

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

responses to pesticide exposure would allow for a more refined 

14
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. 

2.0 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 

pendimethalin on alfalfa, lupine for seed, almond, beech nut, brazil nut, butternut, 

cashew, chestnut, chinquapin, filbert (hazelnut), hickory nut, macadamia nut (bushnut), 

mayhaw (hawthorn), pecan, pistachio, walnut (English/black), calamondin, citron 

(citrus), citrus, citrus hybrids other than tangelo, grapefruit, kumquat, lemon, lime, 

orange, pummelo (shaddock), tangelo, tangerines, Brassica (head and stem) vegetables, 

broccoli, broccoli (Chinese), brussels sprouts, cabbage, cabbage (Chinese), cauliflower, 

kohlrabi, mustard cabbage (gai choy - pak-choi), corn (field, pop, sweet), sunflower, 

cotton, forest trees (all species), Christmas tree plantations, apple, apricot, cherry, 

crabapple, fig, loquat, nectarine, peach, pear, pepino (melon pear), plum, plum 

(Japanese), pomegranate, prune, quince, small fruits, garlic, leek, grapes, eggplant, olive, 

onion (green, scallions, spring), shallot, potato (white/Irish), nonagricultural rights-ofway/fencerows/hedgerows, artichoke, asparagus, beans (dry, succulent/lima/snap), carrot 

(including tops), garbanzos (including chick peas), legume vegetables, lentils, peanuts, 

peas (dry, early dwarf, pigeon, southern/cowpea), pepper, strawberry, tomato, 

groundcherry (strawberry tomato, tomatillo), airports/landing fields, 

commercial/industrial lawns, golf course turf, industrial areas (outdoor), ornamental 

and/or shade trees, ornamental ground cover, ornamental herbaceous plants, ornamental 

lawns and turf, ornamental nonflowering plants, ornamental sod farm (turf), ornamental 

woody shrubs and vines, recreation area lawns, residential lawns, shelterbelt plantings, 

sorghum, wheat, and rice. In addition, this assessment evaluates whether use on these 

sites is expected to result in effects to the species’ designated critical habitat. This 

ecological risk assessment has been prepared consistent with a settlement agreement in 

the case Center for Biological Diversity (CBD) vs. EPA et al. (Case No. 02-1580­

JSW(JL)) entered in Federal District Court for the Northern District of California on 

October 20, 2006. 

15
In this assessment, direct and indirect effects to the CRLF and potential effects 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, AgDRIFT, and AGDISP, 

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

include an analysis of the usage data, consideration of available monitoring data, a spatial 

analysis, and use of the T-HERPS model. 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 pendimethalin is based on an action area. The action area is the area 

directly or indirectly affected by the federal action. It is acknowledged that the action 

area for a national-level FIFRA regulatory decision associated with a use of 

pendimethalin 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 pendimethalin 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 CRLF 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 CRLF or upon the PCEs of the species’ 

designated critical habitat, a “no effect” determination is made for use of pendimethalin 

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

or indirect effects to individual CRLF 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 pendimethalin. 

If a determination is made that use of pendimethalin 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 

16
and other taxonomic groups upon which these species depend (e.g., aquatic and terrestrial 

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

information, including spatial analysis (to determine the geographical proximity of CRLF 

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

pendimethalin on the PCEs is also used to determine whether effects to designated 

critical habitat may occur. Based on the refined information, the Agency uses the best 

available information to distinguish those actions that “may affect, but are not likely to 

adversely affect” from those actions that “may affect and are likely to adversely affect” 

the CRLF or affect 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 pendimethalin is expected to directly impact living organisms within the action 

area (defined in Section 2.7), critical habitat analysis for pendimethalin 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 

pendimethalin 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 

As described above, the herbicide pendimethalin has a wide variety of outdoor uses, 

including both in agricultural and non-agricultural settings. 

The end result of the EPA pesticide registration process (i.e., the FIFRA regulatory 

action) is an approved product label. The label is a legal document that stipulates how 

and where a given pesticide may be used. Product labels (also known as end-use labels) 

describe the formulation type (e.g., liquid), acceptable methods of application, approved 

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

pendimethalin in accordance with the approved product labels for California is “the 

action” relevant to this ecological risk assessment. 

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

risk assessment and effects determination addresses currently registered uses of 

pendimethalin 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. 

Degradates were not considered, as pendimethalin is relatively stable to all degradation 

processes measured. The half-lives used as model inputs reflect this stability and lack of 

17
degradate production. Sixty-nine measurements of one potential degradate (4­

hydroxypendamthalin) in California waters produced no detections. 

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). 

Pendimethalin has registered products that contain multiple active ingredients. Analysis 

of the available open literature and acute oral mammalian LD50 data for multiple active 

ingredient products relative to the single active ingredient is provided in Appendix B. 

The results of this analysis show that an assessment based on the toxicity of the single 

active ingredient of pendimethalin is appropriate. 

2.3 Previous Assessments 

A Reregistration Eligibility Decision (RED) ecological risk chapter was completed in 

1997. The RED concluded that pendimethalin would not represent a high acute risk to 

birds or a high acute or chronic risk to mammals. The chronic risk to birds could not be 

determined because avian reproduction studies had not been submitted. Chronic risk 

Levels of Concern (LOCs) for fish were exceeded by a small margin. But it was 

presumed that overall, pendimethalin did not represent a high risk to aquatic animals and 

plants, including estuarine organisms. The use of pendimethalin may adversely affect 

endangered species of terrestrial and semi-aquatic plants, aquatic plants and invertebrates 

including mollusks, fish, and birds (specifically grazers). The risk to nontarget terrestrial 

and semi-aquatic plants was expected to be moderate. 

On December 1, 2004, EPA initiated consultation with the National Marine Fisheries 

Service (NMFS) requesting concurrence in a determination relative to potential effects 

from pendimethalin uses to Pacific salmon and steelhead. In the assessment supporting 

that consultation, EPA concluded that pendimethalin has moderate to high laboratory 

toxicity to fish and aquatic invertebrates and somewhat less toxicity on aquatic plants. 

OPP determined that any effects of concern would be directly on listed salmon and 

steelhead and would vary for different uses and species. It was further determined that 

pendimethalin would not affect salmon and steelhead food or cover, nor adversely 

modify their Critical Habitat from any of the registered uses except a single aerial use - 

aerial application to corn. OPP's conclusions relative to 26 listed Pacific salmon and 

steelhead was that uses of pendimethalin were not likely to adversely affect four species 

and would have no effect on twenty-two species reviewed. A response to this request is 

anticipated from NMFS in 2012. 

18
2.4 Stressor Source and Distribution 

2.4.1 Environmental Fate Assessment 

The environmental fate properties of pendimethalin indicate that it is persistent, with 

half-lives in relevant compartments of 172 days (soil) and 208 to 330 days (anaerobic and 

aerobic aquatic systems). Only the aqueous photolysis half-life is relatively short (42 

days) but photolysis may not occur because of the strong tendency of pendimethalin to 

sorb to sediment (log Kow = 5.18, average Koc = 17,040 mL/g O.C.). Pendimethalin 

may be bioaccumulative in aquatic organisms. A whole-body bioconcentration factor 

(BCF) of 5100 from a 35-day exposure was measured in bluegill sunfish. BCFs were 

lower in catfish (about 1600) and negligible in guppy. The most rapid dissipation process 

for pendimethalin appears to be volatilization from moist soil (half-life about 12.5 days). 

Thus, pendimethalin may be transported away from the site of application before it 

degrades, making atmospheric transport a potentially important exposure mechanism. 

However, EPISuite v4.0 (http://www.epa.gov/oppt/exposure/pubs/episuite.htm), an 

EPA-sponsored environmental fate estimation program, estimates of atmospheric halflife (4.2 hours due to reaction with hydroxyl radical, AOPWin program) and sorption to 

airborne particles (1 to 7%, AEROWin program) indicate that long-range transport may 

not be a concern. The tendency of pendimethalin to partition to sediment, and its long 

half-life there, make transport by movement of sediment in flowing water systems a 

concern. 

Table 2.1 lists the environmental fate properties of pendimethalin, along with the major 

and minor degradates detected in the submitted environmental fate and transport studies. 

19
Table 2-1 Summary of Pendimethalin Environmental Fate Properties 

Study Value (units) Major Degradates 

Minor Degradates 

MRID # Study Status 

Hydrolysis Stable None 00106777 acceptable 

Direct Aqueous 

Photolysis 

Dark corrected, continuous irradiation 

half-life 21 days; 12-hour light/dark 

half-life 42 days. 

16.5-day half-life 

37 minor, unidentified 

degradates 

2,6-dinitro-3,4-

dimethylaniline (9.3% of 

applied) 

00153763 

43808201 

acceptable 

Soil Photolysis Stable None 00153764 acceptable 

Aerobic Soil 

Metabolism 

172 day (Deviation from IPG, as 

explained in 1997 RED) 

Range 42-1322 days 

2,6-dinitro3,4-xylidine 

4-[(1-ethylpropyl)amino]-2-

methyl-3,5-dinitrobenzyl 

alcohol 

4-[(1-ethylpropyl)amino]-2-

methyl3,5-dinitro-o-toluic 

acid 

40185104 acceptable 

Anaerobic Soil 

Metabolism 

Stable 

98% parent at 60 days 

2,6-dinitro3,4-xylidine 

4-[(1-ethylpropyl)amino]-2-

methyl-3,5-dinitrobenzyl 

alcohol 

4-[(1-ethylpropyl)amino]-2-

methyl3,5-dinitro-o-toluic 

acid 

40185105 acceptable 

Anaerobic 

Aquatic 

Metabolism 

208-day half-life 

(upper 90th %ile confidence bound on 

mean) 

None 40813501 

43154702 

acceptable 

Aerobic Aquatic 

Metabolism 

330 days (single value) None 47385201 acceptable 

Kd-ads / Kd-des 

(mL/g) 

Koc- ads / Koc-des 

(mL/g) 

30 – 854 (U.S. soils) 

61 – 285 (Japanese soils) 

17,040 mL/g O.C. (U.S. soils) 

(avg of 5 values, 13000 – 29400) 

7011- 43863 mL/g o.c. (Japanese 

soils) 

n.a. 00153765 

43041901 

acceptable 

Laboratory 

Volatility 

Volatilization half-life 12.5 days from 

moist loam soil 

None measured 00153766 

Fish 

Bioaccumulation 

Bioconcentration factor BCF = 5100, 

35-day exposure 

4-[(1-ethylpropyl)amino]-2-

methyl-3,5-dinitrobenzyl 

alcohol (3.1%) 

00156726 

00158235 

acceptable 

20
Study Value (units) Major Degradates 

Minor Degradates 

MRID # Study Status 

Terrestrial Field 

Dissipation 

34-day half-life in California almond 

orchard, sandy loam soil. No leaching 

below 6 inches. 

41722504 acceptable 

Aquatic Field 

Dissipation 

No data 

2.4.2 Environmental Transport Assessment 

Potential transport mechanisms include pesticide surface water runoff, spray drift, and 

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

more distant ecosystems. Surface water runoff and spray drift are expected to be the 

major routes of exposure for pendimethalin. 

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

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

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

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

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

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

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

via secondary drift depends on the pendimethalin’s ability to be mobilized into air and its 

eventual removal through wet and dry deposition of gases/particles and photochemical 

reactions in the atmosphere. Therefore, physicochemical properties of pendimethalin that 

describe its potential to enter the air from water or soil (e.g., Henry’s Law constant and 

vapor pressure), pesticide use data, modeled estimated concentrations in water and air, 

and available air monitoring data from the Central Valley and the Sierra Nevadas are 

considered in evaluating the potential for atmospheric transport of pendimethalin to 

locations where it could impact the CRLF. 

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

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

are used to determine potential exposures to aquatic and terrestrial organisms via spray 

drift. 

2.4.2 Mechanism of Action 

Pendimethalin is a selective herbicide registered for control of broadleaf weeds and 

grassy weed species on a variety of agricultural crops, turf, and ornamentals. 

Pendimethalin disrupts the process of mitosis in the growth of shoots and roots. It acts as 

a microtubule disruptor by inhibiting cell division and cell elongation in plants, and is 

generally applied early in the growing season. Absorption of the herbicide takes place at 

the roots and the shoots. Very little translocation occurs from the site of intake. 

21
2.4.3 Use Characterization 

Pendimethalin is applied as a liquid spray formulation. Pendimethalin can be applied 

either by aerial equipment or using ground equipment to a variety of row crops, orchard 

crops, vineyards, sod, seed, and to rice. It can be broadcasted by air or ground, and be 

banded as a directed spray or applied by irrigation equipment in various crops. It can 

also be left on the soil surface or incorporated. 

Approximately 27 million pounds of pendimethalin were applied in 2002 according to the 

National Center for Food and Agriculture, and the majority was applied to soybeans 

(39.6%), cotton (20.22 %), and corn (19.4 %) (Fig. 2.1). Together, soybeans, cotton, and 

corn account for 79 % of the applied pendimethalin nationwide. This existing use 

information is provided as a basis for considering background exposures from existing 

use. 

Table 2.2 presents the uses and corresponding application rates and methods of 

application considered in this assessment. 

Table 2-2 Pendimethalin Uses Assessed for the CRLF 

Use 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. 

Number of 

Application 

per Year 

Application 

Method 

walnut (english/black), tangerines, tangelo, small fruits, pummelo 

(shaddock), pomegranate, pistachio, pecan, orange, macadamia 

nut (bushnut), lime, lemon, kumquat, hickory nut, grapefruit, 

filbert (hazelnut), citrus hybrids other than tangelo, citron 

(citrus), chinquapin, chestnut, cashew, calamondin, butternut, 

brazil nut, beech nut, almond, grapes. 

6.0 1 ground 

Prune, plum, pear, peach, olive, nectarine, fig, cherry, apricot, 

apple, ornamental and/or shade trees, nonagricultural rights-ofway, fencerows, hedgerows, mulch, industrial areas (outdoor), 

forest trees (all or unspecified), Christmas tree plantations, 

recreation area lawns, loquat, shelterbelt plantings, quince, 

mayhaw, crabapple, alfalfa 

4.0 1 ground, aerial 

Asparagus, artichoke 3.9 1 ground, aerial 

golf course turf, airports/landing fields 3.5 1 ground, aerial 

residential lawns 3.0 1 ground, aerial 

Shallot, onion, corn (sweet, pop, field) 2.0 1 ground, aerial 

Carrot 1.9 1 ground, aerial 

Sunflower 1.73 1 ground, aerial 

Tomato, tomatillo, tobacco, sorghum, potato (white/Irish), lupine 

(grain), garlic, garbanzos (including chick peas) beans, succulent 

(snap or lima) beans, dried-type beans, cotton, cowpea (southern 

pea, black-eyed bean), 

1.485 1 ground, aerial 

22
Use 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. 

Number of 

Application 

per Year 

Application 

Method 

Wheat, pepper, pepino (melon pear), groundcherry, eggplant 1.425 1 ground, aerial 

Strawberry 1.485 2 ground, aerial 

Legume vegetables 1.24 1 ground, aerial 

Rice, mustard cabbage (gai choy, pak-choi), kohlrabi, 

cauliflower, cabbage (Chinese), cabbage, brussels sprouts, 

broccoli (Chinese), broccoli, Brassica (head and stem) vegetables 

1.0 1 ground, aerial 

23
The figure below summarizes pendimethalin use nationwide in 2002. Pendimethalin is 

used primarily on soybeans, cotton, corn, sugarcane, peanuts, rice, sunflower, potatoes, 

tobacco, and onions. Most use is apparently outside of California. Non-agricultural 

uses, which predominate in California, are not represented on this map. The map was 

downloaded from a U.S. Geological Survey (USGS), National Water Quality Assessment 

Program (NAWQA) website. 

http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m1629 

Figure 2.1 Pendimethalin Use in Total Pounds per County 

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

action. The current label for pendimethalin represents the FIFRA regulatory action; 

therefore, labeled use and application rates specified on the label form the basis of this 

assessment. The assessment of use information is critical to the development of the action 

area and selection of appropriate modeling scenarios and inputs. 

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

of both national- and county-level usage information (Kaul and Jones, 2006) using state24
level usage data obtained from USDA-NASS1

, Doane (www.doane.com; the full dataset 

is not provided due to its proprietary nature) and the California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database2

 . CDPR PUR is considered 

a more comprehensive source of usage data than USDA-NASS or EPA proprietary 

databases, and thus the usage data reported for pendimethalin by county in this 

California-specific assessment were generated using CDPR PUR data. Eight years 

(1999-2006) of usage data were included in this analysis. Data from CDPR PUR were 

obtained for every pesticide application made on every use site at the section level 

(approximately one square mile) of the public land survey system. BEAD summarized 

these data to the county level by site, pesticide, and unit treated. Calculating county-level 

usage involved summarizing across all applications made within a section and then across 

all sections within a county for each use site and for each pesticide. The county level 

usage data that were calculated include: average annual pounds applied, average annual 

area treated, and average and maximum application rate across all five years. The units 

of area treated are also provided where available. 

Some uses reported in the CDPR PUR database may be different than those considered in 

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

represent either historic uses that have been canceled, mis-reported uses, or mis-use. 

Historical uses, mis-reported uses, and misuse are not considered part of the federal 

action and, therefore, are not considered in this assessment. 

Table 2.3 below gives the total number of pounds of pendimethalin used in California 

counties from 1994 to 1998 and 1999 to 2006. The figures represent the sum of the 

average number of pounds used in each year of the two periods, as reported in the 

Pesticide use Reporting database (PUR). Summaries of the data were provided by OPP’s 

Biological and Economic Analysis Division (BEAD). The data indicate that the largest 

amounts were used in Kings, Fresno, and Kern counties, all of which exceeded 150,000 

pounds. Thirteen counties (Santa Barbara through Glenn) showed use of between 10,000 

and 100,000 pounds during the period. Twenty-two counties (Contra Costa through El 

Dorado) had usage of between 1,000 and 10,000 pounds. Overall, 54 of 58 California 

counties had reported usage of pendimethalin, indicating that use of the compound is 

geographically widespread. This is a strong line of evidence that the CRLF is potentially 

exposed to pendimethalin. 

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

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

and state. See http://www.usda.gov/nass/pubs/estindx1.htm#agchem. 2

 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. 

25
Table 2-3 Summary of California Department of Pesticide Registration (CDPR) 

Pesticide Use Reporting (PUR) Data from 1994 to 2006 for Currently Registered 

Pendimethalin Uses: By County 

County 

1994 – 

1998 

1999- 

2006 Total 

KINGS 99690 95429 195119 

FRESNO 74212 77816 152028 

KERN 90122 59505 149627 

SANTA BARBARA 820 83486 84306 

TULARE 16738 33549 50287 

LOS ANGELES 4671 41639 46310 

IMPERIAL 15584 24149 39733 

STANISLAUS 9128 24623 33751 

RIVERSIDE 15279 14997 30276 

MERCED 10480 9747 20227 

SAN JOAQUIN 6801 11203 18004 

MADERA 7660 10086 17746 

BUTTE 5758 5974 11732 

SANTA CLARA 5717 5838 11555 

GLENN 4865 6202 11067 

CONTRA COSTA 3222 6083 9305 

ORANGE 5272 3634 8906 

SACRAMENTO 5571 3030 8601 

SAN DIEGO 6588 1854 8442 

SOLANO 2211 4261 6472 

SONOMA 2897 3546 6443 

ALAMEDA 1810 3288 5098 

YOLO 1322 3640 4962 

PLACER 1309 3167 4476 

SAN BERNARDINO 2339 2073 4412 

MONTEREY 2328 2052 4380 

COLUSA 1149 2985 4134 

SAN LUIS OBISPO 1761 1610 3371 

SUTTER 1109 1857 2966 

TEHAMA 1148 1212 2360 

YUBA 371 1368 1739 

SAN MATEO 798 854 1652 

VENTURA 824 809 1633 

SAN BENITO 809 753 1562 

CALAVERAS 609 665 1274 

SHASTA 593 642 1235 

EL DORADO 466 611 1077 

NAPA 418 411 829 

LAKE 367 261 628 

MARIN 563 46 609 

TUOLUMNE 239 361 600 

LASSEN 169 309 478 

MODOC 84 389 473 

26
MENDOCINO 158 293 451 

AMADOR 272 145 417 

SISKIYOU 138 250 388 

NEVADA 307 74 381 

SANTA CRUZ 148 131 279 

SAN FRANCISCO 85 116 201 

MONO 149 39 188 

HUMBOLDT 53 19 72 

INYO 49 11 60 

MARIPOSA 24 4 28 

TRINITY 0 1 1 

Table 2.3b below shows the number of pounds of pendimethalin applied by crop and use 

site, during the periods 1994 to 1998, and 1999 to 2006. The figures are aggregated 

across counties, and are ranked by total numbers of pounds applied in the entire period. 

Only crops or use sites with at least 1,000 pounds usage are shown in the table. 

Cotton is the major use, with nearly two million pounds applied from 1994 to 2006. Six 

use sites (landscape, almond, rights-of-way, garlic, onions, pistachio, and alfalfa) each 

totaled over 100,000 pounds applied. A further 18 uses (nursery through nectarine) each 

had from 10,000 to 100,000 pounds of usage. 

It is notable that non-agricultural uses (landscape and rights-of-way) are among the 

highest uses of pendimethalin. Based on the county rankings, it is concluded that these 

uses represent the greatest part of the usage in urban counties. 

Table 2.3b. Summary of California Department of Pesticide Registration (CDPR) 

Pesticide Use Reporting (PUR) Data from 1994 to 2006 for Currently Registered 

Pendimethalin Uses: By Crop/Use Site 

CROP/USE SITE 1994-1998 lbs 1999-2006 lbs TOTAL lbs 

COTTON 1198354 787909 1986263 

LANDSCAPE MAINTENANCE 213421 224903 438324 

ALMOND 124150 88858 213008 

RIGHTS OF WAY 40510 146505 187015 

GARLIC 100657 70532 171189 

ONION, DRY & GREEN 49023 93909 142932 

PISTACHIO 28480 93363 121843 

ALFALFA 3327 99865 103192 

NURSERY - ALL 43897 44167 88064 

GRAPE, WINE 39711 45880 85591 

POTATO 41086 35707 76793 

GRAPE 34005 25160 59165 

CORN, HUMAN CONSUMPTION 29757 28782 58539 

BEAN, DRIED 14529 38200 52729 

UNCULTIVATED AG / nonag 8370 42817 51187 

WALNUT 17340 29352 46692 

27
CORN (FORAGE - FODDER) 13629 22894 36523 

SUNFLOWER 8257 12354 20611 

ORANGE 2317 17988 20305 

PEACH 3572 13534 17106 

RICE 1237 15149 16386 

BEAN, UNSPECIFIED 11229 4922 16151 

SOIL FUMIGATION/PREPLANT 6852 8718 15570 

PRUNE 12351 3213 15564 

BEAN, SUCCULENT 2817 7853 10670 

NECTARINE 2529 8053 10582 

WHEAT 7703 1143 8846 

PLUM 594 6954 7548 

CHERRY 0 6913 6913 

SAFFLOWER 180 3853 4033 

STRUCTURAL PEST CONTROL 2204 1685 3889 

TOMATO 796 2863 3659 

CELERY 3354 15 3369 

LEMON 817 2288 3105 

APPLE 2369 594 2963 

TURF/SOD 1624 1013 2637 

WATER(INDUSTRIAL) 0 1429 1429 

SUGARCANE 72 1307 1379 

TANGERINE 174 1178 1352 

SORGHUM/MILO 892 281 1173 

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). 

28
Populations currently exist along the northern California coast, northern Transverse 

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

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

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

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

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

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

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

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

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

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

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

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

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

Figure 2.2). Recovery units, core areas, and other known occurrences of the CRLF from 

the CNDDB are described in further detail in 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. 

29
 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

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

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

4. Cosumnes River 4. Cosumnes River

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

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek* 9. 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

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

16. East San Francisco Bay 16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay 18. 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 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence Designations for 

CRLF 

30 
2.5.2 Reproduction 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

live 8 to 10 years (USFWS 2002). Figure 2.3 depicts CRLF annual reproductive timing. 

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 2.3 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 

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

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

31
(Sialis cf. californica), pillbugs (Armadilliadrium vulgare), and water striders (Gerris sp). 

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

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

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

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

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

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

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

2.5.4 Habitat 

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

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

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

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

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

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

1997). Dense vegetation close to water, shading, and water of moderate depth are habitat 

features that appear especially important for CRLF (Hayes and Jennings 1988). 

Breeding sites include streams, deep pools, backwaters within streams and creeks, ponds, 

marshes, sag ponds (land depressions between fault zones that have filled with water), 

dune ponds, and lagoons. Breeding adults have been found near deep (0.7 m) still or slow 

moving water surrounded by dense vegetation (USFWS 2002); however, the largest 

number of tadpoles have been found in shallower pools (0.26 – 0.5 m) (Reis, 1999). Data 

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

generally are not suitable (Hayes and Jennings 1988). 

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

additional research is needed to identify habitat requirements within artificial ponds 

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

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

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

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

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

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

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

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

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

to 77 days (USFWS 2002). 

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

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

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

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

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

32
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 adverse modification with regard to actions carried out, funded, or authorized by a 

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

in the destruction or adverse modification of critical habitat. 

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

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

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

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

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

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

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

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

protected from disturbance or are representative of the historic geographical and 

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

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

• Breeding aquatic habitat; 

• Non-breeding aquatic habitat; 

• Upland habitat; and 

• 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 

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 

33
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 adverse modification standards for designated critical habitat 

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

that alter the PCEs and jeopardize the continued existence of the species. Evaluation of 

actions related to use of pendimethalin 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 pendimethalin is expected to directly impact 

living organisms within the action area, critical habitat analysis for pendimethalin 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. 

34
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 pendimethalin is likely to encompass considerable portions of 

the United States based on the large array of agricultural and its non-agricultural uses. 

However, the scope of this assessment limits consideration of the overall action area to 

those portions that may be applicable to the protection of the CRLF and its designated 

critical habitat within the state of California. The Agency’s approach to defining the 

action area under the provisions of the Overview Document (USEPA 2004) considers the 

results of the risk assessment process to establish boundaries for that action area with the 

understanding that exposures below the Agency’s defined Levels of Concern (LOCs) 

constitute a no-effect threshold. For the purposes of this assessment, attention will be 

focused on the footprint of the action (i.e., the area where pesticide application occurs), 

plus all areas where offsite transport (i.e., spray drift, downstream dilution, etc.) may 

result in potential exposure within the state of California that exceeds the Agency’s 

LOCs. 

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

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

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

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

transport within the state of California. Specific measures of ecological effect that define 

the action area include any direct and indirect toxic effect and any potential modification 

of its critical habitat, including reduction in survival, growth, and fecundity as well as the 

full suite of sublethal effects available in the effects literature. Therefore, the action area 

extends to a point where environmental exposures are below any measured lethal or 

sublethal effect threshold for any biological entity at the whole organism, organ, tissue, 

and cellular level of organization. In situations where it is not possible to determine the 

threshold for an observed effect, the action area is not spatially limited and is assumed to 

be the entire state of California. 

The definition of action area requires a stepwise approach that begins with an 

understanding of the federal action. The federal action is defined by the currently labeled 

uses for pendimethalin. An analysis of labeled uses and review of available product 

labels was completed. Several of the current labels are special local needs (SLN) labels 

for other states and some uses on other labels are restricted to specific states other than 

California and are, therefore, excluded from this assessment. The agricultural and 

nonagricultural uses that are considered as part of the federal action evaluated in this 

assessment can be found in Table 2.2. 

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

of pendimethalin 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 

35
represent the labeled uses described above. A map representing all the land cover types 

that make up the initial area of concern for pendimethalin is presented in 

Figure 2.4. 

More information regarding which specific uses are represented for each land cover types 

can be found in Appendix D. 

36
Figure 2.4. Initial area of concern, or “footprint” of potential use, for 

Pendimethalin 

37 
Once the initial area of concern is defined, the next step is to define the potential 

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

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

listed species LOCs. 

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

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

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

The action area is determined by the footprint of the action plus all offsite areas where 

exposure of one or more taxonomic groups to pendimethalin exceeds the Agency’s 

LOCs. The spatial extent at which the Agency’s LOCs are not exceeded is based on the 

potential exposure level and the most sensitive effects endpoints The most sensitive for 

terrestrial environments was the plant EC25 of 0.01 lb/acre for ryegrass seedling 

emergence, Table 4.5). The most sensitive endpoints for aquatic environments were the 

duckweed EC50 of 12.5 ppb and the diatom EC50 of 5.2 ppb (section 4.1.3.1). 

The routes of exposure that determine the extent of the action area are run-off in 

stormwater or irrigation water (mostly in the sorbed phase) and spray drift. Volatilization 

and redeposition (eg., via rainwater) may also contribute to the extent of the action area, 

bur this could not be quantified. A paper by Vogel et al. (2008) found pendimethalin in 

rainwater in Merced County at a concentration of 0.143 ppb maximum. This is an 

additional line of evidence that pendimethalin may be transported by air outside the 

immediate use site. 

The AgDRIFT model (Version 2.01) is used to define how far from the initial area of 

concern an effect to a given species may be expected via spray drift. The spray drift 

analysis for pendimethalin using the most sensitive endpoint (ryegrass seedling 

emergence EC25 of 0.01 lb/acre) suggests that effects are expected beyond 1,000 feet 

from the edge of pendimethalin treated areas 

In addition to the buffered area from the spray drift analysis, the final action area also 

considers the downstream extent of pendimethalin that exceeds the LOC. Based on this 

analysis, the action area includes streams as far as 272 kilometers downstream from 

forestry use areas. 

An evaluation of usage information was conducted to determine the area where use of 

pendimethalin may impact the CRLF. This analysis is used to characterize where 

predicted exposures are most likely to occur, but does not preclude use in other portions 

of the action area. A more detailed review of the county-level use information was also 

completed. These data suggest that pendimethalin is widely used both in agricultural and 

non-agricultural settings in 54 of 58 California counties. Thus, it is likely that 

pendimethalin use sites and CRLF habitat areas intersect. 

38
Based on these considerations, the action area is considered to be the entire state of 

California. 

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.”3 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 

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

are exposed to pendimethalin (e.g., direct contact, etc.). 

2.8.1 Assessment Endpoints for the CRLF 

Assessment endpoints for the CRLF include direct toxic effects on the survival, 

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

the prey base or modification of its habitat. In addition, potential modification of critical 

habitat is assessed by evaluating potential effects to PCEs, which are components of the 

habitat areas that provide essential life cycle needs of the CRLF. Each assessment 

endpoint requires one or more “measures of ecological effect,” defined as changes in the 

attributes of an assessment endpoint or changes in a surrogate entity or attribute in 

response to exposure to a pesticide. Specific measures of ecological effect are generally 

evaluated based on acute and chronic toxicity information from registrant-submitted 

guideline tests that are performed on a limited number of organisms. Additional 

ecological effects data from the open literature are also considered. It should be noted 

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

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

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

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

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

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

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

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 4.0 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 pendimethalin is provided in table 2-4 

below. 

3

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

39
Table 2-4 Assessment Endpoints and Measures of Ecological Effects 

Assessment Endpoint Measures of Ecological Effects4 

Aquatic-Phase CRLF 

(Eggs, larvae, juveniles, and adults)a 

Direct Effects 

1. Survival, growth, and reproduction of CRLF 

1a. Most sensitive fish acute LC50 (guideline or 

ECOTOX) if no suitable amphibian data are 

available 

1b. Most sensitive fish chronic NOAEC (guideline 

or ECOTOX) if no suitable amphibian data are 

available 

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. Most sensitive fish, aquatic invertebrate, and 

aquatic plant EC50 or LC50 (guideline or ECOTOX) 

2b. Most sensitive aquatic invertebrate and fish 

chronic NOAEC (guideline or ECOTOX) 

3. Survival, growth, and reproduction of CRLF 

individuals via indirect effects on habitat, cover, 

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

aquatic plant community) 

3a. Vascular plant acute EC50 (duckweed guideline 

test or ECOTOX vascular plant) 

3b. Non-vascular plant acute EC50 (freshwater algae 

or diatom, or ECOTOX non-vascular) 

4. Survival, growth, and reproduction of CRLF 

individuals via effects to riparian vegetation 

4a. Distribution of EC25 values for monocots 

(seedling emergence, vegetative vigor, or 

ECOTOX) 

4b. Distribution of EC25 values for dicots (seedling 

emergence, vegetative vigor, or ECOTOX) 

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. Most sensitive birdb

 or terrestrial-phase 

amphibian acute LC50 or LD50 (guideline or 

ECOTOX) 

5b. Most sensitive birdb

 or terrestrial-phase 

amphibian chronic NOAEC (guideline or 

ECOTOX) 

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. Most sensitive terrestrial invertebrate and 

vertebrate acute EC50 or LC50 (guideline or 

ECOTOX)c 

6b. Most sensitive terrestrial invertebrate and 

vertebrate chronic NOAEC (guideline or ECOTOX) 

7. Survival, growth, and reproduction of CRLF 

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

riparian and upland vegetation) 

7a. Distribution of EC25 for monocots (seedling 

emergence, vegetative vigor, or ECOTOX 

7b. Distribution of EC25 for dicots (seedling 

emergence, vegetative vigor, or ECOTOX) a

 Adult frogs are no longer in the “aquatic phase” of the amphibian life cycle; however, submerged adult

frogs are considered “aquatic” for the purposes of this assessment because exposure pathways in the water 

are considerably different that exposure pathways on land. b

 Birds are used as surrogates for terrestrial phase amphibians. 

4

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

Appendix A. 

40
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 pendimethalin that may alter the PCEs of the CRLF’s critical habitat. PCEs 

for the CRLF were previously described in Section 2.6. Actions that may modify critical 

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

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 pendimethalin effects data are available. Adverse 

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 pendimethalin on critical habitat of 

the CRLF are described in Table 2-55. Some components of these PCEs are associated 

with physical abiotic features (e.g., presence and/or depth of a water body, or distance 

between two sites), which are not expected to be measurably altered by use of pesticides. 

Assessment endpoints used for the analysis of designated critical habitat are based on the 

adverse modification standard established by USFWS (2006). 

41
Table 2-5 Summary of Assessment Endpoints and Measures of Ecological Effect for 

Primary Constituent Elements of Designated Critical Habitata 

Assessment Endpoint Measures of Ecological Effect 

Aquatic-Phase CRLF PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat) 

Alteration of channel/pond morphology or geometry a. Most sensitive aquatic plant EC50 (guideline or 

and/or increase in sediment deposition within the ECOTOX) 

stream channel or pond: aquatic habitat (including b. Distribution of EC25 values for terrestrial monocots 

riparian vegetation) provides for shelter, foraging, (seedling emergence, vegetative vigor, or ECOTOX) 

predator avoidance, and aquatic dispersal for juvenile c. Distribution of EC25 values for terrestrial dicots 

and adult CRLFs. (seedling emergence, vegetative vigor, or ECOTOX) 

Alteration in water chemistry/quality including 

temperature, turbidity, and oxygen content necessary 

for normal growth and viability of juvenile and adult 

CRLFs and their food source. 

a. Most sensitive EC50 values for aquatic plants (guideline 

or ECOTOX) 

b. Distribution of EC25 values for terrestrial monocots 

(seedling emergence or vegetative vigor, or ECOTOX) 

c. Distribution of EC25 values for terrestrial dicots 

(seedling emergence, vegetative vigor, or ECOTOX) 

Alteration of other chemical characteristics necessary 

for normal growth and viability of CRLFs and their 

food source. 

a. Most sensitive EC50 or LC50 values for fish or aquaticphase amphibians and aquatic invertebrates (guideline or 

ECOTOX) 

b. Most sensitive NOAEC values for fish or aquatic-phase 

amphibians and aquatic invertebrates (guideline or 

ECOTOX) 

Reduction and/or modification of aquatic-based food 

sources for pre-metamorphs (e.g., algae) 

a. Most sensitive aquatic plant EC50 (guideline or 

ECOTOX) 

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 

a. Distribution of EC25 values for monocots (seedling 

emergence, vegetative vigor, or ECOTOX) 

Elimination and/or disturbance of dispersal habitat: b. Distribution of EC25 values for dicots (seedling 

Upland or riparian dispersal habitat within designated emergence, vegetative vigor, or ECOTOX) 

units and between occupied locations within 0.7 mi of c. Most sensitive food source acute EC50/LC50 and NOAEC 

each other that allow for movement between sites values for terrestrial vertebrates (mammals) and 

including both natural and altered sites which do not invertebrates, birds or terrestrial-phase amphibians, and 

contain barriers to dispersal freshwater fish. 

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. 

42 
2.9 Conceptual Model 

2.9.1 Risk Hypotheses 

Risk hypotheses are specific assumptions about potential adverse effects (i.e.,changes in 

assessment endpoints) and may be based on theory and logic, empirical data, 

mathematical models, or probability models (U.S. EPA, 1998). For this assessment, the 

risk is stressor-linked, where the stressor is the release of pendimethalin to the 

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

assessment: 

The labeled use of pendimethalin 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; 

• affect 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); 

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

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

• affect 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. 

• affect 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. 

• affect 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 pendimethalin release mechanisms, biological receptor types, and effects 

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

exposures are shown in Figure 2.5 and Figure 2.6, respectively, which include the 

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

43
Exposure routes shown in dashed lines are not quantitatively considered because the 

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

modification to designated critical habitat is expected to be negligible. 

Stressor 

Source 

Receptors 

Attribute 

Change 

Pendimethalin applied to use site 

Direct 

application Spray drift and 

volatilization 

Birds/terrestrialphase amphibians/ 

reptiles/mammals 

Terrestrial 

insects 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain 

Reduction in prey 

Modification of PCEs 

related to prey availability 

Habitat integrity 

Reduction in primary productivity 

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Terrestrial/riparian plants 

grasses/forbs, fruit, seeds 

(trees, shrubs) 

Runoff 

Mammals/ 

birds 

Exposure 

Media 

Soil 

Ingestion 

Ingestion 

Ingestion 

Ingestion 

Dermal uptake/Ingestion 

Long range 

atmospheric 

transport 

Root uptake 

Wet/dry deposition 

Terrestrial-phase 

amphibians 

Ingestion 

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

CRLF 

44
Stressor 

Source 

Exposure 

Media 

Uptake/gills Receptors or integument 

Fish/aquatic-phase 

amphibians 

**Piscivorous mammals 

and birds 

Food chain 

Reduction in algae 

Reduction in prey 

Modification of PCEs 

 related to prey availability 

Habitat integrity 

Reduction in primary 

productivity 

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Aquatic Animals 

Invertebrates 

Vertebrates 

Ingestion Ingestion 

Uptake/gills 

or integument 

Aquatic Plants 

Non-vascular 

Vascular 

Uptake/cell, 

roots, leaves 

Pendimethalin applied to use site 

Spray drift and 

volatilization 

Surface water/ 

Sediment 

Runoff Long range 

atmospheric 

transport 

Wet/dry deposition 

Soil Groundwater 

Riparian plants 

terrestrial 

exposure 

pathways see 

Figure 2.5 

Attribute 

Change 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

** Route of exposure includes only ingestion of aquatic fish and invertebrates 

Figure 2.6 Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF 

2.10 Analysis Plan 

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

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

environmental fate, and ecological effects of pendimethalin 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 pendimethalin is 

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

below) or actual calculated risk quotient value. 

45
2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 

2.10.1.1 Measures of Exposure 

The environmental fate properties of pendimethalin along with available monitoring data 

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

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

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

quantitative estimates of pendimethalin 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 pendimethalin 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 Tier 1 Rice model is used to estimate pendimethalin 

concentrations in rice paddy water on the day of application. The model used to predict 

terrestrial EECs on food items is T-REX. The model used to derive EECs relevant to 

terrestrial and wetland plants is TerrPlant. These models are parameterized using 

relevant reviewed registrant-submitted environmental fate data. 

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

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

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

bodies adjacent to application sites receiving pendimethalin 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 

pendimethalin. 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 in aquatic environments via bioaccumulation and foodchain exposure was 

quantified with the KABAM (Kow-based bioaccumulation) model (documented at 

http://www.epa.gov/oppefed1/models/water/kabam/kabam_user_guide.html). 

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 

46
exposed to spray drift are derived using the T-REX model (version 1.3.1, 12/07/2006). 

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

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

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

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

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

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

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

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

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

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

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

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

insects. 

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

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

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

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

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

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

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

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

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

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

to estimate avian food intake. 

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

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

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

Spray drift models, AGDISP and/or AgDRIFT are used to assess exposures of terrestrial 

phase CRLF and its prey to pendimethalin deposited on terrestrial habitats by spray drift. 

AGDISP (version 8.13; dated 12/14/2004) (Teske and Curbishley, 2003) is used to 

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

drift analysis, the downstream extent of pendimethalin that exceeds the LOC for the 

effects determination is also considered. 

Although volatility is a potential route of exposure, volatility and subsequent deposition 

will not be quantified because there are currently no valid models that would be useful to 

quantify this potential route of exposure to terrestrial environments. Pendimethalin is a 

semi-volatile compound that may be lost from moist soil. The volatilization half-life 

from moist soil was found to be 12.5 days (MRID 00153766). Pendimethalin has been 

observed at low concentrations (0.143 ppb maximum) in rainwater in California (Vogel 

et al., 2008). This effect was accounted for in the PRZM-EXAMS modeling by invoking 

47
the PRZM volatilization routine, thus allowing volatilization to compete with runoff and 

metabolism as a dissipation route. Comparison of the highest EEC scenario (forestry) 

shows that the effect is small (16.6 ppb with volatility invoked and 18.1 ppb without). 

The relatively slow volatilization rate from even moist soil is not believed to result in as 

large or immediate an aquatic exposure as spray drift at the time of application. Thus, 

airborne exposure is adequately quantified by the spray drift analysis. Terrestrial 

exposure of plants and animals is accounted for by the AgDrift and AgDISP effect area 

analysis. 

2.10.1.2 Measures of Effect 

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

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

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

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

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

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

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

Research Laboratory's Mid-Continent Ecology Division. 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

48
and fecundity, and do not include the full suite of sublethal effects used to define the 

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

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

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

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

assessment endpoints of survival, growth, and fecundity. 

2.10.1.3 Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects characterization 

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

pendimethalin, 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 pendimethalin risks, the risk quotient (RQ) method is used to compare exposure and 

measured toxicity values. EECs are divided by acute and chronic toxicity values. The 

resulting RQs are then compared to the Agency’s levels of concern (LOCs) (USEPA, 

2004) (see Appendix C). 

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

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

estimated exposures directly to the CRLF of pendimethalin 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 pendimethalin resulting from a 

particular use are sufficient to exceed the listed species LOC, then the effects 

determination for that use is a “may affect.” If the RQ being considered also exceeds the 

non-listed species acute risk LOC, then the effects determination is a LAA. If the acute 

RQ is between the listed species LOC and the non-listed acute risk species LOC, then 

further lines of evidence (i.e. probability of individual effects, species sensitivity 

distributions) are considered in distinguishing between a determination of NLAA and a 

LAA. When considering indirect effects to the CRLF due to effects to algae as dietary 

items or plants as habitat, the non-listed species LOC for plants is used because the CRLF 

does not have an obligate relationship with any particular aquatic and/or terrestrial plant. 

If the RQ being considered for a particular use exceeds the non-listed species LOC for 

plants, the effects determination is “may affect”. Further information on LOCs is 

provided in Appendix C. 

3.0 Exposure Assessment 

Pendimethalin is formulated as granules and emulsifiable concentrate. Application 

equipment includes ground application, and aerial application. Risks from ground boom 

49
and aerial applications are expected to result in the highest off-target levels of 

pendimethalin due to generally higher spray drift levels. Ground boom and aerial modes 

of application tend to use lower volumes of application applied in finer sprays than 

applications coincident with sprayers and spreaders and thus have a higher potential for 

off-target movement via spray drift. 

3.1 Label Application Rates and Intervals 

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

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

products. While technical products, which contain pendimethalin 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 grasses and certain broadleaf weeds in agricultural 

crops, orchards, landscapes and rights-of-way. The formulated product labels legally 

limit pendimethalin’s potential use to only those sites that are specified on the labels. 

There are no known pending mitigations that would influence the exposure assessment. 

Currently registered agricultural and non-agricultural uses of pendimethalin within 

California are summarized in Table 3-1. 

Table 3-1 Pendimethalin Uses, Scenarios, and Application Information for the 

CRLF risk assessment1 

Scenario Uses Represented 

by Scenario 

Application Rate Application Method 

Scenario Uses Represented 

Application rate 

(lb/acre) Application method 

CAalfalfa_WirrigOP alfalfa 3.98 air 

CAalmond_WirrigSTD nut trees 5.985 ground 

CAalmond_WirrigSTD nut trees 3.96 air 

Cacitrus_WirrigSTD citrus crops 5.985 ground 

Cacitrus_WirrigSTD citrus crops 3.96 air 

CAcolecropRLF cole crops 0.99 air 

CAcornOP 

field corn, sweet 

corn, popcorn 1.98 air 

Cacotton_WirrigSTD cotton 1.98 air 

CAForestryRLF forestry 3.96 air 

Cafruit_WirrigSTD fruit trees 3.96 air 

CAgarlicRLF garlic 1.485 air 

Cagrapes_WirrigSTD grapes 5.845 ground 

Cagrapes_WirrigSTD grapes 3.96 air 

CAmelonsRLF eggplant 1.485 air 

CAOliveRLF olives 3.96 air 

CAOnion_WirrigSTD onion 1.98 air 

CApotatoRLF potato, white/Irish 1.485 air 

CArowcropRLF artichoke/asparagus 3.895 air 

CAStrawberrynoplasticRLF strawberry 1.485 air 

50
Scenario Uses Represented 

by Scenario 

Application Rate Application Method 

Catomato_WirrigSTD tomato, tomatillo 1.485 ground 

CAtomatoSTD groundcherry 1.485 air 

CARightofWayfRLF, 

CAImperviousRLF rights-of-way 3.96 air 

CAwheatRLF wheat 1.485 air 

Tier 1 Rice Model rice 0.99 air 

1 Uses assessed based on memorandum from SRRD (Verification Memorandum for Pendimethalin for 

Red Legged Frog Assessment) dated 2/17/2009. 

3.2 Aquatic Exposure Assessment 

3.2.1 Modeling Approach 

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

that represent high exposure sites for pendimethalin use. 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, 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 pendimethalin were 

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

application intervals, and the first application date for each crop. 

The Tier 1 Rice Model calculates the water column concentration in a flooded rice paddy 

on the day of application, accounting only for sorption to sediment as a dissipation 

mechanism. 

KABAM is used to estimate potential bioaccumulation of hydrophobic organic pesticides 

in freshwater aquatic ecosystems and risks to mammals and birds consuming aquatic 

organisms which have bioaccumulated these pesticides. This tool can also be used to 

estimate pesticide concentrations in fish tissues consumed by humans (i.e., filets). The 

bioaccumulation portion of KABAM is based upon work by Arnot and Gobas (2004) 

who parameterized a bioaccumulation model based on PCBs and some pesticides (e.g., 

51
lindane, DDT) in freshwater aquatic ecosystems. KABAM relies on a chemical's octanolwater partition coefficient (KOW) to estimate uptake and elimination constants through 

respiration and diet of organisms in different trophic levels. Pesticide tissue residues are 

calculated for different levels of an aquatic food web. The model then uses pesticide 

tissue concentrations in aquatic animals to estimate dose- and dietary-based exposures 

and associated risks to mammals and birds consuming aquatic organisms, using an 

approach that is similar to the TREX model (USEPA 2008). Input parameters for 

KABAM are given in Appendix K. 

3.2.2 Model Inputs 

Pendimethalin is a herbicide used on a wide variety of food, non-food crops, and nonagricultural sites. Pendimethalin environmental fate data used for generating model 

parameters is listed in Table 2.1 in Section 2.4.1. The input parameters for PRZM and 

EXAMS are in Table 3-2. Application dates were generally one to two weeks before the 

crop emergence date in the PRZM scenario, as pendimethalin is a pre-emergence/preplant herbicide. 

Table 3-2 Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Pendimethalin Endangered Species Assessment for the CRLF 1 

Fate Property Value (unit) MRID (or source) 

Molecular Weight 281.31 

Henry’s constant 8.6E-7 atm-m3/mol VP/solubility 

Vapor Pressure 3.0E-5 mmHg 00153766 

Solubility in Water 3.0 mg/L at 25oC 

00153762; 10x actual value as 

per Input Parameter Guidance 

(IPG) 

Photolysis in Water 42 day half-life (12 hours 

light/dark) 

00153763 

Aerobic Soil Metabolism Half-lives 172 days 40185104; deviation from IPG 

as explained in 1997 RED 

Hydrolysis Stable 00106777 

Aerobic Aquatic Metabolism (water 

column) 

330 day half-life (total system) 

3x factor not used due to long halflife from single study 

47385201 

Anaerobic Aquatic Metabolism 

(benthic) 208 day half-life 40813501; upper 90th %ile 

confidence bound 

Koc 17040 mL/g o.c. 43041901 (average of 5 

values) 

Application rate and frequency 

See table 3.1 

Application intervals 

Chemical Application Method (CAM) Soil appl;ied 

52
Fate Property Value (unit) MRID (or source) 

Application Efficiency 0.99 ground 

0.95 aerial 

IPG 

Spray Drift Fraction 0.01 ground 

0.05 aerial 

IPG 

Air Diffusion coefficient 3427 cm2/day = 1.55/(MW)^0.65 

Enthalpy of Vaporization 20 kcal/mol PRZM default 

1 – Inputs determined in accordance with EFED “Guidance for Chemistry and Management Practice Input 

Parameters for Use in Modeling the Environmental Fate and Transport of Pesticides” dated February 28, 

2002 

3.2.3 Results 

The aquatic EECs for the various scenarios and application practices are listed in Table 

3-3. Most peak exposures were 16 ppb or below. Most exposures were dominated by 

spray drift. 

Table 3-3 Aquatic EECs (μg/L) for Pendimethalin Uses in California 

PRZM Scenario 

Application 

Rate (lb/A) 

Aerial/ 

Ground Specific Crop 

Peak 

EEC 

(ppb) 

21-day 

EEC 

(ppb) 

60-day 

EEC 

(ppb) 

CAcolecropRLF 0.99 A 

Several cole 

crops 3.16 1.29 0.98 

Cacitrus_WirrigSTD a 5.985 G 

Several citrus 

crops 3.42 0.94 0.58 

Cagrapes_WirrigSTD, 

CAwinegrapesRLF 5.845 G grapes 4.06 1.31 0.95 

CApotatoRLF 1.485 A 

potato, 

white/Irish 4.14 0.99 0.57 

CAtomatoSTD 1.485 A groundcherry 4.21 1.10 0.66 

CAmelonsRLF 1.485 A eggplant 4.22 1.04 0.62 

CAgarlicRLF 1.485 A garlic 4.31 1.15 0.73 

CAStrawberrynoplasticRLF 1.485 A strawberry 4.74 1.92 1.36 

CAOnion_WirrigSTD 1.98 A onion 5.55 1.35 0.80 

Cacotton_WirrigSTD 1.98 A cotton 5.75 1.54 0.99 

CAwheatRLF 1.485 A wheat 6.10 2.81 2.13 

CAalmond_WirrigSTD 5.985 G several 6.56 2.38 1.61 

CAcornOP 1.98 A several 6.84 2.00 1.54 

Cacitrus_WirrigSTD a 3.96 A several 11.06 2.65 1.52 

CAOliveRLF 3.96 A olives 11.16 2.78 1.70 

Cafruit_WirrigSTD b 3.96 A several 11.18 3.03 1.85 

CAalfalfa_WirrigOP 3.98 A alfalfa 11.28 3.11 1.95 

Cagrapes_WirrigSTD, 

CAwinegrapesRLF 3.96 A grapes 11.44 2.94 1.83 

CAalmond_WirrigSTD c 3.96 A several 11.71 3.68 2.30 

CArowcropRLF d 3.895 A 

artichoke/asp 

aragus 11.92 4.04 2.86 

53
CAForestryRLF e 3.96 A several 16.60 6.41 4.93 

Tier 1 Rice Model 0.99 A rice 48 --- --- 

CARightofWayfRLF, 

CAImperviousRLF f 3.96 A several 1.74 0.55 0.36 

a

 This scenario represents calamondin, citron (citrus), citrus hybrids other than tangelo, grapefruit, 

kumquat, lemon, lime, orange, pummelo (shaddock), and tangelo. b

 This scenario represents apple, apricot, cherry, crabapple, fig, loquat, nectarine, peach, pear, pepino

(melon pear), plum, pomegranate, prune, quince, and small fruits. c

 This scenario represents almond, beech nut, brazil nut, butternut, cashew, chestnut, chinquapin, filbert

(hazelnut), hickory nut, macadamia nut (bushnut), mayhaw (hawthorn), pecan, pistachio, walnut (English 

or black) . 

d This scenario represents artichoke, asparagus, beans, carrot (including tops), garbanzos (including chick 

peas), legume vegetables, peanuts (unspecified), peas, and pepper. e

 This scenario represents forestry and Christmas tree plantations. f These combined scenarios represents airports/landing fields, commercial/industrial lawns, golf course 

turf, industrial areas (outdoor), mulch, ornamental and/or shade trees, ornamental ground cover, ornamental

herbaceous plants, ornamental lawns and turf, ornamental non-flowering plants, ornamental sod farm (turf), 

ornamental woody shrubs and vines, paved areas (private roads/sidewalks), Rights-of-way, fencerows,

hedges recreation area lawns, residential lawns, and shelterbelt plantings. Combined EEC is 50% 

Impervious scenario with 1% of application rate and no drift, and 50% Rights-of-way scenario.

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 

pendimethalin data from the USGS NAWQA program (http://water.usgs.gov/nawqa) and 

data from the California Department of Pesticide Regulation (CDPR). 

3.2.4.1 USGS NAWQA Surface Water Data 

Data for pendimethalin and its degradate hydroxypendamethalin were downloaded from 

the NAWQA Data Warehouse site on March 30, 2009. The data was restricted to 

California sampling stations. There were 604 measurements of pendimethalin in the 

Sacramento River and San Joaquin-Tulare River basins. There was measureable 

pendimethalin in 319 of the 604 samples (53% positive). 

The highest concentration recorded (0.679 ppb) was at a station called “Highline Cn Spill 

Nr Hilmar Ca” in the San Joaquin-Tulare River Basin. All 9 measurements at this site 

were positive for pendimethalin, with a range of 0.262 to 0.679 ppb. 

All 8 measurements of hydroxypendimethalin in surface water were in the San JoaquinTulare basin. All were non-detect (<0.143 ppb). 

3.2.4.2 USGS NAWQA Groundwater Data 

54
There were 589 measurements of pendimethalin in the NAWQA data for the Sacramento 

River and San Joaquin-Tulare River Basins. There was only one positive result (0.0138 

ppb) in Stanislaus County (San Joaquin-Tulare basin). 

All 61 measurements of pendimethalin in ground water were in the San Joaquin-Tulare 

basin. All were non-detect (<0.143 ppb). 

3.2.4.3 California Department of Pesticide Regulation (CPR) Data 

Data for pendimethalin were downloaded from the California surface water monitoring 

database on March 19, 2009. There were 565 measurements of pendimethalin in the 

Sacramento River valley (33 were positive), and 1802 measurements (326 were positive) 

in the San Joaquin River valley. The maximum concentration in the Sacramento River 

valley was 0.7 ppb, and it was 3.5 ppb in the San Joaquin valley. The data for all stations 

in the two valleys are given in Figure 3.1 below. 

Pendimethalin [ug/L] in Sacramento Valley Streams 

0.8 

0.7 

0.6 

0.5 

0.4 Conc [ug/L] 

0.3 

0.2 

0.1 

0

5/7/1990 1/31/1993 10/28/1995 7/24/1998 4/19/2001 1/14/2004 10/10/2006 7/6/2009

date 

Pendimethalin, ppb

55
Pendimethalin [ug/L] in San Joaquin Valley Streams 

ppb Pendimethalin

4 

3.5 

3 

2.5 

2 Conc [ug/L]

1.5 

1 

0.5 

0

5/7/1990 1/31/1993 10/28/1995 7/24/1998 4/19/2001 1/14/2004 10/10/2006

date 

Figure 3.1 California DPR surface water monitoring data 

3.2.4.4 Atmospheric Monitoring Data 

Pendimethalin has been detected in air in an agricultural setting (Merced River basin 

downstream from McSwain dam) in California (Vogel et al. 2008). Of 23 rainwater 

samples taken, 78% were positive, with a maximum concentration of 0.143 μg/L, and a 

median of 0.021 μg/L. Sampling was conducted in 2003 and 2004. 

As explained above in section 2.10.1.1, volatilization and redeposition is not quantifiable 

with currently approved models. However, analysis of water modeling results with and 

with soil volatilization routines invoked indicate that the volatilization effect is small for 

pendimethalin, and airborne exposure is dominated by spray drift at the time of 

application. 

3.2.4.5 Spray Drift Buffer and Downstream Dilution Analysis for 

Action Area 

Because pendimethalin use is documented in 54 of 58 California counties, and because it 

is semi-volatile and has been detected in rainwater, the Action Area is considered to be 

the entire state. Spray drift and downstream dilution analysis for the Action Area were 

therefore not conducted. 

56
3.3 Terrestrial Animal Exposure Assessment 

T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of 

pendimethalin for the CRLF and its potential prey (e.g. small mammals and terrestrial 

insects) inhabiting terrestrial areas. EECs used to represent the CRLF are also used to 

represent exposure values for frogs serving as potential prey of CRLF adults. T-REX 

simulates a 1-year time period. For this assessment, spray applications of pendimethalin 

are considered, as discussed in below. 

Terrestrial EECs for foliar formulations of pendimethalin were derived for the uses 

summarized in Table 3.4. Given that no data on interception and subsequent dissipation 

from foliar surfaces is available for pendimethalin, a default foliar dissipation half-life of 

35 days is used based on the work of Willis and McDowell (1987). Use of specific input 

values, including number of applications, application rate and application interval are 

provided in Table 3.4. Outputs from T-REX is available in Appendix E. 

Table 3-4 Input Parameters for Foliar Applications Used to Derive Terrestrial 

EECs for Pendimethalin with T-REX 

Use Sites Application rate 

(lbs ai/A) 

Number of 

Applications 

walnut (english/black), tangerines, tangelo, small fruits, pummelo 

(shaddock), pomegranate, pistachio, pecan, orange, macadamia nut 

(bushnut), lime, lemon, kumquat, hickory nut, grapefruit, filbert 

(hazelnut), citrus hybrids other than tangelo, citron (citrus), chinquapin, 

chestnut, cashew, calamondin, butternut, brazil nut, beech nut, almond, 

grapes. 

6.0 1 

Prune, plum, pear, peach, olive, nectarine, fig, cherry, apricot, apple, 

ornamental and/or shade trees, nonagricultural rights-of-way, 

fencerows, hedgerows, mulch, industrial areas (outdoor), forest trees 

(all or unspecified), Christmas tree plantations, recreation area lawns, 

loquat, shelterbelt plantings, quince, mayhaw, crabapple, alfalfa 

4.0 1 

Asparagus, artichoke 3.9 1 

golf course turf, airports/landing fields 3.5 1 

residential lawns 3.0 1 

Shallot, onion, corn (sweet, pop, field) 2.0 1 

Carrot 1.9 1 

Sunflower 1.73 1 

Tomato, tomatillo, tobacco, sorghum, potato (white/Irish), lupine 

(grain), garlic, garbanzos (including chick peas) beans, succulent (snap 

or lima) beans, dried-type beans, cotton, cowpea (southern pea, blackeyed bean), 

1.485 1 

Wheat, pepper, pepino (melon pear), groundcherry, eggplant 1.425 1 

Strawberry 1.485 2 (30 day interval) 

Legume vegetables 1.24 1 

Rice, mustard cabbage (gai choy, pak-choi), kohlrabi, cauliflower, 

cabbage (Chinese), cabbage, brussels sprouts, broccoli (Chinese), 

broccoli, Brassica (head and stem) vegetables 

1.0 1 

T-REX is also used to calculate EECs for terrestrial insects exposed to pendimethalin. 

Dietary-based EECs calculated by T-REX for small and large insects (units of a.i./g) are 

used to bound an estimate of exposure to terrestrial insects. Available acute contact 

57 
toxicity data for bees exposed to pendimethalin (in units of μg a.i./bee), are converted to 

μg a.i./g (of bee) by multiplying by 1 bee/0.128 g. The EECs are later compared to the 

adjusted acute contact toxicity data for bees in order to derive RQs. 

For modeling purposes, exposures of the CRLF to pendimethalin 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 3.5). Dietary-based EECs for small and large 

insects reported by T-REX as well as the resulting adjusted EECs are available in Table 

3.6. An example output from T-REX v. 1.3.1 is available in Appendix E. 

Table 3-5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Pendimethalin 

Application rate 

EECs for CRLF (including other 

frog prey) 

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) 

6.0 810 922 1,440 1374 

4.0 540 615 960 915 

3.9 526 600 936 892 

3.5 472 538 840 801 

3.0 405 461 720 686 

2.0 270 307 480 458 

1.9 256 292 456 435 

1.73 234 267 415 396 

1.485 200 228 356 340 

1.425 192 219 242 326 

1.485 (2 applications) 301 342 535 510 

1.24 167 191 298 284 

1.0 135 154 240 229 

Table 3-6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via 

Effects to Terrestrial Invertebrate Prey Items 

Application Rate Small Insect Large Insect 

6.0 810 90 

4.0 540 60 

3.9 527 59 

3.5 473 53 

3.0 405 45 

2.0 270 30 

1.9 257 29 

1.73 234 26 

58
1.485 200 22 

1.425 192 21 

1.485 (2 applications) 342 38 

1.24 167 19 

1.0 135 15 

3.4 Terrestrial Plant Exposure Assessment 

TerrPlant (Version 1.1.2) is used to calculate EECs for non-target plant species inhabiting 

dry and semi-aquatic areas. Parameter values for application rate and drift assumption 

are based upon the use and related application method (Table 3.7). A runoff value of 

0.01 is utilized based on pendimethalin’s solubility, which is 0.28 mg/L. For aerial and 

ground application methods, drift is assumed to be 5% and 1%, respectively. EECs 

relevant to terrestrial plants consider pesticide concentrations in drift and in runoff. 

These EECs are listed by use in Table 3.7. An example output from TerrPlant v.1.2.2 is 

available in Appendix F. 

Table 3-7 TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and 

Semi-aquatic Areas Exposed to Pendimethalin via Runoff and Drift 

Application 

rate 

(lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Spray 

drift 

EEC 

(lbs 

a.i./A) 

Dry area 

EEC 

(lbs 

a.i./A) 

Semi-aquatic 

area EEC 

(lbs a.i./A) 

6.0 Foliar - ground 1 0.060 0.120 0.660 

4.0 Foliar - ground 1 0.040 0.080 0.440 

4.0 Foliar - aerial 5 0.200 0.24 0.600 

3.9 Foliar - ground 1 0.039 0.078 0.429 

3.9 Foliar - aerial 5 0.195 0.234 0.585 

3.5 Foliar - ground 1 0.035 0.070 0.385 

3.5 Foliar - aerial 5 0.175 0.210 0.525 

3.0 Foliar - ground 1 0.030 0.060 0.330 

3.0 Foliar - aerial 5 0.150 0.180 0.450 

2.0 Foliar - ground 1 0.020 0.040 0.220 

2.0 Foliar - aerial 5 0.100 0.120 0.300 

1.9 Foliar - ground 1 0.019 0.038 0.209 

1.9 Foliar - aerial 5 0.095 0.114 0.285 

1.73 Foliar - ground 1 0.017 0.035 0.190 

1.73 Foliar - aerial 5 0.087 0.104 0.260 

1.485 Foliar - ground 1 0.015 0.030 0.163 

1.485 Foliar - aerial 5 0.074 0.089 0.223 

1.24 Foliar - ground 1 0.012 0.025 0.136 

1.24 Foliar - aerial 5 0.062 0.074 0.186 

1.0 Foliar - ground 1 0.010 0.020 0.110 

1.0 Foliar - aerial 5 0.050 0.060 0.150 

59
4.0 Effects Assessment 

This assessment evaluates the potential for pendimethalin to directly or indirectly affect 

the CRLF or affect 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 pendimethalin. 

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 freshwater fish (also as a surrogate for aquatic-phase amphibians), freshwater 

invertebrates, aquatic plants, birds (surrogate for terrestrial-phase amphibians), mammals, 

terrestrial invertebrates, and terrestrial plants. 

Toxicity endpoints are established based on data generated from guideline studies 

submitted by the registrant, and from open literature studies that meet the criteria for 

inclusion into the ECOTOX database maintained by EPA/Office of Research and 

Development (ORD) (U.S. EPA, 2004). Open literature data presented in this assessment 

were obtained from ECOTOX on 10/31/2008. In order to be included in the ECOTOX 

database, papers must meet the following minimum criteria: 

(1) the toxic effects are related to single chemical exposure; 

(2) the toxic effects are on an aquatic or terrestrial plant or animal species; 

(3) there is a biological effect on live, whole organisms; 

(4) a concurrent environmental chemical concentration/dose or application 

rate is reported; and 

(5) there is an explicit duration of exposure. 

Data that pass the ECOTOX screen are evaluated along with the registrant-submitted 

data, and may be incorporated qualitatively or quantitatively into this endangered species 

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 

60
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 pendimethalin. 

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 G. Appendix G also includes a 

rationale for rejection of those studies that did not pass the ECOTOX screen and those 

that were not evaluated as part of this endangered species risk assessment. 

A detailed spreadsheet of the available ECOTOX open literature data, including the full 

suite of lethal and sublethal endpoints is presented in Appendix G. Appendix I also 

includes a summary of the human health effects data for pendimethalin. 

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 pendimethalin. A summary of the available aquatic 

and terrestrial ecotoxicity information, use of the probit dose response relationship, and 

the incident information for pendimethalin are provided in Sections 4.1 through 4.4, 

respectively. 

There are no available toxicity information on the degradates of pendimethalin. 

A detailed summary of the available ecotoxicity information for all pendimethalin 

formulated products are presented in Appendix A. 

In regards to mixtures of other products containing pendimethalin, only three products 

(EPA Reg. Nos. 241-331, 241-376 and 5905-495) have definitive product LD50 values 

with associated confidence intervals. Although there are no 95% confidence intervals 

associated with the pendimethalin technical, an evaluation of the three products listed 

above show that for EPA Reg. Nos. 241-331 and 241-376, the product LD50 values can be 

attributed solely to the toxicity of pendimethalin. When these product LD50s (3506 and 

2500 mg/kg, respectively) and associated confidence intervals (892-1223 and 504-800 

mg/kg, respectively) are adjusted for the percent pendimethalin (30.24% and 25.4%, 

respectively), the adjusted LD50 values for EPA Reg. No. 241-331 (1060 mg/kg, CI: 

892-1223) and EPA Reg. No. 241-376 (635 mg/kg, CI: 504-800) are not toxicologically 

distinct from the LD50 of pendimethalin (1050 mg/kg). Similarly, for EPA Reg. No. 

5905-495, the toxicity can be attributed to propanil. When the LD50 for this product 

(1110 mg/kg) and its confidence interval (913-1360 mg/kg) are adjusted for the percent 

propanil (33.7%), the adjusted LD50 value of 374 mg/kg (CI: 308-458 mg/kg) is within a 

factor of two of the confidence interval for propanil (1080 mg/kg; CI: 868-1343 mg/kg) 

and the difference is not considered to be toxicologically significant. 

61
 

Because the active ingredients are not expected to have similar mechanisms of action, 

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

dose-addition would be inappropriate. Consequently, an assessment based on the toxicity 

of pendimethalin is the only reasonable approach that employs the available data to 

address the potential acute risks of the formulated products. Additional information can 

be found in Appendix B. 

4.1 Evaluation of Aquatic Ecotoxicity Studies 

Table 4-1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of both the submitted studies and the open literature, as previously 

discussed. A brief summary of submitted and open literature data considered relevant to 

this ecological risk assessment for the CRLF is presented below. Additional information 

is provided in Appendix A. 

Table 4-1 Freshwater Aquatic Toxicity Profile for Pendimethalin 

Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Describe effect 

(i.e. mortality, 

growth, 

reproduction) 

Citation 

MRID # 

(Author & 

Date) 

Study 

Classification 

Acute Direct 

Toxicity to AquaticPhase CRLF 

Rainbow 

trout 

LC50 = 138 ppb mortality 00046291 

Sleight, 

1972 

Acceptable 

Chronic Direct 

Toxicity to AquaticPhase CRLF 

Fathead 

minnow 

NOAEC = 6.3 

ppb 

reduction of egg 

production 

00037940 

EG&G 

Bionomics, 

1975 

Acceptable 

Indirect Toxicity to 

Aquatic-Phase 

CRLF via Acute 

Toxicity to 

Freshwater 

Invertebrates (i.e. 

prey items) 

Daphnia 

magna 

EC50 = 280 ppb mortality 00059738 

LeBlanc, 

1976 

Acceptable 

Indirect Toxicity to 

Aquatic-Phase 

CRLF via Chronic 

Toxicity to 

Freshwater 

Invertebrates (i.e. 

prey items) 

Waterflea 

(Daphnia 

magna) 

NOAEC = 14.5 

ppb 

reduced 

production of 

Daphnia young 

00100504 

Graney, 

1981 

Acceptable 

Indirect Toxicity to 

Aquatic-Phase 

CRLF via Toxicity 

to Non-vascular 

Aquatic Plants 

Marine 

diatom 

Skeletonem 

a costatum 

EC50 = 5.2 ppb growth 42372205 

Hughes, 

1992 

Acceptable 

Indirect Toxicity to 

Aquatic-Phase 

CRLF via Toxicity 

to Vascular Aquatic 

Duckweed 

(Lemna 

gibba) 

EC50 = 12..5 

ppb 

growth 42137101 

Hughes, 

1992 

Acceptable 

62
Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Describe effect 

(i.e. mortality, 

growth, 

reproduction) 

Citation 

MRID # 

(Author & 

Date) 

Study 

Classification 

Plants 

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

4-2 (U.S. EPA, 2004). Toxicity categories for aquatic plants have not been defined. 

Table 4-2 Categories of Acute Toxicity for Fish and Aquatic Invertebrates 

LC50 (ppm) Toxicity Category 

< 0.1 Very highly toxic 

> 0.1 - 1 Highly toxic 

> 1 - 10 Moderately toxic 

> 10 - 100 Slightly toxic 

> 100 Practically nontoxic 

4.1.1 Toxicity to Freshwater Fish 

Given that no pendimethalin toxicity data are available for aquatic-phase amphibians; 

freshwater fish data were used as a surrogate to estimate direct acute and chronic risks to 

the CRLF. Freshwater fish toxicity data were also used to assess potential indirect effects 

of pendimethalin to the CRLF. Effects to freshwater fish resulting from exposure to 

pendimethalin 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). 

A summary of acute and chronic freshwater fish data, including data from the open 

literature, is provided below in Sections 4.1.1.1 through 4.1.1.3. 

4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies 

Three acute freshwater studies using the technical pendimethalin and seven using 

formulations were submitted to the Agency. For the technical pendimethalin studies, the 

LC50 for the three species of fish range from 138 ppb to 418 ppb. For the formulated 

product studies, three species of fish range from 520 ppb formulation to 86,600 ppb 

formulation. 

The species for the three technical pendimethalin studies are bluegill sunfish, rainbow 

trout, and channel catfish. The LC50 for the species are 199 ppb, 138 ppb, and 418 ppb, 

respectively. 

The species for the seven formulated product studies are bluegill sunfish, rainbow trout, 

and channel catfish. The LC50 for the formulation studies are listed below:

LC50 (ppm LC50 (ppm a.i.

AI 1 formulation) calculated) MRID

63
Bluegill sunfish Prowl 3E 

Lepomis macrochirus 34.4% 1.04 ppm 0.358 ppm 00037927 

Rainbow trout Prowl 3E 

Oncorhynchus mykiss 34.4% 1.0 ppm 0.344 ppm 00037927 

Bluegill sunfish Prowl 4E 

Lepomis macrochirus 42.3% 0.92 ppm 0.389 ppm 00037927 

Rainbow trout Prowl 4E 

Oncorhynchus mykiss 42.3% 0.52 ppm 0.220 ppm 00037927 

Bluegill sunfish Avenge 2AS 

Lepomis macrochirus 31.8% 90.4 ppm 28.747 ppm 00037927 

Rainbow trout Avenge 2AS 

Oncorhynchus mykiss 31.8% 86.6 ppm 27.539 ppm 00037927 

Channel catfish Prowl 4E 

Ictalurus punctatus 42.3% 1.9 ppm 0.804 ppm 00131773 

1 AI refers to active ingredient percentage. The formulated product is also displayed. 

The endpoint selected for the assessment is the rainbow trout study using technical grade 

pendimethalin and it had an LC50 of 138 ppb (C.I. 110-170) since it is the most sensitive 

of the freshwater fish. The MRID number is 00046291. 

4.1.1.2 Freshwater Fish: Chronic Exposure (Early Life Stage and 

Reproduction) Studies 

One chronic freshwater fish study (MRID00037940) was submitted to the Agency. This 

study is a Fathead minnow (Pimephales promelas) full life cycle study using a technical 

grade pendimethalin. The 288-day NOAEC is 6.3 ppb (0.0063 ppm). The NOAEC is 

based on reduction of egg production. The LOAEC = 9.8 ppb (0.0098 ppm) based on 

reduction of egg production. Reduced hatchability was also observed at 22 and 43 ppb. 

4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open 

Literature Information 

The ecotoxicity studies from the ECOTOX database provided endpoints that are less 

sensitive than the submitted rainbow trout study (MRID 00046291) which had an LC50 of 

138 ppb. No useful chronic freshwater invertebrate studies were found in the ECOTOX 

database with NOAEC more sensitive than the submitted fathead minnow study (MRID 

00037940). 

4.1.1.4 Aquatic-phase Amphibian: Acute and Chronic Studies 

No studies are available. 

64
4.1.2 Toxicity to Freshwater Invertebrates 

Freshwater invertebrate toxicity data were used to assess potential indirect effects of 

pendimethalin to the CRLF. Effects to freshwater invertebrates resulting from exposure 

to pendimethalin could indirectly affect the CRLF via reduction in available food items. 

As discussed in Section 2.5.3, the main food source for juvenile aquatic- and terrestrialphase CRLFs is thought to be aquatic invertebrates found along the shoreline and on the 

water surface, including aquatic sowbugs, larval alderflies and water striders. 

A summary of acute and chronic freshwater invertebrate data, including data published in 

the open literature, is provided below in Sections 4.1.2.1 through 4.1.2.3. 

4.1.2.1 Freshwater Invertebrates: Acute Exposure (Mortality) Studies 

Three acute freshwater invertebrate studies were submitted to the Agency: two studies 

using the waterflea Daphnia magna and one using crayfish, Procambarus simulans. 

A Daphnia magna study using technical grade pendimethalin had an EC50 of 280 ppb 

(C.I. 230-330) and is considered to be an acceptable study. The other Daphnia magna 

study used the formulation containing 45.6% active ingredient with the EC50 = 5.1 ppm 

(5100 ppb) formulation. This study is considered to be acceptable for formulated study. 

The crayfish study is categorized as supplemental due to the precipitate being observed in 

concentrations of 1.0 ppm and more; the dissolved O2 concentration is less than 40% of 

saturation; and DMSO was used as a solvent which tends to increase mortalities rather 

than decrease them. This EC50 for this study is greater than 1.0 ppm (1000 ppb). 

Acute toxicity of pendimethalin technical to freshwater invertebrates is categorized as 

highly toxic and moderately toxic. Pendimethalin formulation is categorized as 

moderately toxic. 

The most sensitive freshwater invertebrate species tested is Daphnia magna, with a 48­

hour EC50 value of 280 μg/L (MRID: 00059738), is used for deriving risk quotients for 

the CRLF prey. 

4.1.2.2 Freshwater Invertebrates: Chronic Exposure (Reproduction) 

Studies 

One chronic freshwater invertebrate study (MRID 00100504) was submitted to the 

Agency. This study is a Daphnia magna life cycle study using a technical grade 

pendimethalin. The 21-day NOAEC is 14.5 ppb (0.0145 ppm). The NOAEC is based on 

reduced production of Daphnia. LOAEC = 0.0172 ppm (17.2 ppb). Mortality was 

observed in the study with 100% mortality at the 2 highest dose levels of 35.8 and 74.2 

ppb. No appreciable mortality observed at 3 lowest concentration levels of 4.3, 8.2, and 

14.5 ppb. Reduction of productivity of 50% occurred at 22.1 and 17.5 ppb concentration 

levels. 

65
4.1.2.3 Freshwater Invertebrates: Sublethal Effects and Open 

Literature Data 

The ecotoxicity studies from the ECOTOX database provided endpoints that are less 

sensitive than the submitted Daphnia magna study (MRID 00059738) which had an EC50 

of 280 ppb. No useful chronic freshwater invertebrate studies were found in the 

ECOTOX database with NOAEC more sensitive than the submitted Daphnia magna 

study (MRID 00100504). 

4.1.3 Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether pendimethalin may 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. 

Only laboratory studies were used to determine whether pendimethalin may cause direct 

effects to aquatic plants since there are no available field studies to evaluate the effects of 

pendimethalin to aquatic plants. A summary of the laboratory data for aquatic plants is 

provided in Sections 4.1.3.1 and 4.1.3.2 . 

4.1.3.1 Aquatic Plants: Laboratory Data 

Aquatic plant toxicity data submitted to the Agency show a range of EC50 values from 5.2 

ppb to >174 ppb. The most sensitive species tested are the green algae and marine 

diatom and the least sensitive species is the cyanobacteria (blue-green algae). Below is a 

listing of the species tested and their toxicity value: 

EC50 (ppb) NOAEL MRID 

Green algae (Selenastrum capricornutum) 5.4 ppb 3.0 42372204 

Marine diatom (Skeletonema costatum) 5.2 ppb 0.7 42372205 

Freshwater diatom (Navicula pelliculosa) 6.7 ppb 3.2 42372206 

Blue-green algae (Anabaena flos-aquae) >174 ppb 98 42372207 

Duckweed (Lemna gibba) 12.5 ppb 5.6 42137101 

The endpoint selected for the assessment is the marine diatom study using technical grade 

pendimethalin had an EC50 of 5.2 ppb since it is most sensitive of the aquatic plants 

tested. In addition, the duckweed is the only vascular aquatic plant tested and will be 

used for assessing aquatic habitat. The EC50 for the duckweed is 12.5 ppb. 

66
4.1.3.2 Freshwater Field Studies 

No studies are available. 

4.2 Toxicity of Pendimethalin to Terrestrial Organisms 

Table 4.3 below summarizes the most sensitive terrestrial toxicity endpoints for the 

CRLF, based on an evaluation of both the submitted studies and the open literature. A 

brief summary of submitted and open literature data considered relevant to this ecological 

risk assessment for the CRLF is presented below. 

Table 4-3. Terrestrial Toxicity Profile for Pendimethalin 

Assessment 

Endpoint 

Species Toxicity 

Value Used in 

Risk 

Assessment 

Describe effect 

(i.e. mortality, 

growth, 

reproduction) 

Citation 

MRID # 

(Author & 

Date) 

Study 

Classification 

Acute Dosebased Direct 

Toxicity to 

TerrestrialPhase CRLF 

Mallard 

duck 

LD50 = 1421 

mg/kg-bw 

mortality 00059739 

Fink, 1976 

Acceptable 

Acute Dietarybased Direct 

Toxicity to 

TerrestrialPhase CRLF 

Bobwhite 

quail 

LC50 = 4187 

ppm 

mortality 00026675 

Fink, 1973 

Acceptable 

Chronic Direct 

Toxicity to 

TerrestrialPhase CRLF 

Mallard 

duck 

NOAEC = 141 

ppm 

14-day survivor 

bodyweight 

44907601 

Beavers, 1996 

Acceptable 

Indirect 

Toxicity to 

TerrestrialPhase CRLF 

(via acute 

toxicity to 

mammalian 

prey items) 

Rat LD50 = 1050 

mg/kg-bw 

mortality 00026657 Acceptable 

Indirect 

Toxicity to 

TerrestrialPhase CRLF 

(via chronic 

toxicity to 

mammalian 

prey items) 

Rat NOAEL = 25 

mg/kg-bw 

(500 ppm) 

decreases in the 

number of pups 

born and pup 

weights 

41725203 Acceptable 

Indirect 

Toxicity to 

TerrestrialHoney bee 

LD50 = >49.8 

μgm/bee 

mortality 00099890 

Atkins, 

Acceptable 

67
Assessment 

Endpoint 

Species Toxicity 

Value Used in 

Risk 

Assessment 

Describe effect 

(i.e. mortality, 

growth, 

reproduction) 

Citation 

MRID # 

(Author & 

Date) 

Study 

Classification 

Phase CRLF 

(via acute 

toxicity to 

terrestrial 

invertebrate 

prey items) 

Indirect 

Toxicity to 

Terrestrial- and 

Aquatic-Phase 

Seedling 

Emergence 

Monocots 

Ryegrass EC25 

= 0.01 lb ai/A 

Dry weight 42372201 

Chetram, 1992 

Acceptable 

CRLF (via 

toxicity to 

terrestrial 

plants) 

Seedling 

Emergence 

Dicots 

Lettuce EC25 = 

0.09 lb ai/A 

Dry weight 42372201 

Chetram, 1992 

Acceptable 

Vegetative 

Vigor 

Monocots 

Ryegrass 

EC25 = 0.034 

lb ai/A 

Dry weight 42372203 

Chetram, 1992 

Acceptable 

Vegetative 

Vigor 

Dicots 

Lettuce 

EC25 = 0.10 lb 

ai/A 

Dry weight 42372203 

Chetram, 1992 

Acceptable 

Acute toxicity to terrestrial animals is categorized using the classification system shown 

in Table 4-44 (U.S. EPA, 2004). Toxicity categories for terrestrial plants have not been 

defined. 

Table 4-4 Categories of Acute Toxicity for Avian and Mammalian Studies 

Toxicity Category Oral LD50 Dietary LC50 

Very highly toxic < 10 mg/kg < 50 ppm 

Highly toxic 10 - 50 mg/kg 50 - 500 ppm 

Moderately toxic 51 - 500 mg/kg 501 - 1000 ppm 

Slightly toxic 501 - 2000 mg/kg 1001 - 5000 ppm 

Practically non-toxic > 2000 mg/kg > 5000 ppm 

4.2.1 Toxicity to Birds 

As specified in the Overview Document, the Agency uses birds as a surrogate for 

terrestrial-phase amphibians when amphibian toxicity data are not available (U.S. EPA, 

2004). No terrestrial-phase amphibian data are available for pendimethalin; therefore, 

acute and chronic avian toxicity data are used to assess the potential direct effects of 

pendimethalin to terrestrial-phase CRLFs. 

68
4.2.1.1 Birds: Acute Exposure (Mortality) Studies 

Two subacute LC50 dietary avian studies and one acute LD50 oral acute avian study using 

the technical pendimethalin were submitted to the Agency. For the LC50 dietary studies, 

the LC50 for the bobwhite quail is 4187 ppm and the mallard duck is >4640 ppm. For the 

acute LD50 for the mallard, the LD50 is 1421 mg/kg-bw. 

The endpoint selected for the assessment is the bobwhite quail’s LC50 of 4187 ppm 

(MRID 00026674) and the mallard’s LD50 of 1421 mg/kg-bw (00059739). 

Pendimethalin technical is categorized as slightly toxic on subacute dietary and acute oral 

basis to birds. No avian studies using pendimethalin formulated product have been 

submitted to the Agency. 

4.2.1.2 Birds: Chronic Exposure (Growth, Reproduction) Studies 

Two avian reproduction studies were submitted to the Agency. No treatment related 

effects were observed in the bobwhite quail and the LOAEL is greater than 1410 ppm. 

The mallard duck study (MRID 44907601) showed the parameter of 14-day survivor 

bodyweight was reduced at 1410 ppm. The NOAEL for the mallard study is 410 ppm. 

No other treatment related effects were observed. This study is classified as acceptable. 

4.2.1.3 Birds: Open Literature Studies 

 No studies more sensitive than the submitted avian studies were found in the ECOTOX 

database. 

4.2.2 Toxicity to Mammals 

Mammalian toxicity data are used to assess potential indirect effects of pendimethalin to 

the terrestrial-phase CRLF. Effects to small mammals resulting from exposure to 

pendimethalin could also indirectly affect the CRLF via reduction in available food. As 

discussed in Section 2.5.3, over 50% of the prey mass of the CRLF may consist of 

vertebrates such as mice, frogs, and fish (Hayes and Tennant, 1985). 

Most recent data and information on mammalian toxicity comes from Registration 

Division document entitled “Pendimethalin. Human Health Risk Assessment for the 

Proposed Food Uses of the Herbicide on Artichoke, Globe; Asparagus; Brassica Head 

and Stem Vegetables, Subgroup 5A; and Grape (PP#6E7129)”, dated August 22, 2007. 

The document was authored by Debra Rate, Ph.D., Biologist, Alternative Risk Integration 

and Assessment (ARIA), Risk Integration, Minor Use, and Emergency Response Branch 

(RIMUERB), Registration Division (RD) (7505P) to Barbara Madden/ Susan Stanton 

RIMUERB/RD (7505P). This is found in Appendix I. 

69
4.2.2.1 Mammals: Acute Exposure (Mortality) Studies 

The Agency’s rat acute oral assessment endpoint for risk assessment purpose shows the 

LD50 to be 1250 mg/kg (male) and 1050 mg/kg (female) (MRID 00026657). 

Pendimethalin is considered to be slightly toxic to mammals on an acute oral basis. 

4.2.2.2 Mammals: Chronic Exposure (Growth, Reproduction) Studies 

A 2-generation reproduction study (MRID 41725203) with pendimethalin was reviewed 

by HED. The parental systemic NOAEL was 25 mg/kg/day (500 ppm), based on 

decreased body weight gain and food consumption at the LOAEL of 125 mg/kg/day 

(2500 ppm). The reproductive/offspring NOAEL is 25 mg/kg/day (500 ppm), based on 

decreases in the number of pups born and pup weight at the LOAEL of 125 mg/kg/day 

(2500 ppm). Parental and reproductive NOAELs and LOAELs were based on a generic 

ratio (1:20) of dietary intake of chemical. 

The HED Cancer Peer Review Committee classified pendimethalin as a “Group C” 

(possible human) carcinogen, based on thyroid follicular cell adenomas in rats. The 

committee recommended a non-quantitative approach (non-linear, RfD approach) since 

mode of action studies are available that demonstrate that the thyroid tumors are due to a 

thyroid-pituitary imbalance, and also since pendimethalin was shown to be nonmutagenic in mammalian somatic cells and germ cells. 

4.2.2.3 Mammals: Open Literature Studies 

 No studies more sensitive than the submitted mammalian studies were found in the 

ECOTOX database. 

4.2.3 Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of 

pendimethalin to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting 

from exposure to pendimethalin may also indirectly affect the CRLF via reduction in 

available food. 

4.2.3.1 Terrestrial Invertebrates: Acute Exposure (Mortality) Studies 

Only one acute oral bee toxicity study (MRID ) was submitted to the Agency. This study 

was authored by Dr. Atkins in 1974. The species tested is the honey bee, Apis mellifera. 

The LD50 acute oral is greater than 49.8 μg/bee with no mortality observed at the highest 

tested dose. Pendimethalin is classified as practically non-toxic to the honey bee. 

70
4.2.3.2 Terrestrial Invertebrates: Open Literature Studies 

No studies more sensitive than the submitted bee study were found in the ECOTOX 

database. 

4.2.4 Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for pendimethalin to affect 

riparian zone and upland vegetation within the action area for the CRLF. Impacts to 

riparian and upland (i.e., grassland, woodland) vegetation may 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 in of 

upland and riparian habitat that provides shelter, foraging, predator avoidance and 

dispersal for juvenile and adult CRLFs. 

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 pendimethalin, 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. 

The results of the Tier II seedling emergence and vegetative vigor toxicity tests on nontarget plants are summarized below in Table 4-55. 

Table 4-5 Non-target Terrestrial Plant Seedling Emergence and Vegetative Vigor 

Toxicity (Tier II) Data 

Crop Species NOAEC 

(lb ai/A) 

EC25 

(lb ai/A) 

Most sensitive 

parameter 

Seedling Emergence 

Monocots Oat, Avena sativa 0.25 1.0 Plant height 

71
Crop Species NOAEC 

(lb ai/A) 

EC25 

(lb ai/A) 

Most sensitive 

parameter 

Ryegrass, Lolium perenne 0.065 1 0.01 Dry weight 

Corn, Zea mays 0.5 0.68 Plant height 

Onion, Allium cepa 0.06 0.08 Dry weight 

Dicots Soybean, Glycine max 2.0 4.7 Dry weight 

Lettuce, Lactuca sativa 0.063 0.09 Dry weight 

Radish, Rhaphanus sativus 0.13 0.86 Plant height 

Tomato, Lycopersicon 

esculentum 0.13 0.2 

Dry weight 

Cucumber, Cucumis sativus 0.25 2.4 Plant height 

Cabbage, Brassica oleracea 0.25 0.44 Plant height 

Vegetative Vigor 

Monocots Oat, Avena sativa 0.5 0.78 Dry weight

 Ryegrass, Lolium perenne 0.0008 0.034 Dry weight

 Corn, Zea mays 2.0 2.8 Plant height

 Onion, Allium cepa 0.50 0.56 Plant height 

Dicots Soybean, Glycine max 0.13 0.27 Dry weight

 Lettuce, Lactuca sativa 0.003 0.10 Dry weight

 Radish, Rhaphanus sativus >4.0 >4.0 n/a 

 Tomato, Lycopersicon 

esculentum 0.13 0.5 

Dry weight 

 Cucumber, Cucumis sativus >4.0 >4.0 n/a 

 Cabbage, Brassica oleracea 2.0 4.8 Dry weight 

1 EC05 was used in lieu of NOEAC since study was unable to determine NOAEC value 

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

Endangered Species Levels of Concern 

The Agency uses the probit dose response relationship as a tool for providing additional 

information on the potential for acute direct effects to individual listed species and 

aquatic animals that may indirectly affect the listed species of concern (U.S. EPA, 2004). 

As part of the risk characterization, an interpretation of acute RQ for listed species is 

discussed. This interpretation is presented in terms of the chance of an individual event 

(i.e., mortality or immobilization) should exposure at the EEC actually occur for a species 

with sensitivity to pendimethalin 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 

72
estimate) as the slope parameter for the spreadsheet. In addition, the acute RQ is entered 

as the desired threshold. 

4.4 Incident Database Review 

EPA maintains an incident database system (Ecological Incident Information System or 

EIIS) to track and evaluate accidental kills associated with pesticide use. The likelihood 

that a particular pesticide caused the incident is classified as “highly probable”, 

“probable”, “possible”, or “unlikely”, based on the information contained in the incident 

report. 

A review of the EIIS database for ecological incidents involving pendimethalin was 

completed on April 16, 2009. The results of this review for terrestrial, plant, and aquatic 

incidents are discussed below in Sections 4.4.1 through 4.4.3, respectively. A complete 

list of the incidents involving pendimethalin including associated uncertainties is 

included as Appendix H. 

In the years since EPA has maintained the database, a total of 69 incidents were reported, 

of which 65 related to terrestrial plants. 

4.4.1 Terrestrial Incidents 

There were two incidents involving pendimethalin with alleged adverse effects to 

terrestrial animals (American Robin, Rock dove, Ground squirrel, and an “unknown” 

bird.) 

In a 1998 Kansas incident (I007495-001) twelve robins and rock doves were killed. It is 

not known as to what site the pendimethalin was applied but was applied nearby. No 

toxicological analysis was reportedly taken; therefore no confirmation was made as to 

whether pendimethalin was responsible for the deaths of the 12 birds. Pendimethalin is 

slightly toxic on an acute oral and subacute dietary basis to birds. It is uncertain that 

pendimethalin was responsible for the bird deaths. 

There was also an incomplete incident report (I002343-001) involving surface application 

of pendimethalin and fertilizer granules to turf with mortality of unknown numbers of 

ground squirrel and an “unknown” bird. The corpses were analyzed for 

organophosphates (OP) but not for pendimethalin. No OPs were found. Pendimethalin is 

slightly toxic on an acute oral and subacute dietary basis to birds and small mammals. It 

is uncertain and not likely that pendimethalin is responsible for the deaths of the squirrels 

and the bird. 

73
4.4.2 Plant Incidents 

Most of these plant incidents related to the treated crop itself, including corn, soybean, 

potatoes, lawns, azalea bushes, and peanuts. Some of the incidents occur from spray drift 

and runoff. Of the 65 reported alleged incidents involving pendimethalin, four of the 

incidents were from spray drift to nearby non-target plants. The severity of these plant 

incidents ranged from plant damage (most incidents) to mortality. The following 

reported alleged incidents are from spray drift or runoff from treated sites to non-target 

plants: 

I014702-053 - In Wisconsin, pendimethalin was applied with clomazone to soybean. 

The two herbicides drifted over to raspberry plants and trees causing some damage to 

these plants. The damage symptoms observed were more consistent with clomazone 

damage than pendimethalin. Pendimethalin may have contributed to plant damage but 

the symptoms of plant damage to non-target plants are more consistent with clomazone. 

It is probable that pendimethalin may be involved with non-target plant damage. 

B000621-001 – In 1986 in Illinois, pendimethalin was applied with clomazone to 

soybean field. The two herbicides drifted over to damage nearby lawns, fruit trees, and 

vegetable gardens. The damage symptoms observed were more consistent with 

clomazone damage than pendimethalin. Pendimethalin may have contributed to plant 

damage but the symptoms of plant damage to non-target plants are more consistent with 

clomazone. It is probable that pendimethalin may be involved with non-target plant 

damage. 

I0009262-054 – In 1996 in California, a greenhouse floor was sprayed with 

pendimethalin. Vapor from the spray drifted onto potted azaleas plants sitting on the 

benches above the floor. 172,636 potted azaleas were unmarketable. It is likely that 

pendimethalin contributed to damage to azaleas from volatility. 

I013587-052 – In 1999 in Washington State, pendimethalin was applied to residential 

ornamentals. Large tree limbs were observed dying. Pendimethalin was analyzed and 

found present in the foliage of the trees. 

4.4.3 Aquatic Incidents 

There are two aquatic incidents reported involving mortality of 300 and 996 fish, 

respectively. The fish involved were bass and bluegill (300 fish) and minnow (996 fish), 

but the incident reports considered pendimethalin involvement “unlikely.” 

In 1998 in Ohio, (incident I007677-001) pendimethalin and Lorsban were applied to a 

corn field. One inch of rain fell shortly afterwards and runoff from the corn field ran into 

a pond. The distance from the field to the pond is from 14-70 feet. Approximately 300 

bass and bluegills were later found dead. Fish and water samples were taken but findings 

74
were not submitted. Lorsban (chlorpyrifos) is much more toxic to aquatic animals than 

pendimethalin by three orders of magnitude and is more soluble in water. It is more 

likely that Lorsban is the cause of the mortality to the fish. Pendimethalin is not expected 

to kill fish outright from runoff from one inch of rain on a corn field based on its EEC 

and toxicity. 

In 1993 in Minnesota, an incident involving the deaths of 996 minnows was reported. 

There is very little information in this report which indicated that “250 -300 gallons 

yellow weed killer, Prowl” contributed to the deaths of the minnows in the Schwerin 

Creek. This information comes from a Minnesota Department of Natural Resources 

report showing a table of various spill incidents involving pesticides reported to 

Minnesota. It is likely that a significant spill of concentrated pendimethalin is the cause 

of deaths of the minnow in the creek. 

75
5.0 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 

pendimethalin risk characterization provides an estimation (Section 5.1) and a description 

(Section 5.2) of the likelihood of adverse effects; articulates risk assessment assumptions, 

limitations, and uncertainties; and synthesizes an overall conclusion regarding the 

likelihood of adverse effects to the CRLF or its designated critical habitat (i.e., “no 

effect,” “likely to adversely affect,” or “may affect, but not likely to adversely affect”). 

5.1 Risk Estimation 

Risk is estimated by calculating the ratio of exposure to toxicity. This ratio is the risk 

quotient (RQ), which is then compared to pre-established acute and chronic levels of 

concern (LOCs) for each category evaluated (Appendix C). For acute exposures to the 

CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC 

is 0.05. For acute exposures to the CRLF and mammals, the LOC is 0.1. The LOC for 

chronic exposures to CRLF and its prey, as well as exposures to plants is 1.0. 

Risk to the aquatic-phase CRLF is estimated by calculating the ratio of exposure to 

toxicity using 1-in-10 year EECs based on the label-recommended pendimethalin usage 

scenarios summarized in Table 3-3 and the appropriate aquatic toxicity endpoint from 

Table 4-1. Risks to the terrestrial-phase CRLF and its prey (e.g. terrestrial insects, small 

mammals and terrestrial-phase frogs) are estimated based on exposures resulting from 

applications of pendimethalin (Tables 3.5 – 3.6) and the appropriate toxicity endpoint 

from Table 4.3. Exposures are also derived for terrestrial plants, as discussed in Section 

3.7 and toxicity summarized in Section 4.2.4, based on the highest application rates of 

pendimethalin use within the action area. 

5.1.1 Exposures in the Aquatic Habitat 

5.1.1.1 Direct Effects to Aquatic-Phase CRLF 

Direct effects to the aquatic-phase CRLF are based on peak EECs in the standard pond 

and the lowest acute toxicity value for freshwater fish. In order to assess direct chronic 

risks to the CRLF, 60-day EECs and the lowest chronic toxicity value for freshwater fish 

are used. Based on exceedances of the acute LOC (0.05) for listed species, 

pendimethalin may directly affect the aquatic-phase of the CRLF. The results of the RQ 

calculations are in Table 5.1 below. 

76
Table 5-1 Summary of Direct Effect RQsa

 for the Aquatic-phase CRLF 

EEC Scenario Peak 

EEC 

(μg/L) b 

60-day 

EEC 

(μg/L) b 

Acute 

RQ 

Chronic 

RQ 

Probability of 

Individual 

Effect at ES 

LOC 

Probability of 

Individual 

Effect at RQ 

CAForestryRLF 

(forest trees and 

Christmas tree 

plantations) 

16.6 4.9 0.12 0.8 e 1 in 5.85E+04 

(1 in 30.5 

 to 1 in 1.73E+16 )c 

CArowcropRLF g 11.9 2.9 0.09 <1.0 e 1 in 7.91E+05 

(1 in 54.8 to 1 in 

4.10E+20 )c 

Rice model 

(Rice) 

48 Not 

available 

0.35 Not 

applicable 

1 in 49.8 

(1 in 5.53 to 1 in 

4.91E+04 )c 

Cagrapes_WirrigSTD, 

CAwinegrapesRLF 

(grapes) 

11.4 1.8 0.08 <1.0 e 

4.18E+08 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

CAalmond_WirrigSTDh 11.7 2.3 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

CATurfRLF j 11.5 1.9 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

CAalfalfa_WirrigOP 

(alfalfa, lupine) 

11.3 2.0 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

Cafruit_WirrigSTD i 11.2 1.9 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

CAOliveRLF 

(olives) 

11.2 1.7 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

Cacitrus_WirrigSTD K 11.1 1.5 0.08 <1.0 e 1 in 2.51E+06 

(1 in 70.8 to 1 in 

3.64E+22 )c 

CAcornOP 

(corn and sunflower) 

6.8 d 1.5 0.05 <1.0 e 1 in 4.18E+08 

(1 in 216 to 1 in 

1.75E+31 )c 

a

 RQs associated with acute and chronic direct toxicity to the CRLF are also used to assess potential 

indirect effects to the CRLF based on a reduction in freshwater fish and frogs as food items. The toxicity 

value used as surrogate species is rainbow trout LC50 of 138 ppb (μg/L) and Fathead minnow NOAEC of 

6.3 ppb (μg/L). b

 The highest EEC based on foliar use of pendimethalin on forestry at 4.0 lb ai/A (see Table 3-3). c

 A probit slope value for the acute rainbow trout toxicity test is not available; therefore, the effect 

probability was calculated based on a default slope assumption of 4.5 with upper and lower 95% 

confidence intervals of 2 and 9 (Urban and Cook, 1986). Not calculated for chronic endpoints. d

 Peak EEC below 6.8 ppb will result in RQ < acute endangered species LOC of 0.05. e

 RQ < chronic LOC of 1.0. g

 This scenario represents artichoke, asparagus, beans, carrot (including tops), garbanzos (including chick 

peas), legume vegetables, peanuts (unspecified), peas, and pepper. h This scenario represents almond, beech nut, brazil nut, butternut, cashew, chestnut, chinquapin, filbert 

(hazelnut), hickory nut, macadamia nut (bushnut), mayhaw (hawthorn), pecan, pistachio, walnut (English 

or black) i

 This scenario represents apple, apricot, cherry, crabapple, fig, loquat, nectarine, peach, pear, pepino

(melon pear), plum, pomegranate, prune, quince, and small fruits. j

 This scenario represents commercial/industrial lawns, golf course turf, ornamental lawns and turf,

ornamental sod farm (turf), recreation area lawns, and residential lawns. 

77
K This scenario represents calamondin, citron (citrus), citrus hybrids other than tangelo, grapefruit, 

kumquat, lemon, lime, orange, pummelo (shaddock), and tangelo. 

5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey 

(non-vascular aquatic plants, aquatic invertebrates, fish, and 

frogs) 

a) Non-vascular Aquatic Plants 

Indirect effects of pendimethalin to the aquatic-phase CRLF (tadpoles) via reduction in 

non-vascular aquatic plants in its diet are based on peak EECs from the standard pond 

and the lowest toxicity value (EC50) for aquatic non-vascular plants. Based on 

exceedances of the acute LOC (1.0) for aquatic plants, pendimethalin is likely to 

indirectly affect the CRLF via reduction in non-vascular plants. The results of the RQ 

calculations are in Table 5.2 below. 

Table 5-2 Summary of RQs Used to Estimate Indirect Effects to the CRLF via 

Effects to Non-Vascular Aquatic Plants a

 (diet of CRLF in tadpole life stage and 

habitat of aquatic-phase CRLF) 

Uses Application rate (lb 

ai/A) and type 

Peak EEC 

(μg/L) 

Indirect effects RQ* 

 (food and habitat) 

Rice model (Rice) 1 lb ai/A (aerial) 48 9.2 

CAForestryRLF (forest trees and 

Christmas tree plantations) 

4 lb ai/A (aerial) 16.6 b 

3.2 

CArowcropRLF e 4 lb ai/A (aerial) 11.9 2.3 

CAalmond_WirrigSTDf 4 lb ai/A (aerial) 11.7 2.3 

CATurfRLF h 4 lb ai/A (aerial) 11.5 2.2 

Cagrapes_WirrigSTD, 

CAwinegrapesRLF (grapes) 

4 lb ai/A (aerial) 11.4 2.2 

CAalmond_WirrigSTD f 6 lb ai/A (ground) 6.6 1.3 

CAalfalfa_WirrigOP (alfalfa, lupine) 4 lb ai/A (aerial) 11.3 2.2 

Cafruit_WirrigSTD g 4 lb ai/A (aerial) 11.2 2.2 

CAOliveRLF (olives) 4 lb ai/A (aerial) 11.2 2.2 

Cacitrus_WirrigSTD i 4 lb ai/A (aerial) 11.1 2.1 

CAcornOP (corn and sunflower) 2 lb ai/A (aerial) 6.8 1.3 

CAwheatRLF (sorghum and wheat) 1.5 lb ai/A (aerial) 6.1 1.2 

CAcotton_WirrigSTD (cotton) 2 lb ai/A (aerial) 5.8 1.1 

CAOnion_WirrigSTD (onion and 

shallot) 2 lb ai/A (aerial) 5.6 c 1.1 

* LOC exceedances (RQ > 1) are bolded and shaded. RQ = use-specific peak EEC/ marine diatom 

(Skeletonema costatum) EC50 of 5.2 ppb. a

 The toxicity value used as surrogate species is marine diatom (Skeletonema costatum) EC50 of 5.2 ppb

(μg/L). b

 The highest EEC based on foliar use of pendimethalin is on forestry at 4.0 lb ai/A (see Table 3-3). c

 Peak EEC below 5.2 ppb will result in RQ < acute LOC of 1.0.

78 
e

 This scenario represents artichoke, asparagus, beans, carrot (including tops), garbanzos (including chick 

peas), legume vegetables, peanuts (unspecified), peas, and pepper. f This scenario represents almond, beech nut, brazil nut, butternut, cashew, chestnut, chinquapin, filbert

(hazelnut), hickory nut, macadamia nut (bushnut), mayhaw (hawthorn), pecan, pistachio, walnut (English 

or black) g

 This scenario represents apple, apricot, cherry, crabapple, fig, loquat, nectarine, peach, pear, pepino

(melon pear), plum, pomegranate, prune, quince, and small fruits. h This scenario represents commercial/industrial lawns, golf course turf, ornamental lawns and turf,

ornamental sod farm (turf), recreation area lawns, and residential lawns. i This scenario represents calamondin, citron (citrus), citrus hybrids other than tangelo, grapefruit, kumquat, 

lemon, lime, orange, pummelo (shaddock), and tangelo. 

b) Aquatic Invertebrates 

Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in 

aquatic habitats are based on peak EECs in the standard pond and the lowest acute 

toxicity value for freshwater invertebrates. For chronic risks, 21-day EECs and the lowest 

chronic toxicity value for invertebrates are used to derive RQs. A summary of the acute 

and chronic RQ values for exposure to aquatic invertebrates (as prey items of aquaticphase CRLFs) is provided in Table 5-3. Based on the acute LOC (0.5) and chronic LOC 

(1.0) not being exceeded, pendimethalin has no effect, indirectly on the CRLF based on 

effects to aquatic invertebrates. The table shows only the use sites that exceed the LOC 

plus the highest EEC that does not exceed the LOC. 

Table 5-3 Summary of Acute and Chronic RQsa

 Used to Estimate Indirect Effects to 

the CRLF via Direct Effects on Aquatic Invertebrates as Dietary Food Items (prey 

of CRLF juveniles and adults in aquatic habitats) 

EEC Scenario Peak 

EEC 

(μg/L)b 

21-day 

EEC 

(μg/L)b 

Acute 

RQ 

Chronic 

RQ 

Probability of 

Individual 

Effect at ES 

LOC 

Probability of 

Individual Effect at 

RQ 

Rice model 

(Rice) 

48 Not 

available 

0.17 f Not 

applicable 

1 in 3,740 (1 in 16.2 to 

1 in 4.62E+11)c 

CAForestryRLF 

(Forest trees and 

Christmas tree 

plantations) 

16.6 6.4 0.06 b 0.4 e 1 in 5.22E+07 (1 in 

138 to 1 in 5.04E+27)c 

CArowcropRLF h 

( artichoke/asparagus) 

11.9 4.0 0.04 f <1.0 e Not applicable 

a

 RQs associated with acute and chronic direct toxicity to the CRLF are also used to assess potential 

indirect effects to the CRLF based on a reduction in freshwater fish and frogs as food items. The toxicity 

value used as surrogate species is Daphnia magna EC50 of 280 ppb (μg/L) and Daphnia magna NOAEC of 

14.5 ppb (μg/L). b

 The highest EEC based on foliar use of pendimethalin on forestry and Christmas trees at 4.0 lb ai/A (see 

Table 3-3). c

 A probit slope value for the acute Daphnia magna toxicity test is not available; therefore, the effect

probability was calculated based on a default slope assumption of 4.5 with upper and lower 95% 

confidence intervals of 2 and 9 (Urban and Cook, 1986). Not calculated for chronic endpoints. d

 Peak EEC below 5.2 ppb will result in RQ < acute endangered species LOC of 0.05. e

 RQ < chronic LOC of 1.0. f RQ < acute LOC of 0.5.

79
h

 This scenario represents artichoke, asparagus, beans, carrot (including tops), garbanzos (including chick 

peas), legume vegetables, peanuts (unspecified), peas, and pepper. 

c) 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 (Table 5.1) are used to 

assess potential indirect effects to the CRLF based on a reduction in freshwater fish and 

frogs as food items. Based on listed species LOC exceedances using the surrogate 

rainbow trout acute LC50 toxicity endpoint and the fathead minnow chronic NOAEC 

endpoint, pendimethalin may indirectly affect the CRLF via reduction in freshwater fish 

and frogs as food items. 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or 

Primary Productivity (Freshwater Aquatic Plants) 

Indirect effects to the CRLF via direct toxicity to aquatic plants are estimated using the 

most sensitive non-vascular and vascular plant toxicity endpoints. Because there are no 

obligate relationships between the CRLF and any aquatic plant species, the most sensitive 

EC50 values, rather than NOAEC values, were used to derive RQs. Based on vascular 

aquatic plant LOC (1.0) exceedances, pendimethalin may indirectly affect the CRLF via 

reduction in vascular plants from uses on forestry and Christmas tree plantations and 

from rice use. Table 5-4 shows only the use sites that exceed the LOC plus the highest 

EEC that does not exceed the LOC. 

Table 5-4 Summary of RQs Used to Estimate Indirect Effects to the CRLF via 

Effects to Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a 

Uses Application rate (lb 

ai/A) and type 

Peak EEC 

(μg/L) 

Indirect effects RQ* 

 (food and habitat) 

Rice model (Rice) 1 lb ai/A (aerial) 48 3.8 

CAForestryRLF (forest trees and 

Christmas tree plantations) 

4 lb ai/A (aerial) 16.6 1.3 

a

 RQs used to estimate indirect effects to the CRLF via toxicity to vascular aquatic plants are summarized in 

Table 5.2. The toxicity value used as surrogate species is duckweed (Lemna gibba) EC50 of 12.5 ppb (μg/L). b

 The highest EEC based on foliar use of pendimethalin on forestry at 4.0 lb ai/A (see Table 3-3). c

 Peak EEC below 12.5 ppb will result in RQ < acute LOC of 1.0. 

* = LOC exceedances (RQ > 1) are bolded. RQ = use-specific peak EEC / duckweed EC50 of 12.5 ppb. 

5.1.2 Exposures in the Terrestrial Habitat 

5.1.2.1 Direct Effects to Terrestrial-phase CRLF 

80 
Potential direct acute effects to the terrestrial-phase CRLF are derived by considering 

dose- and dietary-based EECs modeled in T-REX for a small bird (20 g) consuming 

small invertebrates (Table 3.5) and acute oral and subacute dietary toxicity endpoints for 

avian species. 

Potential direct chronic effects of pendimethalin to the terrestrial-phase CRLF are derived 

by considering dietary-based exposures modeled in T-REX for a small bird (20g) 

consuming small invertebrates. Chronic effects are estimated using the lowest available 

toxicity data for birds. EECs are divided by toxicity values to estimate chronic dietarybased RQs. 

Based on RQ exceedances of listed species acute LOC (0.1) and chronic LOC (1.0) from 

all use sites, pendimethalin may directly affect the terrestrial-phase of the CRLF. 

Table 5-5 Summary of Acute RQs* Used to Estimate Direct Effects to the 

Terrestrial-phase CRLF 

Use Site Application rate 

(lbs ai/A) 

Dietary-based 

Acute RQ1 

Dosebased 

Acute RQ1 

Probability of Individual Effect at 

RQ 

walnut (english/black), tangerines, tangelo, small fruits, 

pummelo (shaddock), pomegranate, pistachio, pecan, orange, 

macadamia nut (bushnut), lime, lemon, kumquat, hickory nut, 

grapefruit, filbert (hazelnut), citrus hybrids other than tangelo, 

citron (citrus), chinquapin, chestnut, cashew, calamondin, 

butternut, brazil nut, beech nut, almond, grapes. 

6.0 0.2 1.25 

Dose-based: 

1 in 1.5 (1 in 1.73 to 1 in 1.24 )2 

Dietary-based: 

1 in 1.5 (1 in 1.73 to 1 in 1.24 )2 

Prune, plum, pear, peach, olive, nectarine, fig, cherry, apricot, 

apple, ornamental and/or shade trees, nonagricultural rights-ofway, fencerows, hedgerows, mulch, industrial areas (outdoor), 

forest trees (all or unspecified), Christmas tree plantations, 

recreation area lawns, loquat, shelterbelt plantings, quince, 

mayhaw, crabapple, alfalfa 

4.0 0.1 0.8 

Dose-based: 

1 in 3.02 (1 in 2.36 to 1 in 5.22 )2 

Dietary-based: 

1 in 2.94E+05 (1 in 44 to 8.86E+18)2 

Asparagus, artichoke 3.9 0.1 0.8 

Dose-based: 

1 in 3.02 (1 in 2.36 to 1 in 5.22 )2 

Dietary-based: 

1 in 2.94E+05 (1 in 44 to 8.86E+18)2 

golf course turf, airports/landing fields 3.5 0.1 0.7 

Dose-based: 

1 in 4.12 (1 in 2.64 to 1 in 12.2 )2 

Dietary-based: 

1 in 2.94E+05 (1 in 44 to 8.86E+18)2 

residential lawns 3.0 0.1 0.6 

Dose-based: 

1 in 1.5 (1 in 1.73 to 1 in 1.24 )2 

Dietary-based: 

1 in 2.94E+05 (1 in 44 to 8.86E+18)2 

Shallot, onion, corn (sweet, pop, field) 2.0 <0.1 0.4 

Dose-based: 

1 in 27.3 (1 in 4.69 to 1 in 5.85E+03)2 

Dietary-based: n/a 

Carrot 1.9 <0.1 0.4 

Dose-based: 

1 in 27.3 (1 in 4.69 to 1 in 5.85E+03)2 

Dietary-based: n/a 

Sunflower 1.73 <0.1 0.4 

Dose-based: 

1 in 27.3 (1 in 4.69 to 1 in 5.85E+03)2 

Dietary-based: n/a 

Tomato, tomatillo, tobacco, sorghum, potato (white/Irish), 

lupine (grain), garlic, garbanzos (including chick peas) beans, 

succulent (snap or lima) beans, dried-type beans, cotton, 

1.485 <0.1 0.3 

Dose-based: 

1 in 107 (1 in 6.76 to 7.91E+05)2 

Dietary-based: n/a 

81
Use Site Application rate 

(lbs ai/A) 

Dietary-based 

Acute RQ1 

Dosebased 

Acute RQ1 

Probability of Individual Effect at 

RQ 

cowpea (southern pea, black-eyed bean), 

Wheat, pepper, pepino (melon pear), groundcherry, eggplant 1.425 <0.1 0.3 

Dose-based: 

1 in 107 (1 in 6.76 to 7.91E+05)2 

Dietary-based: n/a 

Strawberry 

1.485 (2 

applications w/ 

30-day interval) 

<0.1 0.5 

Dose-based: 

1 in 15.5 (1 in 4 to 1 in 832)2 

Dietary-based: n/a 

Legume vegetables 1.24 <0.1 0.3 

Dose-based: 

1 in 107 (1 in 6.76 to 7.91E+05)2 

Dietary-based: n/a 

Rice, mustard cabbage (gai choy, pak-choi), kohlrabi, 

cauliflower, cabbage (Chinese), cabbage, brussels sprouts, 

broccoli (Chinese), broccoli, Brassica (head and stem) 

vegetables 

1.0 <0.1 0.2 

Dose-based: 

1.21E+03 (1 in 12.3 to 6.33E+09)2 

Dietary-based: n/a 

* = LOC exceedances (acute RQ > 0.1) are bolded and shaded. 1

 Based on mallard LD50 of 1421 mg/kg-bw and bobwhite LC50 of 4187 ppm. 2

 A probit slope value for the acute mallard duck and bobwhite quail toxicity test is not available; therefore, the effect probability was calculated based on a 

default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). Not calculated for chronic endpoints. 

Table 5-6 Summary of Chronic RQs* Used to Estimate Direct Effects to the 

Terrestrial-phase CRLF 

Use Site Application rate 

(lbs ai/A) 

Dietary-based Chronic 

RQ1 

walnut (english/black), tangerines, tangelo, small 

fruits, pummelo (shaddock), pomegranate, pistachio, 

pecan, orange, macadamia nut (bushnut), lime, 

lemon, kumquat, hickory nut, grapefruit, filbert 

(hazelnut), citrus hybrids other than tangelo, citron 

(citrus), chinquapin, chestnut, cashew, calamondin, 

butternut, brazil nut, beech nut, almond, grapes. 

6.0 5.7 

Prune, plum, pear, peach, olive, nectarine, fig, 

cherry, apricot, apple, ornamental and/or shade trees, 

nonagricultural rights-of-way, fencerows, 

hedgerows, mulch, industrial areas (outdoor), forest 

trees (all or unspecified), Christmas tree plantations, 

recreation area lawns, loquat, shelterbelt plantings, 

quince, mayhaw, crabapple, alfalfa 

4.0 3.8 

Asparagus, artichoke 3.9 3.7 

golf course turf, airports/landing fields 3.5 3.35 

residential lawns 3.0 2.9 

Shallot, onion, corn (sweet, pop, field) 2.0 1.9 

Carrot 1.9 1.8 

Sunflower 1.73 1.7 

Tomato, tomatillo, tobacco, sorghum, potato 

(white/Irish), lupine (grain), garlic, garbanzos 

(including chick peas) beans, succulent (snap or 

lima) beans, dried-type beans, cotton, cowpea 

(southern pea, black-eyed bean), 

1.485 1.4 

Wheat, pepper, pepino (melon pear), groundcherry, 

eggplant 1.425 1.4 

82
Use Site Application rate 

(lbs ai/A) 

Dietary-based Chronic 

RQ1 

Strawberry 1.485 (2 applications w/ 30­

day interval) 2.1 

Legume vegetables 1.24 1.2 

Rice, mustard cabbage (gai choy, pak-choi), 

kohlrabi, cauliflower, cabbage (Chinese), cabbage, 

brussels sprouts, broccoli (Chinese), broccoli, 

Brassica (head and stem) vegetables 

1.0 1.0 

* = LOC exceedances (chronic RQ > 1) are bolded and shaded. 1 Based on bobwhite NOAEC of 141 ppm. 

5.1.2.2 Indirect Effects to Terrestrial-Phase CRLF via Reduction in 

Prey (terrestrial invertebrates, mammals, and frogs) 

5.1.2.2.1 Terrestrial Invertebrates 

In order to assess the risks of pendimethalin to terrestrial invertebrates, which are 

considered prey of CRLF in terrestrial habitats, the honey bee is used as a surrogate for 

terrestrial invertebrates. The toxicity value for terrestrial invertebrates is calculated by 

multiplying the lowest available acute contact LD50 of greater than 49.8 μg a.i./bee by 1 

bee/0.128g, which is based on the weight of an adult honey bee. EECs (μg a.i./g of bee) 

calculated by T-REX for small and large insects are divided by the calculated toxicity 

value for terrestrial invertebrates, which is >388.4 μg a.i./g of bee. Based on RQ ranges 

from 2.09 to 0.05, pendimethalin may indirectly affect the CRLF via reduction in 

terrestrial invertebrate prey items 

Table 5-7 Summary of RQs Used to Estimate Indirect Effects to the Terrestrialphase CRLF via Direct Effects on Terrestrial Invertebrates as Dietary Food Items 

Use Small Insect 

RQ* 

Large 

Insect RQ* 

Probability of 

Individual Effect at RQ 

of Small Insect 

walnut (english/black), tangerines, tangelo, small 

fruits, pummelo (shaddock), pomegranate, 

pistachio, pecan, orange, macadamia nut 

(bushnut), lime, lemon, kumquat, hickory nut, 

grapefruit, filbert (hazelnut), citrus hybrids other 

than tangelo, citron (citrus), chinquapin, chestnut, 

cashew, calamondin, butternut, brazil nut, beech 

nut, almond, grapes. (6.0 lb ai/A) 

2.09 0.23 1 in 1.08 (1 in 1.35 to 1 

in 1.0 )a 

Prune, plum, pear, peach, olive, nectarine, fig, 

cherry, apricot, apple, ornamental and/or shade 

trees, nonagricultural rights-of-way, fencerows, 

hedgerows, mulch, industrial areas (outdoor), 

forest trees (all or unspecified), Christmas tree 

plantations, recreation area lawns, loquat, 

shelterbelt plantings, quince, mayhaw, crabapple, 

alfalfa (4.0 lb ai/A) 

1.39 0.15 1 in 1.35 (1 in 1.11 to 1 

in 1.63 )a 

Asparagus, artichoke (3.9 lb ai/A) 1.36 0.15 1 in 1.38 (1 in 1.13 to 1 

in 1.65)a 

83
golf course turf, airports/landing fields (3.5 lb 

ai/A) 1.22 0.14 1 in 1.54 (1 in 1.28 to 1 

in 1.76)a 

residential lawns (3.0 lb ai/A) 1.04 0.12 1 in 1.88 (1 in 1.78 to 1 

in 1.95)a 

Shallot, onion, corn (sweet, pop, field) (2.0 lb 

ai/A) 0.70 0.08 1 in 4.12 (1 in 2.64 to 1 

in 12.2)a 

Carrot (1.9 lb ai/A) 0.66 0.07 1 in 4.8 (1 in 2.78 to 1 in 

19.2)a 

Sunflower (1.73 lb ai/A) 0.60 0.07 1 in 6.29 (1 in 3.04 to 1 

in 43.6)a 

Tomato, tomatillo, tobacco, sorghum, potato 

(white/Irish), lupine (grain), garlic, garbanzos 

(including chick peas) beans, succulent (snap or 

lima) beans, dried-type beans, cotton, cowpea 

(southern pea, black-eyed bean), (1.485 lb ai/A) 

0.51 0.06 1 in 10.6 (1 in 3.58 to 1 

in 236)a 

Wheat, pepper, pepino (melon pear), 

groundcherry, eggplant 

(1.425 lb ai/A) 

0.49 0.05 1 in 12.2 (1 in 3.73 to 1 

in 377)a 

Strawberry (1.485 lb ai/A (2 applications w/ 30­

day interval)) 0.88 0.10 1 in 249 (1 in 2.19 to 1 

in 3.24)a 

Legume vegetables (0.124 lb ai/A) 0.43 0.05 1 in 20.2 (1 in 4.31 to 1 

in 2,060)a 

Rice, mustard cabbage (gai choy, pak-choi), 

kohlrabi, cauliflower, cabbage (Chinese), 

cabbage, brussels sprouts, broccoli (Chinese), 

broccoli, Brassica (head and stem) vegetables 

(1.0 lb ai/A) 

0.35 0.04 1 in 49.8 (1 in 5.53 to 1 

in 4.91E+04)a 

* = LOC exceedances (RQ > 0.05) are bolded and shaded. Because a definitive endpoint was not 

established for terrestrial invertebrates (i.e., the value is greater than the highest test concentration), the 

RQ represents an upper bound value. 

a A probit slope value for the acute honey bee toxicity test is not available; therefore, the effect 

probability was calculated based on a default slope assumption of 4.5 with upper and lower 95% 

confidence intervals of 2 and 9 (Urban and Cook, 1986). Probability of Individual Effect at RQ of 0.05 

is 1 in 4.18E+08 (1 in 216 to 1 in 1.75E+31). 

5.1.2.2.2 Mammals 

Risks associated with ingestion of small mammals by large terrestrial-phase CRLFs are 

derived for dietary-based and dose-based exposures modeled in T-REX for a small 

mammal (15g) consuming short grass. Acute and chronic effects are estimated using the 

most sensitive mammalian toxicity data. EECs are divided by the toxicity value to 

estimate acute and chronic dose-based RQs as well as chronic dietary-based RQs. Based 

on RQ ranges from 25.0 to 6.2 for chronic effects, pendimethalin is likely to indirectly 

affect the CRLF via reduction in small mammal prey items. 

84
Table 5-8 Summary of Acute and Chronic RQs* Used to Estimate Indirect Effects 

to the Terrestrial-phase CRLF via Direct Effects on Small Mammals as Dietary 

Food Items 

Use 

(Application Rate) 

Chronic RQ Acute RQ 

Probability of Individual 

Effect at RQ 

Dosebased 

Chronic 

RQ1 

Dietarybased 

Chronic 

RQ2 

Dosebased 

Acute 

RQ3 

walnut (english/black), tangerines, tangelo, small 

fruits, pummelo (shaddock), pomegranate, 

pistachio, pecan, orange, macadamia nut 

(bushnut), lime, lemon, kumquat, hickory nut, 

grapefruit, filbert (hazelnut), citrus hybrids other 

than tangelo, citron (citrus), chinquapin, 

chestnut, cashew, calamondin, butternut, brazil 

nut, beech nut, almond, grapes. (6.0 lb ai/A) 

25.0 2.9 0.6 

Dose-based: 

1 in 6.29 (1 in 1.73 to 1 in 1.24 )2 

Prune, plum, pear, peach, olive, nectarine, fig, 

cherry, apricot, apple, ornamental and/or shade 

trees, nonagricultural rights-of-way, fencerows, 

hedgerows, mulch, industrial areas (outdoor), 

forest trees (all or unspecified), Christmas tree 

plantations, recreation area lawns, loquat, 

shelterbelt plantings, quince, mayhaw, crabapple, 

alfalfa (4.0 lb ai/A) 

16.7 1.9 0.4 

Dose-based: 

1 in 27.3 (1 in 4.69 to 1 in 

5.85E+03)2 

Asparagus, artichoke (3.9 lb ai/A) 16.2 1.9 0.4 

Dose-based: 

1 in 27.3 (1 in 4.69 to 1 in 

5.85E+03)2 

golf course turf, airports/landing fields (3.5 lb 

ai/A) 14.6 1.7 0.35 

Dose-based: 

1 in 49.8 (1 in 5.53 to 1 in 

4.91E+04)2 

residential lawns (3.0 lb ai/A) 12.5 1.4 0.3 

Dose-based: 

1 in 107 (1 in 6.76 to 1 in 

7.91E+05)2 

Shallot, onion, corn (sweet, pop, field) (2.0 lb 

ai/A) 8.3 1.0 0.2 

Dose-based: 

1 in 1.21E+03 (1 in 12.3 to 1 

in 6.33E+09)2 

Carrot (1.9 lb ai/A) 7.9 0.9 0.2 

Dose-based: 

1 in 1.21E+03 (1 in 12.3 to 1 

in 6.33E+09)2 

Sunflower (1.73 lb ai/A) 7.2 0.8 0.2 

Dose-based: 

1 in 1.21E+03 (1 in 12.3 to 1 

in 6.33E+09)2 

Tomato, tomatillo, tobacco, sorghum, potato 

(white/Irish), lupine (grain), garlic, garbanzos 

(including chick peas) beans, succulent (snap or 

lima) beans, dried-type beans, cotton, cowpea 

(southern pea, black-eyed bean), (1.485 lb ai/A) 

6.2 0.7 0.2 

Dose-based: 

1 in 1.21E+03 (1 in 12.3 to 1 

in 6.33E+09)2 

Wheat, pepper, pepino (melon pear), 

groundcherry, eggplant 

(1.425 lb ai/A) 

5.9 0.7 0.1 1 in 2.94E+05 (1 in 44 to 1 in 

8.86E+18)2 

Strawberry (1.485 lb ai/A (2 applications w/ 30­

day interval)) 9.3 1.1 0.2 

Dose-based: 

1 in 1.21E+03 (1 in 12.3 to 1 

in 6.33E+09)2 

Legume vegetables (0.124 lb ai/A) 5.2 0.6 0.1 1 in 2.94E+05 (1 in 44 to 1 in 

8.86E+18)2 

Rice, mustard cabbage (gai choy, pak-choi), 

kohlrabi, cauliflower, cabbage (Chinese), 4.2 0.5 0.1 1 in 2.94E+05 (1 in 44 to 1 in 

8.86E+18)2 

85
Chronic RQ Acute RQ 

cabbage, brussels sprouts, broccoli (Chinese), 

broccoli, Brassica (head and stem) vegetables 

(1.0 lb ai/A) 

* = LOC exceedances (acute RQ > 0.1 and chronic RQ > 1) are bolded and shaded. 1

 Based on dose-based EEC and pendimethalin rat NOAEL = 25 mg/kg-diet/day (500 ppm). 

2 Based on dietary-based EEC and pendimethalin rat NOAEC = 25 mg/kg-diet/day (500 ppm). 

3 Based on dose-based EEC and pendimethalin rat acute oral LD50 = 1050 mg/kg-bw. 

5.1.2.2.3 Frogs 

An additional prey item of the adult terrestrial-phase CRLF is other species of frogs. In 

order to assess risks to these organisms, dietary-based and dose-based exposures modeled 

in T-REX for a small bird (20g) consuming small invertebrates are used. See Section 

5.1.2.1 and associated tables (Table 5-65 and 5.6) for results. Based on dose-based acute 

RQs the range from 1.25 to 0.2 and chronic RQs that range from 5.7 to 1.0, 

pendimethalin may indirectly affect the CRLF via reduction in frogs as prey items. 

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 direct effects on riparian and upland 

vegetation are assessed using RQs from terrestrial plant seedling emergence and 

vegetative vigor EC25 data as a screen. Based on LOC exceedances from all use sites, 

pendimethalin may indirectly affect the CRLF via reduction in terrestrial plants. 

Example output from TerrPlant v.1.2.2 is provided in Appendix F. 

Table 5-9 RQs* for Monocots Inhabiting Dry and Semi-Aquatic Areas Exposed to 

Pendimethalin via Runoff and Drift 

Use 

Application 

rate 

(lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Spray drift 

RQ 

Dry area 

RQ 

Semi-aquatic 

area RQ 

walnut (english/black), tangerines, 

tangelo, small fruits, pummelo 

(shaddock), pomegranate, pistachio, 

pecan, orange, macadamia nut 

(bushnut), lime, lemon, kumquat, 

hickory nut, grapefruit, filbert 

(hazelnut), citrus hybrids other than 

tangelo, citron (citrus), chinquapin, 

chestnut, cashew, calamondin, 

butternut, brazil nut, beech nut, 

almond, grapes. 

6.0 lb ai/A ground 1 6.0 12.0 66.0 

Prune, plum, pear, peach, olive, 

nectarine, fig, cherry, apricot, apple, 

ornamental and/or shade trees, 

nonagricultural rights-of-way, 

fencerows, hedgerows, mulch, 

4.0 lb ai/A ground 1 4.0 8.0 44.0 

86
Use 

Application 

rate 

(lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Spray drift 

RQ 

Dry area 

RQ 

Semi-aquatic 

area RQ 

industrial areas (outdoor), forest trees 

(all or unspecified), Christmas tree 

plantations, recreation area lawns, 

loquat, shelterbelt plantings, quince, 

mayhaw, crabapple, alfalfa 

aerial 5 20.0 24.0 60.0 

Asparagus, artichoke 3.9 lb ai/A ground 1 3.9 7.8 42.9 

aerial 5 19.5 23.4 58.5 

golf course turf, airports/landing fields 3.5 lb ai/A ground 1 3.5 7.0 38.5 

aerial 5 17.5 21.0 52.5 

residential lawns 3.0 lb ai/A ground 1 3.0 6.00 33.0 

aerial 5 15.0 18.0 43.0 

Shallot, onion, corn (sweet, pop, field) 2.0 lb ai/A ground 1 2.0 4.0 22.0 

aerial 5 10.0 12.0 30.0 

Carrot 

1.9 lb ai/A ground 1 1.9 3.8 20.9 

aerial 5 9.5 11.4 28.5 

Sunflower 

1.73 lb ai/A ground 1 1.7 3.5 19.0 

aerial 5 8.7 10.4 26.0 

Tomato, tomatillo, tobacco, sorghum, 

potato (white/Irish), lupine (grain), 

garlic, garbanzos (including chick 

peas) beans, succulent (snap or lima) 

beans, dried-type beans, cotton, 

cowpea (southern pea, black-eyed 

bean) 

1.485 lb ai/A 

ground 1 

1.5 3.0 16.3 

aerial 5 

7.4 8.9 22.3 

Wheat, pepper, pepino (melon pear), 

groundcherry, eggplant, strawberry 

1.425 lb ai/A ground 1 1.4 2.9 15.7 

aerial 5 7.12 8.6 21.4 

Legume vegetables 

1.24 lb ai/A ground 1 1.2 2.5 13.6 

aerial 5 6.2 7.4 18.6 

Rice, mustard cabbage (gai choy, pakchoi), kohlrabi, cauliflower, cabbage 

(Chinese), cabbage, brussels sprouts, 

broccoli (Chinese), broccoli, Brassica 

(head and stem) vegetables 

1.0 lb ai/A 

ground 1 1.0 2.0 11.0 

aerial 5 5.0 6.0 15.0 

* = LOC exceedances (RQ > 1) are bolded and shaded. 

Table 5-10 RQs* for Dicots Inhabiting Dry and Semi-Aquatic Areas Exposed to 

Pendimethalin via Runoff and Drift 

Use 

Application 

rate 

(lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Spray drift 

RQ 

Dry area 

RQ 

Semi-aquatic 

area RQ 

walnut (english/black), tangerines, 

tangelo, small fruits, pummelo 

(shaddock), pomegranate, pistachio, 

pecan, orange, macadamia nut 

(bushnut), lime, lemon, kumquat, 

hickory nut, grapefruit, filbert 

(hazelnut), citrus hybrids other than 

tangelo, citron (citrus), chinquapin, 

chestnut, cashew, calamondin, 

butternut, brazil nut, beech nut, 

almond, grapes. 

6.0 lb ai/A ground 1 

0.7 1.3 7.3 

87
Use 

Application 

rate 

(lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Spray drift 

RQ 

Dry area 

RQ 

Semi-aquatic 

area RQ 

Prune, plum, pear, peach, olive, 

nectarine, fig, cherry, apricot, apple, 

ornamental and/or shade trees, 

nonagricultural rights-of-way, 

fencerows, hedgerows, mulch, 

industrial areas (outdoor), forest trees 

(all or unspecified), Christmas tree 

plantations, recreation area lawns, 

loquat, shelterbelt plantings, quince, 

mayhaw, crabapple, alfalfa 

4.0 lb ai/A 

ground 1 

0.4 0.9 5.9 

aerial 5 

2.2 2.7 6.7 

Asparagus, artichoke 3.9 lb ai/A ground 1 0.4 0.9 4.8 

aerial 5 2.2 2.6 6.5 

golf course turf, airports/landing fields 3.5 lb ai/A ground 1 0.4 0.8 4.3 

aerial 5 1.9 2.3 5.8 

residential lawns 3.0 lb ai/A ground 1 0.3 0.7 3.7 

aerial 5 1.7 2.0 5.0 

Shallot, onion, corn (sweet, pop, field) 2.0 lb ai/A ground 1 0.2 0.4 2.4 

aerial 5 1.1 1.3 3.3 

Carrot 

1.9 lb ai/A ground 1 0.2 0.4 2.3 

aerial 5 1.1 1.3 3.2 

Sunflower 

1.73 lb ai/A ground 1 0.2 0.4 2.1 

aerial 5 1.0 1.2 2.9 

Tomato, tomatillo, tobacco, sorghum, 

potato (white/Irish), lupine (grain), 

garlic, garbanzos (including chick 

peas) beans, succulent (snap or lima) 

beans, dried-type beans, cotton, 

cowpea (southern pea, black-eyed 

bean) 

1.485 lb ai/A 

ground 1 

0.2 0.3 1.8 

aerial 5 

0.8 1.0 2.5 

Wheat, pepper, pepino (melon pear), 

groundcherry, eggplant, strawberry 

1.425 lb ai/A ground 1 0.2 0.3 1.7 

aerial 5 0.8 1.0 2.4 

Legume vegetables 

1.24 lb ai/A ground 1 0.1 0.3 1.5 

aerial 5 0.7 0.8 2.1 

Rice, mustard cabbage (gai choy, pakchoi), kohlrabi, cauliflower, cabbage 

(Chinese), cabbage, brussels sprouts, 

broccoli (Chinese), broccoli, Brassica 

(head and stem) vegetables 

1.0 lb ai/A 

ground 1 0.1 0.2 1.2 

aerial 5 

0.6 0.7 1.7 

* = LOC exceedances (RQ > 1) are bolded and shaded. 

5.1.3 Primary Constituent Elements of Designated Critical Habitat 

5.1.3.1 Aquatic-Phase (Aquatic Breeding Habitat and Aquatic NonBreeding Habitat) 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

88
• 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). 

Based on the risk estimation for potential effects to aquatic and/or terrestrial plants 

provided in Sections 5.1.1.2, 5.1.1.3, and 5.1.2.3, pendimethalin may 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.” To assess 

the impact of pendimethalin on this PCE (i.e., alteration of food sources), acute and 

chronic freshwater fish and invertebrate toxicity endpoints, as well as endpoints for 

aquatic non-vascular plants, are used as measures of effects. RQs for these endpoints 

were calculated in Sections 5.1.1.1 and 5.1.1.2. Based on RQ exceedances of LOC, 

pendimethalin may 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. 

5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat) 

The first two assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or dripline surrounding aquatic and riparian habitat that are comprised 

of grasslands, woodlands, and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance 

• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal 

habitat within designated units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites including both natural and 

altered sites which do not contain barriers to dispersal 

The risk estimation for terrestrial-phase PCEs of designated habitat related to potential 

effects on terrestrial plants is provided in Section 5.1.2.3. Based on exceedances of LOC 

for non-target terrestrial plants, all use sites of pendimethalin may affect the first and 

second terrestrial - phase PCEs. 

89
The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of pendimethalin 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 5.1.2.3. Based on RQ exceedances of LOC from all use sites, pendimethalin may 

affect the third terrestrial - phase PCE. 

The fourth terrestrial-phase PC is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.1.2.1. Based on RQ exceedances of LOC from all use sites, pendimethalin may 

affect the forth terrestrial - phase PCE. 

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. 

Based on the RQs presented in the Risk Estimation (Section 5.1) a preliminary effects 

determination is “may affect” for the CRLF and critical habitat. 

The direct or indirect effect LOCs are exceeded and 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 pendimethalin. A summary of the results of 

the risk estimation results are provided in Error! Not a valid bookmark selfreference.1 for direct and indirect effects to the CRLF and in Table 5-122 for the PCEs 

of designated critical habitat for the CRLF. 

Table 5-11 Risk Estimation Summary for Pendimethalin - 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 

Yes 

There are acute LOC exceedances for fish, the surrogate for the 

aquatic-phase CRLF. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects to 

food supply (i.e., freshwater 

invertebrates, non-vascular plants) 

Yes 

There are LOC exceedances for non-target aquatic non-vascular 

plants. There are no acute and chronic LOC exceedances for 

aquatic invertebrates. 

Indirect Effects 

Survival, growth, and reproduction Yes 

There are exceedances of the LOC for both vascular and nonvascular aquatic plants. 

90
Assessment Endpoint 

LOC 

Exceedances 

(Y/N) 

Description of Results of Risk Estimation 

of CRLF individuals via effects on 

habitat, cover, and/or primary 

productivity (i.e., aquatic plant 

community) 

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. 

Yes 

There are LOC exceedances for non-target aquatic vascular and 

terrestrial plants. Effects are possible for riparian vegetation. 

Terrestrial Phase 

(Juveniles and adults) 

Direct Effects 

Survival, growth, and reproduction 

of CRLF individuals via direct 

effects on terrestrial phase adults and 

juveniles 

Yes 

There are acute and chronic LOC exceedances to birds, the 

surrogate for the terrestrial-phase 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 

There are acute and chronic LOC exceedances to birds, the 

surrogate for the terrestrial-phase amphibians. 

There are LOC exceedances (>0.05) for terrestrial invertebrates. 

There are acute and chronic LOC exceedances for small 

mammals. 

Indirect Effects 

Survival, growth, and reproduction 

of CRLF individuals via effects on 

habitat (i.e., riparian vegetation) 

Yes 

There are LOC exceedances for non-target aquatic vascular and 

terrestrial plants. Effects are possible for riparian vegetation. 

Table 5-12 Risk Estimation Summary for Pendimethalin – PCEs of Designated 

Critical Habitat for the CRLF 

Assessment Endpoint Habitat Effects 

(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. 

Yes 

There are LOC exceedances for non-target aquatic 

vascular and terrestrial plants. Effects are possible to 

non-target terrestrial and aquatic vascular plants 

inhabiting riparian areas. 

91
Assessment Endpoint Habitat Effects 

(Y/N) Description of Results of Risk Estimation 

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. 

Yes 

There are LOC exceedances for non-target aquatic 

vascular and terrestrial plants. Alteration in water 

chemistry/quality is possible due to effects on 

aquatic/terrestrial plant communities in riparian 

areas. 

Alteration of other chemical characteristics 

necessary for normal growth and viability of 

CRLFs and their food source. 

Yes 

Due to potential effects on non-target aquatic plants 

and terrestrial riparian plant species, alteration in 

chemical characteristics of aquatic habitat (e.g. 

dissolved oxygen) may occur. 

Reduction and/or modification of aquaticbased food sources for pre-metamorphs 

(e.g., algae) 

Yes 

There are LOC exceedances for non-vascular aquatic 

plants (i.e. algae). 

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 

Yes 

There are LOC exceedances for non-target 

terrestrial plants. 

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 

Yes 

There are LOC exceedances for non-target 

terrestrial plants. These effects may alter plant 

communities necessary for dispersal habitat. 

Reduction and/or modification of food 

sources for terrestrial phase juveniles and 

adults 

Yes 

Acute and chronic effects are possible on amphibians 

and mammal that serve as prey for CRLFs 

Alteration of chemical characteristics 

necessary for normal growth and viability of 

juvenile and adult CRLFs and their food 

source. 

Yes 

Changes in aquatic vegetation and riparian 

communities may alter the chemical characteristic of 

CRLF habitat 

Following a “may affect” determination, additional information is considered to refine 

the potential for exposure at the predicted levels based on the life history characteristics 

(i.e., habitat range, feeding preferences, etc.) of the CRLF. Based on the best available 

information, the Agency uses the refined evaluation to distinguish those actions that 

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

adversely affect” the CRLF and its designated critical habitat. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the CRLF and its designated critical habitat include the following: 

92
• 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 considers life stages of the frog that are obligatory aquatic organisms, 

including eggs and larvae. It also considers submerged terrestrial-phase juveniles and 

adults, which spend a portion of their time in water bodies that may receive runoff and 

spray drift containing pendimethalin. 

This assessment used the most sensitive acute and chronic toxicity values across fish 

species tested since there was no amphibian data available. RQs were based on acute 

studies in rainbow trout (LC50 = 138 ug/L, MRID 00046291) and a chronic NOAEC in 

fathead minnow (NOAEC = 6.3 ug/L, MRID 00037940). 

The highest acute RQ was 0.12 which exceeds the Agency’s listed species acute LOC of 

0.05. Based on a default probit slope value of 4.5 for the rainbow trout with 95% 

confidence intervals of 2 to 9, the associated probability of an individual effect would be 

approximately 1 in 5.85E+04. 

The most sensitive chronic toxicity value is based on a life cycle study in fathead 

minnow. The chronic RQ (0.8) was based on a NOAEC of 6.3 ug/L which does not 

exceed the Agency’s chronic LOC of 1.0. The NOAEC is from an acceptable life-cycle 

fathead minnow study (MRID 00024377). Most sensitive endpoint was reduction in egg 

production that was observed at LOAEC of 44 ug/L 

93
Eleven aquatic EEC scenarios when compared to the acute toxicity for fish as a surrogate 

for aquatic-phase CRLF have acute RQs that exceeded the listed species LOC (0.05). 

None of the use sites has exceeded the chronic LOC for the CRLF. For the scenarios that 

exceeded the acute LOC for listed species, the probability of individual effect at acute 

RQ ranges from 1 in 5.85E+04 to 1 in 2.51E+06. The acute and chronic RQs are 

presented in Section 5.1.1.1. Direct effects to the aquatic-phase CRLF are based on peak 

EECs in the standard pond and the lowest acute toxicity value for freshwater fish. In 

order to assess direct chronic risks to the CRLF, 60-day EECs and the lowest chronic 

toxicity value for freshwater fish are used. Based on exceedances of the acute LOC for 

listed species (0.05), pendimethalin is likely to directly affect the aquatic-phase of the 

CRLF (LAA). 

Incident data provided to the Agency indicate that there are two alleged fish kill incidents 

that involve pendimethalin. However, these incidents do not show that pendimethalin 

registered use contributed to the alleged fish kill incidents. One of the alleged incidents 

is a spill in which an up to an estimated 300 gallons of undiluted Prowl was spilled into a 

creek. Another alleged incident involved pendimethalin applied as part of a mixture with 

Chlorpyrifos, which is highly toxic to fish. Further information on the pendimethalin 

incidents are provided in Appendix H. 

Monitoring data indicate that EECs modeled in PRZM-EXAMS are conservative. The 

highest detected pendimethalin is 0.679 ppb in San Joaquin-Tulare River Basin. The 

peak EECs range from 16.6 ppb to 6.8 ppb for those scenarios that exceed the listed 

species acute LOC (0.05). 

Based on the acute listed species LOC exceedances for CRLF and the widespread use 

sites that exceed the acute listed species LOC, pendimethalin is Likely to Adversely 

Affect (LAA) the aquatic-phase CRLF directly. 

5.2.1.2 Terrestrial-Phase CRLF 

Acute and chronic LOCs were exceeded for birds (surrogate for terrestrial-phase CRLF). 

Dose-based acute RQs ranged from 1.25 (6.0 lb ai/A application rate) to 0.2 (1.0 lb ai/A 

application rate). Dietary-based acute RQs ranged from 0.2 (6.0 lb ai/A application rate) 

to <0.1 (less than 2 lb ai/A application rate). The RQs that exceed the endangered species 

LOC (0.1) are associated with a probability of an individual effect of approximately 1 in 

1.5 to 1.21E+03 based on a default probit slope of 4.5. 

Avian chronic RQs ranged from 1.0 (1 lb ai/A) to 5.7 (6.0 lb ai/A). No treatment related 

effects were observed in the bobwhite quail and the LOAEL is greater than 1410 ppm. 

The mallard duck study (MRID 44907601) showed the parameter of 14-day survivor 

bodyweight was reduced at 1410 ppm (LOAEL). The NOAEL for the mallard study is 

410 ppm. No other treatment related effects were observed. Avian body weight 

reduction may not necessary be a good parameter to measure effects to frog due to 

different physiological and caloric requirements. Therefore it is uncertain as to whether 

94
there is a potential chronic risk to CRLF. Since the chronic LOC is exceeded for birds 

which are surrogates for CRLF, chronic risk can not be precluded to CRLF. Therefore, 

the direct effects determination from chronic effects to CRLF is LAA. 

Birds were used as surrogate species for terrestrial phase CRLFs. Terrestrial phase 

amphibians are poikilotherms, which mean that their body temperature varies with 

environmental temperature; while birds are homeotherms (temperature is regulated, 

constant, and largely independent of environmental temperatures). As a consequence, the 

caloric requirements of terrestrial phase amphibians are markedly lower than birds. 

Therefore, on a daily dietary intake basis, birds consume more food than terrestrial phase 

amphibians. This can be seen when comparing the caloric requirements for free living 

iguanid lizards (used in this case as a surrogate for terrestrial phase amphibians) to song 

birds (U.S. EPA, 1993): 

iguanid FMR (kcal/day)= 0.0535 (bw g)0.799 

passerine FMR (kcal/day) = 2.123 (bw g)0.749 

With relatively comparable slopes to the allometric functions, one can see that, given a 

comparable body weight, the free living metabolic rate (FMR) of birds can be 40 times 

higher than reptiles, though the requirement differences narrow with high body weights. 

Because the existing risk assessment process is driven by the dietary route of exposure, a 

finding of safety for birds, with their much higher feeding rates and, therefore, higher 

potential dietary exposure is reasoned to be protective of terrestrial phase amphibians. 

For this not to be the case, terrestrial phase amphibians would have to be 40 times more 

sensitive than birds for the differences in dietary uptake to be negated. However, existing 

dietary toxicity studies in amphibians are lacking. To quantify the potential differences 

in food intake between birds and terrestrial phase CRLF, food intake equations for the 

iguanid lizard replaced the food intake equation in T-REX for birds, and additional food 

items of the CRLF were evaluated. These functions were encompassed in a model called 

T-HERPS. T-HERPS is available at: 

http://www.epa.gov/oppefed1/models/terrestrial/index.htm. 

Results of this analysis are presented in Table 5.13 (use pattern (1.0 lb ai/A) with lowest 

EECs and RQs) through Table 5.16 (use pattern (6.0 lb ai/A) with highest EECs and 

RQs). 

95
Table 5-13 Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based 

Risk Quotients (1 lbs a.i./acre, 1 application) 

Size 

Class 

(grams) 

Adjusted 

LD50 

EECs and RQs 

Broadleaf 

Plants/ 

Small Insects 

Fruits/Pods/ 

Seeds/ 

Large Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ 

1.4 1421.00 5.24 0.00 0.58 0.00 N/A N/A N/A N/A N/A N/A 

37 1421.00 5.15 0.00 0.57 0.00 92.77 0.07 5.80 0.00 0.18 0.00 

238 1421.00 3.38 0.00 0.38 0.00 14.42 0.01 0.90 0.00 0.12 0.00 

Table 5-14 Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based 

Risk Quotients (1.24 lbs a.i./acre, 1 application) 

Size 

Class 

(grams) 

Adjusted 

LD50 

EECs and RQs 

Broadleaf 

Plants/ 

Small Insects 

Fruits/Pods/ 

Seeds/ 

Large Insects 

Small Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ 

1.4 1421.00 6.50 0.00 0.72 0.00 N/A N/A N/A N/A N/A N/A 

37 1421.00 6.39 0.00 0.71 0.00 115.03 0.08 7.19 0.01 0.22 0.00 

238 1421.00 4.19 0.00 0.47 0.00 17.88 0.01 1.12 0.00 0.15 0.00 

Table 5-15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based 

Risk Quotients (1.425 lbs a.i./acre, 1 application) 

Size 

Class 

(grams) 

Adjusted 

LD50 

EECs and RQs 

Broadleaf 

Plants/ 

Small Insects 

Fruits/Pods/ 

Seeds/ 

Large Insects 

Small Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ 

1.4 1421.00 7.79 0.01 0.87 0.00 N/A N/A N/A N/A N/A N/A 

37 1421.00 7.65 0.01 0.85 0.00 137.76 0.10 8.61 0.01 0.27 0.00 

238 1421.00 5.02 0.00 0.56 0.00 21.42 0.02 1.34 0.00 0.17 0.00 

Table 5-16. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based 

Risk Quotients (6 lbs a.i./Acre, 1 application) 

Size 

Class 

(grams) 

Adjusted 

LD50 

EECs and RQs 

Broadleaf 

Plants/ 

Small Insects 

Fruits/Pods/ 

Seeds/ 

Large Insects 

Small Herbivore 

Mammals 

Small 

Insectivore 

Mammal 

Small 

Amphibians 

EEC RQ EEC RQ EEC RQ EEC RQ EEC RQ 

96
1.4 1421.00 31.47 0.02 3.50 0.00 N/A N/A N/A N/A N/A N/A 

37 1421.00 30.93 0.02 3.44 0.00 556.59 0.39 34.79 0.02 1.07 0.00 

238 1421.00 20.27 0.01 2.25 0.00 86.53 0.06 5.41 0.00 0.70 0.00 

These data suggest that dietary exposures from consumption of insects by terrestrial 

phase CRLFs is likely to result in exposures that do not exceed LOCs. However, the 

endangered species LOC was exceeded for large terrestrial phase CRLFs that consume 

small herbivorous mammals in use sites that receive only 1.425 lbs a.i./acre or more of 

pendimethalin applications. The California mouse is a prey item of the CRLF, and this 

mouse reportedly eats grasses. However, some assumptions included in the assessment 

of small mammals as a food item are conservative because the CRLF eats a variety of 

food items and LOCs were not exceeded for CRLFs that eat insectivorous mammals. 

The RQs for a 37-gram and a 238-gram CRLF that consumes a small herbivore mammal 

that recently consumed contaminated short grass are found below in Table 5.17. 

Table 5-17 RQ Comparisons among Weight Class and Applications Rates for 

CRLF Consuming Small Herbivore Mammal That Recently Consumed 

Contaminated Short Grass 

Use site 

application rate CRLF weight class 

Dose-based RQ for 

small herbivorous 

mammals 

6.0 37 gram 0.39 

6.0 238 gram 0.06 

1.425 37 gram 0.10 

1.425 238 gram 0.02 

1.24 37 gram 0.08 

1.24 238 gram 0.01 

1.0 37 gram 0.07 

1.0 238 gram 0.01 

RQs remain above the endangered species LOC for CRLFs that consume herbivorous 

mammals for all uses except where applications are less than 1.425 lb ai/A. Refinements 

incorporating dietary habits of CRLFs did not preclude potential risks for CRLFs that 

consume herbivorous mammals. 

Current assessment methods of potential risks from chronic exposures for birds do not 

consider food intake levels. Therefore, T-HERPS does not quantify potential effects of 

reduced food intake of terrestrial phase CRLFs relative to birds for chronic risk 

assessments. However, chronic RQs would be expected to be reduced by a similar 

magnitude seen in the acute analysis. 

Incident data provided to the Agency indicate that there are two alleged terrestrial 

incidents in which birds and squirrels were killed. In both of these incidents there was no 

chemical analysis for pendimethalin made of the corpses. Therefore, the allegation that 

pendimethalin was responsible for the deaths of the birds and squirrels was not confirmed 

but yet pendimethalin can not be precluded. 

97
Acute and chronic LOCs were exceeded for birds (as surrogate for CRLF). The available 

toxicity data suggest that amphibians are less sensitive than birds to pendimethalin, and 

considering factors such as lower food intake of terrestrial phase amphibians relative to 

birds reduces EECs and RQs, but does not reduce RQs to levels that are below LOCs. 

Potential effects to CRLFs that consume herbivorous mammals cannot be precluded. 

Based on the weight-of-evidence, there is a potential direct impact to the terrestrial-phase 

of the CRLF based on the avian acute and chronic toxicity endpoints as a surrogate for 

terrestrial-phase CRLF. The effects determination of likely to adversely affect (LAA) 

terrestrial-phase CRLF from direct effects is based on acute and chronic listed species 

LOC exceedances for birds. 

5.2.2 Indirect Effects (via Reductions in Prey Base) 

5.2.2.1 Algae (non-vascular plants) 

As discussed in Section 2.5.3, the diet of CRLF tadpoles is composed primarily of 

unicellular aquatic plants (i.e., algae and diatoms) and detritus. Because the aquatic 

nonvascular plant RQs exceed the LOC for 15 out of 25 pendimethalin use sites, 

pendimethalin is considered likely to indirectly affect the aquatic-phase CRLF via effects 

on aquatic nonvascular plants. 

There were 604 measurements of pendimethalin in the Sacramento River and San 

Joaquin-Tulare River basins. There was measurable pendimethalin in 319 of the 604 

samples (53% positive). The highest concentration recorded (0.679 ppb) in the San 

Joaquin-Tulare River Basin. All 9 measurements at this site were positive for 

pendimethalin, with a range of 0.262 to 0.679 ppb. LOCs that were exceeded have EECs 

from the modeled PRZM_EXAMS ranging from peak of 48 ppb (rice model scenario) to 

5.6 ppb. EECs that fall below 5.2 ppb did not exceed the LOC for aquatic plants. The 

lowest LOC exceedance which has a peak EEC of 5.6 also has a 60-day EEC of 0.8 ppb 

which is similar to the peak monitoring concentration detected in surface water in 

California. 

There are four aquatic non-vascular plant studies submitted to the Agency. The EC50 

results of these studies range from 5.2 ppb to >174 ppb. Three of the four species have 

similar results. The EC50 data on the species are below: 

Species tested EC50 MRID Source 

Green algae (Selenastrum capricornutum) 5.4 ppb 42372204 

Marine diatom (Skeletonema costatum) 5.2 ppb 42372205 

Freshwater diatom (Navicula pelliculosa) 6.7 ppb 42372206 

Bluegreen algae (Anabaena flos-aquae) >174 ppb 42372207 

98
Based on the Agency’s LOC for aquatic plants being exceeded and that 3 out of 4 species 

have sensitive EC50 similarities; there is a potential indirect impact to the CRLF from 

reduction of aquatic food items. The effects determination for the CRLF based on 

potential reductions in algae is “likely to adversely affect” (LAA). 

5.2.2.2 Aquatic Invertebrates 

The potential for pendimethalin to elicit indirect effects to the CRLF via effects on 

freshwater invertebrate food items is dependent on several factors including: (1) the 

potential magnitude of effect on freshwater invertebrate individuals and populations; and 

(2) the number of prey species potentially affected relative to the expected number of 

species needed to maintain the dietary needs of the CRLF. Together, these data provide a 

basis to evaluate whether the number of individuals within a prey species is likely to be 

reduced such that it may indirectly affect the CRLF. 

There are no aquatic invertebrate’s chronic (1.0) or acute (0.5) exceedances of LOC 

found in section 5.1.1.2. There are two scenarios (rice and forestry) in which the RQ lies 

between the listed species LOC of 0.05 and the acute LOC of 0.5. The probability of 

individual effect at RQ for RQ of 0.17 is 1 in 3740 and for the listed species of 0.06 is 1 

in 5.22E+07. The scenario with an RQ of 0.17 used a conservative scenario (Rice 

model) which is a direct instantaneous deposition to a water column. It does not take into 

account runoffs, dilution, or holding patterns before releasing into the environment. Thus 

the EEC may be an overestimation of the amount of pendimethalin concentration found 

waters from rice fields that are released into environs. The probability of individual 

effect at RQ analysis indicates that the chance of adverse effect to the aquatic invertebrate 

population is not significant. 

Other aquatic invertebrate acute toxicity studies were submitted to the Agency. Another 

waterflea (Daphnia magna) study tested with a formulation of 45.6% active ingredient of 

pendimethalin. This study showed an EC50 of 5.1 ppm formulated product. A submitted 

crayfish (Procambarus simulans) study showed an EC50 greater than 1.0 ppm (1,000 

ppb). This study is considered to be supplemental due to precipitation was seen of 

concentrations over 1 ppm, the dissolved O2 concentration is less than 40% of saturation, 

and DMSO was used as a solvent which tends to increase mortalities rather than decrease 

them. Both studies show that other aquatic invertebrates have orders of magnitude less 

sensitivity than the Daphnia magna surrogate. 

There were 604 measurements of pendimethalin in the Sacramento River and San 

Joaquin-Tulare River basins. There was measurable pendimethalin in 319 of the 604 

samples (53% positive). The highest concentration recorded (0.679 ppb) in the San 

Joaquin-Tulare River Basin. All 9 measurements at this site were positive for 

pendimethalin, with a range of 0.262 to 0.679 ppb which is well below the toxicity level 

of Daphnia magna EC50 of 280 ppb by three orders of magnitude. 

99
Based on the weight-of-evidence of chronic and acute LOC not being exceeded for 

aquatic invertebrates; the probit slope analysis showing probability of effect being 

insignificant; and species sensitivity among other aquatic invertebrates being less 

sensitive than the surrogate invertebrate; the effects determination for the CRLF based on 

potential reductions in aquatic invertebrates is “not likely to adversely affect” (NLAA). 

5.2.2.3 Fish and Aquatic-phase Frogs 

In the absence of toxicity data for amphibians, the most sensitive acute and chronic 

toxicity values across fish species tested. RQs were based on acute studies in rainbow 

trout (LC50 = 138 ug/L, MRID 00046291) and a chronic NOAEC in fathead minnow 

(NOAEC = 6.3 ug/L, MRID 00037940). 

The highest acute RQ from the rice model was 0.0.35. The acute LOC is 0.5. Based on a 

default probit slope value of 4.5 for the rainbow trout with 95% confidence intervals of 2 

to 9, the associated probability of an individual effect would be approximately 1 in 49.8. 

The scenario with an RQ of 0.12 used a conservative scenario (Rice model) which is a 

direct instantaneous deposition to a water column. It does not take into account runoffs, 

dilution, or holding patterns before releasing into the environment. Thus the EEC may be 

an overestimation of the amount of pendimethalin concentration found waters from rice 

fields that are released into environs. 

The next highest RQ is 0.12 from a forestry and Christmas tree use sites. The associated 

probability of an individual effect would be approximately 1 in 5.85E+04. Eleven 

scenarios have LOCs that are between the acute LOC (0.5) and the listed species LOC 

(0.05) and their RQs range from 0.12 to 0.05. The probability of individual effect at RQ 

for RQs of 0.17 to 0.05 is 1 in 5.85E+04 to 1 in 4.18E+08. The probability of individual 

effect at RQ analysis indicates that the chance of adverse effect to the aquatic invertebrate 

population is not be significant. 

The most sensitive chronic toxicity value is based on a life cycle study in fathead 

minnow. The chronic RQ (0.8) is based on a NOAEC of 6.3 ug/L from an acceptable 

life-cycle study (MRID 00024377). Most sensitive endpoint was reduction in egg 

production that was observed at LOAEC of 44 ug/L. 

Based on no acute or chronic exceedances and the low probability of an individual effect; 

the effects determination for the CRLF based on potential reductions in fish and other 

amphibians as food items of the aquatic-phase CRLF is “not likely to adversely affect” 

(NLAA). 

5.2.2.4 Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. 

100
Toxicity data from honey bee study (MRID 00099890) show the LD50 to be greater than 

49.8 μg/Bee. No mortalities were observed at the highest concentration. 

Based upon an average fresh weight per honey bee of 128 milligrams, the LD50 of honey 

bees (μg /bee) can be multiplied by 7.8 to determine the ppm toxicity. The toxicity 

equivalence in ppm would be 388 ppm. According to the Hoerger, Kenaga5

 nomogram 

modified by Fletcher6, the highest EEC exposure from 6 lb ai/A application to large 

insects would be 90 ppm and to small insects would be 810 ppm. The EEC of small 

insects is about 2X more than the highest dose tested. The EEC of the largest insect is 

less than 2X the dose tested. Although there are no mortalities of the honey bee at the 

highest dose tested and that under the Agency’s guidelines, pendimethalin is considered 

to be practically non-toxic to honey bee; the honey bee study did not test up to the highest 

dose that could be expected from 6 lb ai/A directly onto small insects at the site of 

application. 

There is some uncertainty due to the highest concentration dose tested on the bee was 

well below the equivalent of 6 lb ai/A maximum application. There was no mortality at 

the highest dose tested, yet it is uncertain if mortality will exist if a higher dose was used 

in the bee study that would reflect the maximum application rate. Therefore, potential 

risk can not be precluded to terrestrial invertebrates as a food item. Because potential 

risk can not be precluded to terrestrial invertebrates as a food item; the effects 

determination for the CRLF based on potential reductions in terrestrial invertebrates as 

prey items is “likely to adversely affect” (LAA). 

5.2.2.5 Mammals 

Terrestrial phase CRLFs consume small mammals. This assessment used a 15-gram 

herbivorous mammal to determine if there could be a potential reduction in mammal 

abundance. Acute dose-based RQs for a 15-gram mammal ranged from 0.1 to 0.6 

depending on the use pattern. Assuming a probit slope of 4.5, the probability of an 

individual effect would be approximately 1 in 6.9. Assuming that probability of an 

individual effect provides insight into the potential for reductions in a local population of 

small mammals, a probability of 1 in 6.9 (15.9%) would result in a measurable impact to 

mammal abundance and would, therefore, constitute a significant effect. 

Reproduction RQs ranged from 5.2 to 25.0. The toxicity endpoint used in the RQs was 

based on a parental systemic and reproductive NOAEL of 25 mg/kg-bw/day. The 

5

 Hoerger, F. and E.E. Kenaga. 1972. Pesticide residues on plants: correlation of representative data as a 

basis for estimation of their magnitude in the environment. IN: F. Coulston and F. Corte, eds., 

Environmental Quality and Safety: Chemistry, Toxicology and Technology. Vol 1. Georg Theime 

Publishers, Stuttgart, Germany. pp. 9-28 

6 Fletcher, J.S., J.E. Nellesson and T. G. Pfleeger. 1994. Literature review and evaluation of the EPA foodchain (Kenaga) nomogram, an instrument for estimating pesticide residues on plants. Environ. Tox. and 

Chem. 13(9):1383-1391 

101
parental systemic endpoint was based on decreased body weight gain and food 

consumption at the LOAEL of 125 mg/kg/day (500 ppm). The reproductive/offspring 

NOAEL is 25 mg/kg/day (500 ppm), based on decreases in the number of pups born and 

pup weight at the LOAEL of 125 mg/kg/day (2500 ppm) which may be a significant 

population effect. The RQs indicate that potential exposures exceed the NOAEL for all 

uses. Due to the chronic LOC being exceeded for small mammal prey and the 

reproductive endpoint is based on decreasing numbers of pups born and pup weight, the 

effects determination for the potential for pendimethalin to indirectly affect the CRLF 

from potential impacts to mammalian prey is likely to adversely affect (LAA). 

5.2.2.6 Terrestrial-phase Amphibians 

The effects determination for terrestrial phase CRLF consuming small mammalian prey 

was “likely to adversely affect” (LAA). This determination was based solely on frogs 

that consume potentially contaminated herbivore mammals. Terrestrial amphibian prey 

of the CRLF includes small amphibians such as tree frogs that do not prey on mammals. 

Therefore, the mammalian food group is not relevant in the evaluation of potential 

reductions in amphibian prey abundance. 

Avian (as surrogate for frog and CRLF) listed species RQs exceeded LOCs (0.1) (Section 

5.1). Acute LOCs (0.5) were not exceeded for insectivorous amphibians using the THERPS model (Table 5.16) at the highest rate of application (6 lb ai/A). Therefore, 

reductions from acute effects in amphibian prey as a food item at levels likely to affect 

the CRLF are not likely to occur, and the effects determination for the CRLF based on 

potential reductions in terrestrial amphibians is “not likely to adversely affect” (NLAA). 

Since the chronic LOC have been exceeded, the effects determination for the CRLF 

based on potential reductions in terrestrial-phase amphibians as food item is “likely to 

adversely affect” (LAA) from chronic LOC exceedances. 

5.2.3 Indirect Effects (via Habitat Effects) 

5.2.3.1 Aquatic Plants (Vascular and Non-vascular) 

Aquatic plants serve several important functions in aquatic ecosystems. Non-vascular 

aquatic plants are primary producers and provide the autochthonous energy base for 

aquatic ecosystems. Vascular plants provide structure as attachment sites and refugia for 

many aquatic invertebrates, fish, and juvenile organisms, such as fish and frogs. In 

addition, vascular plants also provide primary productivity and oxygen to the aquatic 

ecosystem. Rooted plants help reduce sediment loading and provide stability to 

nearshore areas and lower streambanks. In addition, vascular aquatic plants are important 

as attachment sites for egg masses of CRLFs. 

102
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. Section 5.2.2.1 indicates that pendimethalin is likely to adversely impact nontarget aquatic plants. 

LOCs are exceeded for aquatic vascular plants and algae from exposure to pendimethalin 

from runoff or spray drift. The non-vascular RQs that exceed LOC range from 1.1 to 9.2 

and exceed the LOC for 15 out of 25 pendimethalin aquatic model scenarios. The 

vascular plant LOC is exceeded for 2 of the 25 aquatic model scenarios with the RQs of 

1.3 and 3.8. 

There were 604 measurements of pendimethalin in the Sacramento River and San 

Joaquin-Tulare River basins. There was measurable pendimethalin in 319 of the 604 

samples (53% positive). The highest concentration recorded (0.679 ppb) were found in 

the San Joaquin-Tulare River Basin. All 9 measurements at this site were positive for 

pendimethalin, with a range of 0.262 to 0.679 ppb. LOCs that were exceeded have EECs 

from the modeled PRZM_EXAMS ranging from peak of 48 ppb (rice model scenario) to 

5.6 ppb. EECs that fall below 5.2 ppb did not exceed the LOC for aquatic nonvascular 

plants and 12.5 ppb for the aquatic vascular plants. 

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. Based on the LOC exceedances, there is a potential indirect impact to the 

aquatic-phase-CRLF from aquatic habitat degradation. The effects determination for the 

aquatic-phase CRLF is “likely to adversely affect” (LAA). 

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. 

The terrestrial non-target plant LOC is exceeded for all use sites. The RQs are variable 

according to the method of application used (aerial or ground application) and the amount 

of active ingredient used. Table 5.18 below shows the range of RQs from different 

application methods. 

Table 5-18. Range of Risk Quotients for Non-Target Terrestrial Plants from 

Pendimethalin Usage 

Application Spray Runoff to Low-Lying Runoff to dry 

103
Application Rate Method Drift RQ semi-aquatic areas adjacent areas 

6.0 lb ai/A ground 6.0 66.0 12.0 

4.0 lb ai/A ground 4.0 44.0 8.0 

3.5 lb ai/A ground 3.5 38.5 7.0 

2.0 lb ai/A ground 2.0 22.0 4.0 

1.485 lb ai/A ground 1.5 16.3 3.0 

1.0 lb ai/A ground 1.0 11.0 2.0 

4.0 lb ai/A aerial 20.0 60.0 24.0 

3.5 lb ai/A aerial 17.5 52.5 21.0 

2.0 lb ai/A aerial 10.0 30.0 12.0 

1.485 lb ai/A aerial 7.4 22.3 8.9 

1.0 lb ai/A aerial 5.0 15.0 6.0 

Bold denotes that the RQ has exceeded the LOC. 

Although the endpoint used for runoff (seedling emergence) is 3X more sensitive than the 

spray drift toxicity endpoint (vegetative vigor), there is more LOC exceedances from 

aerial application than from runoff. This is due to the fate characteristics of 

pendimethalin. Pendimethalin binds more closely to the soil and is usually carried with 

soil during a runoff event rather than with water. 

The most sensitive monocot appears to be more sensitive than the most sensitive dicot 

tested by an order of magnitude. Differences of responses between the dicots and 

monocots are mixed and do overlap. Overall, monocots and dicots are both sensitive to 

pendimethalin. The herbicidal mode of action of pendimethalin is that it disrupts the 

process of mitosis in the growth of shoots and roots. It acts as a microtubule disruptor by 

inhibiting cell division and cell elongation in plants. Absorption of the chemical into 

plant is via roots and shoots. 

Based on the weight-of-evidence, the Agency concludes that there is a potential indirect 

impact to CRLF by terrestrial habitat degradation from pendimethalin exposure and 

therefore pendimethalin is Likely to Adversely Affect (LAA) the CRLF both riparian and 

upland vegetation. 

If aerial application was to be eliminated at least 1000 feet of CRLF habitat, potential risk 

would be reduced. 

5.2.4 Effects to Designated Critical Habitat 

Risk conclusions for the designated critical habitat are the same as those for indirect 

effects. Agency concludes that there is a potential indirect impact to CRLF by terrestrial 

and aquatic habitat degradation from pendimethalin exposure. 

5.2.4.1 Aquatic-Phase PCEs 

104
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). 

Conclusions for potential indirect effects to the CRLF via direct effects to aquatic and 

terrestrial plants are used to determine whether modification to critical habitat may occur. 

There is a potential for habitat effects via impacts to aquatic plants (Sections 5.2.2.1 and 

5.2.3.1) and terrestrial plants (5.2.3.2) 

The remaining aquatic-phase PCE is “alteration of other chemical characteristics 

necessary for normal growth and viability of CRLFs and their food source.” 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. There is a 

potential for habitat effects via impacts to aquatic-phase CRLFs (Sections 5.2.1.1) and 

effects to freshwater invertebrates and fish as food items (Sections 5.2.2.2 and 5.2.2.3). 

5.2.4.2 Terrestrial-Phase PCEs 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or drip line surrounding aquatic and riparian habitat that are comprised 

of grasslands, woodlands, and/or wetland/riparian plant species that provides the 

CRLF shelter, forage, and predator avoidance. 

• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal 

habitat within designated units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites including both natural and 

altered sites which do not contain barriers to dispersal. 

There is a potential for habitat effects via impacts to terrestrial plants (5.2.3.2). 

105
The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of pendimethalin on this 

PCE, acute and chronic toxicity endpoints for terrestrial invertebrates, mammals, and 

terrestrial-phase frogs are used as measures of effects. There is a potential for habitat 

effects via indirect effects to terrestrial-phase CRLFs via reduction in prey base (Section 

5.2.2.4 for terrestrial invertebrates, Section 5.2.2.5 for mammals, and 5.2.2.6 for frogs). 

The fourth terrestrial-phase PCE is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. There is a potential for habitat modification via direct (Section 5.2.1.2) and 

indirect effects (Sections 5.2.2.4, 5.2.2.5, and 5.2.2.6) to terrestrial-phase CRLFs. 

5.2.5 Spatial Extent of Potential Effects 

An LAA effects determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of pendimethalin’s use pattern is identified, using 

land cover data that correspond to pendimethalin’s use pattern. The spatial extent of the 

effects determination also includes areas beyond the initial area of concern that may be 

impacted by runoff and/or spray drift. The identified direct/indirect effects and/or 

modification to critical habitat are anticipated to occur only for those currently occupied 

core habitat areas, CNDDB occurrence sections, and designated critical habitat for the 

CRLF that overlap with the initial area of concern plus at least 1000 feet from its 

boundary. It is assumed that non-flowing waterbodies (or potential CRLF habitat) are 

included within this area. 

In addition to the spray drift buffer, the results of the downstream dilution extent analysis 

result in a distance of 272 kilometers which represents the maximum continuous distance 

of downstream dilution from the edge of the initial area of concern (based on the forestry 

use; see Appendix D). If any of these streams reaches flow into CRLF habitat, there is 

potential to affect either the CRLF or modify its habitat. These lotic aquatic habitats 

within the CRLF core areas and critical habitats potentially contain concentrations of 

pendimethalin sufficient to result in LAA determination or effects to critical habitat. 

The determination of the buffer distance and downstream dilution for spatial extent of the 

effects determination is described below. 

5.2.5.1 Spray Drift 

In order to determine terrestrial and aquatic habitats of concern due to pendimethalin 

exposures through spray drift, it is necessary to estimate the distance that spray 

applications can drift from the treated area and still be present at concentrations that 

exceed levels of concern. An analysis of spray drift distances was completed using the 

AgDrift model. 

106
In order to characterize the spatial extent of the effects determination that is relevant to 

the CRLF (i.e. NLAA versus LAA), an analysis was conducted using the most sensitive 

non-endangered plant EC25 of 0.01 lbs ai/acre (ryegrass seedling emergence). Typically 

the NOAEC is used when there is an obligate relationship between the species being 

assessed and endangered plants (or other taxa). However, there is no obligate 

relationship between the CRLF and any endangered plant; therefore the LAA/NLAA 

determination is based on the area defined by the non-listed species LOC (i.e., 

EEC/EC50). 

The most sensitive effect of concern was the lowest EC25 for seedling emergence, 0.01 

lb/acre for ryegrass. Tier 1 ground spray analysis (terrestrial assessment) was performed 

with the AgDrift model. The following settings were used: High boom, ASAE very fine 

to fine spray, point deposition, and an application rate of 6 lb/acre. The resulting buffer 

distance for ground spray was 899 feet. 

A similar analysis was performed using AgDrift, using the Tier 1 aerial (terrestrial) 

assessment with ASAE very fine to fine spray, point deposition, and an application rate 

of 4 lb/acre. The resulting buffer exceeds the limit of the AgDrift model (approximately 

1000 feet). A Tier 2 AgDrift aerial analysis was also conducted, with the same result 

(over 1000 feet). A summary of the modeled distances by application rate is presented in 

Table 5.19. 

Table 5-19 Summary of AgDrift Predicted Terrestrial Spray Drift Distances 

Application Rate 

(method) 

Uses Represented EC25 

Distance (ft) 

6.0 (ground) 899 

4.0 (aerial) Over 1000 

Given that the greatest buffer distance is 1,000 feet for terrestrial plants, this value was 

used to define the spatial extent of the effects determination (i.e., this buffer distance is 

added to the initial area of concern). 

Similar to the analysis described above, the buffer distance needed to get below the most 

sensitive aquatic LOC was determined. This distance identifies those locations where 

water bodies can be impacted by spray drift deposition alone (no runoff considered) 

resulting in concentrations above the LOC. The most sensitive aquatic endpoint is for 

aquatic non-vascular plants (Skeletonema) with an EC50 value of 5.2 μg/L. The analysis 

yields a much lower ground-spray buffer distance than the terrestrial buffer with a 

distance of 79 feet (based on the non-listed LOC using the EC50 value). However, the 

Tier 1 and 2 aerial analysis again yields a distance of over 1,000 feet. The results of the 

analysis are presented in Table 5.20. 

Table 5-20 Summary of AgDrift Predicted Aquatic Spray Drift Distances 

Application Rate 

(method) 

Uses Represented EC50 

Distance (ft) 

107
Application Rate 

(method) 

Uses Represented EC50 

Distance (ft) 

6.0 (ground) 79 

4.0 (aerial) Over 1000 

5.2.5.2 Downstream Dilution Analysis 

The downstream extent of exposure in streams and rivers is where the EEC could 

potentially be above levels that would exceed the most sensitive LOC. To complete this 

assessment, the greatest ratio of aquatic RQ to LOC was estimated. Using an assumption 

of uniform runoff across the landscape, it is assumed that streams flowing through treated 

areas (i.e. the initial area of concern) are represented by the modeled EECs; as those 

waters move downstream, it is assumed that the influx of non-impacted water will dilute 

the concentrations of pendimethalin present. 

Using an EC50 value of 5.2 ug/L for non-vascular aquatic plants (the most sensitive 

species is Skeletonema costatum) and a maximum peak EEC for applications to rice of 48 

ug/L yields an RQ/LOC ratio of 9.2 (9.2/1), which is applied to the pasture/hay and 

cultivated crop land cover class. RQ/LOC ratios for other use categories range from 2.3 

to 3.2. Using the downstream dilution approach (described in more detail in Appendix 

D) yields a target percent crop area (PCA) of 11% for the ratio of 9.2. This value has 

been input into the downstream dilution approach and results in a distance of 47 

kilometers. However, the forestry RQ/LOC ratio of 3.2 yields a downstream dilution 

distance of 272 kilometers, which represents the maximum continuous distance of 

downstream dilution from the edge of the initial area of concern, for the forestry land 

cover class. Similar to the spray drift buffer described above, the LAA/NLAA 

determination is based on the area defined by the point where concentrations exceed the 

EC50 value. 

5.2.5.3 Overlap between CRLF habitat and Spatial Extent of Potential 

Effects 

An LAA effects determination is made to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat 

and the area overlaps with the core areas, critical habitat and available occurrence data 

for CRLF. 

For pendimethalin, the use patterns fall into the following land cover classes: 

Cultivated Crops; Developed, High Intensity; Developed, Low Intensity; Developed, 

Medium Intensity; Developed, Open Space; Forest; Open Water; Orchards and 

vineyards; Pasture/Hay; Wetlands; Turf; and Rights-of-way. The overlap area also 

includes areas beyond the initial area of concern that may be impacted by runoff and/or 

spray drift, where it overlaps with CRLF habitat. Appendix D provides maps of the 

initial area of concern, along with CRLF habitat areas, including currently occupied core 

108
areas, CNDDB occurrence sections, and designated critical habitat. It is expected that 

any additional areas of CRLF habitat that are located at least 1000 ft (to account for 

offsite migration via spray drift) and 272 kilometers of stream reach (to account for 

downstream dilution) outside the initial area of concern may also be impacted and are 

part of the full spatial extent of the LAA and effects to critical habitat determination. 

109
Figure 2.6. Overlap Map: CRLF Habitat and Pendimethalin Initial Area of 

Concern 

110
In order to confirm that uses of pendimethalin have the potential to affect CRLF through 

direct applications to target areas and runoff and spray drift to non-target areas, it is 

necessary to determine whether or not the spatial extent of potential effects based on 

agricultural and orchard crops and turf use of pendimethalin overlap with CRLF habitats. 

Spatial analysis using ArcGIS 9.1 indicates that lotic aquatic habitats within the CRLF 

core areas and critical habitats potentially contain concentrations of pendimethalin 

sufficient to result in RQ values that exceed LOCs. In addition, terrestrial habitats (and 

potentially lentic aquatic habitats) of the final action areas for agricultural, forestry, 

rights-of ways, ornamental, and turf uses of pendimethalin overlap with the core areas, 

critical habitat and available occurrence data for CRLF (Figure 2.6). Thus, uses of 

pendimethalin on agricultural and orchard crops and turf use could result in exposures of 

the CRLF to pendimethalin in aquatic and terrestrial habitats. 

6.0 Uncertainties 

6.1 Exposure Assessment Uncertainties 

6.1.1 Maximum Use Scenario 

The screening-level risk assessment focuses on characterizing potential ecological risks 

resulting from a maximum use scenario, which is determined from labeled statements of 

maximum application rate and number of applications with the shortest time interval 

between applications. The frequency at which actual uses approach this maximum use 

scenario may be dependant on pest resistance, timing of applications, cultural practices, 

and market forces. 

6.1.2 Aquatic Exposure Modeling of Pendimethalin 

The standard ecological water body scenario (EXAMS pond) used to calculate potential 

aquatic exposure to pesticides is intended to represent conservative estimates, and to 

avoid underestimations of the actual exposure. The standard scenario consists of 

application to a 10-hectare field bordering a 1-hectare, 2-meter deep (20,000 m3

) pond 

with no outlet. Exposure estimates generated using the EXAMS pond are intended to 

represent a wide variety of vulnerable water bodies that occur at the top of watersheds 

including prairie pot holes, playa lakes, wetlands, vernal pools, man-made and natural 

ponds, and intermittent and lower order streams. As a group, there are factors that make 

these water bodies more or less vulnerable than the EXAMS pond. Static water bodies 

that have larger ratios of pesticide-treated drainage area to water body volume would be 

expected to have higher peak EECs than the EXAMS pond. These water bodies will be 

either smaller in size or have larger drainage areas. Smaller water bodies have limited 

storage capacity and thus may overflow and carry pesticide in the discharge, whereas the 

EXAMS pond has no discharge. As watershed size increases beyond 10-hectares, it 

becomes increasingly unlikely that the entire watershed is planted with a single crop that 

is all treated simultaneously with the pesticide. Headwater streams can also have peak 

concentrations higher than the EXAMS pond, but they likely persist for only short 

periods of time and are then carried and dissipated downstream. 

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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). 

In general, the linked PRZM/EXAMS model produces estimated aquatic concentrations 

that are expected to be exceeded once within a ten-year period. The Pesticide Root Zone 

Model is a process or “simulation” model that calculates what happens to a pesticide in 

an agricultural field on a day-to-day basis. It considers factors such as rainfall and plant 

transpiration of water, as well as how and when the pesticide is applied. It has two major 

components: hydrology and chemical transport. Water movement is simulated by the use 

of generalized soil parameters, including field capacity, wilting point, and saturation 

water content. The chemical transport component can simulate pesticide application on 

the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in 

the soil are estimated by simultaneously considering the processes of pesticide uptake by 

plants, surface runoff, erosion, decay, volatilization, foliar wash-off, advection, 

dispersion, and retardation. 

Uncertainties associated with each of these individual components add to the overall 

uncertainty of the modeled concentrations. Additionally, model inputs from the 

environmental fate degradation studies are chosen to represent the upper confidence 

bound on the mean values that are not expected to be exceeded in the environment 

approximately 90 percent of the time. Mobility input values are chosen to be 

representative of conditions in the environment. The natural variation in soils adds to the 

uncertainty of modeled values. Factors such as application date, crop emergence date, 

and canopy cover can also affect estimated concentrations, adding to the uncertainty of 

modeled values. Factors within the ambient environment such as soil temperatures, 

sunlight intensity, antecedent soil moisture, and surface water temperatures can cause 

actual aquatic concentrations to differ for the modeled values. 

Unlike spray drift, tools are currently not available to evaluate the effectiveness of 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 

112
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 pendimethalin 

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 pendimethalin use areas. 

The monitoring data available are within same order of magnitude as the PRZM-EXAMS 

predictions, or slightly lower, for the agricultural and forestry scenarios. The monitoring 

data indicated that the maximum observed concentration in Sacramento River valley 

streams was 0.7 ppb, and in San Joaquin Valley streams it was 3.5 ppb. PRZM-EXAMS 

prediction (not including the scenarios for impervious surfaces) estimated pendimethalin 

to be in the surface waters at peak concentrations from 3 to 16 ppb. The impervious 

scenario was much higher at 143 ppb. 

6.1.3 Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Thirteen years of data (1994 

– 2006) were included in this analysis. Because statistical methodology for identifying 

outliers, in terms of area treated and pounds applied, was not provided by CDPR for 

years before 2002, these data are less accurate. No methodology for removing outliers 

was provided by CDPR for 2001 and earlier pesticide data; therefore, this information 

was included separately 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 be instances of misuse and misreporting. The Agency 

made use of the most current, verifiable information; in cases where there were 

discrepancies, the most conservative information was used. 

6.1.4 Terrestrial Exposure Modeling of Pendimethalin 

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 

113
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. 

6.1.5 Spray Drift Modeling 

Although there may be multiple pendimethalin applications at a single site, it is unlikely 

that the same organism would be exposed to the maximum amount of spray drift from 

every application made. In order for an organism to receive the maximum concentration 

of pendimethalin from multiple applications, each application of pendimethalin 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 

114
topography, cover, and meteorological conditions over the transport distance are not 

accounted for by the AgDRIFT/AGDISP model (i.e., it models spray drift from aerial and 

ground applications in a flat area with little to no ground cover and a steady, constant 

wind speed and direction). Therefore, in most cases, the drift estimates from 

AgDRIFT/AGDISP may overestimate exposure even from single applications, especially 

as the distance increases from the site of application, since the model does not account for 

potential obstructions (e.g., large hills, berms, buildings, trees, etc.). Furthermore, 

conservative assumptions are often made regarding the 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. 

6.1.6 Volatility 

Pendimethalin is a semi-volatile compound that may be lost from moist soil. The 

volatilization half-life from moist soil was found to be 12.5 days (MRID 00153766). 

Pendimethalin has been observed in rainwater in California (Vogel et al., 2008). This 

effect was accounted for in the PRZM-EXAMS modeling by invoking the PRZM 

volatilization routine, thus allowing voltilization to compete with runoff and metabolism 

as a dissipation route. Comparison of the highest EEC scenario (forestry) shows that the 

effect is small (16.6 ppb with volatility invoked and 18.1 ppb without). The relatively 

slow volatilization rate from even moist soil is not believed to result in as large or 

immediate an aquatic exposure as spray drift at the time of application. Terrestrial 

exposure of plants and animals is accounted for at least in part by the AgDrift and 

AgDISP effect area analysis. 

6.1.7 Bioaccumulation 

Pendimethalin is believed to be bioaccumulative, as it has a logKow of 5.18, and a 

whole-body BCF of 5100x in bluegill sunfish (MRID 00156726). PRZM-EXAMS 

modeling indicates that pendimethalin may accumulate in sediments as well. Thus, foodchain exposure via accumulation in aquatic organisms may be a completed exposure 

pathway. However, several factors indicate that such exposure may not be a concern. 

First, additional bioaccumulation data in catfish (approximate BCF = 1600x), guppies (no 

accumulation, with metabolism) (MRID 00046293) and crayfish (MRID 00071124) 

whole body BCF = 1.5x) indicate that bioaccumulation is not the only response of aquatic 

organisms to pendimethalin. Second, the bluegill sunfish exposed to pendimethalin 

accumulated 15 ppm body burden with no ill effects, followed by depuration. 

KABAM modeling (Appendix K) indicated some marginal chronic risk for large river 

otters (chronic dose-based RQ 1.3, but dietary-based RQ 0.169) and marginal acute risk 

for small CRLF (acute dose-based RQ 0.14 to 0.38, but dietary-based RQ 0.007 to 0.008. 

However, the assumption of no metabolism in aquatic organisms is probably incorrect, so 

the results from KABAM showing marginal risks are therefore discountable. 

115
6.2 Effects Assessment Uncertainties 

6.2.1 Age Class and Sensitivity of Effects Thresholds 

It is generally recognized that test organism age may have a significant impact on the 

observed sensitivity to a toxicant. The acute toxicity data for fish are collected on 

juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on 

recommended immature age classes (e.g., first instar for daphnids, second instar for 

amphipods, stoneflies, mayflies, and third instar for midges). 

Testing of juveniles may overestimate toxicity at older age classes for pesticide active 

ingredients that act directly without metabolic transformation because younger age 

classes may not have the enzymatic systems associated with detoxifying xenobiotics. In 

so far as the available toxicity data may provide ranges of sensitivity information with 

respect to age class, this assessment uses the most sensitive life-stage information as 

measures of effect for surrogate aquatic animals, and is therefore, considered as 

protective of the CRLF. 

6.2.2 Use of Surrogate Species Effects Data 

Guideline toxicity tests and open literature data on pendimethalin are not available for 

frogs or any other aquatic-phase amphibian; therefore, freshwater fish are used as 

surrogate species for aquatic-phase amphibians. Therefore, 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. 

6.2.3 Sublethal Effects 

When assessing acute risk, the screening risk assessment relies on the acute mortality 

endpoint as well as a suite of sublethal responses to the pesticide, as determined by the 

testing of species response to chronic exposure conditions and subsequent chronic risk 

assessment. Consideration of additional sublethal data in the effects determination t is 

exercised on a case-by-case basis and only after careful consideration of the nature of the 

sublethal effect measured and the extent and quality of available data to support 

establishing a plausible relationship between the measure of effect (sublethal endpoint) 

116
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 pendimethalin on CRLF may be underestimated. 

6.2.4 Location of Wildlife Species 

For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

6.2.5 Toxicity to Honeybees. 

Pendimethalin is rated as practically non-toxic to honeybees. The honeybee contact 

toxicity data is used as a surrogate assessment endpoint for all terrestrial invertebrates. 

Because the calculated exposure (TREX EEC for large and small insects) exceeds the 

highest tested dose, even though there were no mortalities at the highest dose, there is 

some residual uncertainty about potential effects to terrestrial invertebrates. This results 

in a May Affect determination, however, it is considered discountable. 

7.0 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 pendimethalin to the CRLF and its 

designated critical habitat. 

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

Affect/May Affect, determination for the CRLF from the use of pendimethalin. The 

Agency has determined that there is the potential for effects to CRLF designated critical 

habitat from the use of the chemical. All of the uses might affect the frog or its critical 

habitat. The LAA determination is from direct and indirect effects to CRLF from the 

use of pendimethalin. Given the LAA determination for the CRLF and potential effects 

to designated critical habitat, a description of the baseline status and cumulative effects 

for the CRLF is provided in Attachment 2. 

The LAA effects determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of pendimethalin’s use pattern is identified, using 

land cover data that correspond to pendimethalin’s use pattern. The spatial extent of the 

117
LAA effects determination also includes areas beyond the initial area of concern that may 

be impacted by runoff and/or spray drift. The identified direct and indirect effects and 

modification to critical habitat are anticipated to occur only for those currently occupied 

core habitat areas, CNDDB occurrence sections, and designated critical habitat for the 

CRLF that overlap with the initial area of concern plus at least 1000 feet from its 

boundary (refer to analysis in Section 5.1.4). It is assumed that non-flowing waterbodies 

(or potential CRLF habitat) are included within this area. 

In addition to the spray drift buffer, the results of the downstream dilution extent analysis 

result in a distance of 272 kilometers which represents the maximum continuous distance 

of downstream dilution from the edge of the initial area of concern (refer to analysis in 

Section 5.1.4). If any of these streams reaches flow into CRLF habitat, there is potential 

to affect either the CRLF or affect its habitat. These lotic aquatic habitats within the 

CRLF core areas and critical habitats potentially contain concentrations of pendimethalin 

sufficient to result in LAA determination or effects to critical habitat. 

Appendix K provides maps of the initial area of concern, along with CRLF habitat areas, 

including currently occupied core areas, CNDDB occurrence sections, and designated 

critical habitat. It is expected that any additional areas of CRLF habitat that are located at 

least 1000 ft (to account for offsite migration via spray drift) and 272 kilometers of 

stream distance (to account for downstream dilution) outside the initial area of concern 

may also be impacted and are part of the full spatial extent of the LAA and critical habitat 

effects determination. 

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

critical habitat, given the uncertainties discussed in Section 6, is presented in Table 7.0-1 

and Table 7-2. 

Table 7-1 Effects Determination Summary for Pendimethalin Use and the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1 

Basis for Determination 

Survival, growth, Potential for Direct Effects 

and/or reproduction 

of CRLF 

individuals 

LAA1 Aquatic-phase (Eggs, Larvae, and Adults): 

Acute LOCs were exceeded for fish or aquatic-phase amphibians 

Terrestrial-phase (Juveniles and Adults): 

Acute and chronic LOCs were exceeded for birds. The available toxicity 

data suggest that amphibians are less sensitive than birds to pendimethalin, 

and considering factors such as lower food intake of terrestrial phase 

amphibians relative to birds reduces EECs and RQs, but does not reduce 

RQs to levels that are below LOCs. Likely to adversely affect CRLF. 

Potential for Indirect Effects 

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

LOC is exceeded only for non-vascular aquatic plants. RQs are below LOC 

for freshwater invertebrates (chronic and acute effects) and fish or frogs 

(chronic and acute effects). Pendimethalin could potentially impact 

terrestrial and aquatic plants to an extent that could result in indirect effects 

to the CRLF or modification of critical habitat. 

Terrestrial prey items, riparian habitat: 

118
Assessment 

Endpoint 

Effects 

Determination 1 

Basis for Determination 

Chronic LOC is exceeded for mammal and birds (surrogate for frog). 

Pendimethalin is practically non-toxic to honeybees with no mortalities observed 

in testing. LOC is exceeded for terrestrial invertebrates. Uncertainty in toxicity 

to insects from EEC that is above highest bee concentration tested does not 

preclude potential risk. Pendimethalin may adversely affect insects. Acute 

LOCs were not exceeded for insectivorous amphibians using the T-HERPS 

model at the highest rate of application. 

LOC is exceeded for non-target terrestrial plants and thereby critical habitat 

could be affected as a result of these potential impacts. 1

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

affect (LAA) 

Table 7-2 Effects Determination Summary for Pendimethalin Use and CRLF 

Critical Habitat Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 

Basis for Determination 

Modification of 

aquatic-phase PCE 

Habitat Effects 

Effects to riparian vegetation (terrestrial plants) and aquatic non-vascular and 

vascular plants result in LOC exceedances. These effects may indirectly affect 

the CRLF via reduction in food supply, changes in available cover, physical 

parameters of the waterbody (e.g. increase temperature or turbidity) 

LOC is exceeded for effects to non-vascular aquatic plants, freshwater 

invertebrates (chronic and acute effects) and fish or frogs (chronic and acute 

effects). 

Modification of 

terrestrial-phase 

PCE 

Effects to riparian vegetation (terrestrial plants) result in LOC exceedances. 

Effects may result in changes in community composition or relative abundance 

of riparian plant species, possibly altering terrestrial – phase CRLF habitat. 

Chronic LOC is exceeded for mammal. Although there are NLAA 

determinations for other prey items, small mammals constitute up to half of the 

food intake for CRLF and reduction in small mammalian populations may be 

significant. 

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 

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 

119
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 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. 

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8.0 References 

Alvarez, J. 2000. Letter to the U.S. Fish and Wildlife Service providing comments on 

the Draft California Red-legged Frog Recovery Plan. 

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