Risks of Diuron Use to Federally Threatened 

California Red-legged Frog 

(Rana aurora draytonii) 

Pesticide Effects Determination 

Environmental Fate and Effects Division 

Office of Pesticide Programs 

Washington, D.C. 20460 

February 18, 2009 
Primary Authors: 

 Ron Dean, Biologist 

 Tiffany Mason, Environmental Engineer 

 Bill Shaughnessy, PhD, Environmental Scientist 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division (7507C) 

 

Secondary Review: 

 William P. Eckel, PhD, Senior Scientist 

 Environmental Risk Branch II 

 Environmental Fate and Effects Division (7507P) 

Jean Holmes, Senior Scientist 

Environmental Risk Branch II 

Environmental Fate and Effects Division (7507P) 

Branch Chief, Environmental Risk Branch II: 

 Tom Bailey, PhD, Branch Chief 

 Environmental Fate and Effects Division (7507P) 

i
Table of Contents 

1.0 Executive Summary.................................................................................................1 

2.0 Problem Formulation .............................................................................................11 

2.1 Purpose...........................................................................................................................11 

2.2 Scope..............................................................................................................................13 

2.3 Previous Assessments ....................................................................................................14 

2.4 Stressor Source and Distribution ...................................................................................15 

2.4.1 Environmental Fate Properties...............................................................................15 

2.4.2 Environmental Transport Mechanisms..................................................................17 

2.4.3 Mechanism of Action.............................................................................................21 

2.4.4 Use Characterization..............................................................................................21 

2.5 Assessed Species............................................................................................................25 

2.5.1 Distribution ............................................................................................................25 

2.5.2 Reproduction..........................................................................................................28 

2.5.3 Diet.........................................................................................................................28 

2.5.4 Habitat....................................................................................................................29 

2.6 Designated Critical Habitat............................................................................................30 

2.7 Action Area....................................................................................................................32 

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

2.8.1. Assessment Endpoints for the CRLF.....................................................................36 

2.8.2 Assessment Endpoints for Designated Critical Habitat.........................................37 

2.9 Conceptual Model..........................................................................................................40 

2.9.1 Risk Hypotheses.....................................................................................................40 

2.9.2 Diagram..................................................................................................................40 

2.10 Analysis Plan .................................................................................................................44 

2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model ......................44 

2.10.1.1 Measures of Exposure............................................................................ 44 

2.10.1.2 Measures of Effect ................................................................................. 46 

2.10.1.3 Integration of Exposure and Effects ...................................................... 47 

2.10.2 Data Limitations.....................................................................................................48 

3.0 Exposure Assessment.............................................................................................48 

3.1 Label Application Rates and Intervals...........................................................................49 

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

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

3.2.2 Model Inputs ..........................................................................................................49 

3.2.3 Results....................................................................................................................50 

3.2.4 Existing Monitoring Data ......................................................................................52 

3.2.6 Downstream Dilution Analysis..............................................................................53 

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

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

3.4.1 Spray Drift Buffer Analysis...................................................................................59 

4.0 Effects Assessment ................................................................................................61 

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

4.1.1 Toxicity to Freshwater Fish ...................................................................................65 

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

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4.1.3 Toxicity to Aquatic Plants .....................................................................................65 

4.2 Toxicity of Diuron to Terrestrial Organisms .................................................................66 

4.2.1 Toxicity to Birds ....................................................................................................67 

4.2.2 Toxicity to Mammals.............................................................................................68 

4.2.3 Toxicity to Terrestrial Invertebrates ......................................................................68 

4.2.4 Toxicity to Terrestrial Plants .................................................................................68 

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

Species Levels of Concern.........................................................................................................69 

4.4 Incident Database Review..............................................................................................70 

5.0 Risk Characterization.............................................................................................70 

5.1 Risk Estimation..............................................................................................................70 

5.1.1 Exposures in the Aquatic Habitat ..........................................................................71 

5.1.1.1 Direct Effects to Aquatic-Phase CRLF................................................................. 71 

5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey (non-vascular 

aquatic plants, aquatic invertebrates, fish, and frogs)........................................... 79 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and Primary Productivity 

(Freshwater Aquatic Plants).................................................................................. 86 

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

Effects to Vascular Aquatic Plants (habitat of aquatic-phase CRLF) a...............86 

5.1.2 Exposures in the Terrestrial Habitat ......................................................................89 

5.1.2.1 Direct Effects to Terrestrial-phase CRLF............................................................. 89 

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

invertebrates, mammals, and frogs)...................................................................... 91 

5.1.2.3 Indirect Effects to CRLF via Reduction in Terrestrial Plant Community 

(Riparian and Upland Habitat).............................................................................. 93 

5.1.3 Primary Constituent Elements of Designated Critical Habitat ..............................94 

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

Habitat) ................................................................................................................ 94 

5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat) ................................... 95 

5.2 Risk Description.............................................................................................................96 

5.2.1 Direct Effects .......................................................................................................100 

5.2.1.1 Aquatic-Phase CRLF .......................................................................................... 100 

5.2.1.2 Terrestrial-Phase CRLF ...................................................................................... 101 

5.2.2 Indirect Effects (via Reductions in Prey Base)....................................................104 

5.2.2.1 Algae (non-vascular plants) ................................................................................ 104 

5.2.2.2 Aquatic Invertebrates.......................................................................................... 105 

5.2.2.3 Fish and Aquatic-phase Frogs............................................................................. 106 

5.2.2.4 Terrestrial Invertebrates...................................................................................... 106 

5.2.2.5 Mammals............................................................................................................. 107 

5.2.2.6 Terrestrial-phase Amphibians............................................................................. 107 

5.2.3 Indirect Effects (via Habitat Effects) ...................................................................108 

5.2.3.1 Aquatic Plants (Vascular and Non-vascular)...................................................... 108 

5.2.3.2 Terrestrial Plants ................................................................................................. 108 

5.2.4 Modification to Designated Critical Habitat........................................................109 

5.2.4.1 Aquatic-Phase PCEs ........................................................................................... 109 

5.2.4.2 Terrestrial-Phase PCEs ....................................................................................... 110 

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

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

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

6.1.2 Aquatic Exposure Modeling of Diuron................................................................112 

6.1.3 Action Area Uncertainties....................................................................................113 

6.1.4 Usage Uncertainties .............................................................................................114 

6.1.5 Terrestrial Exposure Modeling of Diuron............................................................114 

6.2 Effects Assessment Uncertainties................................................................................115 

6.2.1 Age Class and Sensitivity of Effects Thresholds.................................................115 

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

6.2.3 Sublethal Effects ..................................................................................................117 

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

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

8.0 References............................................................................................................122 

Appendices 

Appendix A Multi-ai Product Analysis 

Appendix B Chemical Structure of Parent and Degradates 

Appendix C Use Rates and Methods 

Appendix D CDPR PUR Usage Data 

Appendix E RQ Method and LOCs 

Appendix F Spatial Summary of Diuron 

Appendix G PRZM/EXAMS Output 

Appendix H T-REX Example Output 

Appendix I TerrPlant Example Output 

Appendix J Bibliography of ECOTOX Open Literature Not Evaluated 

Appendix K Accepted ECOTOX Data Table Spreadsheet 

Appendix L Ecological Effects Data 

Appendix M Diuron Incidents 

Appendix N T-Herps Example 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 

iv
List of Tables 

Table 1.1 Effects Determination Summary for Effects of Diuron on the CRLF............... 5 

Table 1.2 Effects Determination Summary for the Critical Habitat Impact Analysis....... 6 

Table 1.3 Diuron Use-specific Direct Effects Determinations1 for the CRLF .................. 7 

Table 1.4 Diuron Use-specific Indirect Effects Determinations1

 Based on Effects to Prey8 

Table 2.1 Summary of Diuron Half-lives ....................................................................... 17 

Table 2.2 Active Use Sites for Diuron as Applicable to California................................. 24 

Table 3.1 Summary of PRZM/EXAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Diuron Endangered Species Assessment for the CRLF................... 50 

Table 3.2 Table Aquatic EECs (ppb) for Diuron Uses in California............................... 51 

Table 3.3 T-REX Model Input Parameters...................................................................... 54 

Table 3.4 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Diuron ................................................................ 55 

Table 3.5 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to 

Terrestrial Invertebrate Prey Items ................................................................................... 56 

Table 3.6 TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and Semiaquatic Areas Exposed to diuron via Runoff and Drift..................................................... 58 

Table 3.7 Summary of AgDRIFT Predicted Spray Drift Buffer for Terrestrial Plants ... 60 

Table 4.1 Freshwater Aquatic Toxicity Profile for Diuron.............................................. 63 

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

Table 4.3 Terrestrial Toxicity Profile for Diuron ............................................................. 66 

Table 4.4 Categories of Acute Toxicity for Avian and Mammalian Studies.................... 67 

Table 4.5 Terrestrial Plant Toxicity Profile for Diuron .................................................. 69 

Table 5.1. Summary of Direct Effect RQs for the Aquatic-phase CRLF........................ 72 

Table 5.2 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via 

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

aquatic-phase CRLF)* ...................................................................................................... 80 

Table 5.3 Summary of Acute and Chronic RQs 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)* ......................................................................... 83 

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

Effects to Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a ............................. 86 

Table 5.5 Summary of Acute RQs Used to Estimate Direct Effects to the Terrestrialphase CRLF ...................................................................................................................... 90 

Table 5.6 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 .. 92 

Table 5.7 RQs*† for Non-Target Plants Inhabiting Dry and Semi-Aquatic Areas 

Exposed to Diuron via Runoff and Drift........................................................................... 93 

Table 5.8 Risk Estimation Summary for Diuron - Direct and Indirect Effects to CRLF . 97 

Table 5.9 Risk Estimation Summary for Diuron – PCEs of Designated Critical Habitat 

for the CRLF..................................................................................................................... 98 

Table 5.10. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for Dietarybased Exposures of the CRLF and its Prey to Diuron1

................................................... 102 

v
Table 5.11. Revised Acute Dietary-based RQs for CRLF consuming different food items 

(RQs calculated using T-HERPS)*................................................................................. 103 

Table 7.1 Effects Determination Summary for Effects of Diuron on the CRLF........... 119 

Table 7.2 Effects Determination Summary for the Critical Habitat Impact Analysis... 120 

vi
List of Figures 

Figure 2.1 Diuron Agricultural Use in Total Pounds per County..................................... 23 

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

CRLF................................................................................................................................. 27 

Figure 2.3 – CRLF Reproductive Events by Month......................................................... 28 

Figure 2.4 Initial area of concern, or “footprint” of potential use, for diuron ................. 34 

Figure 2.5 Conceptual Model for Aquatic-Phase of the CRLF ....................................... 41 

Figure 2.6 Conceptual Model for Terrestrial-Phase of the CRLF ................................... 42 

Figure 2.7 Conceptual Model for Pesticide Effects on Aquatic Component of CRLF 

Critical Habitat.................................................................................................................. 43 

Figure 2.8 Conceptual Model for Pesticide Effects on Terrestrial Component of CRLF 

Critical Habitat.................................................................................................................. 44 

vii
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), from FIFRA regulatory 

actions regarding use of diuron on agricultural and non-agricultural sites. In addition, 

this assessment evaluates whether these actions can be expected to result in modification 

of the species’ designated critical habitat. This assessment was completed in accordance 

with the U.S. Fish and Wildlife Service (USFWS) and National Marine Fisheries Service 

(NMFS) Endangered Species Consultation Handbook (USFWS/NMFS, 1998) and 

procedures outlined in the Agency’s Overview Document (U.S. EPA, 2004). 

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

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

mountain ranges. A total of 243 streams or drainages are believed to be currently 

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

Santa Barbara counties (USFWS, 1996) in California. 

Diuron (N-(3,4-dichlorophenyl)-N,N-dimethylurea) is a systemic substituted phenylurea 

herbicide. In the U.S. diuron is used on a variety of fruit and nut crops, grains, cotton, 

corn, sorghum, mint, gum, asparagus, sugarcane, seed crops, coffee, hay, cut flowers, and 

for fallow, idle cropland use, and impervious surfaces such as paved areas. It may be 

used in irrigation and drainage systems when water is not present. Diuron is available in 

wettable powder, flowable, liquid suspension, and soluble concentrate formulations for 

use in California. Technical diuron is a white, crystalline, and odorless solid. Diuron is 

registered for pre- and post emergent control using ground and aerial equipment. Diuron 

is often applied in combination with other herbicides such as bromacil, hexazinone, 

paraquat, thiadiazuron, imazapyr, monosodium, sodium chlorate, sodium metaborate, and 

copper sulfate (U.S. EPA, 2004a). This assessment evaluates only those registered uses 

which allow use in California. 

Diuron is easily taken up by plants and rapidly translocated. Diuron primarily functions 

by inhibiting the Hill reaction in photosynthesis, limiting the production of high-energy 

compounds such as adenosine triphosphate (ATP) used for various metabolic processes. 

Diuron is generally considered to be both mobile and persistent. It is relatively stable in 

neutral water. In addition to aquatic photolysis, microbial degradation is the primary 

factor in the degradation of diuron in aquatic environments. 

Diuron is not expected to concentrate in aquatic organisms. Diuron is regulated as a 

known ground water contaminant in California, where it has been detected in the 2 to 3 

ppb range (online at http://www.cdpr.ca.gov). Because diuron has a low Henry’s law 

constant and a low vapor pressure, volatilization is considered insignificant. 

At sufficient levels diuron residues in soil are toxic to plants. Residue levels are lower in 

soils with low organic content. Residue half-lives are from one month to one year for the 

parent, five months for a major degradate , and one month for a minor degradate. 

1
Degradate formation is summarized below. Some pineapple fields contained residues 

three years after the last application. 

Diuron degrades in the environment to four major (>10% of the applied parent) and four 

minor (<10% of the applied parent) metabolites and impurities. The major metabolites 

are: carbon dioxide (CO2), N-(3,4-dichlorophenyl)-N-methylurea (DCPMU), N'-(3-

chlorophenyl)-N,N-dimethylurea (MCPDMU), and 1,1-dimethyl-3-phenylurea (PDMU). 

The minor metabolites include: 4-dichlorophenylurea (DCPU); 3,4-dichloroaniline (3,4-

DCA), N-(3-chlorophenyl)-N-methylurea (CPMU), and 3,3',4,4'-tetrachlorobenzene 

(TCAB). 

There were no acceptable toxicity studies on the major degradates identified in the 

ECOTOX database. As a result, they will be assigned the same toxicity as the parent for 

purposes of this assessment. Because the concentrations of parent diuron exceed levels 

of concern by themselves (both in modeling and monitoring data), the non-inclusion of 

degradates does not cause a different outcome for the "may affect" determination. 

Therefore, this assessment only addresses ecological risks associated with exposures to 

the parent compound. 

Likewise, the minor degradates, 3,4-DCA and TCAB, will not be assessed further. 3,4 

DCA is not a significant residue (< 1%) in any metabolism or hydrolysis study, and 

TCAB is also a minor degradate (0.038 ppm) and is only found in the soil photolysis 

study which is a minor degradation route for diuron. 

Diuron contains two impurities from the manufacturing process, TCAB and 3,3',4,4'-

tetrachloroazoxybenzene (TCAOB), both ‘dioxin-like’ substances. TCAB levels between 

0.15 and 28 ppm have been found in diuron samples tested. TCAOB is present at lower 

levels. Since, these impurities have only been found at fairly low levels, they are not 

residues of concern for diuron and will not be evaluated in this assessment. 

Diuron is classified as a 'known/likely' carcinogen. EPA is required under the FFDCA, 

as amended by FQPA, to develop a screening program to determine whether certain 

substances (including all pesticide active and other ingredients) “may have an effect in 

humans that is similar to an effect produced by a naturally occurring estrogen, or other 

such endocrine effects as the Administrator may designate.” Following the 

recommendations of its Endocrine Disruptor Screening and Testing Advisory Committee 

(EDSTAC), EPA determined that there were scientific bases for including, as part of the 

program, androgen and thyroid hormone systems, in addition to the estrogen hormone 

system. EPA also adopted EDSTAC’s recommendation that the Program include 

evaluations of potential effects in wildlife. When the appropriate screening and/or testing 

protocols being considered under the Agency’s Endocrine Disrupter Screening Program 

(EDSP) have been developed and vetted, diuron may be subjected to additional screening 

and/or testing to better characterize effects related to endocrine disruption. 

As noted in the 2003 Registration Eligibility Decision (RED), diuron has been 

characterized as a “known/likely” carcinogen, based on urinary bladder carcinomas in 

both sexes of the Wistar rat, kidney carcinomas in the male rat (a rare tumor), and 

2
mammary gland carcinomas in the female NMRI mouse. Because of this, the spatial 

extent of the action area (i.e., the boundary where exposures and potential effects are less 

than the Agency’s LOC) for diuron cannot be determined. Therefore, it is assumed that 

the action area encompasses the entire state of California, regardless of the spatial extent 

(i.e., initial area of concern or footprint) of the pesticide use(s). 

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

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

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

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

environmental concentrations resulting from different diuron uses range from 3.76 to 

4911 ppb for use on grasses grown for seed, and paved areas (impervious surfaces), 

respectively. 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 

(CDPR). Particularly in the CDPR monitoring data, diuron is frequently detected and 

exceeds levels of concern with values up to 160 ppb. 

To estimate on-site diuron dietary exposures to the terrestrial-phase CRLF, and its 

potential prey resulting from uses involving diuron applications, the T-REX model is 

used for spray treatment. The AgDRIFT model is also used to estimate deposition of 

diuron on off-site terrestrial and aquatic habitats from spray drift. The TerrPlant model is 

used to estimate diuron exposures to terrestrial-phase CRLF habitat, including plants 

inhabiting semi-aquatic and dry areas, from site run-off and spray drift. The T-HERPS 

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

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

on the survival, reproduction, and growth of the CRLF itself. Direct effects to the CRLF 

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

generally used as a surrogate for aquatic-phase amphibians. 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 CRLFs 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 monocotyledonous (monocot) and dicotyledonous (dicot) plants. 

Risk quotients (RQs) are derived as estimates of potential risk. Acute and chronic RQs 

for animals and listed and non-listed plant RQs are compared to the Agency’s levels of 

concern (LOCs) to identify instances where diuron use within the action area has the 

3
potential to adversely affect the CRLF via direct toxicity or indirectly based on 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) or to modify its critical habitat. When RQs for each particular type of effect 

are below their respective LOCs, the pesticide is determined to have “no effect” on the 

CRLF or to modify its critical habitat. Where RQs exceed LOCs, a potential to cause 

adverse effects is identified, leading to a conclusion of “may affect” for the CRLF and a 

potential to modify its critical habitat. If a determination is made that use of diuron 

within the action area “may affect” the CRLF, 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. Similarly for 

critical habitat additional information is considered to refine the potential for exposure 

and effects to distinguish those actions that do or do result in modification of its critical 

habitat. 

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

Adversely Affect determination for diuron exposure to the CRLF based on direct and 

indirect effects to the aquatic- and terrestrial-phase CRLF. Further, registered uses of 

diuron are anticipated to result in modification to critical habitat of the CRLF. 

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. Use-specific determinations for 

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

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

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

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

cumulative effects for the CRLF is provided in Attachment II. 

4
Table 1.1 Effects Determination Summary for Effects of Diuron on the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects 

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

amphibian acute LOCs for Listed species (0.05) are exceeded for most 

uses of diuron in California. Acute RQs that exceed the Agency’s LOC 

range from 12.28 (paved areas) to 0.06 (papaya and walnut). Chronic 

RQs that exceed the Agency’s LOC for chronic exposure (1.0) range 

from 131.85 (paved areas) to 1.26 (banana, plantain). 

LAA 

Terrestrial-phase (Juveniles and Adults): Refined acute dietary-based 

RQs for CRLFs consuming large insects and small herbivore mammals 

exceed the acute listed species LOC (0.1) for all uses of diuron except 

Sorghum and Field Corn. The acute dietary-based RQs for CRLFs 

consuming large insects exceed the acute listed species LOC for 

Agricultural Rights-of-Way, Fencerows etc. No acute dietary based 

LOCs were exceeded for CLRF consuming small insectivore mammals 

and small terrestrial phase amphibians for any diuron use. 

Potential for Indirect Effects 

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

productivity: LOCs for non-vascular plants are exceeded for all uses. 

The RQs range from 2046.25 (paved areas) to 1.57 (grasses grown for 

seed). 

Aquatic invertebrates acute LOCs are exceeded. The acute RQs that 

exceed the LOC range from 30.69 (paved areas) to 0.05 (cotton aerial 

application). Chronic LOCs are exceeded for several uses. RQs that 

exceed the Agency’s LOC range from 21.21or paved areas to 1.02 for 

bermudagrass. 

RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for 

most uses. These range from 460.46 (paved areas) to 1.30 (hazelnut). 

Survival, 

growth, and/or 

reproduction of 

CRLF 

individuals 

LAA 

Terrestrial prey items, riparian habitat: 

For small mammals, chronic dose based RQs exceed the Agency’s 

LOC (1.0) for most uses (Table 5.6). The dose based chronic RQs that 

exceed the LOC range from 8.29 (Agricultural Rights-of-Way, 

Fencerows, Hedgerows, Airports, etc.) to 1.31 (walnut). The dietary 

based chronic RQs do not exceed the LOC for any uses. The RQs that 

exceed the acute dose based LOC range from 0.41 (Agricultural Rightsof-Way, Fencerows, Hedgerows, Airports, etc.) to 0.13 (Bermuda 

grass, Blackberry, Boysenberry, Spearmint). For terrestrial phase 

amphibians, the chronic LOC is exceeded for all uses of diuron and the 

acute LOCs are exceeded for all uses except sorghum. 

LOCs are exceeded for terrestrial riparian plants and for aquatic plants 

from exposure to diuron from runoff or spray drift. Alteration of 

riparian and vascular plants may result in alteration of temperature, 

turbidity, and oxygen content. RQs for vascular aquatic plants exceed 

the Agency’s LOC (1.0) for most uses. These range from 460.46 

(paved areas) to 1.30 (hazelnut). 

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

adversely affect (LAA) 

5
Table 1.2 Effects Determination Summary for the Critical Habitat Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase 

PCE 

HM Due to aquatic vascular and terrestrial plant communities being reduced 

from all use sites, there is potential for alteration of channel/pond 

morphology or geometry and/or increase in sediment deposition within 

the stream channel or pond. These plant communities provide for 

shelter, foraging, predator avoidance, and aquatic dispersal for juvenile 

and adult CRLFs. In addition, there is potential for 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. 

LOCs are exceeded for terrestrial riparian plants and for aquatic 

vascular plants from exposure to diuron from runoff or spray drift. 

LOCs for non-vascular plants are exceeded for all uses. 

Modification of 

terrestrial-phase 

PCE 

HM 

The use of diuron at all use sites may create the following modification 

of PCE: elimination and/or disturbance of upland habitat; ability of 

habitat to support food source of CRLFs, elimination and/or 

disturbance of dispersal habitat, reduction and/or modification of food 

sources for terrestrial phase juveniles and adults, and alteration of 

chemical characteristics necessary for normal growth and viability of 

juvenile and adult CRLFs and their food source. 

The RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for 

most uses of diuron in California. RQs for vascular aquatic plants 

exceed the Agency’s LOC (1.0) for most uses. These range from 

460.46 (paved areas) to 1.30 (hazelnut). Use of diuron on most use 

sites will exceed acute and chronic LOCs for many prey food items of 

the CRLF. Food sources are reduced and the CRLF is therefore 

indirectly affected. 

The RQs for non-target terrestrial monocot and dicot plants inhabiting 

semi-aquatic and upland dry areas exceed the Agency’s LOC (1.0) for 

all uses except for monocot plants exposed to sorghum applications 

(Table 5.7). These exceedances range from 300.00 (semi-aquatic 

plants exposed to non-agricultural aerial applications) to 0.19 (dicot 

plants inhabiting dry areas exposed to sorghum applications). Several 

diuron uses result in plant LOC exceedances from spray drift. These 

exceedances range from 12.00 (dicot plants exposed to field corn 

applications) to 1.13 (monocot plants exposed to cotton applications). 1

 Habitat Modification (HM) or No effect (NE) 

6
7

Table 1.3 Diuron Use-specific Direct Effects Determinations1

 for the CRLF 

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

AGRICULTURAL RIGHTS-OF-WAY / FENCEROWS/ 

HEDGEROWS LAA LAA LAA LAA

ALFALFA Ground NE NE LAA LAA

ALFALFA Aerial NE NE LAA LAA

APPLE NE NE LAA LAA

ARTICHOKE LAA NE LAA LAA

ASPARGUS LAA NE LAA LAA

BANANA/PLANTAIN LAA LAA LAA LAA

BLACKBERRY/BOYSENBERRY LAA LAA LAA LAA

BLUEBERRY NE NE LAA LAA

CITRUS NE NE LAA LAA

CORN, FIELD NE NE NE LAA

DEWBERRY NE NE LAA LAA

FILBERT (HAZELNUT) NE NE LAA LAA

GRAPE NE NE LAA LAA

LOGANBERRY, RASPBERRY (BLACK/RED) NE NE LAA LAA

OLIVE NE NE LAA LAA

PAPAYA NE NE LAA LAA

PEPPERMINT NE NE LAA LAA

PEACH NE NE LAA LAA

PEAR NE NE LAA LAA

PECAN LAA NE LAA LAA

SORGHUM NE NE NE LAA

SPEARMINT LAA LAA LAA LAA

WALNUT (ENGLISH/BLACK) LAA NE LAA LAA

WHEAT Pre Harvest NE NE LAA LAA

WHEAT Post Harvest NE NE LAA LAA

COTTON Ground NE NE LAA LAA

COTTON Aerial NE NE LAA LAA

AIRPORTS/ LANDING FIELDS, DRAINAGE 

SYSTEMS, INDUSTRIAL AREAS (OUTDOOR), 

SEWAGE DISPOSAL AREAS 

LAA LAA LAA LAA

BERMUDAGRASS LAA LAA LAA LAA

GRASSES GROWN FOR SEED NE NE LAA LAA

IRRIGATION SYSTEMS LAA LAA LAA LAA

ORNAMENTAL HERBACEOUS PLANTS LAA NE LAA LAA

NON-AGRICULTURAL RIGHTS OF WAY Ground LAA LAA LAA LAA

NON-AGRICULTURAL RIGHTS OF WAY Aerial LAA LAA LAA LAA

PAVED AREAS (PRIVATE 

ROADS/SIDEWALKS) LAA LAA LAA LAA

UNCULTIVATED AGRICULTIRAL AREAS Ground LAA LAA LAA LAA

UNCULTIVATED AGRICULTIRAL AREAS Aerial LAA LAA LAA LAA

UNCULTIVATED NON-AGRICULTURAL AREAS LAA LAA LAA LAA 1

 NE = No effect; MA/NLAA = May affect, but not likely to adversely affect; LAA = May affect and likely 

to adversely affect

* No acceptable chronic data are available for amphibians or their avian surrogates by which to determine 

chronic direct risks to the CRLF, therefore an LAA determination is made in the absence of these data and 

because supporting information on linuron, a similar herbicide, exhibits chronic toxicity to these species.
Table 1.4 Diuron Use-specific Indirect Effects Determinations1 Based on Effects to Prey 

Aquatic Invertebrates Aquatic-phase frogs and fish Terrestrial-phase fro

Use(s) Algae 

Acute Chronic 

Terrestrial 

Invertebrates 

(Acute) Acute Chronic Acute Chroni

AGRICULTURAL RIGHTS-OFWAY / FENCEROWS/ HEDGEROWS 

LAA LAA LAA NLAA LAA LAA LAA LAA 

ALFALFA Ground LAA LAA NE NLAA NE NE LAA LAA ALFALFA Aerial LAA LAA NE NLAA NE NE LAA LAA APPLE LAA LAA NE NLAA NE NE LAA LAA ARTICHOKE LAA LAA NE NLAA LAA NE LAA LAA ASPARGUS LAA LAA NE NLAA LAA NE LAA LAA BANANA/PLANTAIN LAA LAA NE NLAA LAA LAA LAA LAA BLACKBERRY/ BOYSENBERRY LAA LAA NE NLAA LAA LAA LAA LAA BLUEBERRY LAA NE NE NLAA NE NE LAA LAA CITRUS LAA NE NE NLAA NE NE LAA LAA CORN, FIELD LAA NE NE NLAA NE NE NE LAA DEWBERRY LAA LAA NE NLAA NE NE LAA LAA FILBERT (HAZELNUT) LAA LAA NE NLAA NE NE LAA LAA GRAPE LAA LAA NE NLAA NE NE LAA LAA LOGANBERRY, RASPBERRY (BLACK/RED) 

LAA NE NE NLAA NE NE LAA LAA 

OLIVE LAA NE NE NLAA NE NE LAA LAA PAPAYA LAA LAA NE NLAA LAA NE LAA LAA PEPPERMINT LAA NE NE NLAA NE NE LAA LAA PEACH LAA LAA NE NLAA NE NE LAA LAA PEAR LAA LAA NE NLAA NE NE LAA LAA PECAN LAA LAA NE NLAA NE NE LAA LAA SORGHUM LAA LAA NE NLAA NE NE NE LAA SPEARMINT LAA LAA NE NLAA LAA LAA LAA LAA WALNUT (ENGLISH/BLACK) LAA LAA NE NLAA NE NE LAA LAA WHEAT Pre Harvest LAA LAA NE NLAA NE NE LAA LAA 

8
9

Aquatic Invertebrates Aquatic-phase frogs and fish Terrestrial-phase fro

Use(s) Algae 

Acute Chronic 

Terrestrial 

Invertebrates 

(Acute) Acute Chronic Acute Chroni

WHEAT Post Harvest LAA LAA NE NLAA NE NE LAA LAA COTTON Ground LAA NE NE NLAA NE NE LAA LAA COTTON Aerial LAA LAA NE NLAA NE NE LAA LAA AIRPORTS/ LANDING FIELDS, DRAINAGE SYSTEMS, INDUSTRIAL AREAS (OUTDOOR), SEWAGE DISPOSAL AREAS 

LAA LAA LAA NLAA LAA LAA LAA LAA 

BERMUDAGRASS LAA LAA LAA NLAA NE NE LAA LAA GRASSES GROWN FOR SEED LAA NE NE NLAA NE NE LAA LAA IRRIGATION SYSTEMS LAA LAA NE NLAA LAA LAA LAA LAA ORNAMENTAL HERBACEOUS PLANTS LAA LAA NE NLAA LAA LAA LAA LAA NON-AGRICULTURAL RIGHTS OF WAY Ground LAA LAA LAA NLAA LAA LAA LAA LAA NON-AGRICULTURAL RIGHTS OF WAY Aerial LAA LAA LAA NLAA LAA LAA LAA LAA PAVED AREAS (PRIVATE ROADS/SIDEWALKS) LAA LAA LAA NLAA LAA LAA LAA LAA UNCULTIVATED AGRICULTIRAL AREAS Ground 

LAA LAA LAA NLAA LAA LAA LAA LAA 

UNCULTIVATED AGRICULTIRAL AREAS Aerial LAA LAA LAA NLAA LAA LAA LAA LAA UNCULTIVATED NONAGRICULTURAL AREAS LAA LAA LAA NLAA LAA LAA LAA LAA 1 NE = No effect; NLAA = May affect, but not likely to adversely affect; LAA = May affect and likely to adversely affect

* No acceptable chronic data are available for amphibians or their avian surrogates by which to determine chronic direct risks to the CRdetermination is made in the absence of these data and because linuron, a similar herbicide, exhibits chronic toxicity to these species in diuron surface water levels.
Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

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

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

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

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

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

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

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

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

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

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

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

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

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

action area. This information would allow for quantitative extrapolation 

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

proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

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

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

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

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

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

repeated exposure events and the inherent demographic characteristics of 

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

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

responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and 

together with the information described above, a more complete prediction 

of effects to individual frogs and potential modification to critical habitat. 

10
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 

diuron on terrestrial food, feed, forestry, non-food, and outdoor residential sites. In 

addition, this assessment evaluates whether diuron use on these crops is expected to result 

in modification of the species’ designated critical habitat. This ecological risk 

assessment has been prepared consistent with a settlement agreement in the case Center 

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

entered in Federal District Court for the Northern District of California on October 20, 

2006. 

Diuron is a broad-spectrum residual herbicide registered for pre-emergent and postemergent control of both broadleaf and annual grassy weeds. When diuron is used on 

pre-emergent weeds, it allows seeds to germinate normally, but causes them to lose their 

green color, after which they soon die of starvation (Ferrell et al., 2004). In the U.S. 

diuron is used on a variety of fruit and nut crops, grains, cotton, corn, sorghum, mint, 

gum, asparagus, sugarcane, seed crops, coffee, hay, cut flowers, and for fallow and idle 

cropland use. It may be used in irrigation and drainage systems when water is not 

present. 

It is also used to control weeds on hard surfaces, such as, roads, railway tracks, and paths 

(at around 3 kg/ha), and to control weeds in crops, such as, pear and apple trees, forestry, 

ornamental trees and shrubs, pineapples, sugar cane, cotton, alfalfa and wheat (at lower 

rates of around 1.8 kg/ha). It can be used for both pre-emergent and knockdown weed 

control. Its use in some locations is becoming limited due to the development of resistant 

weed species. In some products it is combined with other active ingredients such as 

glyphosate, bromacil, hexazinone, amitrole and 2,4-D. In some countries diuron is also 

registered for aquatic weed control, as a cotton defoliant, and for use in home aquaria and 

fish ponds. It is used as a booster biocide in antifouling paints where its activity 

enhances the efficacy of copper in these products. 

11
In this assessment, direct and indirect effects to the CRLF and potential modification to 

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

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

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

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

include 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 diuron is based on an action area. The action area is the area directly or 

indirectly affected by the federal action, as indicated by the exceedance of the Agency’s 

LOCs. It is acknowledged that the action area for a national-level FIFRA regulatory

decision associated with a use of diuron 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 diuron in 

accordance with current labels: 

• “No effect”; 

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

• “May affect and likely to adversely affect”. 

Designated critical habitat identifies specific areas that have the physical and biological 

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

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

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

and dispersal habitat. 

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

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

designated critical habitat, a “no effect” determination is made for use of diuron as it 

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

indirect effects to individual CRLFs are anticipated or effects may impact the PCEs of the 

CRLFs designated critical habitat, a preliminary “may affect” determination is made for 

the FIFRA regulatory action regarding diuron. 

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

CRLF “may affect” this species or its designated critical habitat, additional information is 

considered to refine the potential for exposure and for effects to the CRLF and other 

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

12
vertebrates and invertebrates, aquatic plants, riparian vegetation, etc.). Additional 

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

habitat and diuron use sites) and further evaluation of the potential impact of diuron on 

the PCEs is also used to determine whether modification of designated critical habitat 

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

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

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

or the PCEs of its designated critical habitat. This information is presented as part of the 

Risk Characterization in Section 5 of this document. 

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

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

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

(defined in Section 2.7), critical habitat analysis for diuron 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 diuron that may alter the PCEs of the 

CRLFs critical habitat form the basis of the critical habitat impact analysis. Actions that 

may affect the CRLFs designated critical habitat have been identified by the Services and 

are discussed further in Section 2.6. 

2.2 Scope 

Diuron is registered in California for a variety of agricultural and non-agricultural uses 

(see Table 2.2). 

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

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

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

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

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

use or potential use of diuron in accordance with the approved product labels for 

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

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

assessment and effects determination addresses currently registered uses of diuron 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. 

The major route of dissipation for diuron is microbial degradation in water. Here the 

major metabolism degradate is N’-(3-chlorophenyl)-N,N-dimethylurea (MCPDMU). 

Diuron also degrades through photolysis in both water and soil; here the major degradates 

13
are CO2 and N’-(3,4-dichlorophenyl)-N-methylurea (DCPMU), respectively. A more 

detailed discussion of the major and minor degradates formed in the decomposition of 

diuron is presented in Section 2.4. 

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). 

Diuron has registered products that contain multiple active ingredients. However, there 

are no available data on mixtures containing diuron in the open literature. There are no 

product LD50 values, with associated 95% Confidence Intervals (CIs) available for diuron 

among the data submitted to the Agency. The assessment will be based on the toxicity of 

a single active ingredient of diuron. Analysis of the available open literature and acute 

mammalian toxicity data for multiple active ingredient products relative to the single 

active ingredient is provided in Appendix A. 

As discussed in USEPA (2000) a quantitative component-based evaluation of mixture 

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

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

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

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

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

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

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

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

on the toxicity of diuron is the only reasonable approach that employs the available data 

to address the potential acute risks of the formulated products.

2.3 Previous Assessments 

Diuron has been registered in the United States since 1967 for use as an herbicide, 

mildewcide and algaecide. A Registration Standard, titled "Guidance for the 

Reregistration of Pesticide Products Containing Diuron as the Active Ingredient" 

was released in 1983. The Registration Standard involved a thorough review of 

the scientific data base underlying pesticide registrations and an identification of 

essential but missing studies which may not have been required when the product 

was initially registered or studies that were considered insufficient. Subsequent 

Data Call-Ins (DCIs) were issued in 1990, and 1995 for diuron. 

14
In 2003, the Office of Pesticide Programs (OPP) requested the initiation of ESA section 

7(a) (2) consultation regarding the potential impact of diuron on the Evolutionary 

Significant Units (ESU) of the Pacific salmon and steelhead. OPP determined that the 

use of diuron on certain crops (peaches, walnuts, filberts) may affect 10 ESUs at 

application rates above 3.2 lb ai/A, may affect 7 ESUs at application rates above 2.2 lb 

ai/A, and will have no effect on 10 ESUs at any labeled agricultural rate. The use of 

diuron for non-crop sites, especially rights-of-way, may affect 25 ESUs, may affect but is 

not likely to adversely affect one ESU, and will have no effect on one ESU. 

To mitigate risks of concern posed by the use of diuron, EPA in its Reregistration 

Eligibility Decision (RED) for diuron, dated September 2003, noted a number of label 

amendments identified by EPA or agreed to by the registrant(s) to address the worker, 

residential and ecological concerns; these included: 

• All wettable powder products will be voluntarily canceled. 

• Reduction in application rate and increased treatment intervals, and limit on the 

number of applications for some crops. 

• Use of the backpack sprayer is prohibited. 

• Implement use of PPE and engineering controls for some workers. 

• Eliminate aerial applications except for rights-of-way, alfalfa, cotton, winter 

barley, winter wheat, sugarcane, and grass seed crops. 

• Implement best management practices to reduce spray drift. 

2.4 Stressor Source and Distribution 

2.4.1 Environmental Fate Properties 

The environmental fate database for diuron is essentially complete. Diuron is stable in 

neutral media at normal temperatures, and is hydrolyzed by acid and alkalis. It is stable 

towards oxidation and moisture under normal conditions and decomposes at 180-190° C 

(Helliwell et al., 1998). 

Diuron is mobile and has the potential to leach to groundwater and to contaminate surface 

waters. An upgradable adsorption/desorption/leaching study (MRID 44490501) showed 

that diuron has an average Koc value of 920 and Freundlich Kadss 

values (7.9-28). Sorption 

of diuron to soil is highly correlated with soil organic matter. In addition, diuron has 

relatively low water solubility (42 ppm) and low volatility (6.9 x 10

-8

mm Hg at 25° C). 

Although stable to hydrolysis at pH 5, 7, and 9, the minor degradate 3,4-dichloroaniline 

(3,4-DCA) was identified in all hydrolysis test solutions (0.5% of applied). The 

calculated half-lives for diuron in aqueous and soil photolysis studies were found to be 43 

and 173 days respectively. 

In soil, the photolysis degradate were all minor, N'-(3,4-dichlorophenyl)-N-methylurea 

(DCPMU) at 3.6 percent, demethylated DCPMU (DCMU), 3,4-dichloroaniline (3,4-

DCA), and 3,3',4,4'-tetrachlorobenzene (TCAB) at 0.7 percent, 0.370 ppm, and 0.038 

15
ppm respectively. The calculated half-lives in aerobic and anaerobic soil metabolism 

studies were 372 (aerobic) and 1,000 (anaerobic) days. Under aerobic conditions, the 

major degradate was DCPMU (20.9-22.5 % of the amount applied at 365 days), and 

minor degradates were DCPU and CO2. Under anaerobic conditions, the only major 

degradate identified was DCPMU, which accounted for a maximum of 10.3% of applied 

(at 45 days). 

In contrast to its persistence in laboratory studies of hydrolysis, aqueous and soil 

photolysis, and aerobic and anaerobic soil metabolism, diuron degraded relatively quickly 

in aquatic metabolism laboratory studies, with a half-life of 33 days under aerobic 

conditions and of 5 days under anaerobic conditions. The major metabolism degradate 

under aerobic conditions was N'-(3-chlorophenyl)-N,N-dimethylurea (MCPDMU) which 

reached 25 % of the applied dose by the end of the study and was evenly distributed 

between the soil and aqueous phase. Other degradates identified were DCPMU (9.2 

percent) and CPMU (8 percent), and were primarily associated with the soil phase. The 

three major degradates under anaerobic conditions were MCPDMU which was mainly 

associated with the aqueous phase, 1,1-dimethyl-3-phenylurea (PDMU), and CPMU. 

In terrestrial field dissipation studies in FL, MS, and CA with sand, silt loam, and silty 

clay loam soils, diuron dissipated in bare ground plots with half-lives of 73, 139, and 

133 days, respectively. The major degradate DCPMU dissipated in the same plots 

with half-lives of 217, 1733, and 630 days. In aquatic field dissipation studies, halflives were 115-177 days and the major degradate was DCPMU. These studies were 

conducted in drainage channel berms and soil, and thus should be compared 

quantitatively to soil laboratory studies, not aquatic studies. 

The environmental degradates of toxicological concern to humans and other non-target 

species include 3,4-DCA and TCAB. However, EPA’s Metabolism Assessment Review 

Committee (MARC) has concluded that residues of 3,4-DCA is not a residue of concern 

for diuron as 3,4-DCA (<1%) was not a significant residue in any metabolism or 

hydrolysis study. Moreover, TCAB (0.038 ppm) residue was only found in the soil 

photolysis study which is a minor route of degradation for diuron.

 

Degradation half-lives for the parent are presented in Table 2.1. This table also presents 

the major and minor degradates of the parent compound. The chemical structures of 

diuron and the metabolites (DCPMU, DCPU, DCA, and MCPDMU) are presented in 

Appendix B. 

16
Table 2.1 Summary of Diuron Half-lives 

Study Value (units) Major Degradates 

Minor Degradates

MRID # 

Hydrolysis Stable at pH 5, 7, 9 3,4-DCA (~2%) 41418804 

Direct 

Aqueous 

Photolysis 

T 1⁄2 = 43 days CO2 41418805 

Soil 

Photolysis 

T 1⁄2 = 173 days DCPMU (3.6%) 

DCMU (≤ 0.7%), 3,4-

DCA (0.370 ppm), TCAB 

(0.038 ppm) 

41719302 

Aerobic Soil 

Metabolism T 1⁄2 = 372 days 

DCPMU (22.5%) 

DCPU (3.4%), CO2

(3.36%) 

41719303 

Anaerobic 

Soil 

Metabolism 

T 1⁄2 = 1,000 days DCPMU (10.3 %) 41418806 

Anaerobic 

Aquatic 

Metabolism 

T 1⁄2 = 5 MCPDMU (83%) 

PDMU (13%), mCPMU 

(23%) 

42661901 

Aerobic 

Aquatic 

Metabolism 

T 1⁄2 = 33 MCPDMU (25%) 

DCPMU (9.2%), CPMU 

(8%)

42260501 

Kd-ads / Kd-des 

(mL/g) 

14 

Material balances were 

not reported. 44490501 

Terrestrial 

Field 

Dissipation 

Sand = 73 days 

Silt Loam = 139 days 

Silty Clay Loam = 133 

days 

DCPMU (Sand = 217 

days, Silt Loam = 1733 

days, Silty Clay Loam = 

630 days) 

44654001 

44865001 

2.4.2 Environmental Transport Mechanisms 

AIR 

Diuron is applied by broadcast or band spray on soil surface using ground or aerial 

equipment, suggesting that there is a possibility of drift. Drift is quantitatively 

considered in the exposure assessment. However, diuron is non-volatile, as indicated by 

its low vapor pressure of 6.90 x10-8 mm Hg (25o C), and a low Henry’s law constant of 

5.10 x 10-10 atm m3 mol-1. These properties indicate that diuron is unlikely to be 

dispersed in air over a large area and has a low tendency to volatilize from water or moist 

soils. Volatilization is insignificant except when diuron is exposed on the soil surface for 

several days or weeks under hot, dry conditions (Hess and Warren, 2002). 

17
WATER 

Diuron’s KOC (468-1666) indicates a relatively low tendency to sorb to soils and 

sediments, while its hydrolysis and aqueous photolysis half-lives are relatively long. 

Consequently diuron is both mobile and relatively persistent, and is therefore prone to 

off-site movement in surface runoff, and migration to ground water. As discussed in the 

field monitoring studies below, diuron in runoff water is above Agency LOCs for direct 

effects on the CRLF (fish LC50 of 400 to 710 ppb). 

Andrieux et al. (1997) assessed losses of diuron via runoff from vineyards under natural 

Mediterranean climate conditions. During the growing season of 1994 and 1995, runoff 

and diuron concentrations were monitored at two field sites, one tilled and one under notill management. Despite a time lag of greater than 4 months between application and the 

first runoff event in 1994, diuron concentrations in overland flow exceeded 200 ug/L at 

the no-till in both years of investigations. In 1995, the first strong rainfall following 

application removed 60% of total diuron runoff loss at both sites, although it accounted 

for 17 and 7% of the total seasonal runoff volume at the no-till and tilled site, 

respectively. In 1995, seasonal diuron loss at the no-till sites was 1.71 % and at the tilled 

site was 0.68%. No herbicide residues were detected in soil samples collected 1 week 

prior to chemical spraying in 1994 and 1995. The authors suggested that moderate 

temperatures and moisture conditions during winter facilitated complete microbial 

decomposition. In four-months of soil sampling following herbicide treatment there were 

no detections below a soil depth of 2 cm at the no-till plot. However, at the tilled site, 

pesticides were present in samples at depths of 0-15 cm 17 days after spraying to three 

months. The authors suggested that infiltration increases attributable to tillage effects 

accounted for diuron distribution in the soil profile. The authors concluded that the 

persistence of diuron coupled with tillage may provide the potential for leaching. 

In contrast, no-till practices have been shown to contribute to ground water 

contamination under certain California conditions. 

Braun and Hawkins (1991) conducted a rainfall runoff monitoring study in a citrusgrowing region of Tulare County, California, where growers commonly avoid cultivation 

of citrus row middles. The row middles under such a management practice are typically 

compacted, with low infiltration rates and a high tendency to yield runoff during storm 

events. One method of disposing of excess runoff water in the area is to utilize dry wells 

that allow drainage of run off through the shallow hard-pan soil layer. This mechanism 

allows direct introduction of residues to the subsurface and subsequent potential 

migration to shallow ground water. Although diuron applications had occurred at least 2 

months prior to sampling, relatively high concentrations of diuron in runoff water 

entering dry wells were found, ranging up to 890 μg L-1. The data provide strong 

evidence that the widespread regional presence of diuron in ground water is at least 

partially attributable to contaminated runoff water entering dry wells. 

Spurlock et al. (1997) examined the runoff of diuron, simazine, and bromacil from citrus 

orchard middles under simulated rainfall conditions. Peak diuron runoff concentrations 

were comparable to those reported by Braun and Hawkins (1991), ranging from 600 to 

18
1700 μg L-1. Little difference in runoff concentrations of the three pre-emergent 

herbicides was observed indicating that herbicide substitution would be an ineffective 

mitigation measure. They concluded that low soil permeability arising from hardpan 

layers or compaction drives the runoff process. These same factors are probably 

responsible for diuron off-site movement in runoff water from rights-of-way because 

rights-of-way are generally engineered to maximize water run off. 

Powell et al. (1996) studied off-site movement of diuron in surface water from a highway 

shoulder right-of-way application. Diuron was applied in a spray to a 2.4-meter wide 

strip next to the highway pavement, at a rate of 3.59 kg diuron ha-1. Simulated rain (13 

mm in 1 hr) was applied to plots on treated highway shoulders at three sites. Diuron was 

detected at two of three sites. Concentrations of diuron were as high as 1770 μg L-1 in 

runoff (combined water and sediment), from sampling conducted one day after herbicide 

application. From 0.2 to 5.4% of the diuron applied to highway shoulders moved off-site 

in runoff during the simulations. Simulated rain was applied to treated highway shoulder 

at intervals of 0, 2, and 4 wk after herbicide application. The highest percentages were 

observed when rain was simulated 1 day after herbicide application. Diuron is typically 

applied in winter or early spring during California’s rainy season to control weeds. This 

practice, coupled with relatively high use, is one reason why diuron is commonly 

detected pesticides in California surface water. Diuron was detected in more than half of 

955 surface water samples analyzed and reported in DPR’s surface water database. 

Typical reporting limits for the diuron analyses are 0.05 - 0.10 μg-1. While detected 

concentrations range from 0.01 to 30.6 μg L-1, the majority of concentrations range from 

0.1 to 1 μg L-1. These concentrations were approximately equal to the 20th and 80th 

percentiles of detected concentrations. The highest concentrations and most frequent 

detections occur during December-March, which coincides with both the rainy season 

and the peak diuron application season. 

Microbes are the primary agents in the degradation of diuron in aquatic environments. 

The aerobic biodegradation pathway for diuron is well established, proceeding by 

successive demethylation steps to form DCPMU, DCPU [1-(3,4-dichlorophenyl)urea] 

and DCA (3,4-dichloroaniline). Reductive dechlorination has been observed in anaerobic 

pond sediments and leads to the formation of the dechlorinated product, 3-(3-

chlorophenyl)-1,1 dimethylurea (Field et al., 1997). Boule et al. (1997) reported that the 

major photoproducts observed in the photolysis of diuron [3-(3,4-dichlorophenyl)-1,1-

dimethylurea] in aqueous solution resulted from a heterolytic substitution of chlorine by 

OH. A wavelength effect was observed: at 254 nm the formation of 3-(4-chloro-3-

hydroxyphenyl)-1,1-dimethylurea accounted for more than 90% of the conversion, 

whereas when the solution was irradiated in 'black light' (85% of photons emitted at 365 

nm, about 7% at 344 nm), the major photoproduct was 3-(3-chloro-4-hydroxyphenyl)-

1,1-dimethylurea. However, as noted previously, photolysis is not generally a principal 

route of diuron degradation in aqueous systems. 

SOIL 

Diuron is moderately to highly persistent and mobile in soils. The commonly reported 

average field dissipation half-life is 115 days (MRID 44654001), although such half-lives 

19
are typically highly variable. Phytotoxic residues generally dissipate within a season 

when applied at low selective rates. At higher application rates, residues may persist for 

more than one year (Kidd and James, 1991). Microbial degradation is the primary means 

of diuron dissipation from soil. Photodegradation is not considered a primary dissipation 

route, but losses can be significant if diuron remains on the soil surface for several days 

or weeks (Hess and Warren, 2002). Mobility in the soil is related to organic matter and 

to the type of the residue. The metabolites are less mobile than the parent. Similar to 

many other pesticides, diuron sorption is highly correlated with organic matter (Spurlock 

and Biggar, 1994). Consequently leaching is greatest in low organic matter soils. Other 

soil conditions that favor diuron leaching include high soil permeability to water, such as 

in coarse soils. 

Due to the diuron’s persistence and mobility, the herbicide is commonly detected in 

California’s ground water (Troiano et al., 2001). As of June 2004, there were confirmed 

detections of diuron in 418 California water wells reported in DPR’s well inventory 

database (WIDB). Typical diuron reporting limits in the WIDB are 0.05 μg L-1, and 

detected diuron concentrations range up to 3.96 μg L-1. Diuron’s use is regulated under 

DPR’s ground water protection regulations (online at http://www.cdpr.ca.gov). 

In a right-of-way runoff study by Powell et al. (1996) post-simulation and end of season 

soil cores at the two sites with measurable runoff had detectable diuron residues only in 

the top 15 cm (0.5 ft). However, cores taken prior to application had diuron down to 0.30 

m, presumably from the past year’s application. At a third simulated rainfall site, no 

runoff left the plots. The absence of detectable residues in soil to a depth of 3.0 m (10 ft) 

suggests that much of the herbicide applied to the shoulder may have leached rapidly 

through the coarse gravelly soil. The soil data was insufficient to yield definite 

conclusions about leaching in infiltration areas. However, the levels of diuron found in 

runoff suggest that dry wells, if present, would provide an important conduit for the 

transport of diuron to groundwater. The authors concluded that rights-of-ways with high 

percentages of gravel and sand in the soil may be of concern, since diuron may be 

transported rapidly to depths greater than 0.3 m. 

Bogarets et al. (2000) studied diuron’s microbial degradation and ecotoxicology to 

investigate its fate after application to soils. Quantitative biodegradation assays were 

executed with fungal strains, showing that diuron was degraded but not entirely 

mineralized. A series of tests were carried out to choose the most efficient fungal strain 

for diuron degradation. Among the fungal strains tested, only three were able to 

transform diuron to any extent (up to 50%) after 7 days of incubation: B. bassiana, C. 

elegans, and M. isabellina. No degradation occurred using fungal strains: F. oxysporum 

and G. candidum. Although C. elegans was the most efficient since there was no diuron 

remaining after seven days of incubation. Diuron degradation by the three fungal strains 

led to the formation of two metabolites obtained in different ratios according to the 

microorganism. For the three fungal strains, diuron degradation led to the formation of 

the demethylated products. The identified metabolites were synthesized in sufficient 

amounts to confirm their structures and determine their non-target toxicity using four 

biotests. According to the Microtox test, the metabolites N- (3,4-dichlorophenyl)-N20
methylurea and N-3, 4-dichlorophenylurea presented a three times higher toxicity than 

that of diuron. 

2.4.3 Mechanism of Action 

Diuron is a systemic substituted phenylurea herbicide. Diuron is taken up by plants and 

rapidly translocated. Diuron primarily functions by inhibiting the Hill reaction in 

photosynthesis, limiting the production of high-energy compounds such as adenosine 

triphosphate (ATP) used for various metabolic processes. Diuron binds to the QBbinding niche on D1 protein of the photosystem II complex in chloroplast thylakoid 

membranes, thus blocking electron transport from QA to QB. This process prevents CO2

fixation and the production of ATP and other high energy compounds which are needed 

for plant growth. The inability to reoxidize QA promotes the formation of triplet state 

chlorophyll, which interacts with ground state oxygen to form singlet oxygen. Both 

triplet chlorophyll and singlet oxygen can extract hydrogen from unsaturated lipids, 

producing a lipid radical and initiating a chain reaction of lipid peroxidation. Lipids and 

proteins are attacked and oxidized, resulting in loss of chlorophyll and carotenoids, and in 

leaky membranes which cause cells and cell organelles to dry and disintegrate rapidly 

(Hess and Warren, 2002).

2.4.4 Use Characterization 

There are currently 72 product registrations for diuron including two Special Local Needs 

registrations in California. 

Diuron contains two significant impurities from the manufacturing process 3,3',4,4'-

tetrachloroazobenzene (TCAB) and 3,3',4,4'-tetrachloroazoxybenzene (TCAOB), both 

‘dioxin-like’ substances that are known to cause a skin disease known as chloracne. 

TCAB levels between 0.15 and 28 ppm have been found in diuron samples tested. 

TCAOB is present at lower levels. Since these residues appear to be present at 

insignificant levels, they are not assessed further in this assessment. 

In addition to agricultural uses, diuron also has widespread use in non-agricultural 

applications, especially industrial and rights of way uses, where often in combination 

with other herbicides it provides total vegetation control. These applications include 

along fence lines, pipelines, powerlines, railway lines, roads, footpaths; in timber yards 

and storage areas; and around commercial, industrial and farm buildings, electrical 

substations, and petroleum storage tanks. It has some use as an algaecide in ornamental 

ponds, fountains, and aquaria, but not natural water bodies. 

Diuron is one of the most commonly used pesticides in California. Reported statewide 

use in 2002 was 1,303,745 pounds. Of this reported use, 48 percent was applied to rights 

of way, followed by 18 percent and 14 percent on alfalfa and oranges, respectively. 

Additional 2002 uses in California included grapes (4.3 percent), landscape maintenance 

(3.7 percent), walnuts (2.6 percent), and cotton (1.6 percent). Diuron use has been 

reported in all 58 counties in California.

21
Diuron is often used in combination with other herbicides such as bromacil, hexazinone, 

paraquat, thiadiazuron, imazapyr monosodium, sodium chlorate, sodium metaborate, and 

copper sulfate (U.S. EPA, 2004a). 

Diuron is available in wettable powder, granular, flowable, pelleted/tableted, liquid 

suspension, and soluble concentrate formulations. However, true granular applications 

are not allowed in California and are therefore not assessed further in this document. 

Diuron is applied using the following equipment: groundboom sprayer, aerial equipment, 

chemigation, rights-of-way sprayer, high-pressure handwand, low-pressure handwand, 

tractor-drawn spreader, granular backpack spreader (since true granular applications are 

not allowed in California, this application method will not be assessed), push-type 

spreader, airless sprayer, paintbrush, shaker-type applicator, backpack sprayer, belly 

grinder, and by hand. . 

For agricultural uses, labeled single application rates range from 0.4 to 12 lbs active 

ingredient (ai) per acre (A). For non-agricultural uses labeled rates are 12 lbs ai/acre. 

Diuron may be applied to non-agricultural areas 1 to 2 times per year. 

A national map showing the estimated poundage of diuron used for agricultural purposes 

in 2002 by county is presented in Figure 2.1. The map was obtained from the U.S. 

Geological Survey (USGS), National Water Quality Assessment Program (NAWQA) 

website (URL: http://water.usgs.gov/nawqa/pnsp/usage/maps/show_map.php?year=02&map=m1991). 

22
Source: USGS, 2002 

Figure 2.1 Diuron Agricultural 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 diuron represents the FIFRA regulatory action, therefore, 

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

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

selection of appropriate modeling scenarios and inputs. 

Table 2.2 presents the uses allowed in California that will be focused on in this 

assessment along with the corresponding scenarios used for the Tier II aquatic modeling. 

Please see Section 3.0 for further explanation. 

23
Table 2.2 Active Use Sites for Diuron as Applicable to California 

Crop Scenario 

AGRICULTURAL RIGHTS-OF-WAY/FENCEROWS/HEDGEROWS CA impervious RLF and CA 

residential RLF 

ALFALFA CA alfalfa OP 

APPLE, PEACH, PEAR CA fruit STD 

ARTICHOKE, ASPARGUS CA row crop RLF 

BANANA, PLANTAIN, PAPAYA CA avocado RLF 

BLACKBERRY, BOYSENBERRY, DEWBERRY, LOGANBERRY, 

RASPBERRY (BLACK/RED) OR Berries OP 

BLUEBERRY CA wine grapes RLF 

CITRUS CA citrus STD 

CORN, FIELD CA corn OP 

FILBERT (HAZELNUT), PECAN, WALNUT (ENGLISH/BLACK) CA almond STD 

GRAPE CA grapes STD 

OLIVE CAOliveRLF-V2 

PEPPERMINT, SPEARMINT OR mint STD 

SORGHUM, WHEAT CA wheat RLF 

Non-Food/Feed Crops 

COTTON CA cotton STD 

Non-Crop Land 

AIRPORTS/ LANDING FIELDS, DRAINAGE SYSTEMS, INDUSTRIAL 

AREAS (OUTDOOR), SEWAGE DISPOSAL AREAS. PAVED AREAS 

(PRIVATE ROADS/SIDEWALKS) 

CA impervious RLF 

BERMUDAGRASS CA rangeland hay RLF 

GRASSES GROWN FOR SEED CA turf RLF 

IRRIGATION SYSTEMS CA residential RLF 

ORNAMENTAL HERBACEOUS PLANTS CA Nursery 

NON-AGRICULTURAL RIGHTS OF WAY CA impervious RLF and CA 

residential RLF 

UNCULTIVATED AG, UNCULTIVATED NON-AG CA Rangeland Hay_V2 RLF 

* Uses provided by BEAD – July 23, 2008, Scenarios provided by EFED

24
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 statelevel 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 diuron by county in this Californiaspecific assessment were generated using CDPR PUR data. Four years (2002-2005) of 

usage data were included in this analysis. Available 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. A summary of diuron usage for all California use sites is 

provided in Appendix C, and a summary of the CDPR PUR usage data can be found in 

Appendix D. 

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 I. 

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 

 

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

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

Populations currently exist along the northern California coast, northern Transverse 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

the CNDDB are described in further detail in 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. 

26
Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

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

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

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

Recovery Units

1. Sierra Nevada Foothills and Central Valley

2. North Coast Range Foothills and Western 

Sacramento River Valley

3. North Coast and North San Francisco Bay

4. South and East San Francisco Bay

5. Central Coast

6. Diablo Range and Salinas Valley

7. Northern Transverse Ranges and Tehachapi

Mountains

8. Southern Transverse and Peninsular Ranges

Core Areas

1. Feather River

2. Yuba River- S. Fork Feather River

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

4. Cosumnes River

5. South Fork Calaveras River*

6. Tuolumne River*

7. Piney Creek*

8. Cottonwood Creek

9. Putah Creek – Cache Creek*

10. Lake Berryessa Tributaries

11. Upper Sonoma Creek

12. Petaluma Creek – Sonoma Creek

13. Pt. Reyes Peninsula

14. Belvedere Lagoon

15. Jameson Canyon – Lower Napa River

16. East San Francisco Bay

17. Santa Clara Valley

18. South San Francisco Bay

19. Watsonville Slough-Elkhorn Slough

20. Carmel River – Santa Lucia

21. Gablan Range

22. Estero Bay

23. Arroyo Grange River

24. Santa Maria River – Santa Ynez River

25. Sisquoc River

26. Ventura River – Santa Clara River

27. Santa Monica Bay – Venura Coastal Streams

28. Estrella River

29. San Gabriel Mountain*

30. Forks of the Mojave*

31. Santa Ana Mountain*

32. Santa Rosa Plateau

33. San Luis Ray*

34. Sweetwater*

35. Laguna Mountain*

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

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

Designations for CRLF 

27
Other Known Occurrences from the CNDBB 

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

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

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

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

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

information on the CNDDB. 

2.5.2 Reproduction 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

live 8 to 10 years (USFWS 2002). Figure 2.3 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 = 

Adults and juveniles can be present all year

Young Juveniles

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

Light Blue = Breeding/Egg Masses

Green = 

Orange = 

Adults and juveniles can be present all year

Tadpoles (except those that over-winter)

Young Juveniles

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) 

28
via mouthparts designed for effective grazing of periphyton (Wassersug, 1984, 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2.5.4 Habitat 

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

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

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

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

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

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

1997). Dense vegetation 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 

29
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 

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

exposure (Alvarez 2000). 

2.6 Designated Critical Habitat 

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

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

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

areas (previously discussed in Section 2.5.1) is provided in Table 2.4. 

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

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

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

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

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

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

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

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

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

destruction or adverse modification of critical habitat. 

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

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

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

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

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

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

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

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

protected from disturbance or are representative of the historic geographical and 

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

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

• Breeding aquatic habitat; 

• Non-breeding aquatic habitat; 

• Upland habitat; and 

30
• Dispersal habitat. 

Further description of these habitat types is provided in Attachment 1. 

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

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

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

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

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

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

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

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

rule exempting routine ranching activities associated with livestock ranching from 

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

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

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

see Attachment 1 for a full explanation on this special rule. 

USFWS has established adverse modification standards for designated critical habitat 

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

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

actions related to use of diuron 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) Significant increase in sediment deposition within the stream channel or pond or 

disturbance of upland foraging and dispersal habitat that could result in 

elimination or reduction of habitat necessary for the growth and reproduction of 

the CRLF by increasing the sediment deposition to levels that would adversely 

affect their ability to complete their life cycles. 

(3) Significant alteration of channel/pond morphology or geometry that may lead to 

changes to the hydrologic functioning of the stream or pond and alter the timing, 

duration, water flows, and levels that would degrade or eliminate the CRLF 

and/or its habitat. Such an effect could also lead to increased sedimentation and 

degradation in water quality to levels that are beyond the CRLF’s tolerances. 

(4) Elimination of upland foraging and/or aestivating habitat or dispersal habitat. 

(5) Introduction, spread, or augmentation of non-native aquatic species in stream 

segments or ponds used by the CRLF. 

(6) Alteration or elimination of the CRLF’s food sources or prey base (also 

evaluated as indirect effects to the CRLF). 

31
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 diuron is expected to directly impact living 

organisms within the action area, critical habitat analysis for diuron is limited in a 

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

reasonably linked to biologically mediated processes. 

2.7 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 diuron is likely to encompass considerable portions of the 

United States based on the large array of agricultural uses. However, the scope of this 

assessment limits consideration of the overall action area to those portions that may be 

applicable to the protection of the CRLF and its designated critical habitat within the state 

of California. The Agency’s approach to defining the action area under the provisions of 

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

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

below the Agency’s defined 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 diuron may be expected to have on the 

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

best available information concerning the use of diuron and its fate and transport within 

the state of California. Specific measures of ecological effect for the CRLF that define 

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

modification of its critical habitat, including reduction in survival, growth, and fecundity 

as well as the full suite of sublethal effects available in the effects literature. Therefore, 

the action area extends to a point where environmental exposures are below any 

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

organism, organ, tissue, and cellular level of organization. In situations where it is not 

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

limited and is assumed to be the entire state of California. 

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

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

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

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

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

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

32
diuron, agricultural and non-agricultural uses listed in Table 2.2 are considered part of the 

federal action evaluated in this assessment. 

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

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

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

land cover data for the state of California. The initial area of concern is defined as all 

land cover types and the stream reaches within the land cover areas that represent the 

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

agricultural uses that make up the initial area of concern for diuron is presented in Figure 

2.4. Diuron, in fact, is used in all 58 of California’s counties when you consider nonagricultural uses as well. 

33
Figure 2.4 Initial area of concern, or “footprint” of potential use, for diuron 

34
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. 

As stated in the 2003 Registration Eligibility Decision (RED), diuron has been 

characterized as a “known/likely” human carcinogen, based on urinary bladder 

carcinomas in both sexes of the Wistar rat, kidney carcinomas in the male rat (a rare 

tumor), and mammary gland carcinomas in the female NMRI mouse. Because of this, 

the spatial extent of the action area (i.e., the boundary where exposures and potential 

effects are less than the Agency’s LOC) for diuron cannot be determined. Therefore, it is 

assumed that the action area encompasses the entire state of California, regardless of the 

spatial extent (i.e., initial area of concern or footprint) of the pesticide use(s). This 

determination is supported by the fact that diuron use has been reported in all 58 counties 

in California. 

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 diuron using the most sensitive endpoint (most sensitive plant toxicity study: 

Vegetative Vigor - Tomato (Lycopersicon esculentum)) suggests that a maximum spray 

drift distance of at least 1,000 feet is necessary. The Tier 1 ground analysis allows users 

to evaluate off-site deposition and exposure out to 1,000 ft downwind from the location 

of the application. Further detail on the spray drift analysis is provided in Section 3.2.5. 

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

considers the downstream extent of diuron that exceeds the LOC (discussed in Section 

3.2.6). 

 

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

diuron 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 diuron has historically been used on a wide variety of 

agricultural and non-agricultural uses. 

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 diuron 

 

3

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

35
(e.g., runoff, spray drift, etc.), and the routes by which ecological receptors are exposed 

to diuron (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 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 diuron is provided in Table 2.4. 

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. Amphibian acute LC50 (ECOTOX) or most sensitive 

fish acute LC50 (guideline or ECOTOX) if no suitable 

amphibian data are available 

1b. Amphibian chronic NOAEC (ECOTOX) or most 

sensitive fish chronic NOAEC (guideline or ECOTOX) 

1c. Amphibian early-life stage data (ECOTOX) or most 

sensitive fish early-life stage NOAEC (guideline or 

ECOTOX) 

Indirect Effects and Critical Habitat Effects 

2. Survival, growth, and reproduction of 

CRLF individuals via indirect effects on 

2a. Most sensitive fish, aquatic invertebrate, and aquatic 

plant EC50 or LC50 (guideline or ECOTOX) 

 

4

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

Appendix J. 

36
Assessment Endpoint Measures of Ecological Effects4

aquatic prey food supply (i.e., fish, 

freshwater invertebrates, non-vascular plants) 

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. 

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 diuron 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 diuron effects data are available. 

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

the following, as specified by USFWS (2006): 

1. Alteration of water chemistry/quality including temperature, turbidity, and 

oxygen content necessary for normal growth and viability of juvenile and 

adult CRLFs. 

37
2. Alteration of chemical characteristics necessary for normal growth and 

viability of juvenile and adult CRLFs. 

3. Significant increase in sediment deposition within the stream channel or pond 

or disturbance of upland foraging and dispersal habitat. 

4. Significant alteration of channel/pond morphology or geometry. 

5. Elimination of upland foraging and/or aestivating habitat, as well as dispersal 

habitat. 

6. Introduction, spread, or augmentation of non-native aquatic species in stream 

segments or ponds used by the CRLF. 

7. Alteration or elimination of the CRLF’s food sources or prey base. 

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

are described in Table. 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). 

38
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 

and/or increase in sediment deposition within the 

stream channel or pond: aquatic habitat (including 

riparian vegetation) provides for shelter, foraging, 

predator avoidance, and aquatic dispersal for juvenile 

and adult CRLFs. 

a. Most sensitive aquatic plant EC50 (guideline or 

ECOTOX) 

b. Distribution of EC25 values for terrestrial monocots 

(seedling emergence, vegetative vigor, or ECOTOX) 

c. Distribution of EC25 values for terrestrial dicots 

(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 

Elimination and/or disturbance of dispersal habitat: 

Upland or riparian dispersal habitat within designated 

units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites 

including both natural and altered sites which do not 

contain barriers to dispersal 

Reduction and/or modification of food sources for 

terrestrial phase juveniles and adults 

Alteration of chemical characteristics necessary for 

normal growth and viability of juvenile and adult 

CRLFs and their food source. 

a. Distribution of EC25 values for monocots (seedling 

emergence, vegetative vigor, or ECOTOX) 

b. Distribution of EC25 values for dicots (seedling 

emergence, vegetative vigor, or ECOTOX) 

c. Most sensitive food source acute EC50/LC50 and NOAEC 

values for terrestrial vertebrates (mammals) and 

invertebrates, birds or terrestrial-phase amphibians, and 

freshwater fish. 

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. 

39
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 diuron to the environment. The 

following risk hypotheses are presumed for this endangered species assessment: 

The labeled use of diuron within the action area may: 

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

fecundity; 

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

supply; 

• Indirectly affect the CRLF or modify designated critical habitat by reducing or 

changing the composition of the aquatic plant community in the ponds and streams 

comprising the species’ current range and designated critical habitat, thus affecting 

primary productivity and/or cover; 

• Indirectly affect the CRLF or modify designated critical habitat by reducing or 

changing the composition of the terrestrial plant community (i.e., riparian habitat) 

required to maintain acceptable water quality and habitat in the ponds and streams 

comprising the species’ current range and designated critical habitat; 

• Modify the designated critical habitat of the CRLF by reducing or changing 

breeding and non-breeding aquatic habitat (via modification of water quality parameters, 

habitat morphology, and/or sedimentation); 

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

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

• Modify the designated critical habitat of the CRLF by reducing or changing 

upland habitat within 200 ft of the edge of the riparian vegetation necessary for shelter, 

foraging, and predator avoidance. 

• Modify the designated critical habitat of the CRLF by reducing or changing 

dispersal habitat within designated units and between occupied locations within 0.7 mi of 

each other that allow for movement between sites including both natural and altered sites 

which do not contain barriers to dispersal. 

• Modify the designated critical habitat of the CRLF by altering chemical 

characteristics necessary for normal growth and viability of juvenile and adult CRLFs. 

2.9.2 Diagram 

The conceptual model is a graphic representation of the structure of the risk assessment. 

It specifies the diuron release mechanisms, biological receptor types, and effects 

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

of the CRLF are shown in Figure 2.5 and Figure 2.6, respectively, and the conceptual 

models for the aquatic and terrestrial PCE components of critical habitat are shown in 

40
Figure 2.7 and Figure 2.8, respectively. 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 

Spray drift 

Red-legged 

Frog 

Eggs 

Juveniles 

Individual 

organisms 

Reduced survival 

Reduced growth 

Food sources

Reduction in algae 

Reduction in prey

Habitat quality and channel/pond 

morphology or geometry 

Adverse water quality changes 

Increased sedimentation

Reduced shelter 

Surface water/ 

Sediment 

Runoff

Aquatic Animals

Invertebrates 

Vertebrates

Exposure 

Media 

Uptake/gills 

or 

Ingestion Ingestion 

Long range 

atmospheric 

transport

Wet/dry 

Soil

Uptake/gills 

or 

Aquatic Plants

Non-vascular 

Vascular

Uptake/cell, 

roots, 

Riparian and 

Upland plants

terrestrial exposure 

pathways and 

PCEs 

Community 

Reduced seedling 

emergence or 

vegetative vigor 

Other chemical 

characteristics 

Adversely modified 

chemical characteristics 

Population 

Yield 

Reduced 

yield 

Individual 

organisms 

Reduced survival 

Reduced growth

Groundwater 

diuron applied to use site 

Habitat 

PCEs 

Figure 2.5 Conceptual Model for Aquatic-Phase of the CRLF 

41
Stressor

Source

Attribute 

Change 

Habitat 

PCEs 

Direct 

application Spray drift

Red-legged 

Frog 

Juvenile/Adult 

Terrestrial 

insects 

Food resources

Reduction in food 

sources 

Elimination and/or disturbance of 

upland or dispersal habitat 

Reduction in primary productivity

Reduced shelter 

Restrict movement 

Terrestrial plants 

grasses/forbs, fruit, 

seeds (trees, shrubs)

Runoff 

Mammals

Exposure Soil

Ingestion 

Ingestion 

Ingestion

Ingestion

Dermal 

Long range 

atmospheric 

transport 

Root 

Wet/dry 

Individual 

organisms 

Reduced survival 

Reduced growth 

Other chemical 

characteristics 

Adversely modified 

chemical characteristics 

Population

Reduced survival 

Reduced growth 

Reduced 

reproduction 

Community 

Reduced seedling 

emergence or vegetative 

vigor (Distribution) 

Media 

and 

diuron applied to use site 

Figure 2.6 Conceptual Model for Terrestrial-Phase of the CRLF 

42
Stressor

Source 

Receptor

Attribute 

Change 

diuron applied to use site 

Spray drift 

Red-legged 

Frog 

Eggs/Juveniles 

Larvae/Adult 

Tadpoles 

Individual 

organisms 

Reduced survival 

Reduced growth 

Food sources

Reduction in algae 

Reduction in prey

Habitat quality and channel/pond 

morphology or geometry 

Adverse water quality changes 

Increased sedimentation

Reduced shelter 

Surface water/ 

Sediment 

Runoff

Aquatic Animals

Invertebrates 

Vertebrates

Exposure 

Media 

Uptake/gills 

or 

Ingestion Ingestion 

Long range 

atmospheric 

transport

Wet/dry 

Soil

Uptake/gills 

or 

Aquatic Plants

Non-vascular 

Vascular

Uptake/cell, 

roots, 

Riparian and 

Upland plants

terrestrial exposure 

pathways and 

PCEs 

Community 

Reduced seedling 

emergence or 

vegetative vigor 

Habitat 

PCEs 

Other chemical 

characteristics 

Adversely modified 

chemical characteristics 

Population 

Yield 

Reduced 

yield Individual 

organisms 

Reduced survival 

Reduced growth

Groundwater

Figure 2.7 Conceptual Model for Pesticide Effects on Aquatic Component of CRLF 

Critical Habitat

43
Figure 2.8 Conceptual Model for Pesticide Effects on Terrestrial Component of 

CRLF Critical Habitat 

Stressor 

Source 

Attribute 

Change

Habitat 

PCEs 

Direct 

application Spray drift

Red-legged 

Frog 

Juvenile/Adult 

Terrestrial 

insects 

Food resources

Reduction in food 

sources 

Elimination and/or disturbance of 

upland or dispersal habitat 

Reduction in primary productivity

Reduced shelter 

Restrict movement 

Terrestrial plants 

grasses/forbs, fruit, 

seeds (trees, shrubs)

Runoff 

Mammals

Exposure 

Media 

and 

Soil

Ingestion 

Ingestion 

Ingestion

Ingestion

Dermal 

Long range 

atmospheric 

transport 

Root 

Wet/dry 

Individual 

organisms 

Reduced survival 

Reduced growth 

Other chemical 

characteristics 

Adversely modified 

chemical characteristics 

Population

Reduced survival 

Reduced growth 

Reduced 

reproduction 

Community 

Reduced seedling 

emergence or vegetative 

vigor (Distribution) 

diuron applied to use site 

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 diuron 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 diuron is estimated 

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

actual calculated risk quotient value. 

2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 

2.10.1.1 Measures of Exposure 

The environmental fate properties of diuron along with available monitoring data indicate 

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

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

44
through runoff and spray drift is considered in deriving quantitative estimates of diuron 

exposure to CRLF, its prey and its habitats. The reported vapor pressure of diuron is 6.9 

x 10-8 torr at 25°C; therefore, volatilization is not considered a probable route of 

dissipation. 

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

environmental concentrations (EECs) of diuron 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). 

T-REX is used to predict terrestrial EECs on food items. TerrPlant is used to derive 

EECs relevant to terrestrial and wetland plants. 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 diuron that may occur in surface water bodies 

adjacent to application sites receiving diuron 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 diuron. The measure of 

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

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

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

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

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

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

invertebrates, which are also potential prey items. 

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

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

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

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

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

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

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

the CRLF to diuron 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 

45
for the CRLF and one of its prey items. Estimated exposures of terrestrial insects to 

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

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

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

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

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

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

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

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

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

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

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

to estimate avian food intake. 

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

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

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

minimum incorporation depth. 

Spray drift model, AgDRIFT is used to assess exposures of terrestrial phase CRLF and its 

prey to diuron deposited on terrestrial habitats by spray drift. In addition to the buffered 

area from the spray drift analysis, the downstream extent of diuron that exceeds the LOC 

for the effects determination is also considered. 

2.10.1.2 Measures of Effect 

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

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

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

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

existing data gaps. ECOTOX is a source for locating single chemical toxicity data for 

aquatic life, terrestrial plants, and wildlife. ECOTOX was created and is maintained by 

the USEPA, Office of Research and Development, and the National Health and 

Environmental Effects Research Laboratory's Mid-Continent Ecology Division. 

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

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

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

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

in the aquatic habitat. Terrestrial invertebrates, small mammals, and terrestrial-phase 

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

46
The acute measures of effect used for animals in this screening level assessment are the 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

assessment endpoints of survival, growth, and fecundity. 

2.10.1.3 Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects characterization 

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

diuron, 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 diuron 

risks, the risk quotient (RQ) method is used to compare exposure and measured toxicity 

values. EECs are divided by acute and chronic toxicity values. The resulting RQs are 

then compared to the Agency’s levels of concern (LOCs) (USEPA, 2004) (see Appendix 

E). 

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

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

exposures directly to the CRLF of diuron 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 diuron 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 

47
LOC and the non-listed acute risk species LOC, then further lines of evidence (i.e. 

probability of individual effects, species sensitivity distributions) are considered in 

distinguishing between a determination of NLAA and a LAA. When considering indirect 

effects to the CRLF due to effects to algae as dietary items or plants as habitat, the nonlisted species LOC for plants is used because the CRLF does not have an obligate 

relationship with any particular aquatic and/or terrestrial plant. If the RQ being 

considered for a particular use exceeds the non-listed species LOC for plants, the effects 

determination is “may affect”. Further information on LOCs is provided in Appendix E. 

2.10.2 Data Limitations 

No acceptable toxicity studies have submitted to the Agency, nor were any acceptable 

studies found in the open literature for the chronic effects of diuron on avian or 

amphibian species. Linuron, a similar chemical of the same class, has acceptable chronic 

avian data. The avian reproductive studies for linuron (MRID 42541801, 42541802) 

using mallard duck and bobwhite quail found reproductive effects at 300 ppm ai with an 

NOAEL being established at 100 ppm. 

The endpoints affected for mallard duck reproduction are egg production, adult body 

weight, feed consumption, viable embryos of eggs set, number of viable embryos, and 

number of live embryos. For the Bobwhite Quail the endpoints affected are hatchability 

and offspring survivability. The LC50 for the Bobwhite Quail exposed to linuron was 

1700ppm, where the LC50 for the Bobwhite Quail exposed to linuron was almost identical 

at 1730 ppm (MRID 00022923). Therefore it is assumed that linuron and diuron have a 

similar mode of action and therefore chronic effects from diuron are assumed to be 

similar to those observed from linuron exposure. Chronic risks are therefore assumed to 

amphibian species and their avian surrogates for uses of diuron in California. 

3.0 Exposure Assessment 

Diuron is a broad-spectrum residual herbicide registered for pre-emergent and postemergent control of both broadleaf and annual grassy weeds. In the U.S. diuron is used 

on a variety of fruit and nut crops, grains, cotton, corn, sorghum, mint, gum, asparagus, 

sugarcane, seed crops, coffee, hay, cut flowers, and for fallow and idle cropland use. It 

may be used in irrigation and drainage systems when water is not present. 

It is also used to control weeds on hard surfaces, such as, roads, railway tracks, and paths 

(at around 3 kg/ha), and to control weeds in crops, such as, pear and apple trees, forestry, 

ornamental trees and shrubs, pineapples, sugar cane, cotton, alfalfa and wheat (at lower 

rates of around 1.8 kg/ha). Its use in some locations is becoming limited due to the 

development of resistant weed species. In some products it is formulated with other 

active ingredients such as glyphosate, bromacil, hexazinone, amitrole and 2,4-D. This 

assessment evaluates only those registered uses which allow use in California. 

48
3.1 Label Application Rates and Intervals 

Currently registered agricultural and non-agricultural uses of diuron within California 

include agricultural and non-agricultural crops, as well as industrial and paved areas. 

The uses and applicable PRZM/EXAMS scenarios being assessed were summarized 

in Table 2.2. For a list of the application rates and intervals used, please refer to 

Table 3.2 below. 

3.2 Aquatic Exposure Assessment 

3.2.1 Modeling Approach 

As discussed in Section 2.10.1.1, the models used to predict aquatic EECs are the 

Pesticide Root Zone Model coupled with the Exposure Analysis Model System 

(PRZM/EXAMS). Using PRZM/EXAMS, aquatic exposures are quantitatively estimated 

for all of the assessed uses with scenarios that represent high exposure sites for diuron 

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 diuron were used for 

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

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

developed based on several sources of information including data provided by BEAD, a 

summary of individual applications from the CDPR PUR data, and Crop Profiles 

maintained by the USDA. 

3.2.2 Model Inputs 

The input parameters for PRZM and EXAMS are presented in Table 3.1. 

49
Table 3.1 Summary of PRZM/EXAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Diuron Endangered Species Assessment for the CRLF 

Fate Property Value (unit) MRID (or source) 

Molecular Weight 233.1 g/mol Product Chemistry 

Henry’s constant 5.1E-10 atm-m-3/mol MRID 00017763 

Vapor Pressure 6.9E-8 torr (25°C) 

MRID 43762901, Herbicide 

Handbook 7th Edition, WSSA, 

1994 

Solubility in Water 420 mg/l (42 mg/l x 10 per input 

guidance) 

Product Chemistry, Input 

Parameter Guidance 

Photolysis in Water 43 days MRID 41418805 

Aerobic Soil Metabolism Half-lives 372 days MRID 41719303 

Hydrolysis Stable at pH7 (>500 hrs) MRID 41418804 

Aerobic Aquatic Metabolism (water 

column) 

99 days (33 days x 3 per input 

guidance) 

MRID 42260501, Input 

Parameter Guidance 

Anaerobic Aquatic Metabolism 

(benthic) 

15 days (5 days x 3 per input 

guidance) 

MRID 42661901, Input 

Parameter Guidance 

Koc 920 (avg. of 468, 626, and 1666) MRID 44490501 

Application rate and frequency Various (see Table 3.2) Per Label Instructions 

Application intervals Various (see Table 3.2) Per Label Instructions 

Chemical Application Method (CAM) 1 Label; Input Guidance for Eco 

Assessments 

Application Efficiency 0.99 Ground 

0.95 Aerial 

Per Input Guidance for Eco 

Assessments 

Spray Drift Fraction1 0.01 Ground 

0.05 Aerial 

Per Input Guidance for Eco 

Assessments

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 PRZM/EXAMS model estimated aquatic EECs for the various scenarios and 

application practices are listed in Table 3.2. See Appendix G for a summary of the 

outputs. Peak EECs ranged from 3.76 to 4911 ppb for use on grasses grown for seed and 

paved areas (impervious surfaces), respectively. 

Since some of the application data needed for modeling were not stated on the labels, 

assumptions were made by EFED analysts regarding the maximum number of 

applications allowed per season, and/or the interval between applications. The 

assumptions were as follows: 

50
• For the application intervals that were not stated, the most conservative (minimum) 

known application interval was used (21 days for cotton). 21 days was chosen 

because it was the minimum known interval that was registered. 

• If the maximum number of applications was also missing, the maximum number of 

intervals a year was determine by taking the maximum allowable application of active 

ingredient per year (12 lb ai/yr) and dividing it by the maximum application rate for 

the use in question. The following is an example of a calculation to determine the 

maximum number of applications per year for the use of diuron on blackberries: 

Max No. of Appl.Yr-1 = 12 lb ai. acre -1 yr -1 ÷ 2.4 lb ai. acre -1 appl. -1 = 5 appl. yr-1

 Table 3.2 Aquatic EECs (ppb) for Diuron Uses in California 

Max # of 

Applications 

Interval 

Between 

Apps 

Application 

method Crop 

Application 

Rate 

(lbs/A) 

(Apps/yr) (Days) 

Peak 

EEC 

ppb 

96 

Hour 

EEC 

pbb 

21 

Day 

EEC 

pbb 

60 

Day 

EEC 

pbb 

AGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 12 2 21 Ground 688 672 584 457 

Ground 10.6 10.4 9.1 7.2 ALFALFA 2.4 1 0 

Aircraft 13.7 13.3 11.7 9.4 

APPLE 3.2 2 90 Ground 11.7 11.2 9.6 7.8 

ARTICHOKE 3.2 1 0 Ground 32.0 31.1 27.4 25.1 

ASPARGUS 3.2 1 0 Ground 37.0 35.5 30.2 22.9 

BANANA, PLANTAIN 4.8 2 42 Ground 54.2 52.0 44.3 32.8 

BLACKBERRY, BOYSENBERRY 2.4 5 21 Ground 140 136 121 103 

BLUEBERRY 1.6 2 112 Ground 4.65 4.48 4.05 3.21 

CITRUS 3.2 2 60 Ground 6.83 6.57 5.65 4.88 

CORN, FIELD 0.8 1 0 Ground 7.24 6.95 6.13 4.68 

DEWBERRY 2.4 3 60 Ground 19.7 19.3 16.8 14.3 

FILBERT (HAZELNUT) 2.2 2 150 Ground 18.9 18.4 15.9 12.2 

GRAPE 3.2 2 90 Ground 9.71 9.32 8.10 6.37 

LOGANBERRY, RASPBERRY 

(BLACK/RED) 2.4 1 0 Ground 5.13 4.95 4.31 3.02 

OLIVE 1.6 2 168 Ground 7.17 6.90 5.97 4.86 

PAPAYA 4 1 0 Ground 25.2 24.6 21.2 15.7 

PEPPERMINT 2.4 1 0 Ground 4.17 4.05 3.55 2.79 

PEACH 3 1 0 Ground 11.0 10.5 9.0 7.3 

PEAR 3.2 1 0 Ground 11.7 11.2 9.6 7.8 

PECAN 3.2 1 0 Ground 27.0 26.3 22.8 17.3 

SORGHUM 0.4 2 21 Ground 5.87 5.67 5.44 4.52 

SPEARMINT 2.4 5 21 Ground 107 104 97.1 82.0 

WALNUT (ENGLISH/BLACK) 3 2 150 Ground 25.8 25.1 21.8 16.6 

18.0 17.3 14.9 12.0 WHEAT 1.6 1 0 Ground 

12.8 12.3 11.9 10.3 

Non-Food/Feed Crops 

Ground 8.36 8.13 7.42 5.72 COTTON 1.6 3 21 

Aircraft 14.2 13.9 12.9 10.2 

Non-Crop Land 

AIRPORTS/ LANDING FIELDS, 

DRAINAGE SYSTEMS, 

INDUSTRIAL AREAS 

(OUTDOOR), SEWAGE DISPOSAL 

AREAS 

12 2 21 Ground 

2190 2115 1825 1382 

BERMUDAGRASS 2.4 5 21 Ground 226 219 204 168 

GRASSES GROWN FOR SEED 1.6 1 0 Ground 3.76 3.62 3.20 2.89 

51
Interval Max # of Application Between Applications method Apps 

IRRIGATION SYSTEMS 12 2 21 Ground 52.6 50.7 45.0 37.5 

ORNAMENTAL HERBACEOUS 

PLANTS 4 1 0 Ground 36.5 35.5 30.7 23.6 

NON-AGRICULTURAL RIGHTS Ground 630 619 560 476 

OF WAY 12 2 90 

Aircraft 627 616 552 468 

PAVED AREAS (PRIVATE 

ROADS/SIDEWALKS) 12 1 0 Ground 4911 4725 4241 3428 

Ground 1130 1099 1023 840 UNCULTIVATED AG 12 1 0 

Aircraft 1169 1130 1063 891 

UNCULTIVATED NON-AG 12 2 21 Ground 285 276 252 203 

3.2.4 Existing Monitoring Data 

 3.2.4.1 USGS NAWQA Surface Water Data 

A critical step in the process of characterizing EECs is comparing the modeled estimates 

with available surface water monitoring data. The USGS has collected 347 surface water 

samples from four river basins in California (Sacramento, San Joaquin-Tulare, Santa 

Ana, and Nevada) with a detection limit of 0.01 ppb. Out of these samples, 232 (69%) 

were positive for diuron. The maximum concentration observed was 23.3 ppb in the 

Santa Ana River Basin in San Bernardino County. 

3.2.4.2 USGS NAWQA Groundwater Data 

The USGS collected 428 ground water samples where 75 (17.5%) of these samples 

contained diuron. The three river basins looked at were Sacramento, San Joaquin-Tulare, 

and Santa Ana. The maximum concentration observed was 1.8 ppb in the San JoaquinTulare water basins in Kings County. 

Similar to the most recent analysis of USGS data, an “Analysis of Diuron Monitoring 

Data with [an] emphasis on the USGS NAQWA Surface Water Sampling” in February 

1999, observed the San Joaquin-Tulare basin as well. It was concluded that diuron 

concentrations reached their peaks in the winter and spring seasons (January to June). 

USGS concluded that runoff was more efficient than irrigation return flows in 

transporting pesticides, in this case diuron, into surface water. Although USGS detected 

diuron in more than 10% of the ground water samples, the concentrations were generally 

less than 0.1 ppb. 

. 

 3.2.4.3 California Department of Pesticide Regulation Data 

The California Department of Pesticide Regulation (CDPR) has been collecting surface 

water data on diuron since 1992. In a memo sent to the Special Review and 

Reregistration Division (SRRD) from the Environmental Fate and Effects Division 

(EFED) in 2003, titled “Surface Water Monitoring Data for Diuron”, summarized surface 

water monitoring data for diuron collected by CDPR over the period December 2000 to 

March 2001. There were three locations (82 samples) in the Sacramento River, and two 

locations (54 samples) in the San Joaquin River. One-hundred percent of the samples 

52
taken from the San Joaquin River had detectable diuron, with a maximum concentration 

of 8.45 ppb. The average concentration at the San Joaquin River stations was 1.7 ppb. 

About seventy-five percent of the samples in the Sacramento River had detectable diuron 

with a maximum concentration of 1.42 ppb. The average concentration, assuming that all 

non-detects were equal to the detection limit of 0.05 ppb, was 0.16 ppb. 

Currently, the monitoring data available at CDPR has expanded to include data collected 

from 2004, 2005, and 2006. Between 2004 and 2006, the data indicates that diuron 

concentrations range from non-detect to 160 ppb (San Joaquin County, January 3, 2005, 

and Tulare County, February 21, 2005). Out of 156 sampling locations, 152 locations 

(97.4%) were located within 0.5 miles or less of an agricultural area. The four other 

locations were 8 miles or more from the nearest agricultural area. 

Sixteen percent of the 705 samples collected from 2004 to 2006 were greater than the 

detection limit. The majority of the higher concentrations of diuron came from San 

Joaquin County, and occurred between the end of December and the beginning of 

January. The year 2005, for San Joaquin County, yielded the greatest concentrations 

overall with an average concentration of 10.9 ppb for the year. 

Diuron’s concentrations being greatest in, and mainly only showing up in the monitoring 

data during the winter months is consistent with the labeled use for pre-emergence 

application. In addition, during the winter months, high concentrations may be detected 

due to transport from rainfall runoff. Runoff during the winter in the San Joaquin Valley 

may include large amounts of non-agricultural runoff from coastal ranges. When 

comparing this monitoring data to the results from PRZM/EXAMS, the CDPR data 

further supports that spray drift is a major route of off site movement. 

 3.2.4.4 Atmospheric Monitoring Data 

The California Air Review Board has not conducted ambient air monitoring of diuron. 

Therefore, no atmospheric monitoring data is available. 

3.2.6 Downstream Dilution Analysis 

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 nonimpacted water will dilute the concentrations of diuron present. 

Using the LC50 for Selenastrum capricornutum (the most sensitive species) of 2.40 ppb 

and a maximum peak EEC for applications to non-agricultural rights-ofway/fencerows/hedgerows of 341.6 ug/L yields an RQ/LOC ratio of 142.33. Using the 

downstream dilution approach (described in more detail in Appendix F) yields a target 

percent crop area (PCA). This value has been input into the downstream dilution 

approach resulting in a total of 285 kilometers of stream downstream from the initial area 

53
of concern (footprint of use). Similar to the spray drift buffer described above, the 

LAA/NLAA determination is based on the area defined by the point where 

concentrations exceed the LC50 value, in this case 2.40 ppb. 

See Appendix F for the spatial summary of diuron used to determine the downstream 

dilution. 

3.3 Terrestrial Animal Exposure Assessment 

T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of diuron 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, ground spray applications of diuron are considered, as 

discussed in below. 

Terrestrial EECs for ground spray applications of diuron were derived for the uses 

registered in California. Given that no data on interception and subsequent dissipation 

from foliar surfaces is available for diuron, a default foliar dissipation half-life of 35 days 

is used based on the work of Willis and McDowell (1987). Use specific input values, 

including number of applications, and application rate are shown in Table 3.3. An 

example output from T-REX is available in Appendix H. 

Table 3.3 T-REX Model Input Parameters 

Crop 

Application 

Rate per 

Crop 

Scenario 

(lbs/A) 

Number of 

Applications

Application 

interval 

(days) 

Agricultural Rights-of-Way, Fencerows, 

Hedgerows, Airports, landing Fields, Drainage 

Systems, Outdoor Industrial Areas, Irrigation 

Systems, Non-Agricultural Rights of Way, 

Sewage Disposal Areas, Uncultivated NonAgricultural Areas 

12 2 21 

Alfalfa, Peppermint 2.4 1 n/a 

Apple, Grape 3.2 2 90 

Citrus 3.2 2 60 

Artichoke, Asparagus 3.2 1 n/a 

Banana, Plantain 4.8 2 42 

Bermuda grass, Blackberry, Boysenberry, 

Spearmint 2.4 5 21 

Blueberry 1.6 2 112 

Field Corn 0.8 1 n/a 

Cotton 1.6 3 21 

Dewberry 2.4 3 60 

Filbert (Hazelnut) 2.2 2 150 

Grass Seed, Wheat 1.6 1 n/a 

54
Application 

Crop 

Rate per Application Number of Crop interval Applications Scenario (days) 

(lbs/A) 

Loganberry, Raspberry 2.4 1 n/a 

Olive 1.6 2 168 

Pecan, Pear 3.2 1 n/a 

Ornamental Herbaceous Plants, Papaya 4 1 21 

Uncultivated Agricultural Areas, Paved Areas 12 1 n/a 

Peach 3 1 n/a 

Sorghum 0.4 1 n/a 

Walnut 3 2 150 

T-REX is also used to calculate EECs for terrestrial insects exposed to diuron. Dietarybased EECs calculated by T-REX for small and large insects (units of a.i. /g) are used to 

bound an estimate of exposure to bees. Available acute contact toxicity data for bees 

exposed to diuron (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 diuron 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 3.4). Dietary-based EECs for small and large insects 

reported by T-REX as well as the resulting adjusted EECs are available in Table 3.5. 

Table 3.4 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based 

Exposures of the CRLF and its Prey to Diuron 

EECs for CRLF EECs for Prey 

(small mammals) 

Use Dietarybased EEC 

(ppm) 

Dose-based 

EEC 

(mg/kg-bw) 

Dietarybased EEC 

(ppm) 

Dose-based 

EEC 

(mg/kgbw) 

Agricultural Rights-ofWay, Fencerows, 

Hedgerows, Airports, 

landing Fields, Drainage 

Systems, Outdoor 

Industrial Areas, Irrigation 

Systems, Non-Agricultural 

Rights of Way, Sewage 

Disposal Areas, 

Uncultivated Non2688.80 3062.28 4780.09 5444.05 

55
EECs for CRLF EECs for Prey 

(small mammals) 

Use Dietarybased EEC 

(ppm) 

Dose-based 

EEC 

(mg/kg-bw) 

Dose-based Dietarybased EEC 

(ppm) 

EEC 

(mg/kgbw) 

Agricultural Areas 

Alfalfa, Peppermint, 324.00 369.00 576.00 549.17 

Apple, Grape 504.68 574.78 897.21 855.42 

Artichoke, Asparagus 432.00 874.68 768.00 732.23 

Banana, Plantain 930.06 1059.24 1653.44 1576.43 

Bermuda grass, 

Blackberry, Boysenberry, 

Spearmint 

833.22 948.95 1481.28 1412.29 

Blueberry 239.50 272.77 425.79 405.95 

Citrus, 563.65 641.94 1002.05 955.38 

Field Corn 108.00 123.00 192.00 183.06 

Cotton 452.53 515.38 804.49 767.02 

Dewberry 452.83 515.73 805.03 767.54 

Filbert (Hazelnut) 312.23 355.60 555.07 529.22 

Grass Seed, Wheat 216.00 246.00 384.00 366.11 

Loganberry, Raspberry 324.00 369.00 576.00 549.17 

Olive 223.75 254.83 397.78 379.26 

Ornamental Herbaceous 

plants, Papaya 540.00 615.01 960.00 915.29 

Uncultivated Agricultural 

Areas, Paved Areas 1620.00 1845.02 2880.00 2745.86 

Peach 461.25 461.25 720.00 686.46 

Pear, Pecan 432.00 492.00 768.00 737.23 

Sorghum 54.00 61.50 96.00 91.53 

Walnut 425.76 484.9 756.92 721.66 

Table 3.5 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via 

Effects to Terrestrial Invertebrate Prey Items 

Use Small Insect Large Insect 

Agricultural Rights-of-Way, Fencerows, Hedgerows, 

Airports, landing Fields, Drainage Systems, Outdoor 

Industrial Areas, Irrigation Systems, Non-Agricultural 

Rights of Way, Sewage Disposal Areas, Uncultivated 

Non-Agricultural Areas 2688.80 298.76 

Alfalfa, Peppermint, 324.00 36.00 

Apple, Grape 504.68 56.08 

Citrus, 563.65 62.63 

56
Use Small Insect Large Insect 

Artichoke, Asparagus 432.00 48.00 

Banana, Plantain 930.06 103.34 

Bermuda grass, Blackberry, Boysenberry, Spearmint 833.22 92.58 

Blueberry 239.50 26.61 

Field Corn 108.00 12.00 

Cotton 452.53 50.28 

Dewberry 452.83 50.31 

Filbert (Hazelnut) 312.23 34.69 

Grass Seed, Wheat 216.00 24.00 

Loganberry, Raspberry 324.00 36.00 

Olive 223.75 24.86 

Pecan, Pear 432.00 48.00 

Ornamental Herbaceous plants, Papaya 540.00 60.00 

Uncultivated Agricultural Areas, Paved Areas 1620.00 180.00 

Peach 461.25 51.25 

Sorghum 54.00 6.00 

Walnut 425.76 47.31 

3.4 Terrestrial Plant Exposure Assessment 

TerrPlant (version 1.2.2) is used to calculate EECs for non-target plants inhabiting dry 

and semi-aquatic areas. Parameter values for application rate, drift assumption and 

incorporation depth are based upon the use and related application method. A runoff 

value of 0.02 was selected based on diuron’s solubility of between 10-100 mg/L. Drift 

was assumed to be 5% for aerial applications and 1% for ground applications. 

TerrPlant was used to model EECs from soil and foliar applications relevant to terrestrial 

plants. These EECs are presented in Table 3.6. An example output from TerrPlant 

v.1.2.2 is available in Appendix I.

57
Table 3.6 TerrPlant Inputs and Resulting EECs for Plants Inhabiting Dry and 

Semi-aquatic Areas Exposed to diuron via Runoff and Drift 

Application 

rate 

Spray 

drift 

EEC 

Dry 

area 

EEC 

Semiaquatic 

area 

EEC 

Use (lbs a.i./A) 

Application 

method 

Drift 

Value 

(%) 

(lbs 

a.i./A) 

(lbs 

a.i./A) 

(lbs 

a.i./A) 

Agricultural 

Rights-of-Way, 

Fencerows, 

Hedgerows, 

Airports, landing 

Fields, Drainage 

Systems, Outdoor 

Industrial Areas, 

Irrigation 

Systems, NonAgricultural 

Rights of Way, 

Sewage Disposal 

Areas, 

Uncultivated 

Non-Agricultural 

Areas 

12 Foliar/Ground 1 0.12 0.36 2.52 

Alfalfa, 

Peppermint, 

Spearmint, 

Bermudagrass, 

Blackberry, 

Boysenberry, 

Loganberry, 

Raspberry, 

Dewberry 

2.4 Foliar/Ground 1 0.02 0.07 0.50 

Apple, Citrus, 

Pear, Grape, 

Artichoke, 

Asparagus, Pecan 

3.2 Foliar/Ground 1 0.03 0.10 0.67 

Asparagus 

(Aerial) 3.2 Foliar/Ground 5 0.16 0.22 0.80 

Banana, Plantain 4.8 Foliar/Ground 1 0.05 0.14 1.01 

Blueberry, 

Cotton, Grass 

Seed, Olive, 

Wheat 

1.6 Foliar/Ground 1 0.02 0.05 0.34 

58
Application 

rate 

Spray 

drift 

EEC 

Dry 

area 

EEC 

Semiaquatic 

area 

Drift EEC 

Application Value 

Use (lbs a.i./A) method (%) 

(lbs 

a.i./A) 

(lbs (lbs 

a.i./A) a.i./A) 

Field Corn 0.8 Foliar/Ground 1 0.01 0.02 0.17 

Filbert (Hazelnut) 2.2 Foliar/Ground 1 0.02 0.07 0.46 

Non-agricultural 12 Aerial 5 0.60 0.84 3.00 

Ornamental 

Herbaceous 

Plants, Papaya 

4 Foliar/Ground 1 0.04 0.12 0.84 

Uncultivated 

Agricultural 

Areas, Paved 

Areas 

12 Foliar/Ground 1 0.12 0.36 2.52 

Peach, Walnut 3 Foliar/Ground 1 0.03 0.09 0.63 

Sorghum 0.4 Foliar/Ground 1 0.00 0.01 0.08 

3.4.1 Spray Drift Buffer Analysis 

In order to determine terrestrial and aquatic habitats of concern due to diuron 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 AgDRIFT and can be 

found in Table 3.7. 

AgDRIFT was run for terrestrial analysis only, and due to uncertainties default values 

were used to determine the aquatic analysis. The default values for PRZM/EXAMS 

(used for the aquatic analysis) assumed the most conservative scenario, that a buffer was 

not present. 

For the terrestrial assessment, the AgDRIFT model was run as a Tier I ground analysis, 

as well as Tier I, and II aerial analyses focusing on determining a buffer zone for nonlisted and listed plant species. The following settings beyond the standard default 

settings were implemented: 

• 12 lb ai/A 

• EC25= 0.002 lb ai/A 

• EC05 = 0.001 lb ai/A 

• Nonvolatile Rate (Tier II aerial only) = 25 lb/a 

59
• Spray Volume Rate (Tier II aerial only) = 3 gal/a 

• Active Rate (Tier II aerial only) = 4 lb/a 

The AgDRIFT model was used to evaluate potential distances beyond which exposures 

would be expected to be below LOC. However, due to the limitations imposed by the 

Tier 1 ground analysis, and Tiers I and II aerial analyses, which allow users to evaluate 

off-site deposition and exposure out to 1,000 ft downwind from the location of the 

application, the exact buffer needed for exposures to be below the LOC is uncertain. 

Table 3.7 Summary of AgDRIFT Predicted Spray Drift Buffer for Terrestrial 

Plants 

Tier I Ground Application 

Risk 

Class Risk Description Application Rate 

Toxicity 

Value 

Used 

Fraction 

of 

applied 

Nonvolatile 

Rate (lb/a) 

Minimum 

Spray 

Volume 

Rate 

(gal/a) 

Active 

Rate 

(lb 

ai/a) 

Distance 

NonListed 

Plants 

Potential for 

effects to nontarget, non-listed 

plants from 

exposures 

EC25 = 

0.002 lb 

ai/A 

0.0002 

Listed 

Plants 

Potential for 

effects to nontarget, listed 

plants from 

exposures 

12 

EC05 = 

0.001 lb 

ai/A 

0.0001 

Does not 

apply 

Does not 

apply 

Does 

not 

apply 

> 1,000 

ft 

Tier I Aerial Application 

Risk 

Class Risk Description Application Rate 

Toxicity 

Value 

Used 

Fraction 

of 

applied 

Nonvolatile 

Rate (lb/a) 

Minimum 

Spray 

Volume 

Rate 

(gal/a) 

Active 

Rate 

(lb 

ai/a) 

Distance 

NonListed 

Plants 

Potential for 

effects to nontarget, non-listed 

plants from 

exposures 

EC25 = 

0.002 lb 

ai/A 

0.0002 

Listed 

Plants 

Potential for 

effects to nontarget, listed 

plants from 

exposures 

12 

EC05 = 

0.001 lb 

ai/A 

0.0001 

Does not 

apply 

Does not 

apply 

Does 

not 

apply 

> 1,000 

ft 

Tier II Aerial Application 

Risk 

Class Risk Description Application Rate 

Toxicity 

Value 

Used 

Fraction 

of 

applied 

Nonvolatile 

Rate (lb/a) 

Minimum 

Spray 

Volume 

Rate 

(gal/a) 

Active 

Rate 

(lb 

ai/a) 

Distance 

NonListed 

Plants 

Potential for 

effects to nontarget, non-listed 

plants from 

exposures 

EC25 = 

0.002 lb 

ai/A 

0.0002 

Listed 

Plants 

Potential for 

effects to nontarget, listed 

plants from 

exposures 

12 

EC05 = 

0.001 lb 

ai/A 

0.0001 

25 3 4 > 1,000 

ft 

60
4.0 Effects Assessment 

This assessment evaluates the potential for diuron to directly or indirectly affect the 

CRLF or modify its designated critical habitat. As previously discussed in Section 2.7, 

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

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

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

modification of critical habitat is assessed by evaluating effects to the PCEs, which are 

components of the critical habitat areas that provide essential life cycle needs of the 

CRLF. Direct effects to the aquatic-phase of the CRLF are based on toxicity information 

for freshwater fish, while terrestrial-phase effects are based on avian toxicity data, given 

that birds are generally used as a surrogate for terrestrial-phase amphibians. Because the 

frog’s prey items and habitat requirements are dependent on the availability of freshwater 

fish and invertebrates, small mammals, terrestrial invertebrates, and aquatic and 

terrestrial plants, toxicity information for these taxa are also discussed. Acute (shortterm) and chronic (long-term) toxicity information is characterized based on registrantsubmitted studies and a comprehensive review of the open literature on diuron. 

As described in the Agency’s Overview Document (U.S. EPA, 2004), the most sensitive 

endpoint for each taxon is used for risk estimation. For this assessment, evaluated taxa 

include aquatic-phase amphibians, freshwater fish, freshwater invertebrates, aquatic 

plants, birds (surrogate for terrestrial-phase amphibians), mammals, terrestrial 

invertebrates, and terrestrial plants. 

Toxicity endpoints are established based on data generated from guideline studies 

submitted by the registrant, and from open literature studies that meet the criteria for 

inclusion into the ECOTOX database maintained by EPA/Office of Research and 

Development (ORD) (U.S. EPA, 2004). Open literature data presented in this assessment 

were obtained from as well as ECOTOX information obtained on [11/30/2007]. 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, 

61
endpoints such as behavior modifications are likely to be qualitatively evaluated, unless 

quantitative relationships between modifications and reduction in species survival, 

reproduction, and/or growth are 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 diuron. 

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 J. This section 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 J. This section also 

includes a summary of the human health effects data for diuron. 

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 diuron. A summary of the available aquatic and 

terrestrial ecotoxicity information, use of the probit dose response relationship, and the 

incident information for diuron are provided in Sections 4.1 through 4.4, respectively. 

The terminal residues of concern in plants and animals are diuron (parent) and 

metabolites convertible to 3, 4-DCA including desmethoxy-linuron, norlinuron, 

desmethyl diuron, and hydroxy-norlinuron. The Agency determined that 3, 4-DCA was 

not of regulatory concern in connection with the registered use of diuron due to the very 

low levels at which the chemical is detected in plants and animals (<0.01ppm). The 

Agency concluded that with the possible exception of 3,4-DCA itself, metabolites 

convertible to 3,4-DCA are not likely to be more toxic than the parent compound. This 

risk assessment considers the total residues of diuron as parent diuron for purposes of risk 

assessment 

A detailed summary of the available ecotoxicity information for all diuron degradates and 

formulated products are presented in Appendix L. 

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 

62
are available for a formulated product containing more than one active ingredient, they 

may be used qualitatively or quantitatively5 6

. 

There are no product LD50 values, with associated 95% Confidence Intervals (CIs) 

available for diuron. 

As discussed in USEPA (2000) a quantitative component-based evaluation of mixture 

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

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

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

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

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

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

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

assumption of dose-addition would be inappropriate. 

4.1 Toxicity of Diuron to Aquatic Organisms 

Table 4.1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of both the submitted studies and the open literature, as previously 

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 L. 

Table 4.1 Freshwater Aquatic Toxicity Profile for Diuron 

Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Citation 

MRID # or 

ECOTOX # 

Comment 

Acute Direct 

Toxicity to 

Aquatic-Phase 

CRLF 

Striped Bass 

(Morone 

saxatilis) 

96h LC50= 400 ppb ECOTOX 

102151 Supplemental1

Chronic Direct 

Toxicity to 

Aquatic-Phase 

CRLF 

Fathead 

Minnows 

(Pimephales 

promelas) 

NOAEC = 26 ppb 

ai MRID 

00141636 

Acceptable 

(Based on No. 

of Survivors) 

Indirect 

Toxicity to 

Aquatic-Phase 

Scud 

(Gammmarus 

fasciatus) 

48h EC50 = 160 ppb 

ai 

MRID 

40094602 Acceptable 

 

5

 Overview of the Ecological Risk Assessment Process in the Office of Pesticide Programs, Environmental 

Protection Agency (January 2004) (Overview Document). 

6

 Memorandum to Office of Prevention, Pesticides and Toxic Substance, US EPA conveying an evaluation 

by the U.S. Fish and Wildlife Service and National Marine Fisheries Service of an approach to assessing 

the ecological risks of pesticide products (January 2004). 

63
Table 4.1 Freshwater Aquatic Toxicity Profile for Diuron 

Assessment 

Endpoint 

Species Toxicity Value 

Used in Risk 

Assessment 

Citation Comment 

MRID # or 

ECOTOX # 

CRLF via 

Acute Toxicity 

to Freshwater 

Invertebrates 

(i.e. prey items) 

Indirect 

Toxicity to 

Aquatic-Phase 

CRLF via 

Chronic 

Toxicity to 

Freshwater 

Invertebrates 

(i.e. prey items) 

Water Flea 

(Daphnia 

magna) 

28d NOAEC = 200 

ppb ai 

MRID 

TN2418 Supplemental1

Indirect 

Toxicity to 

Aquatic-Phase 

CRLF via 

Acute Toxicity 

to Non-vascular 

Aquatic Plants 

Green Algae 

(Selenastrum 

capricornutu

m) 

EC50 = 2.40 ppb ai MRID 

42218401 Acceptable 

Indirect 

Toxicity to 

Aquatic-Phase 

CRLF via 

Acute Toxicity 

to Vascular 

Aquatic Plants 

Duckweed

(Lemna 

perpusilla) 

7d EC50 = 15 ppb 

ECOTOX 

8628 

Supplemental1 

1

This study is considered to be supplemental due to lack of raw data and replicates.

Toxicity to aquatic fish and invertebrates is categorized using the system shown in Table 

4.2 (U.S. EPA, 2004). Diuron falls in the range of “highly toxic” (0.1 to 1 ppm or 100 to 

1000 ppb for fish and invertebrates. Toxicity categories for aquatic plants have not been 

defined. 

Table 4.2 Categories of Acute Toxicity for Aquatic Organisms 

LC50 (ppm) Toxicity Category 

< 0.1 Very highly toxic 

> 0.1 - 1 Highly toxic 

> 1 - 10 Moderately toxic 

> 10 - 100 Slightly toxic 

> 100 Practically nontoxic 

64
4.1.1 Toxicity to Freshwater Fish 

Given that no diuron 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 diuron 

to the CRLF. Effects to freshwater fish resulting from exposure to diuron 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). 

Diuron is highly toxic to both warm water and cold water freshwater fish on an acute 

exposure basis. The most sensitive freshwater species tested was the warm water Striped 

Bass species, which exhibited a 96-hour LC50 value of 400 ppb (ECOTOX 102151). 

Another acute study accepted by the Agency showed the cold water Cutthroat Trout 

having an acute 96h LC50 of 710 ppb ai (MRID 40098001). The Fathead Minnow 

exhibited a chronic toxicity NOAEC of 26 ppb based on number of survivors (MRID 

00141636). 

4.1.2 Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of diuron to the CRLF. Effects to freshwater invertebrates resulting from exposure to 

diuron 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. 

Diuron is highly acutely toxic to freshwater invertebrates based on acceptable studies on 

the Scud. This species exhibited a 48-hour EC50 value of 160 ppb (MRID: 40094602). 

Chronic toxicity studies showed the Water Flea as exhibiting a 28d NOAEC of 200 ppb 

(MRID: TN2418). 

4.1.3 Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether diuron 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. 

Two types of studies were used to evaluate the potential of diuron to affect aquatic plants. 

Laboratory and field studies were used to determine whether diuron may cause direct 

effects to aquatic plants. A summary of the laboratory data and freshwater field studies 

for aquatic plants is provided in Sections 4.1.3.1 and 4.1.4. 

The Green Alga Selenastrum capricornutum exhibited the most sensitive non-vascular 

plant acute endpoint with an EC50 of 2.40 ppb. The aquatic vascular plant Lemna 

65
perpusilla exhibited the most sensitive vascular acute endpoint with a 7d EC50 of 15 ppb 

ai (ECOTOX 8628). 

4.2 Toxicity of Diuron to Terrestrial Organisms 

Table 4.3 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 Diuron 

Endpoint Species Toxicity Value Used in 

Risk Assessment 

Citation 

MRID# 

Comment 

Acute Direct 

Toxicity to 

TerrestrialPhase CRLF 

(LD50) 

Mallard Duck 

(Anas 

platyrhynchos) 

14d LD50 = >2000 

mg/kg-bw 

MRID 

00160000 

Acceptable 

No mortality reported. Ataxia 

persisted for 11d after study 

Acute Direct 

Toxicity to 

TerrestrialPhase CRLF 

(LC50) 

Northern 

Bobwhite Quail 

(Colinus 

virgianus) 

9d LC50 = 1730 ppm ai MRID 

00022923 

Acceptable 

Chronic 

Direct 

Toxicity to 

TerrestrialPhase CRLF 

No Acceptable 

Study Available -- -- -- 

Indirect 

Toxicity to 

TerrestrialPhase CRLF 

(via acute 

toxicity to 

mammalian 

prey items) 

Laboratory Rat 

(Rattus 

norvegicus) 

LD50 = 

♂ 5000 mg/kg-bw 

♀ 10000 mg/kg-bw 

MRID 

00146145 Acceptable 

Indirect 

Toxicity to 

TerrestrialPhase CRLF 

(via chronic 

toxicity to 

mammalian 

prey items) 

Laboratory Rat 

(Rattus 

norvegicus) 

NOAEC = 250 ppm MRID 

00146145 

Acceptable 

Indirect 

Toxicity to 

TerrestrialPhase CRLF 

(via acute 

toxicity to 

terrestrial 

invertebrate 

prey items) 

Honey Bee 

(Apis mellifera) 

LD50 >145 μg/bee 

(1133.05 ppm) 

MRID 

00036935 

Acceptable 

(Only 2.7% mortality observed 

at highest level tested) 

Indirect Dicot Seedling EC25 = 0.075 lb ai/A MRID Acceptable 

66
Endpoint Species Toxicity Value Used in 

Risk Assessment 

Citation Comment 

MRID# 

Emergence 

Tomato 

(Lycopersicon 

esculentum) 

44113401 

Dicot Vegetative 

Vigor Tomato 

(Lycopersicon 

esculentum) 

EC25 = 0.002 lb ai/A MRID 

44113401 Acceptable 

Monocot 

Seedling 

Emergence 

Onion 

(Allium cepa) 

EC25 = 0.099 lb ai/A MRID 

44113401 Acceptable 

Toxicity to 

Terrestrialand AquaticPhase CRLF 

(via toxicity 

to terrestrial 

plants) 

Monocot 

Vegetative Vigor 

Wheat 

(Triticum 

aestivum)

EC25 = 0.021 lb ai/A MRID 

44113401 Acceptable 

Acute toxicity to terrestrial animals is categorized using the classification system shown 

in Table 4.4 (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 diuron; therefore, acute and 

chronic avian toxicity data are used to assess the potential direct effects of diuron to 

terrestrial-phase CRLFs. 

There are no data by which to assess chronic toxicity to avian species (surrogates for the 

terrestrial phase frog) either submitted by the registrants or in the public literature. 

Linuron, a similar chemical of the same class, has acceptable chronic avian data. The 

avian reproductive studies for linuron (MRID 42541801, 42541802) using mallard duck 

and bobwhite quail found reproductive effects at 300 ppm ai with an NOAEL being 

established at 100 ppm. The endpoints affected for mallard duck reproduction are egg 

production, adult body weight, feed consumption, viable embryos of eggs set, number of 

viable embryos, and number of live embryos. For the bobwhite quail the endpoints 

67
affected are hatchability and offspring survivability. This suggests that diuron may have 

chronic effects to avian species, but these risks cannot be quantified in the absence of 

data. 

4.2.2 Toxicity to Mammals 

Mammalian toxicity data are used to assess potential indirect effects of diuron to the 

terrestrial-phase CRLF. Effects to small mammals resulting from exposure to diuron 

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). 

An acute oral LD50 of 5000 mg/kg-bw was observed in the Laboratory Rat (MRID 

00146145). The Laboratory Rat NOAEC is 250 ppm (MRID 00146145). 

4.2.3 Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of diuron 

to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting from exposure 

to diuron could also indirectly affect the CRLF via reduction in available food. Diuron is 

classified as practically non-toxic to the Honey Bee. The acute LD50 for Honey Bee is 

>145.03 μg/bee (MRID 00036935). Only 2.7% mortality was observed in this study at 

the highest level tested. The test was therefore not definitive and RQs were not 

calculated. There were no additional acceptable terrestrial invertebrate data from 

registrant submitted studies or from the open literature by which to calculate RQ values. 

4.2.4 Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for diuron to affect 

riparian zone and upland vegetation within the action area for the CRLF. Impacts to 

riparian and upland (i.e., grassland, woodland) vegetation could result in indirect effects 

to both aquatic- and terrestrial-phase CRLFs, as well as modification to designated 

critical habitat PCEs via increased sedimentation, alteration in water quality, and 

reduction 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. 

68
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 diuron, 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 most sensitive endpoints reported for diuron exposure to dicot plants are a vegetative 

vigor EC25 value of 0.002 lb ai/A and a seedling emergence EC25 value of 0.075 lb ai/A as 

expressed by tomato plants (MRID 44113401). The most sensitive endpoints reported 

for diuron exposure to monocot plants are a vegetative vigor EC25 value of 0.021 lb ai/A 

as expressed by Wheat, and a seedling emergence EC25 value of 0.099 lb ai/A as 

expressed by Onion (MRID 44113401). The results of the Tier II seedling emergence 

and vegetative vigor toxicity tests on non-target plants are summarized below in Table 

4.5. 

Table 4.5 Terrestrial Plant Toxicity Profile for Diuron 

Endpoint Species Toxicity Value Used 

in Risk Assessment 

Citation 

MRID# 

Comment 

Dicot Seedling 

Emergence 

Tomato 

(Lycopersicon 

esculentum) 

EC25 = 0.075 lb ai/A MRID 

44113401 Acceptable 

Dicot Vegetative 

Vigor Tomato 

(Lycopersicon 

esculentum) 

EC25 = 0.002 lb ai/A MRID 

44113401 Acceptable 

Monocot 

Seedling 

Emergence 

Onion 

(Allium cepa) 

EC25 = 0.099 lb ai/A MRID 

44113401 Acceptable 

Indirect 

Toxicity to 

Terrestrialand AquaticPhase CRLF 

(via toxicity 

to terrestrial 

plants) 

Monocot 

Vegetative Vigor 

Wheat 

(Triticum 

aestivum)

EC25 = 0.021 lb ai/A MRID 

44113401 Acceptable 

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 diuron on par with the acute toxicity endpoint selected for RQ 

calculation. To accomplish this interpretation, the Agency uses the slope of the dose 

69
response relationship available from the toxicity study used to establish the acute toxicity 

measures of effect for each taxonomic group that is relevant to this assessment. The 

individual effects probability associated with the acute RQ is based on the mean estimate 

of the slope and an assumption of a probit dose response relationship. In addition to a 

single effects probability estimate based on the mean, upper and lower estimates of the 

effects probability are also provided to account for variance in the slope, if available. 

Individual effect probabilities are calculated based on an Excel spreadsheet tool IECV1.1 

(Individual Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP, 

Environmental Fate and Effects Division (June 22, 2004). The model allows for such 

calculations by entering the mean slope estimate (and the 95% confidence bounds of that 

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

as the desired threshold. 

4.4 Incident Database Review 

A review of the EIIS database for ecological incidents involving diuron was completed 

on 12/18/08. Thirty highly probable, probable or possible incidents involving diuron 

have been reported. These uses were either registered uses or undetermined uses. One of 

these incidents resulted in a bird kill, 22 resulted in plant damage and seven resulted in 

fish kills. A detailed description of incidents relating to diuron use is presented in 

Appendix M. These incidents will be used in addition to other lines of evidence to draw 

conclusions regarding the risks of diuron to the CRLF. 

The absence of additional documented incidents does not necessarily mean that such 

incidents did not occur. Mortality incidents must be seen, reported, investigated, and 

submitted to the Agency in order to be recorded in the incident database. Incidents may 

not be noticed because the carcasses decayed, were removed by scavengers, or were in 

out-of-the-way or hard-to-see locations. Due to the voluntary nature of incident reporting, 

an incident may not be reported to appropriate authorities capable of investigating it. 

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 diuron in 

CA. The risk characterization provides an estimation (Section 5.1) and a description 

(Section 5.2) of the likelihood of adverse effects; articulates risk assessment assumptions, 

limitations, and uncertainties; and synthesizes an overall conclusion regarding the 

likelihood of adverse effects to the CRLF or its designated critical habitat (i.e., “no 

effect,” “likely to adversely affect,” or “may affect, but not likely to adversely affect”). 

5.1 Risk Estimation 

Risk is estimated by calculating the ratio of exposure to toxicity. This ratio is the risk 

quotient (RQ), which is then compared to pre-established acute and chronic levels of 

70
concern (LOCs) for each category evaluated (Appendix E). For acute exposures to the 

CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC 

is 0.05. For acute exposures to the CRLF and mammals, the LOC is 0.1. The LOC for 

chronic exposures to CRLF and its prey, as well as acute exposures to plants is 1.0. 

Risk to the aquatic-phase CRLF is estimated by calculating the ratio of exposure to 

toxicity using 1-in-10 year EECs based on the label-recommended diuron usage scenarios 

summarized in Table 3.2 and the appropriate aquatic toxicity endpoint from Table 4.1. 

Risks to the terrestrial-phase CRLF and its prey (e.g. terrestrial insects, small mammals 

and terrestrial-phase frogs) are estimated based on exposures resulting from applications 

of diuron (Tables 3.5 through 3.6) and the appropriate toxicity endpoint from Table 4.6. 

Exposures are also derived for terrestrial plants, as discussed in Section 3.3 and 

summarized in Table 3.7, based on the highest application rates of diuron 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. The RQs for diuron uses results in acute and chronic exceedances of the 

Agency’s LOC for freshwater fish which are surrogates for the aquatic phase for 

amphibians. 

The aquatic phase amphibian acute LOCs for listed species (0.05) are exceeded for most 

uses of diuron in California. Acute RQs that exceed the Agency’s LOC range from 12.28 

(paved areas) to 0.06 (papaya and walnut). Chronic RQs that exceed the Agency’s LOC 

for chronic exposure (1.0) range from 131.85 (paved areas) to 1.26 (banana, plantain). A 

probit slope value for the acute fathead minnow toxicity test is not available; therefore, 

the individual effect probability was estimated based on a default slope assumption of 4.5 

with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986). The 

estimated probability of an individual effect for treatments that exceeded the Agency’s 

acute RQs for listed species ranged from approximately 1 in 1 for paved areas to 

approximately 1 in 1.64 E+4 for banana/plantain (upper and lower 95% confidence 

intervals of 1 in 2.28 E+01 to 1 in 1.29 E+14). Based on these exceedances, diuron 

May Affect the aquatic-phase of the CRLF. Results are presented in Table 5.1. 

71
Table 5.1. Summary of Direct Effect RQs for the Aquatic-phase CRLF 

Use Direct 

Effects 

to 

CRLFa

Surrogate 

Species 

Toxicity 

Value 

(μg/L) 

EEC 

Peak/60d 

(μg/L) 

RQ Probability 

of Individual 

Effect* 

 LOC 

Exceedance 

and Risk 

Interpretation 

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

687.59 1.72 1 in 1.17 (1 

in 1.47 to 1 

in 1.07)b

Yesc AGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/

HEDGEROWS Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

456.81 17.57 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

10.63 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

ALFALFA No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

7.20 0.28 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

13.69 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

ALFALFA(Aerial) No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

9.40 0.36 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

11.68 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

APPLE No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

7.80 0.30 Not 

applicable 

for chronic 

endpoints

No

ARTICHOKE Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

32.00 0.08 1 in 

2.51E+06 (1 

in 1.00 E+16 

to 1 in 7.08 

E+1)b

Yesc

72
Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

25.08 0.96 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

36.99 0.09 1 in 7.90 

E+05 (1 in 

1.00 E+16 to 

1 in 1.00 

E+06)b

Yesc ASPARGUS

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

22.92 0.88 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

54.21 0.14 1 in 1.64 

E+4 (1 in 

2.28 E+01 to 

1 in 1.29 

E+14)b

Yesc BANANA, 

PLANTAIN

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

32.78 1.26 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

139.70 0.35 1 in 

4.98E+01 (1 

in 5.53 to 1 

in 4.91 

E+4)b

Yesc BLACKBERRY, 

BOYSENBERRY

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

102.90 3.96 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

4.65 0.01 <1 in 

1.00E+16 (1 

in 3.16 E+04 

to <1 in 

1.00E+16)b

BLUEBERRY No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

3.21 0.12 Not 

applicable 

for chronic 

endpoints

No

CITRUS Acute 

Direct 

LC50 = 

400

6.83 0.02 1 in 9.48 

E+13 (1 in 

No

73
Toxicity 2.95 E+3 to 

<1 in 1.00 

E+16)b

Chronic 

Direct 

Toxicity

Rainbow 

Trout

NOAEC = 

26

4.88 0.19 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

7.24 0.02 1 in 9.48 

E+13 (1 in 

2.95 E+3 to 

<1 in 1.00 

E+16)b

CORN, FIELD No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

4.68 0.18 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

19.71 0.05 1 in 4.17 

E+08 (1 in 

2.16 E+02 

to <1 in 1.00 

E+16)b

DEWBERRY No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

14.29 0.55 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

18.90 0.05 1 in 4.17 

E+08 (1 in 

2.16 E+02 

to <1 in 1.00 

E+16)b

FILBERT No

(HAZELNUT)

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

12.16 0.47 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

9.71 0.02 1 in 9.48 

E+13 (1 in 

2.95 E+3 to 

<1 in 1.00 

E+16)b

GRAPE No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

6.37 0.24 Not 

applicable 

for chronic 

endpoints

No

LOGANBERRY, 

RASPBERRY 

Acute 

Direct 

Striped 

Bass

LC50 = 

400

5.13 0.01 <1 in 

1.00E+16 (1 

No

74
Toxicity in 3.16 E+04 

to <1 in 

1.00E+16)b

(BLACK/RED)

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

3.02 0.12 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

7.17 0.02 1 in 9.48 

E+13 (1 in 

2.95 E+3 to 

<1 in 1.00 

E+16)b

OLIVE No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

4.86 0.19 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

25.21 0.06 1 in 5.20 

E+07 (1 in 

1.38 E+02 to 

<1 in 1.00 

E+16)b

Yesc PAPAYA

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

15.72 0.60 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

4.17 0.01 <1 in 

1.00E+16 (1 

in 3.16 E+04 

to <1 in 

1.00E+16)b

PEPPERMINT No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

2.79 0.11 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

10.95 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

PEACH No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

7.31 0.28 Not 

applicable 

for chronic 

endpoints

No

PEAR Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

11.66 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

No

75
Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

7.79 0.30 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

27.04 0.07 1 in 9.86 

E+06 (1 in 

9.57 E+1 to 

<1 in 1.00 

E+16)b

Yesc PECAN

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

17.33 0.67 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

5.87 0.01 <1 in 

1.00E+16 (1 

in 3.16 E+04 

to <1 in 

1.00E+16)b

SORGHUM No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

4.52 0.17 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

106.70 0.27 1 in 

1.90E+02 (1 

in 7.83 to 1 

in 6.46 

E+06)b

Yesc SPEARMINT

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

81.95 3.15 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

25.78 0.06 1 in 5.20 

E+07 (1 in 

1.38 E+02 to 

<1 in 1.00 

E+16)b

Yesc WALNUT 

(ENGLISH/BLACK)

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

16.58 0.64 Not 

applicable 

for chronic 

endpoints

No

WHEAT (preharvest)Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

18.03 0.05 1 in 4.17 

E+08 (1 in 

2.16 E+02 

to <1 in 1.00 

E+16)b

No

76
Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

12.02 0.46 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

12.82 0.03 1 in 

2.75E+11 (1 

in 1.00 E+16 

to 1 in 8.63 

E+02)b

WHEAT (post- No

harvest)

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

10.29 0.40 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

8.36 0.02 1 in 9.48 

E+13 (1 in 

2.95 E+3 to 

<1 in 1.00 

E+16)b

COTTON (ground) No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

5.72 0.22 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

14.25 0.04 1 in 6.30 

E+09 (1 in 

3.86 E+02 to 

<1 in 1.00 

E+16)b

COTTON(aerial) No

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

10.24 0.39 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

2190.00 5.48 ~1 in 1b Yesc AIRPORTS/ 

LANDING FIELDS, 

DRAINAGE 

SYSTEMS, 

INDUSTRIAL 

AREAS 

(OUTDOOR), 

SEWAGE 

DISPOSAL AREAS

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

1382.00 53.15 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

226.00 0.57 1 in 7.35 (1 

in 3.20 to 1 

in 7.14 

E+01)b

Yesc BERMUDAGRASS

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

167.90 6.46 Not 

applicable 

for chronic 

endpoints

Yesd

77
Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

3.76 0.01 <1 in 

1.00E+16 (1 

in 3.16 E+04 

to <1 in 

1.00E+16)b

GRASSES GROWN No

FOR SEED

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

2.89 0.11 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

52.56 0.13 1 in 

2.99E+04 (1 

in 2.62 E+01 

to 1 in 1.29 

E+15)b

Yesc IRRIGATION 

SYSTEMS

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

37.45 1.44 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

36.55 0.09 1 in 7.90 

E+05 (1 in 

5.48 E+01 to 

<1 in 1.00 

E+16)b

Yesc ORNAMENTAL 

HERBACEOUS 

PLANTS

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

23.56 0.91 Not 

applicable 

for chronic 

endpoints

No

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

629.91 1.57 1 in 1.23. (1 

in 1.53 to 1 

in 1.04)b

Yesc NONAGRICULTURAL 

RIGHTS OF WAY 

(ground) Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

476.35 18.32 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

626.91 1.57 1 in 1.23. (1 

in 1.53 to 1 

in 1.04)b

Yesc NONAGRICULTURAL 

RIGHTS OF WAY 

(aerial) Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

468.16 18.01 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

4911.00 12.28 ~1 in 1b Yesc PAVED AREAS 

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

3428.00 131.85 Not 

applicable 

for chronic 

endpoints

Yesd

78
Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

1130.00 2.83 ~1 in 1b Yesc UNCULTIVATED 

AG

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

840.40 32.32 Not 

applicable 

for chronic 

endpoints

Yesd

Acute 

Direct 

Toxicity

Striped 

Bass

LC50 = 

400

284.80 0.71 1 in 3.97 (1 

in 2.61 to 1 

in 1.11 

E+01)b

Yesc UNCULTIVATED 

NON-AG

Chronic 

Direct 

Toxicity

Fathead 

Minnow

NOAEC = 

26

202.50 7.79 Not 

applicable 

for chronic 

endpoints

Yesd

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. 

b

 A probit slope value for the acute fathead minnow 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). c

 RQ > acute endangered species LOC of 0.05. 

d

 RQ > chronic LOC of 1.0.

5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey 

(non-vascular aquatic plants, aquatic invertebrates, fish, and frogs) 

Non-vascular Aquatic Plants

Indirect effects of diuron to the aquatic-phase CRLF (tadpoles) via reduction in nonvascular aquatic plants in its diet are based on peak EECs from the standard pond and the 

lowest acute toxicity value for aquatic non-vascular plants. The Agency’s LOC (1.0) is 

exceeded for all current uses of diuron in California. The RQs range from 2046.25 

(paved areas) to 1.57 (grasses grown for seed). Results are presented in Table 5.2. 

Based on these exceedances, diuron May Affect the CRLF via reduction of nonvascular plants. 

79
Table 5.2 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via 

Effects to Non-Vascular Aquatic Plants (diet of CRLF in tadpole life stage and 

habitat of aquatic-phase CRLF)* 

Uses Application 

rate (lb ai/A) 

Application 

Method 

Peak 

EEC 

(μg/L) 

Indirect effects RQ* 

(food and habitat) 

AGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 12 Ground 687.59 286.50 

2.4 Ground 10.63 4.43 

ALFALFA 

 2.4 Aircraft 13.69 5.71 

APPLE 3.2 Ground 11.68 4.87 

ARTICHOKE 3.2 Ground 32.00 13.33 

ASPARGUS 4.8 Ground 36.99 15.41 

BANANA, PLANTAIN 2.4 Ground 54.21 22.59 

BLACKBERRY, BOYSENBERRY 1.6 Ground 139.70 58.21 

BLUEBERRY 2.4 Ground 4.65 1.94 

CITRUS 3.2 Ground 6.83 2.85 

CORN, FIELD 0.8 Ground 7.24 3.02 

DEWBERRY 2.4 Ground 19.71 8.21 

FILBERT (HAZELNUT) 2.2 Ground 18.90 7.87 

80
Peak Indirect effects RQ* Application Application Uses rate (lb ai/A) Method EEC (food and habitat) 

(μg/L) 

GRAPE 3.2 Ground 9.71 4.04 

LOGANBERRY, RASPBERRY 

(BLACK/RED) 2.4 Ground 5.13 2.14 

OLIVE 1.6 Ground 7.17 2.99 

PAPAYA 4 Ground 25.21 10.50 

PEPPERMINT 2.4 Ground 4.17 1.74 

PEACH 3 Ground 10.95 4.56 

PEAR 3.2 Ground 11.66 4.86 

PECAN 3.2 Ground 27.04 11.27 

SORGHUM 4.8 Ground 5.87 2.45 

SPEARMINT 2.4 Ground 106.70 44.46 

WALNUT (ENGLISH/BLACK) 0.4 Ground 25.78 10.74 

2.4 Pre-Harvest 18.03 7.51 

WHEAT 

3 PostHarvest 12.82 5.34 

Ground 8.36 3.48 

COTTON 1.6 

Aircraft 14.25 5.94 

81
Peak Indirect effects RQ* Application Application Uses rate (lb ai/A) Method EEC (food and habitat) 

(μg/L) 

AIRPORTS/ LANDING FIELDS, 

DRAINAGE SYSTEMS, 

INDUSTRIAL AREAS 

(OUTDOOR), SEWAGE DISPOSAL 

AREAS 

12 Ground 2190.00 912.50 

BERMUDAGRASS 2.4 Ground 226.00 94.17 

GRASSES GROWN FOR SEED 12 Ground 3.76 1.57 

IRRIGATION SYSTEMS 1.6 Ground 52.56 21.90 

ORNAMENTAL HERBACEOUS 

PLANTS 12 Ground 36.55 15.23 

12 Ground 629.91 262.46 NON-AGRICULTURAL RIGHTS 

OF WAY 

4 Aircraft 626.91 261.21 

PAVED AREAS (PRIVATE 

ROADS/SIDEWALKS) 12 Ground 4911.00 2046.25 

12 Ground 1130.00 470.83 

UNCULTIVATED AG 

12 Ground 1169.00 487.08 

UNCULTIVATED NON-AG 12 Ground 284.80 118.67 

82
Peak Indirect effects RQ* Application Application Uses rate (lb ai/A) Method EEC (food and habitat) 

(μg/L) 

* LOC exceedances (RQ > 1.0) are bolded and shaded. RQ = use-specific peak EEC/ [Selenastrum capricornutum

EC50 = 2.40 ppb]. 

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. The Agency’s acute 

Listed LOC (0.05) is exceeded for most uses of diuron in California. The acute RQs that 

exceed the LOC range from 30.69 (Paved Areas) to 0.06 (grape). The chronic RQs 

exceed the Agency’s LOC for several used. The chronic values that exceeded range from 

21.21 (Paved Areas) to 1.02 (Bermudagrass). A summary of the acute and chronic RQ 

values for exposure to aquatic invertebrates (as prey items of aquatic-phase CRLFs) is 

provided in Table 5.3. Based on acute Listed LOC exceedances for chronic and acute 

aquatic invertebrates from most use sites, diuron May Affect the CRLF indirectly 

via reduction in freshwater invertebrate prey items. 

Table 5.3 Summary of Acute and Chronic RQs 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)* 

Uses 

Application 

rate (lb 

ai/A) 

Application 

Method 

Peak 

EEC 

(μg/L) 

21 Day 

EEC 

ppb 

Indirect 

Effects 

Acute 

RQ* 

Indirect 

Effects 

Chronic 

RQ** 

AGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 12 Ground 687.59 672.03 4.30 2.92 

2.4 Ground 10.63 10.37 0.07 0.05 

ALFALFA 

 2.4 Aircraft 13.69 13.29 0.09 0.06 

83
APPLE 3.2 Ground 11.68 11.22 0.07 0.05 

ARTICHOKE 3.2 Ground 32.00 31.15 0.20 0.14 

ASPARGUS 4.8 Ground 36.99 35.46 0.23 0.15 

BANANA, PLANTAIN 2.4 Ground 54.21 51.95 0.34 0.22 

BLACKBERRY, BOYSENBERRY 1.6 Ground 139.70 135.80 0.87 0.60 

BLUEBERRY 2.4 Ground 4.65 4.48 0.03 0.02 

CITRUS 3.2 Ground 6.83 6.57 0.04 0.03 

CORN, FIELD 0.8 Ground 7.24 6.95 0.05 0.03 

DEWBERRY 2.4 Ground 19.71 19.25 0.12 0.08 

FILBERT (HAZELNUT) 2.2 Ground 18.90 18.41 0.12 0.08 

GRAPE 3.2 Ground 9.71 9.32 0.06 0.04 

LOGANBERRY, RASPBERRY 

(BLACK/RED) 2.4 Ground 5.13 4.95 0.03 0.02 

OLIVE 1.6 Ground 7.17 6.90 0.04 0.03 

PAPAYA 4 Ground 25.21 24.56 0.16 0.11 

PEPPERMINT 2.4 Ground 4.17 4.05 0.03 0.02 

PEACH 3 Ground 10.95 10.51 0.07 0.05 

PEAR 3.2 Ground 11.66 11.20 0.07 0.05 

PECAN 3.2 Ground 27.04 26.33 0.17 0.11 

SORGHUM 4.8 Ground 5.87 5.67 0.04 0.03 

SPEARMINT 2.4 Ground 106.70 103.70 0.67 0.49 

84
WALNUT (ENGLISH/BLACK) 0.4 Ground 25.78 25.11 0.16 0.11 

2.4 Pre-Harvest 18.03 17.34 0.11 0.07 

WHEAT 

3 PostHarvest 12.82 12.34 0.08 0.06 

Ground 8.36 8.13 0.05 0.04 

COTTON 1.6 

Aircraft 14.25 13.88 0.09 0.06 

AIRPORTS/ LANDING FIELDS, 

DRAINAGE SYSTEMS, 

INDUSTRIAL AREAS 

(OUTDOOR), SEWAGE DISPOSAL 

AREAS 

12 Ground 2190.00 2115.00 13.69 9.13 

BERMUDAGRASS 2.4 Ground 226.00 218.80 1.41 1.02 

GRASSES GROWN FOR SEED 12 Ground 3.76 3.62 0.02 0.02 

IRRIGATION SYSTEMS 1.6 Ground 52.56 50.67 0.33 0.23 

ORNAMENTAL HERBACEOUS 

PLANTS 12 Ground 36.55 35.45 0.23 0.15 

12 Ground 629.91 619.12 3.94 2.80 NON-AGRICULTURAL RIGHTS 

OF WAY 

4 Aircraft 626.91 615.83 3.92 2.76 

PAVED AREAS (PRIVATE 

ROADS/SIDEWALKS) 12 Ground 4911.00 4725.00 30.69 21.21 

12 Ground 1130.00 1099.00 7.06 5.12 

UNCULTIVATED AG 

12 Ground 1169.00 1130.00 7.31 5.31 

UNCULTIVATED NON-AG 12 Ground 284.80 275.70 1.78 1.26 

85
* LOC exceedances (acute RQ > 0.05, chronic RQ > 1.0) are bolded and shaded. RQ = use-specific peak EEC/ 

[Selenastrum capricornutum EC50 = 2.40 ppb]. 

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 chronic and acute LOC exceedances, diuron May 

Affect the CRLF indirectly via reduction in freshwater fish and frogs as food items. 

5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and 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. A summary of acute 

RQs used to estimate indirect effects to the CRLF via effects to vascular aquatic plants is 

presented in Table 5.4. The RQs for vascular aquatic plants exceed the Agency’s LOC 

(1.0) for most uses of diuron in California. These range from 327.40 (paved areas) to 

1.26 (Filbert/Hazelnut). Because of this, and in addition to the LOC exceedances for 

all uses of diuron to non-vascular aquatic plants, it is determined that diuron May 

Affect the CRLF indirectly via reduction in vascular aquatic plants. 

Table 5.4 Summary of Acute 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) 

Application 

Method 

Peak 

EEC 

(μg/L) 

Indirect Effects Acute 

RQ* 

AGRICULTURAL RIGHTS-OFWAY/FENCEROWS/HEDGEROWS 12 Ground 687.59 45.84 

2.4 Ground 10.63 0.71 

ALFALFA 

 2.4 Aircraft 13.69 0.91 

86
APPLE 3.2 Ground 11.68 0.78 

ARTICHOKE 3.2 Ground 32.00 2.13 

ASPARGUS 4.8 Ground 36.99 2.47 

BANANA, PLANTAIN 2.4 Ground 54.21 3.61 

BLACKBERRY, BOYSENBERRY 1.6 Ground 139.70 9.31 

BLUEBERRY 2.4 Ground 4.65 0.31 

CITRUS 3.2 Ground 6.83 0.46 

CORN, FIELD 0.8 Ground 7.24 0.48 

DEWBERRY 2.4 Ground 19.71 1.31 

FILBERT (HAZELNUT) 2.2 Ground 18.90 1.26 

GRAPE 3.2 Ground 9.71 0.65 

LOGANBERRY, RASPBERRY 

(BLACK/RED) 2.4 Ground 5.13 0.34 

OLIVE 1.6 Ground 7.17 0.48 

PAPAYA 4 Ground 25.21 1.68 

PEPPERMINT 2.4 Ground 4.17 0.28 

PEACH 3 Ground 10.95 0.73 

PEAR 3.2 Ground 11.66 0.78 

PECAN 3.2 Ground 27.04 1.80 

87
SORGHUM 4.8 Ground 5.87 0.39 

SPEARMINT 2.4 Ground 106.70 7.11 

WALNUT (ENGLISH/BLACK) 0.4 Ground 25.78 1.72 

2.4 Pre-Harvest 18.03 1.20 

WHEAT 

3 PostHarvest 12.82 0.85 

Ground 8.36 0.56 

COTTON 1.6 

Aircraft 14.25 0.95 

AIRPORTS/ LANDING FIELDS, 

DRAINAGE SYSTEMS, 

INDUSTRIAL AREAS 

(OUTDOOR), SEWAGE DISPOSAL 

AREAS 

12 Ground 2190.00 146.00 

BERMUDAGRASS 2.4 Ground 226.00 15.07 

GRASSES GROWN FOR SEED 12 Ground 3.76 0.25 

IRRIGATION SYSTEMS 1.6 Ground 52.56 3.50 

ORNAMENTAL HERBACEOUS 

PLANTS 12 Ground 36.55 2.44 

12 Ground 629.91 41.99 NON-AGRICULTURAL RIGHTS 

OF WAY 

4 Aircraft 626.91 41.79 

88
PAVED AREAS (PRIVATE 

ROADS/SIDEWALKS) 12 Ground 4911.00 327.40 

12 Ground 1130.00 75.33 

UNCULTIVATED AG 

12 Ground 1169.00 77.93 

UNCULTIVATED NON-AG 12 Ground 284.80 18.99 

a

LOC exceedances (RQ > 1) are bolded and shaded. 

RQ = use-specific peak EEC/ [Lemna major EC50 = 2.40 ppb]. 

5.1.2 Exposures in the Terrestrial Habitat 

5.1.2.1 Direct Effects to Terrestrial-phase CRLF 

As discussed in Section 3.3, potential direct effect determinations to terrestrial-phase 

CRLFs are based on foliar applications of diuron. 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. The Bobwhite Quail 

exhibited the greatest acute sensitivity to diuron and was therefore selected to serve as a 

surrogate for the CRLF. 

Potential direct chronic effects of diuron 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 dietary-based RQs. 

As mentioned, there are no acceptable data for chronic or dose based acute toxicity of 

diuron to amphibians or their avian surrogates. Therefore, no RQs were calculated for 

these values. Linuron, a similar chemical of the same class, has acceptable chronic avian 

data. The avian reproductive studies for linuron (MRID 42541801, 42541802) using 

mallard duck and bobwhite quail found reproductive effects at 300 ppm ai with an 

NOAEL being established at 100 ppm. The endpoints affected for mallard duck 

reproduction are egg production, adult body weight, feed consumption, viable embryos of 

eggs set, number of viable embryos, and number of live embryos. For the bobwhite 

quail the endpoints affected are hatchability and offspring survivability. This suggests 

that diuron may have chronic effects to avian species, but these risks cannot be quantified 

89
in the absence of data. As a result, chronic risks are assumed to avian surrogates for all 

uses of diuron in California. 

The most sensitive dose based LD50 was >2000 ppm ai as exhibited by the Mallard Duck 

(MRID 00160000). It is important to note that although no mortality was observed in this 

study, ataxia persisted for 11d after study concluded. This sublethal effect could 

potentially result in the animals being more susceptible to predation as a result of being 

both conspicuous to predators and by having a diminished ability to avoid capture. The 

calculated acute dietary RQs for the terrestrial phase CRLF exceed the Agency’s LOC 

(0.1) for all uses except sorghum. The RQs in Table 5.5 below are based on the 5-day 

dietary LC50 of 1730 ppm for bobwhite quail. 

The acute dietary based RQs exceed the LOC for all uses except Field Corn and 

Sorghum. The exceedances range from 1.55 (Agricultural Rights-of-Way, Fencerows, 

Hedgerows, Airports, etc.) to 0.12 (grass for seed, wheat). These results are summarized 

in Table 5.5. Based on acute dietary LOC exceedances for the listed bird surrogate 

for amphibians, diuron May Affect the terrestrial-phase of the CRLF directly. 

Table 5.5 Summary of Acute RQs Used to Estimate Direct Effects to the Terrestrialphase CRLF 

Use 

(Application Rate) 

Dietary -based Acute 

RQ2

Agricultural Rights-of-Way, Fencerows, Hedgerows, Airports, 

landing Fields, Drainage Systems, Outdoor Industrial Areas, 

Irrigation Systems, Non-Agricultural Rights of Way, Sewage 

Disposal Areas, Uncultivated Non-Agricultural Areas

1.55 

Alfalfa, Peppermint, 0.19 

Apple, Grape 0.29 

Citrus, 0.33 

Artichoke, Asparagus 0.25 

Banana, Plantain 0.54 

Bermuda grass, Blackberry, Boysenberry, Spearmint 0.48 

Blueberry 0.14 

Field Corn 0.06

Cotton 0.26

Dewberry 0.26 

Filbert (Hazelnut) 0.18 

Grass Seed, Wheat 0.12 

Loganberry, Raspberry 0.19 

Olive 0.13 

Pear, Pecan 0.25 

Ornamental Herbaceous plants, Papaya 0.31 

Uncultivated Agricultural Areas, Paved Areas 0.94 

Peach 0.23 

Sorghum 0.03

90
Use Dietary -based Acute 

(Application Rate) RQ2

Walnut 0.25 

* = LOC exceedances (acute RQ > 0.1) are bolded and shaded. 2

Based on Bobwhite Quail LC50 =1730 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 diuron to terrestrial invertebrates, which are considered 

prey of CRLF in terrestrial habitats, the honeybee 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 >145.03 μg a.i. /bee by 1 bee/0.128g, which is 

based on the weight of an adult honey bee. The toxicity value for terrestrial invertebrates 

is calculated to be >1133.05 ppm, but only 2.77% mortality was observed at the highest 

levels tested. Therefore, this test is not definitive and acute RQs were not calculated. 

However, because the terrestrial EEC’s exceed the highest levels tested for several 

uses, it is determined that diuron May Affect the CRLF indirectly via reduction in 

terrestrial invertebrate prey items. 

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. 

Chronic dose based RQs exceed the Agency’s LOC (1.0) for most uses ( 

Table 5.6). The dose based chronic RQs that exceed the LOC range from 8.29 

(Agricultural Rights-of-Way, Fencerows, Hedgerows, Airports, etc.) to 1.31 (walnut). 

The dietary based chronic RQs do not exceed the LOC for any uses. The RQs that 

exceed the acute dose based LOC range from 0.41 (Agricultural Rights-of-Way, 

Fencerows, Hedgerows, Airports, etc.) to 0.13 (Bermuda grass, Blackberry, Boysenberry, 

Spearmint). Based on acute and chronic LOC exceedances on small mammal prey 

items, diuron May Affect the CRLF indirectly via reduction in small mammal prey 

items. 

91
Table 5.6 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 

Chronic RQ1

Use Acute RQ 

Dose-based 

Chronic RQ 

Dietary-based 

Chronic RQ 

Dose-based 

Acute RQ2

Agricultural Rights-of-Way, Fencerows, 

Hedgerows, Airports, landing Fields, 

Drainage Systems, Outdoor Industrial 

Areas, Irrigation Systems, Non-Agricultural 

Rights of Way, Sewage Disposal Areas, 

Uncultivated Non-Agricultural Areas

8.29 0.96 0.41 

Alfalfa, Peppermint 1.00 0.12 0.05 

Apple, Grape 1.56 0.18 0.08 

Artichoke, Asparagus 1.33 0.15 0.07 

Banana, Plantain 2.87 0.33 0.14

Bermuda grass, Blackberry, Boysenberry, 

Spearmint 2.57 0.30 0.13

Blueberry 0.74 0.09 0.04

Citrus 1.74 0.20 0.09

Field Corn 0.33 0.04 0.02 

Cotton 1.40 0.16 0.07 

Dewberry 1.40 0.16 0.07 

Filbert (Hazelnut) 0.96 0.11 0.05 

Grass Seed, Wheat 0.67 0.08 0.03 

Loganberry, Raspberry 1.00 0.12 0.05 

Olive 0.69 0.08 0.03 

Ornamental Herbaceous Plants, Papaya 1.67 0.19 0.08 

Uncultivated Agricultural Areas, Paved 

Areas 5.00 0.58 0.25 

Peach 1.25 0.14 0.06 

Pear, Pecan 1.33 0.15 0.07

Sorghum 0.17 0.02 0.01 

Walnut 1.31 0.15 0.07

* = LOC exceedances (acute RQ > 0.1 and chronic RQ > 1) are bolded and shaded. 1

 Based on dose-based EEC and diuron rat NOAEC = 250.00 mg/kg-bw. 2

 Based on dose-based EEC and diuron rat acute oral LD50 = 5000.00 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 

92
in T-REX for a small bird (20g) consuming small invertebrates are used. See Section 

5.1.2.1 for results. The chronic LOC is exceeded for all uses of diuron and the acute 

LOCs are exceeded for all uses except sorghum. Based on acute and chronic LOC 

exceedances on the prey item of small frogs, diuron May Affect the CRLF indirectly 

via reduction in small frog 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. Diuron is a systemic herbicide. 

The RQs for non-target terrestrial monocot and dicot plants inhabiting semi-aquatic and 

upland dry areas exceed the Agency’s LOC (1.0) for all uses except for monocot plants 

exposed to sorghum applications (Table 5.7). These exceedances range from 300.00 

(semi-aquatic plants exposed to non-agricultural aerial applications) to 0.19 (dicot plants 

inhabiting dry areas exposed to sorghum applications). 

Several diuron uses result in LOC exceedances from spray drift. These exceedances 

range from 12.00 (dicot plants exposed to field corn applications) to 1.13 (monocot plants 

exposed to cotton applications). Based on LOC exceedances on the non-target 

terrestrial plants at all use sites, diuron May Affect the terrestrial phase CRLF 

indirectly via effects to riparian and upland habitat. 

Table 5.7 RQs*† for Non-Target Plants Inhabiting Dry and Semi-Aquatic Areas 

Exposed to Diuron via Runoff and Drift 

Use 

Application 

rate (lbs 

a.i./A) 

Application 

method 

Drift 

Value 

(%) 

Group Spray drift 

RQ 

Dry 

area RQ 

Semiaquatic 

area RQ 

Monocot 3.64 25.45 5.71 Agricultural Rights-ofWay, Fencerows, 

Hedgerows, Airports, 

landing Fields, Drainage 

Systems, Outdoor 

Industrial Areas, 

Irrigation Systems, 

Non-Agricultural Rights 

of Way, Sewage 

Disposal Areas, 

Uncultivated NonAgricultural Areas 

12.0 Foliar/Ground 1 

Dicot 4.80 33.60 60.00 

Monocot 0.73 5.09 1.14 

Alfalfa, Peppermint, 

Spearmint, 

Bermudagrass, 

Blackberry, 

Boysenberry, 

Loganberry, Raspberry, 

Dewberry 

2.4 Foliar/Ground 1 

Dicot 0.96 6.72 12.00 

93
Application Drift Semi- Application Spray drift Dry Use rate (lbs 

a.i./A) method Value (%) 

Group aquatic RQ area RQ area RQ 

Monocot 0.97 6.79 1.52 

Apple, Citrus, Pear, 

Grape, Artichoke, 

Asparagus, Pecan 

3.2 Foliar/Ground 1 

Dicot 1.28 8.96 16.00 

Monocot 2.26 8.08 7.62 

Asparagus (Aerial) 

3.2 Foliar/Ground 1 

Dicot 2.99 10.67 80.00 

Monocot 1.45 10.18 2.29 

Banana, Plantain 

4.8 Foliar/Ground 1 

Dicot 1.92 13.44 24.00 

Monocot 0.48 3.39 0.76 

Blueberry, Grass Seed, 

Olive, Pear, Wheat 

1.6 Foliar/Ground 1 

Dicot 0.64 4.48 8.00 

Monocot 0.24 1.70 0.38 

Field Corn 0.8 Foliar/Ground 1 

Dicot 12.00 84.00 4.00 

Monocot 1.13 4.04 3.81 Cotton 3 Aerial 5 

Dicot 1.49 5.33 40.00 

Monocot 0.48 3.39 0.76 

Cotton 3 Ground 1 

 Dicot 24.0 168.0 8.00 

Monocot 0.67 4.67 1.05 

Filbert (Hazelnut) 2.2 Foliar/Ground 1 

Dicot 0.88 6.16 11.00 

Monocot 8.48 30.30 28.57 

Non-agricultural 

12 Aerial 5 

Dicot 11.20 40.00 300.00 

Monocot 1.21 8.48 1.90 

Ornamental Herbaceous 

Plants, Papaya 

4 Foliar/Ground 1 

Dicot 1.60 11.20 20.00 

Uncultivated Monocot 3.64 25.45 5.71 

Agricultural Areas, 

Paved Areas 

12 Foliar/Ground 1 

Dicot 4.80 33.60 60.00 

Monocot 0.91 6.36 1.43 

Peach, Walnut 

3 Foliar/Ground 1 

Dicot 1.20 8.40 15.00 

Monocot 0.12 0.85 0.19 

Sorghum 

0.4 Foliar/Ground 1 

Dicot 0.16 1.12 2.00 

* LOC Exceedances (RQ > 1.0) are bolded and shaded. 

† Spray drift RQs consider only spray drift alone, dry area and semi-aquatic RQs consider spray drift as well as runoff. 

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: 

94
• 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 vascular and non-vascular 

aquatic and terrestrial plants provided in Sections 5.1.1.2, 5.1.1.3, and 5.1.2.3, 

diuron may result in Habitat Modification of aquatic-phase PCEs of designated 

habitat related to effects on aquatic and/or terrestrial plants. 

• Aquatic non-vascular plants used as food source and habitat for CRLF may be 

potentially affected from all diuron uses. 

• Reduction of aquatic based food sources may occur from most use sites. 

• Due to aquatic vascular and terrestrial plant communities being reduced from 

most use sites, there is potential for alteration of channel/pond morphology or 

geometry and/or increase in sediment deposition within the stream channel or 

pond. 

• Due to aquatic vascular and terrestrial plant communities being reduced from 

most use sites, there is potential for 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. 

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 diuron on this PCE, acute and chronic freshwater fish and invertebrate 

toxicity endpoints, as well endpoints for aquatic non-vascular plants are used as measures 

of effects. RQs for these endpoints were calculated in Sections 5.1.1.1 and 5.1.1.2. 

Based on acute and chronic LOC exceedances for freshwater fish and aquatic 

invertebrates, diuron may result in Habitat Modification of aquatic-phase PCEs 

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) 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or dripline surrounding aquatic and riparian habitat that are comprised 

95
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. These results will inform the 

effects determination for modification of designated critical habitat for the CRLF. 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of diuron on this PCE, acute 

and chronic toxicity endpoints for birds, mammals, and terrestrial invertebrates are used 

as measures of effects. RQs for these endpoints were calculated in Section 5.1.2.2. 

Based on acute and chronic LOC exceedances for CRLF prey items of small 

mammals, terrestrial invertebrates and other frogs, diuron may result in Habitat 

Modification of the first three terrestrial phase PCEs. 

The fourth terrestrial-phase PC is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.2.1.2. Due to acute and chronic LOC exceedances at all use sites to 

terrestrial-phase CRLFs, diuron may result in Habitat Modification of the fourth 

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. 

If the RQs presented in the Risk Estimation (Section 5.1) show no direct or indirect 

effects for the CRLF, and no modification to PCEs of the CRLF’s designated critical 

habitat, a “no effect” determination is made, based on diuron’s use within the action 

area. However, if direct or indirect effects LOCs are exceeded, or if 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 diuron. Based on 

direct and indirect LOC exceedances for CRLF, the Agency concludes a 

preliminary May Affect determination for the CRLF and critical habitat. A 

summary of the results of the risk estimation is provided in Table 5.8 for direct and 

indirect effects to the CRLF and in Table 5.9 for the PCEs of designated critical habitat 

for the CRLF. 

96
Table 5.8 Risk Estimation Summary for Diuron - 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 

The aquatic phase amphibian acute LOCs for Listed species (0.05) are 

exceeded for most uses of diuron in California. Acute RQs that exceed the 

Agency’s LOC range from 12.28 (paved areas) to 0.06 (papaya and 

walnut). Chronic RQs that exceed the Agency’s LOC for chronic exposure 

(1.0) range from 131.85 (paved areas) to 1.26 (banana, plantain). 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects to food 

supply (i.e., freshwater invertebrates, 

non-vascular plants) YES 

LOCs for non-vascular plants are exceeded for all uses. The RQs range 

from 2046.25 (paved areas) to 1.57 (grasses grown for seed). 

Aquatic invertebrates acute a LOC are exceeded. The acute RQs that 

exceed the LOC range from 30.69 (paved areas) to 0.05 (cotton aerial 

application). Chronic LOCs are exceeded for several uses. RQs that 

exceed the Agency’s LOC range from 21.21or paved areas to 1.02 for 

bermudagrass. 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects on habitat, 

cover, and/or primary productivity (i.e.,

aquatic plant community) 

YES 

RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for most 

uses. These range from 460.46 (paved areas) to 1.30 (hazelnut). 

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 

RQs for non-target terrestrial monocot and dicot plants inhabiting semiaquatic and upland dry areas exceed the Agency’s LOC for all uses 

except for monocot plants exposed to sorghum applications. RQs range 

from 300.00 (semi-aquatic plants exposed to non-agricultural aerial 

applications) to 0.19 (dicot plants inhabiting dry areas exposed to 

sorghum applications). 

Terrestrial Phase 

(Juveniles and adults) 

Direct Effects 

Survival, growth, and reproduction of 

CRLF individuals via direct effects on 

terrestrial phase adults and juveniles 

YES 

The acute dietary based RQs that exceed the LOC range from 1.55 

(Agricultural Rights-of-Way, Fencerows, Hedgerows, Airports, etc.) to 

0.12 (grass for seed, wheat). These results are summarized in Table 5.5. 

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 

Chronic dose based RQs exceed the Agency’s LOC (1.0) for most uses ( 

Table 5.6). The dose based chronic RQs that exceed the LOC range 

from 8.29 (Agricultural Rights-of-Way, Fencerows, Hedgerows, 

Airports, etc.) to 1.31 (walnut). The dietary based chronic RQs do not 

exceed the LOC for any uses. The RQs that exceed the acute dose based 

LOC range from 0.41 (Agricultural Rights-of-Way, Fencerows, 

Hedgerows, Airports, etc.) to 0.13 (Bermuda grass, Blackberry, 

Boysenberry, Spearmint). 

Indirect Effects 

Survival, growth, and reproduction of 

CRLF individuals via effects on habitat 

(i.e., riparian vegetation) YES 

The RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for 

most uses of diuron in California. These range from 373.33 (paved areas) 

to 1.01 (alfalfa aerial). 

The RQs for non-target terrestrial monocot and dicot plants inhabiting 

semi-aquatic and upland dry areas exceed the Agency’s LOC (1.0) for all 

97
uses except for monocot plants exposed to sorghum applications (Table 

5.7). These exceedances range from 300.00 (semi-aquatic plants exposed 

to non-agricultural aerial applications) to 0.19 (dicot plants inhabiting dry 

areas exposed to sorghum applications). Several diuron uses result in 

plant LOC exceedances from spray drift. These exceedances range from 

12.00 (dicot plants exposed to field corn applications) to 1.13 (monocot 

plants exposed to cotton applications). 

Table 5.9 Risk Estimation Summary for Diuron – PCEs of Designated Critical 

Habitat for the CRLF 

Assessment Endpoint LOC Exceedances 

(Y/N) Description of Results of Risk Estimation 

Aquatic Phase PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat) 

Alteration of channel/pond morphology or 

geometry and/or increase in sediment deposition 

within the stream channel or pond: aquatic 

habitat (including riparian vegetation) provides 

for shelter, foraging, predator avoidance, and 

aquatic dispersal for juvenile and adult CRLFs. 

YES

LOCs are exceeded for terrestrial riparian plants and for 

aquatic vascular plants from exposure to diuron from 

runoff or spray drift. 

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

LOCs are exceeded for terrestrial riparian plants and for 

aquatic plants from exposure to diuron from runoff or 

spray drift. Alteration of riparian and vascular plants 

may result in alteration of temperature, turbidity, and 

oxygen content. 

Alteration of other chemical characteristics 

necessary for normal growth and viability of 

CRLFs and their food source. 

YES

LOC is exceeded for indirect effects on terrestrial and 

aquatic-phase CRLF from most diuron applications. 

Reduction and/or modification of aquatic-based 

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

LOCs for non-vascular plants are exceeded for all uses. 

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

The AgDRIFT model was used to evaluate potential 

distances beyond which exposures would be expected to be 

below LOC. However, due to the limitations imposed by 

the Tier 1 ground analysis (allows users to evaluate off-site 

deposition and exposure out to 1,000 ft downwind from the 

location of the application), the exact buffer needed for 

exposures to be below the LOC is uncertain. 

The output from AgDRIFT indicated that the buffer zone 

required would be greater than 1,000 feet. Since the model 

is restricted to accurately discerning a buffer within 1,000 

feet of the application, the exact distance needed for a 

buffer to protect non-listed and listed plants is unknown. 

However, as seen in Table 3.7, the calculated risk quotient 

(RQ) is significantly larger than the level of concern 

(LOC) for non-listed and listed plant species (LOC = 1). 

98
LOC Exceedances Assessment Endpoint Description of Results of Risk Estimation (Y/N) 

This comparison provides awareness to approximately how 

much greater than 1,000 feet the buffer needs to be. 

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

Effects are expected to non-target terrestrial plants over 

1000 ft from use site from ground application. 

Reduction and/or modification of food sources 

for terrestrial phase juveniles and adults YES

LOC is exceeded for indirect effects on terrestrial and 

aquatic-phase CRLF from diuron application 

Alteration of chemical characteristics necessary 

for normal growth and viability of juvenile and 

adult CRLFs and their food source. 

YES

LOC is exceeded for direct effects on terrestrial and 

aquatic-phase CRLF from diuron applications. 

Following a preliminary “may affect” or “habitat modification” 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: 

• Significance of Effect: Insignificant effects are those that cannot be meaningfully 

measured, detected, or evaluated in the context of a level of effect where “take” 

occurs for even a single individual. “Take” in this context means to harass or 

harm, defined as the following: 

 Harm includes significant habitat modification or degradation that 

results in death or injury to listed species by significantly impairing 

behavioral patterns such as breeding, feeding, or sheltering. 

 Harass is defined as actions that create the likelihood of injury to listed 

species to such an extent as to significantly disrupt normal behavior 

patterns which include, but are not limited to, breeding, feeding, or 

sheltering. 

• Likelihood of the Effect Occurring: Discountable effects are those that are 

extremely unlikely to occur. 

• Adverse Nature of Effect: Effects that are wholly beneficial without any adverse 

effects are not considered adverse. 

 

A description of the risk and effects determination for each of the established assessment 

endpoints for the CRLF and its designated critical habitat is provided in Sections 5.2.1 

through 5.2.3. 

99
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 diuron. 

Diuron is considered “highly toxic” to the freshwater fish, which are surrogates for the 

aquatic phase CRLF. The aquatic animal acute LOCs for Listed species (0.05) are 

exceeded for most uses of diuron in California. Acute RQs that exceed the Agency’s 

LOC range from 12.28 (paved areas) to 0.06 (papaya). The probability of an individual 

effect to the aquatic-phase CRLF is based on the slope of the dose response curve for the 

acute endpoint used to derive the RQ. The probability, based on the default slope of 4.5 

is one in 4.18E+08. Because the upper and lower 95% confidence interval of the slope is 

not known, a plausible range of slopes of 2 to 9 were used to obtain reasonable upper and 

lower bounds. The resulting probability of an individual effect to the aquatic-phase 

CRLF range between one in 215.83 and one in 1.75E+31. 

Chronic RQs that exceed the Agency’s LOC for chronic exposure (1.0) range from 

131.85 (paved areas) to 1.26 (banana, plantain). The aquatic chronic animal LOC for 

listed species (1.0) is exceeded for most uses of diuron in California. 

The USGS collected 422 positive ground water samples containing Diuron. The three 

river basins looked at were Sacramento, San Joaquin-Tulare, and Santa Ana. The 

maximum concentration observed was 1.8 ppb in the San Joaquin-Tulare water basins in 

Kings County. 

Monitoring data from CDPR indicate that diuron concentrations range from nondetectable to 160 ppb. The majority of the large concentrations of Diuron came from San 

Joaquin County, and occurred between the end of December and the beginning of 

January. The year 2005, for San Joaquin County, yielded the greatest concentrations 

overall. 

Moreover, the CDPR monitoring data showed that eleven samples (1.56% of the 705 

samples), ranging from 31 ppb to 160 ppb, detected levels of diuron that would result in 

chronic direct effects to the CRLF. Likewise, four samples (0.57%), ranging from 9.5 

ppb to 15 ppb, detected diuron at levels that would result in chronic indirect effects to the 

CRLF. These detects occurred during the months of December, January, and February in 

years 2005 and 2006, and in March for the 2006 year. The San Joaquin County had 5 

detects (3.38% of samples) resulting in chronic direct effects and 1 detect (0.68% of 

samples) resulting in chronic indirect effects, Tulare County had 1 detect (2.22% of 

samples) resulting in a chronic direct effect, and Stanislaus County had 5 detects (6.94% 

of samples) resulting in chronic direct effects and 2 detects (2.78% of samples) resulting 

100
in chronic indirect effects. Consequently, almost 10% of the samples taken from 

Stanislaus County, just over 4% from San Joaquin County, a little over 2% from Tulare 

County contained detects at levels that would cause chronic or indirect effects to the 

CRLF. 

Seven reported incidents resulted in fish kills. Five of these fish kills resulted from direct 

application of diuron to water bodies which is not allowed in California. However, three 

incidents were from drift or runoff of diuron from applications made adjacent to water 

bodies. This demonstrates that diuron may pose a threat to aquatic organisms through 

labeled uses (Appendix J). It is also important to note that the absence of additional 

documented incidents does not necessarily mean that such incidents did not occur. 

Mortality incidents must be seen, reported, investigated, and submitted to the Agency in 

order to be recorded in the incident database. Incidents may not be noticed because the 

carcasses decayed, were removed by scavengers, or were in out-of-the-way or hard-to-see 

locations. Due to the voluntary nature of incident reporting, an incident may not be 

reported to appropriate authorities capable of investigating it. Based on these 

exceedances to the aquatic-phase CRLF, reported diuron incidents resulting in fish 

kills and its monitored presence in surface water, a “May Affect and Likely to 

Adversely Affect (LAA)” determination is made for diuron use in California. 

5.2.1.2 Terrestrial-Phase CRLF 

The RQs representing acute dietary-based exposures exceed the Agency’s LOC for all 

uses except sorghum (Section 5.1.2.1.). These RQs were derived using the T-REX 

model, which estimates exposures that are specific to food intake equations for birds. 

RQs generated for birds are used as surrogates to represent RQs for the terrestrial-phase 

CRLF. Based on these exceedances to the terrestrial-phase CRLF, a “May Affect” 

determination was made. 

The T-HERPS model was therefore employed as a refinement tool to explore amphibianspecific food intake on potential exposures to the terrestrial phase CRLF. The T-HERPS 

model incorporates the same inputs as T-REX with equations adjusted for poikilotherm 

food intake. The EECs generated by T-HERPS are found in Table 5.10. An example 

output from T-HERPS is available in Appendix K. 

101
Table 5.10. Upper-bound Kenega Nomogram T-HERPS EECs (mg/kg-diet) for 

Dietary-based Exposures of the CRLF and its Prey to Diuron1. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small 

Insectivore 

Mammals 

Small 

Terrestrial 

Phase 

Amphibians 

Agricultural Rights-of-Way, 

Fencerows, Hedgerows, Airports, 

landing Fields, Drainage Systems, 

Outdoor Industrial Areas, 

Irrigation Systems, NonAgricultural Rights of Way, 

Sewage Disposal Areas, 

Uncultivated Non-Agricultural 

Areas

2113.70 234.86 2476.10 154.76 73.37 

Alfalfa, Peppermint, 324.00 36.00 379.55 23.72 11.25 

Apple, Grape 504.68 56.08 591.21 36.95 17.52 

Citrus

Artichoke, Asparagus 432.00 48.00 506.07 31.36 15.00 

Banana, Plantain 930.06 103.34 1089.52 68.10 32.28 

Bermuda grass, Blackberry, 

Boysenberry, Spearmint 951.44 105.72 114.57 69.66 33.03 

Blueberry 239.50 23.61 280.57 17.54 8.31 

Field Corn 108.00 12.00 126.52 7.91 3.75 

Cotton 452.53 50.28 530.11 33.13 15.71 

Dewberry 452.83 50.31 530.47 33.15 15.72 

Filbert (Hazelnut) 312.23 34.69 365.76 22.86 10.84 

Grass Seed, Wheat 216.00 24.00 253.03 15.81 7.50 

Loganberry, Raspberry 324.00 36.00 379.55 23.72 11.25 

Olive 223.75 24.86 262.12 16.38 7.77 

Pear, Pecan 432.00 48.00 506.07 31.63 15.00 

Ornamental Herbaceous Plants, 

Papaya 540.00 60.00 632.59 39.54 18.74 

Uncultivated Agricultural Areas, 

Paved Areas 1620.00 180.00 1897.76 118.61 56.23 

Peach 405.00 45.00 474.44 29.65 14.06 

Sorghum 54.00 6.00 63.26 3.95 1.87 

Walnut 425.76 47.31 498.76 31.17 14.78 

1

weights of small herbivore and insectivore mammals are 35g 

102
Acute exposures 

Refined acute dietary-based RQs for CRLFs consuming large insects and small herbivore 

mammals exceed the acute listed species LOC (0.1) for all uses of diuron except 

Sorghum and Field Corn. The acute dietary-based RQs for CRLFs consuming large 

insects exceed the acute listed species LOC for Agricultural Rights-of-Way, Fencerows 

etc. No acute dietary based LOCs were exceeded for CLRF consuming small insectivore 

mammals and small terrestrial phase amphibians for any diuron use. Results are 

presented in Table 5.11. 

Table 5.11. Revised Acute Dietary-based RQs for CRLF consuming different food 

items (RQs calculated using T-HERPS)*. 

Scenario Small 

Insects 

Large 

Insects 

Small 

Herbivore 

Mammals 

Small Insectivore 

Mammals 

Small 

Terrestrial 

Phase 

Amphibians 

Agricultural Rights-ofWay, Fencerows, 

Hedgerows, Airports, 

landing Fields, Drainage 

Systems, Outdoor 

Industrial Areas, Irrigation 

Systems, Non-Agricultural 

Rights of Way, Sewage 

Disposal Areas, 

Uncultivated NonAgricultural Areas

1.22 0.14 1.43 0.09 0.04 

Alfalfa, Peppermint, 0.19 0.02 0.22 0.01 0.01 

Apple, Grape 0.29 0.03 0.34 0.02 0.01 

Citrus 0.33 0.04 0.38 0.02 0.01 

Artichoke, Asparagus 0.25 0.03 0.29 0.02 0.01 

Banana, Plantain 0.54 0.06 0.63 0.04 0.02 

Bermuda grass, 

Blackberry, Boysenberry, 

Spearmint

0.55 0.06 0.64 0.04 0.02 

Blueberry 0.14 0.02 0.16 0.01 <0.01 

Field Corn 0.06 0.01 0.07 <0.01 <0.01 

Cotton 0.26 0.03 0.31 0.02 <0.01 

Dewberry 0.26 0.03 0.31 0.02 0.01 

Filbert (Hazelnut) 0.18 0.02 0.21 0.01 0.01 

Grass Seed, Wheat 0.12 0.01 0.15 0.01 <0.01 

103
Loganberry, Raspberry 0.19 0.02 0.22 0.01 0.01 

Olive 0.13 0.01 0.15 0.01 <0.01 

Ornamental Herbaceous 

plants, Papaya 0.31 0.03 0.37 0.02 0.01 

Uncultivated Agricultural 

Areas, Paved Areas 0.94 0.10 1.10 0.07 0.03 

Peach 0.23 0.03 0.27 0.02 0.01 

Pear, Pecan 0.25 0.03 0.29 0.02 0.01 

Sorghum 0.03 <0.01 0.04 <0.01 <0.01 

Walnut 0.25 0.03 0.29 0.02 0.01 

*RQs exceeding the Listed LOC (0.10) are bolded and shaded 

Based on the dietary probit dose-response slope of 7.22 the chance of individual mortality 

for which RQs exceed the Listed LOC range from approximately one in 6.75E+10 at an 

RQ of 0.12 (Grass Seed and Wheat uses) to approximately one in one at an RQ of 1.22 

(Agricultural Rights-of-Way, etc.). Therefore, based on these refined dietary based 

acute risk quotients and their exceedances of the Agency’s LOC a Likely to 

Adversely Affect (LAA) determination is made for diuron use in California. 

5.2.2 Indirect Effects (via Reductions in Prey Base) 

5.2.2.1 Algae (non-vascular plants) 

 

Indirect effects of diuron to the aquatic-phase CRLF (tadpoles) via reduction in nonvascular aquatic plants in its diet are based on peak EECs from the standard pond and the 

lowest acute toxicity value for aquatic non-vascular plants. The Agency’s LOC (1.0) is 

exceeded for all current uses of diuron in California. The RQs range from 2331.67 for 

Paved Areas to 1.82 for Sorghum (Section 5.1.1.2). 

The fate characteristics indicate that diuron is expected to be persistent in aquatic 

environments with EECs after 60 days being above the Agency’s LOC for non-vascular 

aquatic plants, a primary food source for aquatic-phase CRLF. The application of diuron 

in California is anticipated to be in late winter and early spring. The timing of the 

application would coincide with reproduction of CRLF in aquatic environments as well 

as for the tadpoles to feed on non-vascular aquatic plants. 

Because of non-vascular LOC exceedances from registered uses of diuron, its 

monitored presence in surface water and verified non-target incidents resulting 

from diuron use, based on multiple lines of evidence, the Agency concludes that 

there is a potential indirect impact to the aquatic-phase of the CRLF from reduction 

of food items (algae) and therefore diuron is Likely to Adversely Affect (LAA) the 

CRLF. 

. 

104
5.2.2.2 Aquatic Invertebrates 

As discussed in Section 2.5.3, the diet of CRLF also includes aquatic invertebrates. 

The potential for diuron to elicit indirect effects to the CRLF via effects on freshwater 

invertebrate food items is dependent on several factors including: (1) the potential 

magnitude of effect on freshwater invertebrate individuals and populations; and (2) the 

number of prey species potentially affected relative to the expected number of species 

needed to maintain the dietary needs of the CRLF. Together, these data provide a basis 

to evaluate whether the number of individuals within a prey species is likely to be 

reduced such that it may indirectly affect the CRLF. 

The main food source for juvenile aquatic- and terrestrial-phase CRLFs is thought to be 

aquatic and terrestrial invertebrates found along the shoreline and on the water surface. 

The application of diuron in California is anticipated to be in late winter and early spring. 

The timing of the application would coincide with juvenile aquatic- and terrestrial-phase 

CRLFs that would be feeding on aquatic and terrestrial 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. The Agency’s acute 

Listed LOC (0.05) is exceeded for most uses of diuron in California. The acute RQs that 

exceed the LOC range from 30.69 (Paved Areas) to 0.06 (grape). The chronic RQs 

exceed the Agency’s LOC for several used. The chronic values that exceeded range from 

21.21 (paved areas) to 1.02 (bermudagrass). A summary of the acute and chronic RQ 

values for exposure to aquatic invertebrates (as prey items of aquatic-phase CRLFs) is 

provided in Table 5.3. Based on acute Listed LOC exceedances for chronic and acute 

aquatic invertebrates from most use sites, diuron May Affect the CRLF indirectly via 

reduction in freshwater invertebrate prey items. 

Based on the aquatic invertebrate RQs and a probit dose-response default slope of 4.5 the 

estimated reduction in aquatic invertebrate populations for which RQs exceed the Listed 

LOC range from <0.01% to approximately 100%. To further investigate the estimated 

reduction in aquatic invertebrate populations, a plausible range of slopes of 2 to 9 were 

selected to investigate the upper and lower bounds of this estimate. Using a slope of 2 

led to a range of possible reduction in populations from less than 0.10% to approximately 

100%. Employing a slope of 7 led to a range of possible reduction in aquatic invertebrate 

populations from less than 1.0% to approximately 100%. 

Because aquatic invertebrate populations are expected to be impacted from 

registered uses of diuron in California, and because diuron’s presence has been 

observed in monitored surface water in California, the Agency concludes that there 

is a potential indirect impact to the aquatic-phase of the CRLF from a reduction of 

aquatic invertebrate food items and therefore diuron is Likely to Adversely Affect 

(LAA) the CRLF. 

105
5.2.2.3 Fish and Aquatic-phase Frogs

As discussed in Section 2.5.3, the diet of CRLF also includes small fish and other 

aquatic-phase frogs. 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. The RQs for diuron uses results in acute and chronic 

exceedances of the Agency’s LOC for freshwater fish which are surrogates for the 

aquatic phase for amphibians. 

The aquatic phase amphibian acute LOCs for Listed species (0.05) are exceeded for most 

uses of diuron in California. Acute RQs that exceed the Agency’s LOC range from 13.99 

(paved areas) to 0.053 (papaya). Based on these RQs and a probit dose-response default 

slope of 4.5 the estimated aquatic vertebrate prey item population reduction for which 

RQs exceed the Listed LOC range from approximately 10.0% to approximately 100%. 

To further investigate the estimated aquatic vertebrate prey item population reduction, a 

plausible range of slopes of 2 to 9 were selected to investigate the upper and lower 

bounds of this estimate. Using a slope of 2 led to a range of estimated population 

reduction of approximately 30.0% to approximately 100%. Employing a slope of 9 led to 

a range of possible population reduction of approximately 3.0% to approximately 100%. 

The aquatic phase amphibian acute LOCs for listed species (0.05) are exceeded for most 

uses of diuron in California. Acute RQs that exceed the Agency’s LOC range from 12.28 

(paved areas) to 0.06 (papaya and walnut). Chronic RQs that exceed the Agency’s LOC 

for chronic exposure (1.0) range from 131.85 (paved areas) to 1.26 (banana, plantain). 

The application of diuron in California is anticipated to be in late winter and early spring. 

The timing of the application would coincide with juvenile aquatic- and terrestrial-phase 

CRLFs that would be feeding on small fish, small frogs or tadpoles. Because fish and 

aquatic phase amphibian populations are expected to be impacted from registered 

uses of diuron in California, and because diuron’s presence has been observed in 

monitored surface water in California, the Agency concludes that there is a 

potential indirect impact to the aquatic-phase of the CRLF from a reduction of fish 

and aquatic phase amphibian food items and therefore diuron is Likely to Adversely 

Affect (LAA) the CRLF. 

5.2.2.4 Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. Terrestrial invertebrate toxicity data are used to 

assess potential indirect effects of diuron to the terrestrial-phase CRLF. Effects to 

terrestrial invertebrates resulting from exposure to diuron may also indirectly affect the 

CRLF via reduction in available food. 

Because the estimated EEC’s exceed the highest level tested for three uses for diuron, a 

preliminary “May Affect” determination was made. However, the calculated EEC’s for 

106
Agricultural Rights-of-Way etc (2688.80 ppm) was only roughly twice the highest level 

tested (>1305) at which only a 2.7% mortality was observed. Therefore, it is reasonable 

to assume that the effects to terrestrial invertebrate populations will be negligible for 

these uses of diuron in California. Therefore, based on the weight-of-evidence, the 

Agency concludes that there is a negligible potential impact to terrestrial 

invertebrates that the CRLF consumes, and therefore diuron May Affect but is Not 

Likely to Adversely Affect (NLAA) the CRLF. 

5.2.2.5 Mammals 

Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume 

terrestrial vertebrates, including mice. Small mammals can make up to 50% of the CRLF 

food intake. Acute dose based RQs exceed the Agency’s LOC (1.0) for most uses. The 

dose based RQs that exceed the LOC range from 0.41 (Agricultural Areas, Rights-ofWay, fencelines etc.) to 0.13 (Bermudagrass, Blackberry, Boysenberry, Spearmint). The 

dose based chronic RQs that exceed the LOC range from 8.29 (Agricultural Areas, 

Rights-of-Way, fencelines etc.) to 1.33 (Artichoke, Asparagus, Pear, Pecan).. The dietary 

based chronic RQs do not exceed the LOC for any uses. 

Based on the mammalian dose based acute RQs and a probit dose-response default slope 

of 4.5 the estimated mammalian prey item population reduction for which RQs exceed 

the Listed LOC range from <0.01% to approximately 4.00%. To further investigate the 

estimated reduction in these populations, a plausible range of slopes of 2 to 9 were 

selected to investigate the upper and lower bounds of this estimate. Employing a slope of 

9 led to an estimated population reduction of <0.01% for all RQs. Using a slope of 2 led 

to a range of possible population reduction from approximately 4.00% to approximately 

22.00%. Because mammalian populations are expected to be impacted from 

registered uses of diuron in California, the Agency concludes that there is a 

potential indirect impact to the terrestrial phase of the CRLF from a reduction of 

mammalian food items and therefore diuron is Likely to Adversely Affect (LAA) the 

CRLF. 

5.2.2.6 Terrestrial-phase Amphibians 

Terrestrial-phase adult CRLFs also consume frogs. RQ values representing direct 

exposures of diuron to terrestrial-phase CRLFs are used to represent exposures of diuron 

to frogs in terrestrial habitats. The T-HERPS model was therefore employed as a 

refinement tool to explore amphibian-specific food intake on potential exposures to 

terrestrial phase amphibian food items for the CRLF. The T-HERPS model incorporates 

the same inputs as T-REX with equations adjusted for poikilotherm food intake. 

Acute exposures 

Refined acute dietary-based RQs for terrestrial phase amphibians consuming small 

insects and small herbivore mammals exceed the acute listed species LOC (0.1) for all 

uses of diuron except Sorghum and Field Corn. The acute dietary-based RQ for 

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terrestrial phase amphibians consuming large insects exceeds the acute listed species 

LOC for Agricultural Rights-of-Way, Fencerows etc. Based on these refined dietary 

based risk quotients, terrestrial phase amphibian populations are expected to be 

impacted from registered uses of diuron in California. Therefore the Agency 

concludes that there is a potential indirect impact to the terrestrial phase of the 

CRLF from a reduction of terrestrial phase amphibian food items and therefore 

diuron is Likely to Adversely Affect (LAA) the CRLF. 

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. In addition to energy, vascular plants provide structure as 

attachment sites and refugia for many aquatic invertebrates, fish, and juvenile organisms, 

such as fish and frogs. In addition, vascular plants also provide primary productivity and 

oxygen to the aquatic ecosystem. Rooted plants help reduce sediment loading and 

provide stability to nearshore areas and lower streambanks. In addition, vascular aquatic 

plants are important as attachment sites for egg masses of CRLFs. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production were assessed using RQs from freshwater aquatic vascular and non-vascular 

plant data. 

Indirect effects of diuron to the aquatic-phase CRLF (tadpoles) via reduction in nonvascular aquatic plants in its diet are based on peak EECs from the standard pond and the 

lowest acute toxicity value for aquatic non-vascular plants. The Agency’s LOC (1.0) is 

exceeded for all current uses of diuron in California. The RQs range from 2331.67 

(paved areas) to 1.82 (sorghum). 

Indirect effects to the CRLF via direct toxicity to aquatic plants are estimated using the 

most sensitive non-vascular and vascular plant toxicity endpoints. The RQs for vascular 

aquatic plants exceed the Agency’s LOC (1.0) for most uses of diuron in California. 

These range from 373.33 (paved areas) to 1.01 (alfalfa aerial). Based on LOC 

exceedances in vascular and non-vascular plants, diuron is Likely to Adversely 

Affect (LAA) the CRLF indirectly via habitat degradation through reduction in 

vascular and non-vascular aquatic plants. 

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 

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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. 

Twenty-two incidents have been reported that were either registered use or of 

undetermined legality that resulted in plant damage. The RQs for non-target terrestrial 

monocot and dicot plants inhabiting semi-aquatic and upland dry areas exceed the 

Agency’s LOC (1.0) for all uses except for monocot plants exposed to sorghum 

applications. These exceedances range from 300.00 (semi-aquatic plants exposed to nonagricultural aerial applications) to 0.19 (dicot plants inhabiting dry areas exposed to 

sorghum applications). Several diuron uses result in LOC exceedances from spray drift. 

These exceedances range from 12.00 (dicot plants exposed to field corn applications) to 

1.13 (monocot plants exposed to cotton applications). Based on LOC exceedances in 

vascular and non-vascular plants, and twenty-two reported incidents that resulted 

in plant damage, diuron is Likely to Adversely Affect (LAA) the CRLF indirectly 

via habitat degradation through reduction terrestrial plants. 

5.2.4 Modification 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 

habitat degradation from diuron exposure. 

 5.2.4.1 Aquatic-Phase PCEs 

Three of the four assessment endpoints for the aquatic-phase primary constituent 

elements (PCEs) of designated critical habitat for the CRLF are related to potential 

effects to aquatic and/or terrestrial plants: 

• Alteration of channel/pond morphology or geometry and/or increase in sediment 

deposition within the stream channel or pond: aquatic habitat (including riparian 

vegetation) provides for shelter, foraging, predator avoidance, and aquatic 

dispersal for juvenile and adult CRLFs. 

• Alteration in water chemistry/quality including temperature, turbidity, and 

oxygen content necessary for normal growth and viability of juvenile and adult 

CRLFs and their food source. 

• Reduction and/or modification of aquatic-based food sources for pre-metamorphs 

(e.g., algae). 

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, diuron will result in habitat 

modification based on effects to aquatic-phase PCEs of designated critical habitat. 

• Aquatic non-vascular plants used as food source and habitat for CRLF may be 

potentially affected from all diuron uses. 

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• Reduction of aquatic based food sources may occur from most use sites. 

• Due to aquatic vascular and terrestrial plant communities being reduced from 

most use sites, there is potential for alteration of channel/pond morphology or 

geometry and/or increase in sediment deposition within the stream channel or 

pond. 

• Due to aquatic vascular and terrestrial plant communities being reduced from 

most use sites, there is potential for 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. 

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 diuron on this PCE, acute and chronic freshwater fish and invertebrate 

toxicity endpoints, as well endpoints for aquatic non-vascular plants are used as measures 

of effects. RQs for these endpoints were calculated in Sections 5.1.1.1 and 5.1.1.2. 

Based on acute and chronic LOC exceedances for freshwater fish and aquatic 

invertebrates, diuron will result in habitat modification based on effects to aquaticphase PCEs of designated critical habitat related to effects of alteration of other 

chemical characteristics necessary for normal growth and viability of CRLFs and 

their food source. 

5.2.4.2 Terrestrial-Phase PCEs 

Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical 

habitat for the CRLF are related to potential effects to terrestrial plants: 

• Elimination and/or disturbance of upland habitat; ability of habitat to support food 

source of CRLFs: Upland areas within 200 ft of the edge of the riparian 

vegetation or 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 modification via impacts to terrestrial plants 

(Section 5.2.3.2). 

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. These results will inform the 

effects determination for modification of designated critical habitat for the CRLF. 

The third terrestrial-phase PCE is “reduction and/or modification of food sources for 

terrestrial phase juveniles and adults.” To assess the impact of diuron on this PCE, acute 

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and chronic toxicity endpoints for birds, mammals, and terrestrial invertebrates are used 

as measures of effects. RQs for these endpoints were calculated in Section 5.1.2.2. 

Based on acute and chronic LOC exceedances for CRLF prey items of small 

mammals, terrestrial invertebrates and other frogs, diuron will result in habitat 

modification based on effects to the terrestrial PCE relative to reduction in food 

sources. 

The fourth terrestrial-phase PCE is based on alteration of chemical characteristics 

necessary for normal growth and viability of juvenile and adult CRLFs and their food 

source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in 

Section 5.2.1.2. Due to acute and chronic LOC exceedances at all use sites to 

terrestrial-phase CRLFs, diuron will result in habitat modification based on effects 

to the terrestrial PCE related to alteration of chemical characteristics necessary for 

normal growth and viability. 

5.2.5.1 Downstream Dilution 

The downstream extent of exposure in streams and rivers 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 diuron present. 

Using a EC50 value of 2.40 ug/L for non-vascular aquatic plants (the most sensitive 

species) and a maximum peak EEC for applications to Agricultural Rights-of-Way, 

Fencerows, etc. of 688 ug/L yields an RQ/LOC ratio of 142 (142/1). Using the 

downstream dilution approach (described in more detail in Appendix F) yields a distance 

of 285 kilometers which represents the maximum continuous distance of downstream 

dilution from the edge of the initial area of concern. Similar to the spray drift buffer 

described above, the LAA/NLAA determination is based on the area defined by the point 

where concentrations exceed the EC50 value. 

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. 

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6.1.2 Aquatic Exposure Modeling of Diuron 

The standard ecological water body scenario (EXAMS pond) used to calculate potential 

aquatic exposure to pesticides is intended to represent conservative estimates, and to 

avoid underestimations of the actual exposure. The standard scenario consists of 

application to a 10-hectare field bordering a 1-hectare, 2-meter deep (20,000 m3

) pond 

with no outlet. Exposure estimates generated using the EXAMS pond are intended to 

represent a wide variety of vulnerable water bodies that occur at the top of watersheds 

including prairie pot holes, playa lakes, wetlands, vernal pools, man-made and natural 

ponds, and intermittent and lower order streams. As a group, there are factors that make 

these water bodies more or less vulnerable than the EXAMS pond. Static water bodies 

that have larger ratios of pesticide-treated drainage area to water body volume would be 

expected to have higher peak EECs than the EXAMS pond. These water bodies will be 

either smaller in size or have larger drainage areas. Smaller water bodies have limited 

storage capacity and thus may overflow and carry pesticide in the discharge, whereas the 

EXAMS pond has no discharge. As watershed size increases beyond 10-hectares, it 

becomes increasingly unlikely that the entire watershed is planted with a single crop that 

is all treated simultaneously with the pesticide. Headwater streams can also have peak 

concentrations higher than the EXAMS pond, but they likely persist for only short 

periods of time and are then carried and dissipated downstream. 

The Agency acknowledges that there are some unique aquatic habitats that are not 

accurately captured by this modeling scenario and modeling results may, therefore, 

under- or over-estimate exposure, depending on a number of variables. For example, 

aquatic-phase CRLFs may inhabit water bodies of different size and depth and/or are 

located adjacent to larger or smaller drainage areas than the EXAMS pond. The Agency 

does not currently have sufficient information regarding the hydrology of these aquatic 

habitats to develop a specific alternate scenario for the CRLF. CRLFs prefer habitat with 

perennial (present year-round) or near-perennial water and do not frequently inhabit 

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 

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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 

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 diuron 

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 diuron use areas. 

The monitoring data available are below the PZM-EXAMS predictions by an order of 

magnitude. The monitoring data indicated that the time weighted mean concentration 

ranged from below the 0.001 ppb to 0.86 ppb with a mean concentration of 0.21 ppb. 

PRZM-EXAMS prediction (not including the scenarios for impervious surfaces) 

estimated diuron to be in the surface waters at peak concentrations from 27.5 ppb to 2.6 

ppb and 60 day EEC from 18.3 ppb to 1.8. 

6.1.3 Action Area Uncertainties 

The action area is considered to be the whole State of California since diuron is 

considered to be an animal carcinogen and can not be spatially defined. Diuron use has 

been reported in all 58 counties. 

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6.1.4 Usage Uncertainties

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Four years of data (2002 – 

2005) were included in this analysis because statistical methodology for identifying 

outliers, in terms of area treated and pounds applied, was provided by CDPR for these 

years only. No methodology for removing outliers was provided by CDPR for 2001 and 

earlier pesticide data; therefore, this information was not included in the analysis because 

it may misrepresent actual usage patterns. CDPR PUR documentation indicates that 

errors in the data may include the following: a misplaced decimal; incorrect measures, 

area treated, or units; and reports of diluted pesticide concentrations. In addition, it is 

possible that the data may contain reports for pesticide uses that have been cancelled. 

The CPDR PUR data does not include home owner applied pesticides; therefore, 

residential uses are not likely to be reported. As with all pesticide usage data, there may 

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. 

6.1.5 Terrestrial Exposure Modeling of Diuron 

The Agency relies on the work of Fletcher et al. (1994) for setting the assumed pesticide 

residues in wildlife dietary items. These residue assumptions are believed to reflect a 

realistic upper-bound residue estimate, although the degree to which this assumption 

reflects a specific percentile estimate is difficult to quantify. It is important to note that 

the field measurement efforts used to develop the Fletcher estimates of exposure involve 

highly varied sampling techniques. It is entirely possible that much of these data reflect 

residues averaged over entire above ground plants in the case of grass and forage 

sampling. 

It was assumed that ingestion of food items in the field occurs at rates commensurate 

with those in the laboratory. Although the screening assessment process adjusts dryweight estimates of food intake to reflect the increased mass in fresh-weight wildlife food 

intake estimates, it does not allow for gross energy differences. Direct comparison of a 

laboratory dietary concentration- based effects threshold to a fresh-weight pesticide 

residue estimate would result in an underestimation of field exposure by food 

consumption by a factor of 1.25 – 2.5 for most food items. 

Differences in assimilative efficiency between laboratory and wild diets suggest that 

current screening assessment methods do not account for a potentially important aspect of 

food requirements. Depending upon species and dietary matrix, bird assimilation of wild 

diet energy ranges from 23 – 80%, and mammal’s assimilation ranges from 41 – 85% 

(U.S. Environmental Protection Agency, 1993). If it is assumed that laboratory chow is 

formulated to maximize assimilative efficiency (e.g., a value of 85%), a potential for 

underestimation of exposure may exist by assuming that consumption of food in the wild 

is comparable with consumption during laboratory testing. In the screening process, 

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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. 

6.1.6 Spray Drift Modeling 

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 diuron from multiple applications, each application of diuron would 

have to occur under identical atmospheric conditions (e.g., same wind speed and same 

wind direction) and (if it is an animal) the animal being exposed would have to be located 

in the same location (which receives the maximum amount of spray drift) after each 

application. Additionally, other factors, including variations in topography, cover, and 

meteorological conditions over the transport distance are not accounted for by the 

AgDRIFT model (i.e., it models spray drift from aerial and ground applications in a flat 

area with little to no ground cover and a steady, constant wind speed and direction). 

Therefore, in most cases, the drift estimates from AgDRIFT may overestimate exposure, 

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 made regarding the droplet size distributions 

being modeled (was set to the default ‘ASAE Very Fine to Fine’), the application method 

(i.e., aerial), release heights and wind speeds (also set to the default values of 10 ft and 10 

mph respectively for aerial application; the height for ground application was set to the 

default ‘high boom’). Alterations in any of these inputs would decrease the area of 

potential effect. 

6.2 Effects Assessment Uncertainties 

6.2.1 Age Class and Sensitivity of Effects Thresholds 

 

It is generally recognized that test organism age may have a significant impact on the 

observed sensitivity to a toxicant. The acute toxicity data for fish are collected on 

juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on 

recommended immature age classes (e.g., first instar for daphnids, second instar for 

amphipods, stoneflies, mayflies, and third instar for midges). 

Testing of juveniles may overestimate toxicity at older age classes for pesticide active 

ingredients that act directly without metabolic transformation because younger age 

classes may not have the enzymatic systems associated with detoxifying xenobiotics. In 

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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 

CRLF 

Guideline toxicity tests and open literature data on diuron 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.

Terrestrial Plants 

Plant toxicity data from both registrant-submitted studies and studies in the scientific 

literature were reviewed for this assessment. Registrant-submitted studies are conducted 

under conditions and with species defined in EPA toxicity test guidelines. Sub-lethal 

endpoints such as plant growth, dry weight, and biomass are evaluated for both monocots 

and dicots, and effects are evaluated at both seedling emergence and vegetative life 

stages. Guideline studies generally evaluate toxicity to ten crop species. A drawback to 

these tests is that they are conducted on herbaceous crop species only, and extrapolation 

of effects to other species, such as the woody shrubs and trees and wild herbaceous 

species, contributes uncertainty to risk conclusions. 

Commercial crop species have been selectively bred, and may be more or less resistant to 

particular stressors than wild herbs and forbs. The direction of this uncertainty for 

specific plants and stressors, including diuron, 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. 

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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) 

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. 

6.2.4 Location of Wildlife Species 

For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

7.0 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 diuron to the CRLF and its designated 

critical habitat. 

The Agency makes a Likely to Adversely Affect determination for the CRLF from the 

use of diuron. The Agency has determined that there is the potential for modification of 

CRLF designated critical habitat from the use of the chemical. The direct effects and 

habitat modification effects determinations are summarized in Table 7.1 and Table 7.2

respectively. Given the LAA determination for the CRLF and potential modification of 

designated critical habitat, a description of the baseline status and cumulative effects for 

the CRLF is provided in Attachment II. 

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 iron’s use pattern is identified, using land cover 

data that correspond to iron’s use pattern. The spatial extent of the 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. The AgDRIFT model was used to evaluate 

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potential distances beyond which exposures would be expected to be below LOC. 

However, due to the limitations imposed by the Tier 1 ground analysis (allows users to 

evaluate off-site deposition and exposure out to 1,000 ft downwind from the location of 

the application), the exact buffer needed for exposures to be below the LOC is uncertain. 

The output from AgDRIFT indicated that the buffer zone required would be greater than 

1,000 feet. Since the model is restricted to accurately discerning a buffer within 1,000 

feet of the application, the exact distance needed for a buffer to protect non-listed and 

listed plants is unknown. However, as seen in Table 3.7, the calculated risk quotient 

(RQ) is significantly larger than the level of concern (LOC) for non-listed and listed plant 

species (LOC = 1). This comparison provides awareness to approximately how much 

greater than 1,000 feet the buffer needs to be. It is assumed that non-flowing water 

bodies (or potential CRLF habitat) are included within this area. 

In addition to the spray drift buffer, the results of the downstream dilution extent analysis 

result in a distance of 285 kilometers which represents the maximum continuous distance 

of downstream dilution from the edge of the initial area of concern. If any of these 

streams reaches flow into CRLF habitat, there is potential to affect either the CRLF or 

modify its habitat. These lotic aquatic habitats within the CRLF core areas and critical 

habitats potentially contain concentrations of diuron sufficient to result in LAA 

determination or modification of critical habitat. 

 

Appendix F 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 

285 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/modification of critical habitat effects determination. 

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Table 7.1 Effects Determination Summary for Effects of Diuron on the CRLF 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects 

Aquatic-phase (Eggs, Larvae, and Adults): The aquatic phase amphibian 

acute LOCs for Listed species (0.05) are exceeded for most uses of diuron in 

California. Acute RQs that exceed the Agency’s LOC range from 13.99 

(paved areas) to 0.053 (papaya). Chronic RQs that exceed the Agency’s 

LOC for chronic exposure (1.0) range from 188.58 (paved areas) to 1.0003 

(banana, plantain). 

Terrestrial-phase (Juveniles and Adults): Refined acute dietary-based RQs 

for CRLFs consuming large insects and small herbivore mammals exceed 

the acute listed species LOC (0.1) for all uses of diuron except Sorghum and 

Field Corn. The acute dietary-based RQs for CRLFs consuming large 

insects exceed the acute listed species LOC for Agricultural Rights-of-Way, 

Fencerows etc. and Ornamental Herbaceous Plants. No acute dietary based 

LOCs were exceeded for CLRF consuming small insectivore mammals and

small terrestrial phase amphibians for any diuron use. 

Refined chronic dietary-based RQs for CRLFs consuming insects and 

mammals exceed the chronic listed species LOC for all uses of diuron across 

all food item groups. 

Potential for Indirect Effects 

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

LOCs for non-vascular plants are exceeded for all uses. The RQs range 

from 2331.67 (paved areas) to 1.82 (sorghum). 

Aquatic invertebrates acute a LOC are exceeded. The acute RQs that 

exceed the LOC range from 34.98 (paved areas) to 0.06 (post-harvest 

wheat). Chronic LOCs are exceeded for paved area applications 

(RQ=1.50). 

RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for most 

uses. These range from 373.33 (paved areas) to 1.01 (alfalfa aerial). 

Survival, 

growth, 

and/or 

reproduction 

of CRLF 

individuals 

LAA 

Terrestrial prey items, riparian habitat: 

For small mammal, chronic dose based RQs exceed the Agency’s LOC (1.0) 

for most uses. The dose based chronic RQs that exceed the LOC range from 

5.00 (paved areas) to1.31 (walnut). The acute RQs exceed the Agency’s 

LOC for several uses. The RQs that exceed the acute dose based LOC range 

from 0.25 (paved areas) to 0.12 (Agricultural Rights-of-Way, Fencerows, 

Hedgerows, Airports, etc.). 

LOCs are exceeded for terrestrial riparian plants and for aquatic plants from 

exposure to diuron from runoff or spray drift. Alteration of riparian and 

vascular plants may result in alteration of temperature, turbidity, and oxygen 

content. RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for 

most uses. These range from 373.33 (paved areas) to 1.01 (alfalfa aerial). 

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

to adversely affect (LAA) 

119
Table 7.2 Effects Determination Summary for the Critical Habitat Impact Analysis

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase PCE HM Due to aquatic vascular and terrestrial plant communities being reduced from all 

use sites, there is potential for alteration of channel/pond morphology or 

geometry and/or increase in sediment deposition within the stream channel or 

pond. These plant communities provide for shelter, foraging, predator 

avoidance, and aquatic dispersal for juvenile and adult CRLFs. In addition, there 

is potential for 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. 

LOCs are exceeded for terrestrial riparian plants and for aquatic vascular plants 

from exposure to diuron from runoff or spray drift. LOCs for non-vascular 

plants are exceeded for all uses. 

Modification of 

terrestrial-phase 

PCE 

HM

The use of diuron at all use sites may create the following modification of PCE: 

elimination and/or disturbance of upland habitat; ability of habitat to support 

food source of CRLFs, Elimination and/or disturbance of dispersal habitat, 

reduction and/or modification of food sources for terrestrial phase juveniles and 

adults, and alteration of chemical characteristics necessary for normal growth 

and viability of juvenile and adult CRLFs and their food source. 

The RQs for vascular aquatic plants exceed the Agency’s LOC (1.0) for most 

uses of diuron in California. These range from 373.33 (paved areas) to 1.01 

(alfalfa aerial). Use of diuron on most use sites will exceed acute dietary- and 

dose-based LOC and chronic LOC for prey food items of small mammals, frogs, 

and invertebrates. Food source for CRLF is reduced and CRLF is indirectly 

affected. 1

 Habitat Modification (HM) or No effect (NE)

Based on the conclusions of this assessment, a formal consultation with the U. S. 

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

initiated. 

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

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

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

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

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

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

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

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

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

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

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

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

action area. This information would allow for quantitative extrapolation 

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

120
proportion of the population extant within geographical areas where those 

effects are predicted. Furthermore, such population information would 

allow for a more comprehensive evaluation of the significance of potential 

resource impairment to individuals of the species. 

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

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

does not establish minimal requirements to sustain healthy individuals at 

varying life stages. Such information could be used to establish 

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

establish geographical limits to those effects. This information could be 

used together with the density data discussed above to characterize the 

likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the 

pesticide. Currently, methodologies are limited to predicting exposures 

and likely levels of direct mortality, growth or reproductive impairment 

immediately following exposure to the pesticide. The degree to which 

repeated exposure events and the inherent demographic characteristics of 

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

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

responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and 

together with the information described above, a more complete prediction 

of effects to individual frogs and potential modification to critical habitat. 

121
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