Risks of Diflubenzuron 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 

October 19, 2009 
Primary Authors: 

Stephen Wente 

Fred Jenkins 

Lewis Brown 

Secondary Review: 

Brian Anderson 

Jim Lin 

Ed Odenkirchen 

Branch Chief, Environmental Risk Assessment Branch 1: 

Nancy Andrews
Table of Contents 

1.0 Executive Summary.......................................................................................... 4

2.0 Problem Formulation ..................................................................................... 13

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

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

2.3 Previous Assessments....................................................................................... 17 

2.4 Stressor Source and Distribution ...................................................................... 18 

2.4.1 Environmental Fate Assessment................................................................... 18 

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

2.4.3 Use Characterization..................................................................................... 21 

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

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

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

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

2.5.4 Habitat........................................................................................................... 30 

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

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

2.8 Assessment Endpoints and Measures of Ecological Effect.............................. 34 

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

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

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

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

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

2.10 Analysis Plan .................................................................................................... 41 

2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model.............. 42 

3.0 Exposure Assessment...................................................................................... 46

3.1. Label Application Rates and Intervals.............................................................. 46 

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

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

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

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

3.2.4. Existing Monitoring Data ............................................................................. 57 

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

3.5 Terrestrial Plant Exposure Assessment............................................................. 60 

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

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

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

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

4.1.3. Toxicity to Aquatic Plants ............................................................................ 67 

4.2. Toxicity of diflubenzuron to Terrestrial Organisms......................................... 72 

4.2.1. Toxicity to Birds........................................................................................... 72 

4.2.3. Toxicity to Mammals.................................................................................... 73 

4.2.4. Toxicity to Terrestrial Invertebrates ............................................................. 74 

4.2.5. Toxicity to Terrestrial Plants ........................................................................ 76 
4.3. Use of Probit Slope Response Relationship to Provide Information on the 

Endangered Species Levels of Concern............................................................ 77 

4.4. Incident Database Review ................................................................................ 78 

5.0 Risk Characterization..................................................................................... 79

5.1. Risk Estimation................................................................................................. 79 

5.1.1. Exposures in the Aquatic Habitat ................................................................. 79 

5.1.2. Exposures in the Terrestrial Habitat ............................................................. 83 

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

5.2. Risk Description ............................................................................................... 87 

5.2.1. Direct Effects................................................................................................ 91 

5.2.2. Indirect Effects (via Reductions in Prey Base)............................................. 94 

5.2.3. Indirect Effects (via Habitat Effects)............................................................ 97 

5.2.4. Modification to Designated Critical Habitat................................................. 98 

5.2.5. Spatial Extent of Potential Effects................................................................ 99 

6.0 Uncertainties.................................................................................................. 104

6.1 Exposure Assessment Uncertainties............................................................... 104 

6.1.1 Maximum Use Scenario.............................................................................. 104 

6.1.2 Crops Not Grown in California (include if you had crops that are not grown 

in CA).......................................................................................................... 104 

6.1.3 Aquatic Exposure Modeling of Diflubenzuron........................................... 104 

6.1.4 Usage Uncertainties.................................................................................... 106 

6.1.5 Terrestrial Exposure Modeling of Diflubenzuron....................................... 106 

6.1.6 Spray Drift Modeling.................................................................................. 107 

6.2 Effects Assessment Uncertainties................................................................... 108 

6.2.1 Age Class and Sensitivity of Effects Thresholds........................................ 108 

6.2.2 Use of Surrogate Species Effects Data ....................................................... 108 

6.2.3 Sublethal Effects......................................................................................... 109 

6.2.4 Location of Wildlife Species....................................................................... 109 

7.0 Risk Conclusions........................................................................................... 110

8.0 References...................................................................................................... 113
List of Tables 

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

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

Habitat Impact Analysis.................................................................................... 8 

Table 1-3 Diflubenzuron Use-specific Direct Effects Determinations1 for the CRLF....... 9 

Table 1-4 Diflubenzuron Use-specific Indirect Effects Determinations1 Based on Effects 

to Aquatic Prey ............................................................................................... 10 

Table 2-1 Summary of Diflubenzuron Environmental Fate Properties............................ 19 

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

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

Use Reporting (PUR) Data from 1999 to 2006 for Currently Registered 

Diflubenzuron Uses ........................................................................................ 25 

Table 2-4. Assessment Endpoints and Measures of Ecological Effects .......................... 36 

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

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

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

assessment1 ..................................................................................................... 48 

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

Exposure Inputs for Diflubenzuron Endangered Species Assessment for the 

CRLF 1............................................................................................................. 53 

Table 3-3 Aquatic EECs (μg/L) for Diflubenzuron Uses in California............................ 54 

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

Diflubenzuron with T-REX ............................................................................ 58 

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

Exposures of the CRLF and its Prey to Diflubenzuron. ................................. 59 

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

Terrestrial Invertebrate Prey Items ................................................................. 60 

Table 4-1 Freshwater Aquatic Toxicity Profile for Diflubenzuron .................................. 63 

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

Table 4-3. Diflubenzuron Effects Reported in a Littoral Enclosure Study (MRID 443862-

01). .................................................................................................................. 69 

Table 4-4 Terrestrial Toxicity Profile for Diflubenzuron................................................. 72 

Table 4-5 Categories of Acute Toxicity for Avian and Mammalian Studies................... 72 

Table 4-6. Probit slopes used for taxonomic groups evaluated in this assessment.......... 77 

Table 5-1 Summary of Direct Effect RQ LOCs for the Aquatic-phase CRLF................. 80 
Table 5-2 Summary of 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).......................................... 82 

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

Vascular Aquatic Plants (habitat of aquatic-phase CRLF)a............................ 83 

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

CRLF............................................................................................................... 84 

Table 5-6 Summary of RQs Used to Estimate Indirect Effects to the Terrestrial-phase 

CRLF via Direct Effects on Terrestrial Invertebrates as Dietary Food Items 

based on an LC50 of 234 ppm (30 μg per bee / 0.128 grams per bee)........... 85 

Table 5-7 Summary of Chronic RQs Used to Estimate Indirect Effects to the Terrestrialphase CRLF via Direct Effects on Small Mammals as Dietary Food Items .. 86 

Table 5-8 Risk Estimation Summary for Diflubenzuron - Direct and Indirect Effects to 

CRLF............................................................................................................... 88 

Table 5-9 Risk Estimation Summary for Diflubenzuron – PCEs of Designated Critical 

Habitat for the CRLF ...................................................................................... 89 

Table 5-10. Results from T-HERPS for the Barnyard/Mushroom Use........................... 93 

Table 5-11. Summary of AgDRIFT* Predicted Terrestrial Spray Drift Distances ........ 102 

Table 7-1. Effects Determination Summary for Diflubenzuron Use and the CRLF ...... 110 

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

Habitat Impact Analysis................................................................................ 111 
List of Figures 

Figure 2-1. Diflubenzuron use in total pounds per square mile of agricultural land in 

county.............................................................................................................. 24 

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

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

Figure 2-3 CRLF Reproductive Events by Month........................................................... 29 

Figure 2-4 Conceptual Model for Pesticide Effects on Terrestrial Phase of the CRLF .. 40 

Figure 2-5. Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF. .... 41 

Figure 3-1. Identification of application dates based on analysis of the average pounds of 

diflubenzuron applied in California to citrus via ground application methods 

based on CDPR PUR data (1990-2007).......................................................... 52 

Figure 3-2. Variation in diflubenzuron EECs (peak, 21-day, and 60-day) as function of 

application date (first application date for scenarios with multiple 

applications).................................................................................................... 56 

Figure 3-3. Variation in diflubenzuron EECs (peak, 21-day, and 60-day) as function of 

application date (first application date for scenarios with multiple 

applications) compared to variation in the amount of diflubenzuron applied 

(lbs. ai/day) per day in California for fruit use. .............................................. 56 

Appendices 

Appendix A Summary of Registrant Submitted Ecotoxicity Data 

Appendix B Multi-ai Product Analysis 

Appendix C RQ Method and LOC Description 

Appendix D Exposure Data 

Appendix E Human Health Assessment on Diflubenzuron 

Appendix F Manure Use Assessment Method 

Appendix G1 Accepted ECOTOX Data Table (sorted by effect) 

Appendix G2 Diflubenzuron Bibliography of Acceptable Papers 

Appendix G3 Diflubenzuron Bibliography of Rejected Papers 

Appendix G4 Papers Rejected by Ecotox Because they were Tested Using Mixtures 

Appendix H Bibliography of all Registrant Submitted Data 

Appendix I Description of Ecotox Rejection Codes 

Appendix J Residential Use Assessment Method 

Appendix K Right-of-Way Use Assessment Method 

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 
1.0 Executive Summary 

1.1. Purpose 

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

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

Insecticide, Fungicide, Rodenticide, Act (FIFRA) regulatory actions regarding use of 

diflubenzuron 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 (U.S. FWS) and National Marine Fisheries Service (NMFS) 

Endangered Species Consultation Handbook (U.S. FWS/NMFS 1998) and procedures 

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

The CRLF was listed as a threatened species by U.S. FWS 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 (U.S. FWS 1996) in California. 

1.2. Chemical Assessed 

Diflubenzuron is a benzoylphenylurea insecticide that kills immature insects by 

disrupting the molting process. Formulation types registered include liquid, water 

dispersible granules, wettable powder, effervescent tablets (for ornamental pond 

treatment), and termite bait (in bait stations). 

1.3. Use Characterization 

Labeled uses of diflubenzuron include citrus, cotton, forestry, mushrooms, ornamentals, 

pastures, soybeans, standing water, sewage systems, termite bait stations, and wide-area 

general outdoor treatment sites. All of these uses are considered as part of the federal 

action evaluated in this assessment with the exception of termite bait stations (in which a 

solidified form of the pesticide is placed in an enclosed bait station to minimize loss to 

the environment) and soybeans, which are not grown in California. 

1.4. Environmental Fate and Exposure Summary 

Diflubenzuron appears to be relatively non-persistent and immobile under normal use 

conditions. Diflubenzuron is relatively stable to hydrolysis and photolysis. The major 

route of dissipation appears to be biotic processes (half-life of approximately 2 days for 

aerobic soil metabolism). Available data indicate that it is unlikely that diflubenzuron 

will contaminate ground water or appreciably volatilize and be transported through longrange atmospheric transport processes. However, a degradate of diflubenzuron, 4-

chlorophenylurea (CPU), has a much greater potential to leach through soil. Spray drift 

4
and transport with eroded soil in runoff are thought to be the main mechanisms of 

transport from the site of application. 

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

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

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

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

from 0.01 to 113 μg/L. Diflubenzuron is typically not measured by surface and 

groundwater monitoring programs, therefore, no monitoring data are available for 

comparison with the model estimated diflubenzuron concentrations. 

1.4. Toxicity Summary 

Diflubenzuron is very highly toxic to aquatic invertebrates. The most sensitive LC50

identified for diflubenzuron was 0.0028 μg/L (Ecotox no. 16591). Toxicity is influenced 

by species and life-stage of the test organism. The most sensitive NOAEC was 0.00025 

μg/L (Ecotox No. 9397). Diflubenzuron is less toxic to fish, with a 96-hr LC50 of 

129,000 μg/L (MRID 00056150) and a reproduction NOAEC of 110 μg/L (MRID 

00099755). Diflubenzuron was not toxic to aquatic plants at up to 200 μg/L (MRID 

45252205 and 42940103). 

Diflubenzuron is also practically non-toxic to birds on an acute basis. The most sensitive 

study available produced an LD50 value of 3763 mg/kg-bw in the Red-Winged Blackbird 

(MRID 00038614). 

Using the technical grade active ingredient (TGAI), diflubenzuron is categorized as 

practically non-toxic to avian species on a subacute dietary toxicity basis based on an 

LC50 value of >4640 ppm for the bobwhite quail and mallard duck (MRID 00039080). A 

1 % Granular formulation was also categorized as practically non-toxic to bobwhite quail 

and mallard ducks based on an LC50 value of >20,000 ppm (MRID 00060381). The most 

sensitive NOAEC from a reliable reproduction study was 500 mg/kg-diet (MRID 

4166800102). The NOAEC of 500 mg/kg-food was based on effects on eggshell 

thickness in mallard ducks and reduced egg production in bobwhite quail at 1000 mg/kgdiet. 

The available mammalian acute toxicity data demonstrate that diflubenzuron is 

practically nontoxic to mammals (LD50 > 5000 mg/kg; MRID 00157103). In a 2-

generation reproduction study no effects on reproductive performance were observed at 

any dose level in F0 or F1 males or females. Litter and mean pup weights decreased 

slightly from birth to 21 days postpartum in F1 offspring at 2500 mg/kg/day. The NOEL 

for reproductive performance in parental adults is 2500 mg/kg/day. The NOEL for 

developmental toxicity in progeny is 250 mg/kg/day and the LOAEL is 2500 mg/kg/day, 

based on decreased body weights in F1 pups from birth to 21 days postpartum. (MRID 

43578301). A NOAEL of 250 mg/kg-bw is used in this assessment. 

5
Currently, there are no guideline terrestrial plant toxicity data for diflubenzuron for use in 

this assessment. However, given the mode of toxicity of diflubenzuron as an insect 

growth regulator, the long history of use on numerous agricultural commodities without 

incidence of effects to terrestrial plants that are clearly associated with diflubenzuron, and 

the available efficacy data that reported minimal phytotoxic effects, the available data do 

not suggest that terrestrial plants are likely to be impacted to an extent that indirect 

effects will result to species that depend on them for survival or reproduction. 

This assessment was based on potential risks from diflubenzuron. Several degradates 

have been identified that are of equal or greater toxicity than the parent chemical. These 

degradates were considered in this assessment; however, analysis of potential exposure to 

and toxicity of these degradates suggests that potential risks to these degradates is 

negligible relative to risks posed by parent diflubenzuron. 

1.5. Effects Determination 

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

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

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

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

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

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

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

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

aquatic invertebrates and aquatic plants, toxicity information for these taxonomic groups 

is also discussed. In the terrestrial habitat, indirect effects due to depletion of prey are 

assessed by considering effects to terrestrial insects, small terrestrial mammals, and frogs. 

Indirect effects due to modification of the terrestrial habitat are characterized by available 

data for terrestrial monocots and dicots. However, given the lack of guideline toxicity 

studies on terrestrial plants, additional lines of evidence were used to determine if 

terrestrial plants may be impacted to an extent that could result in indirect effects to the 

CRLF. 

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

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

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

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

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

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

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

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

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

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

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

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

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

6
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 not result in modification of its critical habitat. 

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

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

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

from the use of the chemical. A summary of the risk conclusions and effects 

determinations for the CRLF and its critical habitat is presented in Table 1-1 and 1-2. 

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

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

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

There was an LOC exceedance for risk to fish (CRLF surrogate) 

from chronic exposures to diflubenzuron posed by the rice use. 

However, the risk was determined to be discountable because 

effects were considered to be unlikely to occur for reasons 

discussed in Section 5.2. LOCs were not exceeded for any other 

use. The effects determination for direct effects to aquatic phase 

CRLFs was NE for all uses except rice and NLAA for rice. 

Terrestrial-phase (Juveniles and Adults): 

Direct effects to CRLFs could occur from diflubenzuron’s use on 

barn yards based on acute and chronic LOC exceedances for birds 

and refined analysis for frogs. Direct effects to CRLFs are not 

expected to occur for any other use. The effects determination is 

LAA for barnyard/mushroom use and NE for all other uses. 

Potential for Indirect Effects

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

productivity

Aquatic prey of the CRLF may be impacted based on potential 

impacts to aquatic invertebrate for all uses. Several lines of 

evidence support the conclusion that aquatic invertebrates may be 

impacted by labeled diflubenzuron uses including laboratory 

studies and field studies. The effects determination is LAA for all 

uses based on potential impacts to aquatic invertebrates. 

Survival, growth, 

and/or 

reproduction of 

CRLF individuals 

LAA, all uses 

Terrestrial prey items, riparian habitat 

Terrestrial prey items of the CRLF are expected to be impacted 

based on potential impacts to terrestrial invertebrates. The effects 

determination is LAA for all uses based on potential impacts to 

terrestrial invertebrates. 1

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

affect (LAA)

7
Table 1-2. Effects Determination Summary for Diflubenzuron Use and CRLF Critical Habitat 

Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase PCE There are LOC exceedances for risk to terrestrial and aquatic 

invertebrate prey of the CRLF. 

Modification of 

terrestrial-phase 

PCE 

HM 

There are LOC exceedances for risk to terrestrial and aquatic 

invertebrate prey of the CRLF. 

1

 Habitat Modification or No effect (NE)

Use-specific determinations for direct and indirect effects to the CRLF are provided in 

Table 1-3, 1-4, and 1-5. Further information on the results of the effects determination is 

included as part of the Risk Description in Section 5.2. 

8
Table 1-3 Diflubenzuron Use-specific Direct Effects Determinations1

 for the CRLF 

Aquatic Habitat Terrestrial Habitat 

Use(s) Acute Chronic Acute Chronic 

Barnyard/Mushroom NE NE LAA LAA 

Spray on manure, Manure to field NE NE NE NE 

Beech Nut NE NE NE NE 

Brassica (head and stem) vegetables NE NE NE NE 

Citrus hybrids, grapefruit, orange, pummelo (shaddock), 

and tangerines NE NE NE NE 

Broccoli raab, cabbage - Chinese, collards, and kale NE NE NE NE 

Cotton (unspecified) NE NE NE NE 

Christmas tree plantations and forest trees (softwoods - 

conifers) NE NE NE NE 

Apricot, nectarine, peach, pear, plum, prune, and stone 

fruits NE NE NE NE 

Almond, Brazil nut, cashew, chestnut, chinquapin, filbert 

(hazelnut), hickory nut, macadamia nut (bushnut), pecan, 

tree nuts, and walnut (English/black) 

NE NE NE NE 

Barley, canola\rape, oats, triticale, and wheat NE NE NE NE 

Forest nursery plantings (for transplant purposes), 

ornamental and/or shade trees, ornamental woody shrubs 

and vines, and shelterbelt plantings 

NE NE NE NE 

Grass forage/fodder/hay, pastures, and rangeland NE NE NE NE 

Pistachio NE NE NE NE 

Household/domestic dwellings outdoor premises NE NE NE NE 

Rice/Ornamental Pond NE NLAA NE NE 

Agricultural rights-of-way/fencerows/hedgerows, 

fencerows/hedgerows, and nonagricultural rights-ofway/fencerows/hedgerows 

NE NE NE NE 

Artichoke, mustard, peanuts (unspecified), and pepper NE NE NE NE 

Commercial/institutional/industrial premises/equipment 

(outdoor), nonagricultural outdoor buildings/structures, 

paved areas (private roads/sidewalks), refuse/solid waste 

sites (outdoor), and wide area/general outdoor treatment 

(quarantine/eradication use) 

NE NE NE NE 

Butternut NE NE NE NE 

Agricultural fallow/idleland, golf course turf, and 

nonagricultural uncultivated areas/soils NE NE NE NE 

Ornamental sod farm (turf) and recreational areas NE NE NE NE 

Turnip (greens) NE NE NE NE 1

 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect

9
Table 1-4 Diflubenzuron Use-specific Indirect Effects Determinations1

 Based on Effects to Aquatic 

Prey 

Aquatic Invertebrates 

Aquatic-phase frogs 

and fish 

Use(s) Algae Acute Chronic Acute Chronic 

Barnyard/Mushroom NE LAA LAA NE NE 

Spray on manure, Manure to field NE LAA LAA NE NE 

Beech Nut NE LAA LAA NE NE 

Brassica (head and stem) vegetables NE LAA LAA NE NE 

Citrus hybrids, grapefruit, orange, pummelo 

(shaddock), and tangerines NE LAA LAA NE NE 

Broccoli raab, cabbage - Chinese, collards, 

and kale NE LAA LAA NE NE 

Cotton (unspecified) NE LAA LAA NE NE 

Christmas tree plantations and forest trees 

(softwoods - conifers) NE LAA LAA NE NE 

Apricot, nectarine, peach, pear, plum, prune, 

and stone fruits NE LAA LAA NE NE 

Almond, Brazil nut, cashew, chestnut, 

chinquapin, filbert (hazelnut), hickory nut, 

macadamia nut (bushnut), pecan, tree nuts, 

and walnut (English/black) 

NE LAA LAA NE NE 

Barley, canola\rape, oats, triticale, and wheat NE LAA LAA NE NE 

Forest nursery plantings (for transplant 

purposes), ornamental and/or shade trees, 

ornamental woody shrubs and vines, and 

shelterbelt plantings 

NE LAA LAA NE NE 

Grass forage/fodder/hay, pastures, and 

rangeland NE LAA LAA NE NE 

Pistachio NE LAA LAA NE NE 

Household/domestic dwellings outdoor 

premises NE LAA LAA NE NE 

Rice/Ornamental Pond NE LAA LAA NE NLAA 

Agricultural rights-ofway/fencerows/hedgerows, 

fencerows/hedgerows, and nonagricultural 

rights-of-way/fencerows/hedgerows 

NE LAA LAA NE NE 

Artichoke, mustard, peanuts (unspecified), 

and pepper NE LAA LAA NE NE 

Commercial/institutional/industrial 

premises/equipment (outdoor), 

nonagricultural outdoor buildings/structures, 

paved areas (private roads/sidewalks), 

refuse/solid waste sites (outdoor), and wide 

area/general outdoor treatment 

(quarantine/eradication use) 

NE LAA LAA NE NE 

Butternut NE LAA LAA NE NE 

Agricultural fallow/idleland, golf course turf, 

and nonagricultural uncultivated areas/soils NE LAA LAA NE NE 

Ornamental sod farm (turf) and recreational 

areas 

NE LAA LAA NE NE 

Turnip (greens) NE LAA LAA NE NE 1

 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect

10
Table 1-5. Diflubenzuron Use-specific Indirect Effects Determinations1

 Based on Effects to 

Terrestrial Prey 

Terrestrial-phase 

frogs Small Mammals 

Use(s) 

Terrestrial 

Inverts. 

(Acute) Acute Chronic Acute Chronic 

Barnyard/Mushroom LAA LAA LAA NE NE 

Spray on manure, Manure to field LAA NE NE NE NE 

Beech Nut LAA NE NE NE NE 

Brassica (head and stem) vegetables LAA NE NE NE NE 

Citrus hybrids, grapefruit, orange, pummelo 

(shaddock), and tangerines LAA NE NE NE NE 

Broccoli raab, cabbage - Chinese, collards, 

and kale LAA NE NE NE NE 

Cotton (unspecified) LAA NE NE NE NE 

Christmas tree plantations and forest trees 

(softwoods - conifers) LAA NE NE NE NE 

Apricot, nectarine, peach, pear, plum, prune, 

and stone fruits LAA NE NE NE NE 

Almond, Brazil nut, cashew, chestnut, 

chinquapin, filbert (hazelnut), hickory nut, 

macadamia nut (bushnut), pecan, tree nuts, 

and walnut (English/black) 

LAA NE NE NE NE 

Barley, canola\rape, oats, triticale, and wheat LAA NE NE NE NE 

Forest nursery plantings (for transplant 

purposes), ornamental and/or shade trees, 

ornamental woody shrubs and vines, and 

shelterbelt plantings 

LAA NE NE NE NE 

Grass forage/fodder/hay, pastures, and 

rangeland LAA NE NE NE NE 

Pistachio LAA NE NE NE NE 

Household/domestic dwellings outdoor 

premises LAA NE NE NE NE 

Rice/Ornamental Pond LAA NE NE NE NE 

Agricultural rights-ofway/fencerows/hedgerows, 

fencerows/hedgerows, and nonagricultural 

rights-of-way/fencerows/hedgerows 

LAA NE NE NE NE 

Artichoke, mustard, peanuts (unspecified), and 

pepper 

LAA NE NE NE NE 

Commercial/institutional/industrial 

premises/equipment (outdoor), nonagricultural 

outdoor buildings/structures, paved areas 

(private roads/sidewalks), refuse/solid waste 

sites (outdoor), and wide area/general outdoor 

treatment (quarantine/eradication use) 

LAA NE NE NE NE 

Butternut LAA NE NE NE NE 

Agricultural fallow/idleland, golf course turf, 

and nonagricultural uncultivated areas/soils LAA NE NE NE NE 

Ornamental sod farm (turf) and recreational 

areas 

LAA NE NE NE NE 

Turnip (greens) LAA NE NE NE NE 1

 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect

11
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. 

12
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. Environmental Protection Agency's (EPA’s) Guidance for Ecological 

Risk Assessment (U.S. EPA 1998), the Services’ Endangered Species Consultation 

Handbook (U.S. FWS/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 (U.S. FWS/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 

diflubenzuron on agricultural uses (nuts, cotton, rice, row crops, citrus, fruits, etc.), 

forestry, nursery, residential, right-of-ways, and turf uses. In addition, this assessment 

evaluates whether use on these sites is expected to result in modification of the species’ 

designated critical habitat. This ecological risk assessment has been prepared consistent 

with a settlement agreement in the case Center for Biological Diversity (CBD) vs. EPA et 

al. (Case No. 02-1580-JSW(JL) settlement entered in Federal District Court for the 

Northern District of California on October 20, 2006. 

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

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

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

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

described at length in the Overview Document. Additional refinements were made to 

PRZM/EXAMS scenarios to account for heterogeneous land cover within treatment areas 

in order to more accurately model residential and right-of-way uses. 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 diflubenzuron is based on an action area. The action area is the area 

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

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

national-level FIFRA regulatory decision associated with a use of diflubenzuron 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 

13
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 diflubenzuron on the CRLF 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 diflubenzuron 

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

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

the CRLF’s designated critical habitat, a preliminary “may affect” determination is made 

for the FIFRA regulatory action regarding diflubenzuron. 

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

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

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

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

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

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

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

diflubenzuron 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 diflubenzuron is expected to directly impact living organisms within the action 

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

14
appreciably diminish the value of the habitat. Evaluation of actions related to use of 

diflubenzuron that may alter the PCEs of the CRLF’s critical habitat form the basis of the 

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

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

2.2 Scope 

Diflubenzuron is an insecticide that is registered for both agricultural and nonagricultural uses in California and can be applied through ground, aerial, and airblast 

methods. 

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 diflubenzuron in accordance with the approved product labels for 

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

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

risk assessment and effects determination addresses currently registered uses of 

diflubenzuron in portions of the action area that are reasonably assumed to be 

biologically relevant to the CRLF and its designated critical habitat. 

There are two reasons for considering the action area of diflubenzuron use to include the 

entire state of California. First, because of the large variety of label uses for 

diflubenzuron in the state of California, it would be difficult to exclude any portions of 

this state from potential for diflubenzuron use. Second, a degradate of diflubenzon is a 

suspected carcinogen and is genotoxic. Because there is no concentration threshold 

defined that would result in no genotoxic effects to exposed organisms, it is not possible 

to define a threshold below which effects can occur. Therefore, the action area is 

assumed to be the entire state of California. Further discussion of the action area for the 

CRLF and its critical habitat is provided in Section 2.7. 

2.2.1. Chemicals Assessed 

This assessment evaluates the potential for diflubenzuron to adversely affect the CRLF. 

Several degradates of diflubenzuron have been identified and tested for toxicity. The 

available toxicity data are summarized below and are further described in Section 4 and 

Appendix A. 

Degradates of diflubenzuron include 2,6-diflubenzoic acid (DFBA), 4-chlorophenylurea 

(CPU), 4-chloroaniline (PCA), and 2,6-diflubenzamide (DFBAM). The available data 

suggest that the degradates are several orders of magnitude less toxic than diflubenzuron 

to invertebrates. For example, LC50s in midge for PCA and DFBA are 43 mg/L and >100 

mg/L, respectively, compared with an LC50 of 0.07 mg/L for parent chemical in midge. 

15
Lower toxicity to invertebrates of degradates is consistent with the specific mode of 

action of diflubenzuron as a chitin inhibitor. 

Several degradates have been shown to be of similar toxicity to fish compared with 

parent difbluenzuron. In particular, PCA has been shown to be more toxic than 

diflubenzuron to fish with LC50 values ranging from 2 mg/L to 23 mg/L. DFBA and 

PCPU appear to have similar toxicity relative to parent diflubenzuron with 96-hr LC50

values of approximately 70 mg/L to >100 mg/L in fish. The most sensitive LC50 in fish 

was 127 mg/L for diflubenzuron. 

Although PCA has been shown to be more toxic to fish, it has not formed more than 10% 

of parent compound in available degradation studies. Potential risks resulting from 

exposure to PCA or other degradates were not quantified. It was determined that 

quantifying risks from the degradates would not affect risk conclusions because 

comparing EECs for parent diflubenzuron (which would represent a highly conservative 

exposure estimate for any of the degradates) with the lowest toxicity value across all 

degradates still results in no LOC exceedances for fish. Therefore, degradates were not 

further considered. 

One diflubenzuron formulated product toxicity study provided a lower LC50 than values 

from studies on the TGAI. The lowest LC50 for formulated products was 57 mg/L (see 

Appendix A). EFED evaluates potential risks from formulated products resulting from 

drift only exposure. The highest drift fraction estimated to be deposited into water bodies 

was approximately 4% from aerial spray. The highest application rate for diflubenzuron 

is 8.5 lbs a.i./Acre (barnyard/mushroom use). The resulting drift only EEC is 0.003 mg 

a.i./L assuming a volume of 20,000,000 L in the receiving water. Based on an LC50 of 57 

mg/L, the resulting RQ would be orders of magnitude below concern levels 

(approximately 10-5). Therefore, toxicity values from formulated products were not 

further considered as part of this assessment. 

2.2.2. Evaluation of Mixtures 

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; U.S. 

FWS/NMFS 2004). 

Diflubenzuron has one registered product that contains multiple active ingredients 

(diflubenzuron and permethrin). Analysis of the available open literature and acute oral 

16
mammalian LD50 data for the diflubenzuron and permethrin products relative to the 

single active ingredient is provided in Appendix B. There are no product LD50 values, 

with associated 95% Confidence Intervals (CIs) available for diflubenzuron. 

As discussed in USEPA (2004) 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 single formulated product for diflubenzuron does not have a 95% CI associated with 

the oral LD50 value, it is not possible to undertake a quantitative or qualitative analysis 

for potential interactive effects (See appendix B). 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 

diflubenzuron is the only reasonable approach that employs the available data to address 

the potential acute risks of the formulated products. Studies on mixtures located in the 

open literature are included in Appendix B. 

2.3 Previous Assessments 

A large number of risk assessments have been performed on diflubenzuron by EFED. 

Two recent, comprehensive assessments are described below. 

The Reregistration Eligibility Decision (RED) Science Chapter (6/30/95) identified risks 

to aquatic invertebrates as the major environmental threat of diflubenzuron. Acute and 

chronic levels of concern (LOCs) were exceeded for both freshwater and estuarine and 

marine aquatic invertebrates, for the use of diflubenzuron on cotton, citrus, forest trees 

and forest plantings. 

In an assessment entitled “Section 3 Registration Request for Uses of Diflubenzuron on 

Pears, Stone Fruits, Tree Nuts, Peppers, and Pasture Grass (Chemical # 108201, DP 

Barcode D279732, D279735, D278770)” from 2/5/02, EFED identified acute concerns 

for freshwater invertebrates, but not for fish, birds, or mammals for the uses assessed. 

The document indicates concern for sediment dwelling aquatic organisms due to the 

expectation of diflubenzuron accumulation in sediments, but did not quantify this risk 

due to the lack of chronic toxicity data for these organisms. This document developed the 

critical life stage hypothesis for diflubenzuron effects which is discussed in Section 4.1.2 

of this document. 

A major difference between this assessment and the previous assessments concerns the 

inclusion of open literature toxicity values in the current assessment. This assessment is 

the first to include open literature toxicity data from the Ecotox database (See Appendix 

G). Inclusion of this data resulted in much lower acute and chronic freshwater 

invertebrate endpoints, which result in much greater risk estimates. 

17
2.4 Stressor Source and Distribution 

2.4.1 Environmental Fate Assessment 

Diflubenzuron (DFB) is a member of a larger group of insecticides known as 

benzoylphenylureas. Based on acceptable and supplemental registrant-submitted data, 

diflubenzuron appears to be relatively non-persistent and relatively immobile under 

normal use conditions. The major route of dissipation for diflubenzuron appears to be 

biotic mediated processes (t1⁄2 ≈2-14 days for aerobic soil metabolism). Anaerobic aquatic 

metabolism was reported to be slower (t1⁄2 = 34 days). Aerobic aquatic metabolism halflife in total water sediment system was 26 days. Other laboratory data indicate that 

diflubenzuron is stable to hydrolysis (t1⁄2 ≈30-433 days for pHs 5-9) and photolysis (t1⁄2 = 

80 days for aquatic; t1⁄2 for control < light exposed for soil) and is relative immobile in 

soil (Rf values = 0.01, 0.07, 0.14, and 0.34 for silty clay loam, clay loam, and two sand 

loam soils, respectively). Supplemental and acceptable field data (including forestry 

dissipation data) confirm the laboratory data with reported half-lives of 5.8 to 60 days, 

and diflubenzuron detectable only in the 0-15 cm soil depth segments. However, 

calculated half-lives for CA and OR orchard applications were higher (t1⁄2 = 68.2-78 

days). Diflubenzuron has not been detected in well monitoring data (National SummaryPesticides in Ground Water Data base-A Compilation of Monitoring Studies: 1971-

1991), as well. 

Under aerobic conditions diflubenzuron appears to degrade to 4-chlorophenyl urea (CPU) 

which reached a maximum concentration of 37% of applied at 14 days post-treatment. 

The other major degradate, CO2, was reported to reach a maximum concentration of 

26.3% of applied by day 21 post-treatment. Three minor degradates, 2,6-difluorobenzoic 

acid, 2,6-difluorobenzamide, and p-chloroaniline, each reaching a maximum 

concentration of <10% of applied, were identified in the aerobic soil study. These 

metabolites were detectable in an anaerobic metabolism study, as well. Due to the 

stability of diflubenzuron to abiotic processes, limited data are available on the 

persistence and mobility of diflubenzuron metabolites. However, CPU was reported in 

leachate of a column leaching study (approximately 15 to 30% of applied in MRIDs 

00039477 & 00040777). Even though CPU appears to be mobile under laboratory 

conditions, it has not been reported below the 0 to 15 cm soil depth segment in any field 

data. 

Diflubenzuron does appear to accumulate at low levels and depurate rapidly in fish tissue. 

The reported bioconcentration factors ranged from 34 to 200X for fillet, 78 to 360X for 

whole fish, and 100 to 500X for viscera. In addition, the depuration rate indicates a rapid 

decrease (99%) of accumulated residues in tissue during the 14 day depuration period. 

Table 2-1 lists the environmental fate properties of diflubenzuron, along with the major 

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

18
Table 2-1 Summary of Diflubenzuron Environmental Fate Properties 

Study Value (units) 

Major Degradates 

Minor Degradates MRID # Study Status

14C label 

433 days @ pH 5

117 days @ pH 7

42 days @ pH 9

3

H Label 

187 days @ pH 5

158 days @ pH 7

40 days @ pH 9

Unlabeled 

Stable @ pH 5 

Stable @ pH 7 

44 days @ pH 9

14C and 3

H label 7.5 days @ pH 

12 

CPU – Not Quantified 

DFBA – Not Quantified 00143355 Supplemental

223.6 days @ pH 5 

247 days @ pH 7 

32.5 days @ pH 9 

CPU – Not Quantified 

DFBA – Not Quantified 

40859801 

41087801 Acceptable 

Hydrolysis 

Dark aqueous photolysis control: 

Stable @ pH 7 None Identified 40816301 Supplemental(15 days only)

Direct Aqueous 

Photolysis 

40 days (continuous light) 

80 days (assuming 12 hr. light/dark 

cycle 

CPU – 8% 

DFBA – 4% 

DFBAM – 1% 

2,6-Difluorobenzene – not 

quantified 

Unknown – 6% 

40816301 

41087802 Acceptable 

144 days Not Quantified 40941601 Supplemental

Soil Photolysis 11.3 days 

CPU – 12.9 % (Day l0) 

DFBA – 3.0% (Day 7) 

Unknown SP1 – 0.6% 

(Day 10) 

Unknown PK1 – 0.1% 

(Day 16) 

42251201 Acceptable 

00039473 

00039474 

00040154 

Supplemental

(39474 same 

as 40154) 

00099875 Supplemental Aerobic Soil 

Metabolism 2 to 14 days 

CPU – 37 % (Day l4) 

DFBA – <10% 

DFBAM – <10% 

PCA – <10% 41722801 

Same as 

46888708 

Acceptable 

Anaerobic Soil 

Metabolism 2 to 14 days 

CPU – 37 % (Day 2-l4: 

depending on 

temperature) 

DFBA – 23% (Day 35) 

00040782 

41837601 Acceptable 

Anaerobic 

Aquatic 

Metabolism 

34 days 

CPU – 31 % (Day 42) 

DFBA – 31% (Day 42) 

PCA – 0.4% (Day 18)

41837601 Acceptable 

26 days 

CPU – 30 % (Day l6) 

DFBA – 9% (Day 1) 

DFBAM – 1.6% (Day 2) 

PCA – 2% (Day 16)

Aerobic 44895001 Acceptable 

Aquatic 

Metabolism 5.4 days 

3.7 days 

DFBA – 17% (Day 4) 

CPU – 48 % (Day l6) 46888707 Supplemental

19
Major Degradates 

Study Value (units) Minor Degradates MRID # Study Status

Koc

2878 

3008 

3401 

6918 

6801 

2780 

1938 

Not Applicable 

00039476 

00039477 

00040777 

00157842 

46888704 

46888705 

46895401 

Acceptable 

Terrestrial 

Field 

Dissipation 

The orchard and bare ground halflives (5.8 to 13.2 days). 

CA citrus and the OR apple 

orchards half-life of approximately 

68.2 to 78 days. 

DFBA – up to 0.01 ppm 

CPU – up to 0.06 ppm 

00040156 

40660601 

40660602 

41816502 

41816503 

41821002 

41826701 

42290401 

42290402 

42441101 

Acceptable 

Forestry Field 

Dissipation 30 to 35 days None Detected 

41077201 

41077202 

41922201 

42197701 

Supplemental

Aquatic Field 

Dissipation 2 to 6 days CPU 

00040155 

00161945 

00161947 

40598601 

44399309 

45009601 

45191001 

45197601 

Acceptable 

Accumulation 

Bioconcentration factors of 34 to 

200× for fillet, 78 to 360× for 

whole fish, and 100, to 550× for 

viscera. 

99% depuration within 14 day 

DFBAM – 16% 

(0.28,μg/g) in Fillet; 

12% (0.37 μg/g) in Whole 

Fish; and 

11% (0.55 μg/g) in 

Viscera. 

6 unidentified minor 

metabolites 

42258401 

42258402 Acceptable 

2.4.2 Environmental Transport Assessment 

Surface water runoff and spray drift are expected to be the major routes of exposure for 

diflubenzuron. 

In general, deposition of drifting pesticides is expected to be greatest close to the site of 

application. Computer models of spray drift (AgDRIFT) are used to determine potential 

exposures to aquatic and terrestrial organisms via spray drift. The distance of potential 

impact away from the use sites (action area) is determined by the distance required to fall 

below the LOC for aquatic invertebrates. 

20
2.4.2 Mechanism of Action 

Diflubenzuron is an insect growth regulator and works by preventing the formation of 

chitin, a molecule necessary to the formation of an insect’s cuticle or outer shell. Insects 

exposed to a sufficient dose of diflubenzuron cannot form their protective outer shell and 

die during molting. It is most effective against insect larva, but also acts as an ovicide, 

killing insect eggs (U.S. EPA 1997). 

2.4.3 Use Characterization 

Currently, labeled uses of diflubenzuron include citrus, cotton, forestry, mushrooms, 

ornamentals, pastures, soybeans, standing water, sewage systems, termite bait stations, 

and wide-area general outdoor treatment sites. All of these uses are considered as part of 

the federal action evaluated in this assessment with the exception of termite bait stations 

(in which a solidified form of the pesticide is placed in an enclosed bait station to 

minimize loss to the environment) and soybeans, which are not grown in California 

(NASS 2002). If use patterns indicate that soybeans are grown in CA in the future, the 

conclusions of this assessment may need to be revisited. 

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

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

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

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

area and selection of appropriate modeling scenarios and inputs. 

Table 2-2 presents the uses and corresponding application rates and methods of 

application considered in this assessment. 

21
Table 2-2 Diflubenzuron Uses Assessed for the CRLF 

Use (Application Method – Ground (G), Airblast 

(AB), or Air (A)) 

Max. Single 

Appl. Rate 

(lb ai/A) 

Max. Number of 

Application per 

Year 

Application 

Interval 

Spray on manure, Manure to field G 8.508 NS (17) NS (21) 

G 0.0408 3 7 Beech Nut A 0.0408 3 7 

Brassica (head and stem) vegetables G 0.0313 1 NA 

Citrus hybrids, grapefruit, orange, pummelo G 0.3125 3 21 

(shaddock), and tangerines A 0.125 3 7 

Broccoli raab, cabbage - Chinese, collards, and 

kale G 0.25 4 21 

G 0.3125 3 7 Cotton (unspecified) A 0.3125 3 90 

Christmas tree plantations and forest trees G 0.22048 NS (4) 14 

(softwoods - conifers) A 0.25 1 NA 

G 0.3125 3 7 

AB 0.3125 3 7 Apricot, nectarine, peach, pear, plum, prune, and 

stone fruits 

A 0.3125 3 7 

G 0.3125 3 7 

AB 0.3125 3 7 

Almond, Brazil nut, cashew, chestnut, chinquapin, 

filbert (hazelnut), hickory nut, macadamia nut 

(bushnut), pecan, tree nuts, and walnut 

(English/black) A 0.3125 3 90 

G 0.25 4 21 Barley, canola\rape, oats, triticale, and wheat A 0.125 3 NS (21) 

Forest nursery plantings (for transplant purposes), G 0.25 1 NA 

ornamental and/or shade trees, ornamental woody 

shrubs and vines, and shelterbelt plantings A 0.25 1 NA 

G 0.25 1 NA 

Grass forage/fodder/hay, pastures, and rangeland AB 0.25 1 NA 

A 0.25 1 NA 

Pistachio G 0.75 2 14 

G 0.25 1 NA Household/domestic dwellings outdoor premises A 0.25 6 10 

Rice A 0.625 6 5 

G 0.25 2 5 

AB 0.25 2 5 

Agricultural rights-of-way/fencerows/hedgerows, 

fencerows/hedgerows, and nonagricultural rightsof-way/fencerows/hedgerows A 0.25 2 5 

Artichoke, mustard, peanuts (unspecified), and G 0.3125 3 21 

pepper A 0.3125 3 14 

Commercial/institutional/industrial 

premises/equipment (outdoor), nonagricultural 

outdoor buildings/structures, paved areas (private 

roads/sidewalks), refuse/solid waste sites 

(outdoor), and wide area/general outdoor 

treatment (quarantine/eradication use) 

A 0.25 6 NS (10) 

G 0.25 4 15 Butternut A 0.25 4 15 

G 0.25 2 14 

AB 0.25 2 14 Agricultural fallow/idleland, golf course turf, and 

nonagricultural uncultivated areas/soils A 0.25 2 5 

Ornamental sod farm (turf) and recreational areas G 0.0313 3 7 

22
Use (Application Method – Ground (G), Airblast 

(AB), or Air (A)) 

Max. Single Max. Number of 

Appl. Rate Application per Application 

(lb ai/A) Year Interval 

AB 0.0313 3 7 

A 0.0313 3 7 

Turnip (greens) G 0.25 2 21 

NS = Not specified on label. 

Several of the agricultural crop uses for diflubenzuron may have more than one crop 

cycle per year. Because many diflubenzuron labels specify application limits based on 

crop cycles rather than an annual limitation, the potential exists for more diflubenzuron to 

be applied than is reflected in Table2-2. However, because the persistence of 

diflubenzuron is relatively short compared to the length of a crop cycle, there would be 

little accumulation of the parent compound across crop cycles. Instead the greatest impact 

from multiple crop cycles would likely result from the extension of the time that 

diflubenzuron would be applied both earlier in the spring and later in the fall when more 

runoff would occur. This extension of the time that diflubenzuron may be used is 

addressed in this assessment in Section 3.2.1, which evaluates both early and late 

application dates on diflubenzuron exposure. 

Figure 2-1 provides a visual depiction of the spatial distribution of diflubenzuron use 

across the United States in terms of pounds of diflubenzuron applied per square mile of 

agricultural land in each county. The map was downloaded from a U.S. Geological 

Survey (USGS), National Water Quality Assessment Program (NAWQA) website. Many 

simplifying assumptions were made in order to produce this map. Non-agricultural uses 

of diflubenzuron such as forestry, residential and right-of-way uses are not included in 

this depiction. 

23
Figure 2-1. Diflubenzuron use in total pounds per square mile of agricultural land 

in county. 

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

of both national- and county-level usage information (U.S. EPA 2009) using state-level 

usage data obtained from USDA-NASS1

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

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

Regulation Pesticide Use Reporting (CDPR PUR) database2

 . CDPR PUR is considered 

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

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

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

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

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

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

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

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

 

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. 

24
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 eight years. The units 

of area treated are also provided where available. 

A summary of diflubenzuron usage for all California use sites is provided below in Table 

2-3. 

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

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

Site Name, Application Method (Aerial, 

Ground, or Other), and Years of Data 

Average Pounds 

Applied by Use 

Average App. 

Rate by Use 

Maximum App. 

Rate by Use 

A (3) 988.30 0.24 4.05 Almond G (3) 8007.51 0.22 3.74 

A (1) 17.46 0.25 0.25 Apricot G (3) 100.25 0.19 0.24 

Aquatic Areas, Water Areas (All Or 

Unspec) O (7) 15.82 0.22 0.25 

A (8) 1737.13 0.14 2.23 Artichoke (Globe) (All Or Unspec) G (8) 1982.66 0.14 2.97 

Beets, General G (1) 3.50 5.00 5.00 

A (1) 0.92 0.02 0.02 Broccoli G (1) 2.10 0.02 0.02 

Brussels Sprouts G (2) 1.36 0.41 0.70 

Cherry G (1) 3.75 0.25 0.25 

Christmas Tree Plantations G (6) 0.71 0.08 0.13 

A (3) 45.42 0.12 0.14 Cotton, General G (2) 54.38 0.10 0.13 

Endive (Escarole) G (1) 26.64 0.19 0.19 

A (1) 2.44 0.13 0.13 Grapefruit G (4) 73.25 0.17 0.78 

Landscape Maintenance O (8) 23.11 NR NR 

Nectarine G (3) 38.51 0.09 0.25 

A (3) 0.06 0.32 0.69 

N-Outdr Container/Fld Grwn Plants G (8) 18.14 0.46 24.16 

O (1) 0.00 0.00 0.00 

A (1) 0.03 0.00 0.00 N-Outdr Grwn Cut Flwrs Or Greens G (8) 2.82 0.29 3.57 

A (1) 2.55 0.21 0.59 N-Outdr Grwn Trnsplnt/Prpgtv Mtrl G (6) 9.23 0.20 2.08 

A (2) 31.03 0.13 0.14 Orange (All Or Unspec) G (4) 762.37 0.19 2.50 

Pastures (All Or Unspec) A (1) 0.95 0.02 0.02 

Peach G (4) 167.31 0.20 6.07 

Pear G (3) 75.48 0.22 0.26 

Peppers (Chili Type) (Flavoring And 

Spice Crop) G (1) 0.75 0.13 0.13 

Peppers (Fruiting Vegetable), 

(Bell,Chili, Etc.) G (1) 141.96 0.12 0.13 

Pistachio (Pistache Nut) G (2) 36.87 0.24 0.25 

25
Site Name, Application Method (Aerial, 

Ground, or Other), and Years of Data 

Average Pounds 

Applied by Use 

Average App. Maximum App. 

Rate by Use Rate by Use 

Plum (Includes Wild Plums For A (1) 5.95 0.20 0.20 

Human Consumption) G (3) 94.31 0.19 0.25 

A (1) 5.56 0.25 0.25 Prune G (3) 66.94 0.25 0.95 

Public Health Pest Control O (8) 527.15 NR NR 

A (2) 3.08 0.03 0.03 Rangeland (All Or Unspec) G (1) 2.21 0.03 0.03 

Regulatory Pest Control O (5) 42.22 NR NR 

A (8) 789.74 0.14 3.93 Rice (All Or Unspec) G (1) 4.08 0.09 0.09 

Rights Of Way O (6) 9.98 NR NR 

Soil Application, Preplant-Outdoor 

(Seedbeds,Etc.) G (3) 213.56 11.28 12.61 

Strawberry (All Or Unspec) G (1) 23.50 0.25 0.25 

Structural Pest Control O (8) 284.86 NR NR 

Tangerine (Mandarin, Satsuma, 

Murcott, Etc.) G (2) 30.73 0.16 0.26 

Uncultivated Agricultural Areas (All 

Or Unspec) G (2) 1.39 0.05 0.07 

Uncultivated Non-Ag Areas (All Or 

Unspec) O (2) 8.15 0.06 0.13 

Walnut (English Walnut, Persian A (4) 37.22 0.24 0.38 

Walnut) G (8) 1406.79 0.26 2.02 

2.5 Assessed Species 

The CRLF was federally listed as a threatened species by U.S. FWS effective June 24, 

1996 (U.S. FWS 1996). It is one of two subspecies of the red-legged frog and is the 

largest native frog in the western United States (U.S. FWS 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 U.S. FWS on April 13, 2006 (U.S. 

FWS 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 (U.S. FWS 1996). Its range has been reduced by about 70%, 

and the species currently resides in 22 counties in California (U.S. FWS 1996). The 

species has an elevational range of near sea level to 1,500 meters (5,200 feet) (Jennings 

and Hayes 1994); however, nearly all of the known CRLF populations have been 

documented below 1,050 meters (3,500 feet) (U.S. FWS 2002). 

26
Populations currently exist along the northern California coast, northern Transverse 

Ranges (U.S. FWS 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 (U.S. FWS 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) (U.S. FWS 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. 

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 

U.S. FWS to be important in the preservation of the species. Designated critical habitat is 

generally contained within the core areas, although a number of critical habitat units are 

outside the boundaries of core areas, but within the boundaries of the recovery units. 

Additional information on CRLF occurrences from the CNDDB is used to cover the 

current range of the species not included in core areas and/or designated critical habitat, 

but within the recovery units. 

Other Known Occurrences from the CNDBB 

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

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

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

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

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

information on the CNDDB. 

27
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 

28
2.5.2 Reproduction 

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

marshes, and lagoons (Fellers 2005a). According to the Recovery Plan (U.S. FWS 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, 

U.S. FWS 2002); tadpoles have been observed to over-winter (delay metamorphosis until 

the following year) (Fellers 2005b; U.S. FWS 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 (U.S. FWS 2002). Figure 2-3 depicts CRLF annual reproductive 

timing. 

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

Light Blue = Breeding/Egg Masses

Green = Tadpoles (except those that over-winter)

Orange = Young Juveniles

Adults and juveniles can be present all year

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

Light Blue = Breeding/Egg Masses

Green = 

Orange = Young Juveniles

Adults and juveniles can be present all year

Tadpoles (except those that over-winter)

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 

(U.S. FWS 2002). Tadpoles filter and entrap suspended algae (Seale and Beckvar 1980) 

via mouthparts designed for effective grazing of periphyton (Wassersug 1984; 

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

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

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

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

29
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 (U.S. FWS 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 (U.S. FWS 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 (U.S. 

FWS 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://ecos.fws.gov/speciesProfile/SpeciesReport.do?spcode=D02D). 

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

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

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

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

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

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

to 77 days (U.S. FWS 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 

30
trees or logs, industrial debris, and agricultural features (U.S. FWS 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 U.S. FWS (U.S. FWS 2006; FR 51 19244-19346). A 

summary of the 34 critical habitat units relative to U.S. FWS-designated recovery units 

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

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

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

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

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

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

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

protection under Section 7 of the ESA (Section 7) through prohibition against destruction 

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

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

in the destruction or adverse modification of critical habitat. 

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

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

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

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

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

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

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

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

protected from disturbance or are representative of the historic geographical and 

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

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

• Breeding aquatic habitat; 

• Non-breeding aquatic habitat; 

• Upland habitat; and 

• Dispersal habitat. 

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

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

the habitat may require special management or protection. Therefore, U.S. FWS does not 

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

31
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 the Final Rule (FR) 

listing notice in April 2006 (71 FR 19243, 2006). The FR notice designating critical 

habitat for the CRLF includes a special rule exempting routine ranching activities 

associated with livestock ranching from incidental take prohibitions. The purpose of this 

exemption is to promote the conservation of rangelands, which could be beneficial to the 

CRLF, and to reduce the rate of conversion to other land uses that are incompatible with 

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

U.S. FWS has established adverse modification standards for designated critical habitat 

(U.S. FWS 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 diflubenzuron that may alter the PCEs of the 

CRLF’s critical habitat form the basis of the critical habitat impact analysis. According 

to U.S. FWS (2006), activities that may affect critical habitat and therefore result in 

adverse effects to the CRLF include, but are not limited to the following: 

(1) Significant alteration of water chemistry or temperature to levels beyond the 

tolerances of the CRLF that result in direct or cumulative adverse effects to 

individuals and their life-cycles. 

(2) Alteration of chemical characteristics necessary for normal growth and viability 

of juvenile and adult CRLFs. 

(3) Significant increase in sediment deposition within the stream channel or pond or 

disturbance of upland foraging and dispersal habitat that could result in 

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

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

affect their ability to complete their life cycles. 

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

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

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

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

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

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

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

segments or ponds used by the CRLF. 

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

evaluated as indirect effects to the CRLF). 

As previously noted in Section 2.1, the Agency believes that the analysis of direct and 

indirect effects to listed species provides the basis for an analysis of potential effects on 

the designated critical habitat. Because diflubenzuron is expected to directly impact 

living organisms within the action area, critical habitat analysis for diflubenzuron is 

limited in a practical sense to those PCEs of critical habitat that are biological or that can 

be reasonably linked to biologically mediated processes. 

32
2.7 Action Area 

For listed species assessment purposes, the action area is considered to be the area 

affected directly or indirectly by the federal action and not merely the immediate area 

involved in the action (50 CFR 402.02). It is recognized that the overall action area for 

the national registration of diflubenzuron is likely to encompass considerable portions of 

the United States based on the large array of agricultural uses. However, the scope of this 

assessment limits consideration of the overall action area to those portions that may be 

applicable to the protection of the CRLF and its designated critical habitat within the state 

of California. The Agency’s approach to defining the action area under the provisions of 

the Overview Document (U.S. EPA 2004) considers the results of the risk assessment 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

be the entire state of California. 

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

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

uses for diflubenzuron. 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 nonfood/non-agricultural uses. For those uses relevant to the CRLF, the analysis indicates 

that, for diflubenzuron, all uses are considered as part of the federal action evaluated in 

this assessment except soybeans and termite bait station uses. 

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

of diflubenzuron 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 

33
represent the labeled uses described above. Because diflubenzuron has a diverse set of 

uses that encompass all land cover classes used to define the initial “footprint”, the initial 

area of concern is considered to be statewide. 

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

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

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

listed species LOCs. 

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

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

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

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

exposure of one or more taxonomic groups to diflubenzuron exceeds the Agency’s LOCs. 

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

exposure level and the most sensitive effects endpoint. The most sensitive effects 

endpoint is the acute aquatic invertebrate endpoint. It is expected that invertebrates would 

be the most sensitive since this chemical is an insecticide. This endpoint and all other 

effects endpoints used in this assessment are discussed in Section 4. 

Because diflubenzuron has a relatively short period of persistence relative to seasonal 

variations in rainfall in California, the major diflubenzuron exposure route to the 

organisms of concern (aquatic invertebrates) varies throughout the year. During the dryseason (late spring, summer, and early fall) when little runoff occurs, most of the 

exposure will occur via spray drift. During the wet season (late fall, winter, and early 

spring) when much more runoff occurs, runoff will be a major contributor to aquatic 

EECs and risks. 

The initial footprint for diflubenzuron is the entire state of California given the large 

various types of uses (e.g., agriculture, residential, rights of ways, forestry). In addition, 

given the genotoxicity of a diflubenzuron degradates, defining a threshold below which 

effects can occur is not possible. Therefore, the action area is the entire state of 

California. 

2.8 Assessment Endpoints and Measures of Ecological Effect 

Assessment endpoints are defined as “explicit expressions of the actual environmental 

value that is to be protected.”3

 Selection of the assessment endpoints is based on valued 

entities (e.g., CRLF, organisms important in the life cycle of the CRLF, and the PCEs of 

its designated critical habitat), the ecosystems potentially at risk (e.g., waterbodies, 

riparian vegetation, and upland and dispersal habitats), the migration pathways of 

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

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

 

3

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

34
2.8.1 Assessment Endpoints for the CRLF 

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

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

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

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

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

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

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

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

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

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

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

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

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

to define the action area. According the Overview Document (U.S. EPA 2004), the 

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

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

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

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

A complete discussion of all the toxicity data available for this risk assessment, including 

resulting measures of ecological effect selected for each taxonomic group of concern, is 

included in Section 4.0 of this document. A summary of the assessment endpoints and 

measures of ecological effect selected to characterize potential assessed direct and 

indirect CRLF risks associated with exposure to diflubenzuron is provided in Table 2-4. 

35
Table 2-4. Assessment Endpoints and Measures of Ecological Effects 

Assessment Endpoint Measures of Ecological Effects4

Aquatic-Phase CRLF 

(Eggs, larvae, juveniles, and adults)a

Direct Effects 

1. Survival, growth, and reproduction of CRLF 

1a. Most sensitive fish acute LC50 (guideline study; 

test species: bluegill sunfish) 

1b. Most sensitive fish chronic NOAEC (guideline; 

test species: fathead minnow) 

 

Indirect Effects and Critical Habitat Effects 

2. Survival, growth, and reproduction of CRLF 

individuals via indirect effects on aquatic prey food 

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

2a. Most sensitive fish (guideline study; test 

species: bluegill sunfish), aquatic invertebrate 

(ECOTOX study; test species: mosquito larvae), and 

aquatic plant EC50 (guideline study; test species: 

Duck weed) or LC50 

2b. Most sensitive aquatic invertebrate and fish 

chronic NOAEC (ECOTOX study and guideline 

respectively) 

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) 

3b. Non-vascular plant acute EC50 (guideline study; 

test species: green algae) 

4. Survival, growth, and reproduction of CRLF 

individuals via effects to riparian vegetation 

Currently no acceptable, guideline terrestrial plant 

toxicity data are available. 

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 study; test 

species: Redwinged blackbird) 

5b. Most sensitive birdb or terrestrial-phase 

amphibian chronic NOAEC (guideline study; test 

species: Bobwhite quail) 

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 (guideline study; test species: 

Bobwhite quail) 

7. Survival, growth, and reproduction of CRLF 

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

riparian and upland vegetation) 

Currently no acceptable, guideline terrestrial plant 

toxicity data are available. 

a

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

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

are considerably different that exposure pathways on land. 

b

 Birds are used as surrogates for terrestrial phase amphibians. 

 

4

 Citations for all registrant-submitted and open literature toxicity data reviewed for this assessment are 

included in Appendix A. 

36
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 diflubenzuron 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 diflubenzuron effects data are available. Adverse 

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

in Section 2.6. 

Measures of such possible effects by labeled use of diflubenzuron on critical habitat of 

the CRLF are described in Table 2-5. 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 U.S. FWS (2006). 

37
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 study: 

test species: green algae) 

b. Currently no guideline terrestrial plant toxicity data 

is available. 

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 aquatic plant EC50 

b. Currently, no guideline terrestrial plant toxicity 

data is available. 

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 and 

aquatic invertebrates (guideline and ECOTOX studies 

respectively; respective test species: bluegill sunfish 

and mosquito larvae) 

b. Most sensitive NOAEC values for fish and aquatic 

invertebrates (guideline and ECOTOX respectively; 

respective test species: fathead minnow and water 

flea)

Reduction and/or modification of aquatic-based 

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

a. Most sensitive aquatic plant EC50 (guideline study; 

test species: green algae) 

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 Currently, no guideline terrestrial plant toxicity data 

are available. 

b. 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. 

38
2.9 Conceptual Model 

2.9.1 Risk Hypotheses 

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

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

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

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

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

assessment: 

The labeled use of diflubenzuron 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; or 

• 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 diflubenzuron release mechanisms, biological receptor types, and effects 

endpoints of potential concern. The conceptual models for all of the uses other than fly 

39
control at livestock operations for both terrestrial and aquatic exposures are shown in 

Figure 2-4 and Figure 2-5, respectively, which include the conceptual models for the 

aquatic and terrestrial PCE components of critical habitat. Exposure routes shown in 

dashed lines are not quantitatively considered because the contribution of those potential 

exposure routes to potential risks to the CRLF and modification to designated critical 

habitat is expected to be negligible. 

Diflubenzuron applied to use site 

Stressor 

Source 

Receptors 

Attribute 

Change 

Direct 

application Spray drift

Birds/terrestrialphase amphibians/ 

reptiles/mammals 

Terrestrial 

insects 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain

Reduction in prey 

Modification of PCEs 

related to prey availability 

Habitat integrity 

Reduction in primary productivity

Reduced cover 

Community change 

Modification of PCEs related to 

habitat 

Terrestrial/riparian plants

grasses/forbs, fruit, seeds 

(trees, shrubs) 

Runoff 

Mammals/

birds 

Exposure 

Media 

Soil

Ingestion 

Ingestion

Ingestion 

Ingestion

Dermal uptake/Ingestion

Long range 

atmospheric 

transport 

Root uptake 

Wet/dry deposition 

Terrestrial-phase 

amphibians 

Ingestion 

Figure 2-4 Conceptual Model for Pesticide Effects on Terrestrial Phase of the 

CRLF 

40
41

Diflubenzuron applied to use site Stressor

Source 

Receptors

Attribute 

Change 

Spray drift 

Fish/aquatic-phase 

amphibians 

Exposure 

Media 

Piscivorous mammals 

 and birds 

Individual 

organisms 

Reduced survival 

Reduced growth 

Reduced reproduction 

Food chain

Reduction in algae 

Reduction in prey 

Modification of PCEs 

 related to prey availability 

Habitat integrity 

Reduction in primary 

productivity

Reduced cover 

Community change 

Modification of PCEs related to 

 habitat 

Surface water/ 

Sediment 

Runoff

Aquatic Animals

Invertebrates 

Vertebrates 

Uptake/gills 

or integument 

Ingestion Ingestion

Uptake/gills 

or integument 

Aquatic Plants

Non-vascular 

Vascular

Uptake/cell, 

roots, leaves Riparian plants

terrestrial 

exposure 

pathways see 

Figure 2.4 

Long range 

atmospheric 

transport 

Wet/dry deposition 

Soil Groundwater

Figure 2-5. Conceptual Model for Pesticide Effects on Aquatic Phase of the CRLF. 

Because the assessment methods are distinctly different for fly control at livestock 

operations, a separate conceptual model diagram is included along with the detailed 

description of these methods for this use in Appendix F. Other uses (right-of-ways in 

Appendix J and residential uses in Appendix K) have separate appendices to describe 

there unique assessment methods, but conceptually, are similar enough to use Figure 2-4 

and 5 as the conceptual model diagram for these uses. 

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 diflubenzuron 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 diflubenzuron 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 diflubenzuron along with available monitoring data 

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

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

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

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

Long-range atmospheric transport of diflubenzuron is not considered due to this 

chemical’s physical properties related to volatilization. 

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

environmental concentrations (EECs) of diflubenzuron 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) and GENeric Expected Environmental Concentration (GENEEC). The 

model used to predict terrestrial EECs on food items is T-REX. 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 diflubenzuron that may occur in surface water 

bodies adjacent to application sites receiving diflubenzuron 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 

diflubenzuron. 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. 

The EFED GENEEC (GENeric Expected Environmental Concentration) model (version 

1.2) is used to estimate pesticide concentrations in a 1 hectare by 2 meter deep pond with 

no outlet draining an adjacent 10 hectare field. It provides an upper-bound screening 

concentration value for most types of surface water for up to 56 days after runoff. 

GENEEC is a single runoff event but accounts for spray drift from single or multiple 

applications. The pond receives a pesticide load from spray drift for each application plus 

loading from a single-runoff event, in this case, two days after the last application. The 

42
runoff event transports a maximum of 10% of the pesticide remaining in the top 2.5 cm 

of soil. This amount can be reduced through soil adsorption. The amount of pesticide 

remaining on the field in the top 2.5 cm of soil depends on the application rate, number of 

applications, interval between applications, incorporation depth, and degradation rate in 

the soil. 

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 diflubenzuron through contaminated food are estimated using the EECs for 

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

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

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

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

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

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

diflubenzuron 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. 

AgDRIFT was used to assess aquatic exposures of aquatic phase CRLF and its prey to 

diflubenzuron deposited in aquatic habitats by spray drift and to determine the maximum 

distance from a treated area that effects may occur. 

2.10.1.2 Measures of Effect 

Diflubenzuron is a chitinase inhibitor, impairing the ability of insects to synthesize a 

critical component of their exoskeleton (U.S. EPA, 1997). Chitin synthesis is particularly 

important in the early life stages of insects, as they molt and form a new exoskeleton in 

43
various growth stages. Thus, aquatic guideline tests, which typically run for 48 hours for 

the aquatic invertebrate (Daphnia magna), may not capture a molting stage, and are not 

an appropriate “most sensitive” acute endpoint for assessments. Endpoints derived from 

studies that test the toxic effects of diflubenzuron on the larval/molting stages of 

freshwater invertebrates and that test the chronic exposure of freshwater invertebrates to 

diflubenzuron more appropriately assess the toxicity of this type of chemical. 

Diflubenzuron is fairly persistent in both aquatic and terrestrial environments. For this 

reason, data that evaluated the toxic effects of diflubenzuron at life stages (larval/molting 

stages) most vulnerable to this chemical were used as both acute and chronic endpoints 

for aquatic invertebrate RQs. Likewise, guideline tests on honeybees frequently run for 

only 48 hours and are conducted on adult bees, and, therefore, may not reflect the toxicity 

of diflubenzuron. 

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

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

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

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

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

terrestrial plants, and wildlife. ECOTOX was created and is maintained by the U.S. EPA, 

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 diflubenzuron to birds is similar to or less than the toxicity to 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

44
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 

diflubenzuron, 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 diflubenzuron risks, the risk quotient (RQ) method is used to compare exposure and 

measured toxicity values. EECs are divided by acute and chronic toxicity values. The 

resulting RQs are then compared to the Agency’s levels of concern (LOCs) (U.S. EPA 

2004) (see Appendix C). 

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

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

estimated exposures directly to the CRLF of diflubenzuron 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 diflubenzuron resulting from a 

particular use are sufficient to exceed the listed species LOC, then the effects 

determination for that use is a “may affect.” If the RQ being considered also exceeds the 

non-listed species acute risk LOC, then the effects determination is a LAA. If the acute 

RQ is between the listed species LOC and the non-listed acute risk species LOC, then 

further lines of evidence (i.e., probability of individual effects, species sensitivity 

distributions) are considered in distinguishing between a determination of NLAA and a 

LAA. When considering indirect effects to the CRLF due to effects to algae as dietary 

items or plants as habitat, the non-listed species LOC for plants is used because the CRLF 

does not have an obligate relationship with any particular aquatic and/or terrestrial plant. 

If the RQ being considered for a particular use exceeds the non-listed species LOC for 

plants, the effects determination is “may affect”. Further information on LOCs is 

provided in Appendix C. 

45
2.10.1.4 Data Gaps 

No guideline terrestrial plant toxicity data have been submitted to the Agency. However, 

open literature data and other lines of evidence will be used to evaluate the potential for 

diflubenzuron to indirectly affect CRLFs via impacts to terrestrial plants. 

3.0 Exposure Assessment 

Diflubenzuron formulations include liquid, water dispersible granules, wettable powder, 

effervescent tablets (for ornamental pond treatment), and termite bait (in bait stations). 

Application equipment includes ground, aerial, and airblast applicators and directed 

sprayers. Risks from ground boom and aerial applications are expected to result in the 

highest off-target levels of diflubenzuron due to generally higher spray drift levels. 

Ground boom and aerial modes of application tend to use lower volumes of application 

applied in finer sprays than applications coincident with sprayers and spreaders and thus 

have a higher potential for off-target movement via spray drift. 

3.1. Label Application Rates and Intervals 

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

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

products. While technical products which contain diflubenzuron of high purity are not 

used directly in the environment, they are used to make formulated products, which can 

be applied in specific areas to control insects. The formulated product labels legally limit 

diflubenzuron’s potential use to only those sites that are specified on the labels. 

Currently registered agricultural and non-agricultural uses of diflubenzuron within 

California include: almond, Brazil nut, cashew, chestnut, chinquapin, filbert (hazelnut), 

hickory nut, macadamia nut (bushnut), pecan, tree nuts, walnut (English/black), citrus 

hybrids other than tangelo, grapefruit, orange, pummelo (shaddock), tangerines, broccoli 

raab, cabbage - Chinese, collards, kale, soybeans (unspecified), cotton (unspecified), 

Christmas tree plantations, forest trees (softwoods - conifers), apricot, nectarine, peach, 

pear, plum, prune, stone fruits, commercial/institutional/industrial premises/equipment 

(outdoor), nonagricultural outdoor buildings/structures, paved areas (private 

roads/sidewalks), refuse/solid waste sites (outdoor), wide area/general outdoor treatment 

(quarantine/eradication use), butternut, forest nursery plantings (for transplant purposes), 

ornamental and/or shade trees, ornamental woody shrubs and vines, shelterbelt plantings, 

turnip (greens), household/domestic dwellings outdoor premises, artichoke, mustard, 

peanuts (unspecified), pepper, agricultural fallow/idleland, golf course turf, 

nonagricultural uncultivated areas/soils, barley, canola\rape, oats, triticale, wheat, grass 

forage/fodder/hay, pastures, rangeland, agricultural rights-of-way/fencerows/hedgerows, 

nonagricultural rights-of-way/fencerows/hedgerows, agricultural/farm 

structures/buildings and equipment, barns/barnyards/auction barns, manure, brassica 

(head and stem) vegetables, ornamental sod farm (turf), recreational areas, rice, drainage 

systems, forest plantings (reforestation programs)(tree farms - tree plantations - etc.), 

forest trees (all or unspecified), forest trees (softwoods - conifers), intermittently flooded 

areas/water, lakes/ponds/reservoirs (with human or wildlife use), lakes/ponds/reservoirs 

46
(without human or wildlife use), ornamental herbaceous plants, ornamental 

ponds/aquaria, ornamental woody shrubs and vines, shelterbelt plantings, 

streams/rivers/channeled water, swimming pool water systems, wide area/general outdoor 

treatment (quarantine/eradication use), beech nut, pistachio. The uses being assessed are 

summarized in Table 3-1. 

47
Table 3-1 Diflubenzuron Uses, Scenarios, and Application Information for the CRLF risk 

assessment1

Scenario Uses Represented by Scenario 

Application 

Rate

Number of 

Applications

Application 

Interval 

Application 

Method 

1. Barnyard Spray on manure, Manure to field 8.508 NS (17) NS (21) Ground 

0.0408 3 7 Ground 2. Beech Nut Beech Nut 0.0408 3 7 Air 

3. Brassica Brassica (head and stem) vegetables 0.0313 1 NA Ground 

0.3125 3 21 Ground 4. Citrus Citrus hybrids, grapefruit, orange, 

pummelo (shaddock), and tangerines 0.125 3 7 Air 

5. Cole Crop Broccoli raab, cabbage - Chinese, 

collards, and kale 0.25 4 21 Ground 

0.3125 3 7 Ground 6. Cotton Cotton (unspecified) 0.3125 3 90 Air 

0.22048 NS (4) 14 Ground 7. Forest Christmas tree plantations and forest 

trees (softwoods - conifers) 0.25 1 NA Air 

0.3125 3 7 Ground 

8. Fruit 0.3125 3 7 Airblast Apricot, nectarine, peach, pear, plum, 

prune, and stone fruits 0.3125 3 7 Air 

0.3125 3 7 Ground 

0.3125 3 7 Airblast 9. General 

Nuts 

Almond, Brazil nut, cashew, chestnut, 

chinquapin, filbert (hazelnut), hickory 

nut, macadamia nut (bushnut), pecan, 

tree nuts, and walnut (English/black) 0.3125 3 90 Air 

0.25 4 21 Ground 10. Grains Barley, canola\rape, oats, triticale, and 

wheat 0.125 3 NS (21) Air 

0.25 1 NA Ground 

11. Nursery 

Forest nursery plantings (for 

transplant purposes), ornamental 

and/or shade trees, ornamental woody 

shrubs and vines, and shelterbelt 

plantings 

0.25 1 NA Air 

0.25 1 NA Ground 

12. Pasture 0.25 1 NA Airblast Grass forage/fodder/hay, pastures, and 

rangeland 0.25 1 NA Air 

13. Pistachio Pistachio 0.75 2 14 Ground 

14. 0.25 1 NA Ground 

Residential 

Household/domestic dwellings 

outdoor premises 0.25 6 10 Air 

15. Rice Rice 0.625 6 5 Air 

0.25 2 5 Ground 

0.25 2 5 Airblast 16. Rightsof-way 

Agricultural rights-ofway/fencerows/hedgerows, 

fencerows/hedgerows, and 

nonagricultural rights-ofway/fencerows/hedgerows 0.25 2 5 Air 

17. Row 0.3125 3 21 Ground 

Crop 

Artichoke, mustard, peanuts 

(unspecified), and pepper 0.3125 3 14 Air 

18. Urban

Commercial/institutional/industrial 

premises/equipment (outdoor), 

nonagricultural outdoor 

buildings/structures, paved areas 

(private roads/sidewalks), refuse/solid 

waste sites (outdoor), and wide 

area/general outdoor treatment 

(quarantine/eradication use) 

0.25 6 NS (10) Air 

0.25 4 15 Ground 19. Squash Butternut 0.25 4 15 Air 

48
Scenario Uses Represented by Scenario 

Application 

Rate

Number of 

Applications

Application Application 

Interval Method 

0.25 2 14 Ground 

0.25 2 14 Airblast 

20. Turf 

(High Appl. 

Rate) 

Agricultural fallow/idleland, golf 

course turf, and nonagricultural 

uncultivated areas/soils 0.25 2 5 Air 

0.0313 3 7 Ground 

0.0313 3 7 Airblast 

21. Turf 

(Low Appl. 

Rate) 

Ornamental sod farm (turf) and 

recreational areas 0.0313 3 7 Air 

22. Turnip Turnip (greens) 0.25 2 21 Ground 1

 Uses assessed based on memorandum from SRRD dated 2/24/09. 

NS = Not specified on the label. 

The large number of scenarios is mainly a reflection of the diverse uses of diflubenzuron. 

However, some uses were split from other similar uses because they had much higher or 

lower application rates. 

Some of the scenarios require additional explanation. The barnyard scenario is a direct 

spray of diflubenzuron on manure. This scenario, methods of analysis, and resulting 

EECs are discussed in detail in Appendix F. Similarly, the rationales for the residential 

and right-of-way scenarios are described in appendices J and K. 

In addition, diflubenzuron can be administered to cattle with a gun-like device that shoots 

a “bolus” impregnated with diflubenzuron down the throats of the cattle. This bolus 

slowly dissolves and releases diflubenzuron with the waste. One bolus treats an animal 

for as long as 21 weeks. Similarly, diflubenzuron can be administered through cattle and 

swine feed. 

EFED does not have data indicating the diflubenzuron concentration in the manure 

resulting from all three methods (direct spray on manure, bolus, or feed-through 

treatments). However, the desired result from all three methods is to produce manure that 

has a concentration of diflubenzuron that inhibits the growth of the same species of 

nuisance flies. Therefore, EFED assumes for purposes of this assessment that the 

exposure levels from all three methods of manure treatment would be similar and 

considers the barnyard EECs to apply to all three manure treatment methods. 

The urban scenario provides EECs for the quarantine/eradication use of diflubenzuron if 

used in urban areas. It assumes 100% impervious surfaces in the watershed treated. 

Another non-standard diflubenzuron use is ornamental pond care. Because this use is a 

direct application to water (ornamental ponds) and results in EECs that are roughly 

equivalent to the EECS produced by the tier 1 rice model for the rice uses of 

diflubenzuron, the ornamental pond care use is assessed using the rice model EECs. 

However, an important difference between the rice use and ornamental pond care is 

ornamental ponds are required to not be connected to natural drainage-ways, while rice 

paddy water is assumed to be discharged to waters off of the use site. 

As an example, the ornamental pond care EEC for label registration # 400-469 is 

calculated as: 

49
Lg galLgal

ggg

Volume

pecent Amount

EEC

Treated

Diflubenzuron Applied

μ μ 66

8.31000

101%25 6

= ×

×× =

× =

where: EEC is the ornamental pond concentration after treatment in μg/L; AmountApplied is 

the amount of diflubenzuron applied to each 1000 gallons of pond water treated 

(converted to μg); and volume treated is 1000 gallons (converted to liters). 

The last non-standard use is application of diflubenzuron to the material (often compost) 

in which mushrooms are grown. The compost is treated with diflubenzuron to keep 

nuisance flies from harming the mushroom crop. As mushrooms are grown, the ability of 

the compost material to grow mushrooms decreases. Therefore, the compost material that 

was treated with diflubenzuron must be periodically replaced and the old material 

discarded. Often this material is land applied on farm fields. 

Mushrooms can be grown outdoors or in mushroom houses. When grown outdoors, there 

is potential for exposure to CRLFs while the diflubenzuron-treated compost is in use for 

growing mushrooms as well as after it has been land applied. Alternatively when 

mushrooms are grown indoors, exposure to the diflubenzuron-treated compost would 

only occur after the compost is land applied on farm fields. 

The mushroom use of diflubenzuron is very similar to the barnyard or manure use 

described previously in terms of both exposure at the site of application and at the site of 

disposal through land application as well as the concentration in the compost and manure. 

For the barnyard or manure use (Appendix F), the resultant diflubenzuron concentration 

in the manure is approximately 10 to 60 ppm (Appendix F, Figure F3). For the mushroom 

use, the desired diflubenzuron concentration in the compost is approximately 30 to 50 

ppm. Therefore, mushroom uses will be assessed using the EECs generated for the 

barnyard use. 

An important assumption in the barnyard analysis (appendix F) is that degradation 

behavior and rate of diflubenzuron in manure is similar to the degradation behavior and 

rate of diflubenzuron in soil. This assumption is made because EFED does not have 

diflubenzuron fate data in manure. Similarly, the assumption is made that the behavior 

and rate of diflubenzuron degradation in mushroom compost is similar to the degradation 

behavior and rate of diflubenzuron in soil because EFED does not have diflubenzuron 

fate data in mushroom compost. 

50
3.2. Aquatic Exposure Assessment 

3.2.1. Modeling Approach 

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

that represent high exposure sites for diflubenzuron 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 diflubenzuron were 

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

application intervals, buffer widths and resulting spray drift values modeled from 

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

developed based on the use information provided by BEAD and a summary of individual 

applications by use from the CDPR PUR data. Because diflubenzuron persists in fields 

for only a short time after application (aerobic soil metabolism half-life of 2 to 14 days) 

and most runoff in the semi-arid portions of California occur mainly in the winter and 

early spring, application date will greatly affect the estimated exposure to diflubenzuron. 

As an example of identifying scenario application dates based on PUR data, Figure 3-1 

depicts variability in diflubenzuron application by date within a calendar year for ground 

citrus applications based on CDPR PUR data from 1990 through 2007. The curvy indigo 

(dark blue) line indicates the average pounds of diflubenzuron applied on each day. This 

average is calculated as a 15-day moving average of the average total pounds applied for 

each day of the year. The solid vertical line indicates the date on which this 15-day 

average is highest. The dashed vertical lines the first and last application dates which 

were calculated to bracket the highest daily average. Similar analyses for other uses are 

included in Appendix D, Figure D3. Because PUR data only identifies applications as 

ground or air, airblast application dates are based on the highest 15-day average of 

ground applications. 

51
0

1

2

3

4

5

6

J J MAM J J A S OND J

Calender Date

Average Diflubenzuron Applied per 

Day (lbs. ai/Day)

F

Figure 3-1. Identification of application dates based on analysis of the average 

pounds of diflubenzuron applied in California to citrus via ground application 

methods based on CDPR PUR data (1990-2007). 

This method of selecting application dates works well for those uses that are wellrepresented in the PUR data set. However many of the uses that can be legally applied in 

California (and therefore, must be assessed in this document), do not appear in the PUR 

data set. For those uses that do not occur in the PUR, the model was run for each 

potential date of application and the date that resulted in the highest exposure was used in 

order to ensure a conservative exposure estimate. This method is described at the end of 

Section 3.2.3. 

3.2.2. Model Inputs 

Diflubenzuron is an insecticide used on a wide variety of food and non-food uses. 

Diflubenzuron environmental fate data used for generating model parameters is listed in 

Table 2-2. The input parameters for PRZM and EXAMS are in Table 3-2. 

52
Table 3-2 Summary of PRZM/EZAMS Environmental Fate Data Used for Aquatic 

Exposure Inputs for Diflubenzuron Endangered Species Assessment for the CRLF 1

Fate Property Value (unit) MRID (or source) 

Molecular Weight 311 g/mol Product Chemistry 

Henry’s constant 1.87×10-09 atm-m3/mol Product Chemistry 

Vapor Pressure 9.00×10-10 torr Product Chemistry 

Solubility in Water 0.2 mg/L Product Chemistry 

Photolysis in Water 80 days 

40816301 

41087802 

Aerobic Soil Metabolism Half-lives 4.7 days 

00039473 

00039474 

41722801 

Hydrolysis 

Stable @ pH 5 

119 days @ pH 7 

32 days @ pH 9 

00143355 

40859801 

41087801 

40816301 

Aerobic Aquatic Metabolism (water 

column) 23.4 days 

Combined Data 

Mean = 11.70 days 

SD = 12.41 days, n=3 

t90, n-1 = 1.638 

tinput = (mean of t1⁄2) + 

(t90,n-1×SD)/n1⁄2

Anaerobic Aquatic Metabolism 

(benthic) 102 days 34 days×3 

Koc 3961 Mean from Table 2-1 Kocs 

Application rate and frequency (See Table 3-1) 

Label or Maximum from LUIS 

Report 

Application intervals (See Table 3-1) 

Label or Minimum from LUIS 

Report 

Chemical Application Method (CAM) 2 Input Parameter Guidance1

Application Efficiency 0.99 (Ground/Airblast) or .95 (Air) Input Parameter Guidance1 

Spray Drift Fraction2 0.0007 (Ground), .015 (Airblast), 

or .039 (Air) AgDRIFT Model 

1

 Inputs determined in accordance with EFED “Guidance for Chemistry and Management Practice Input 

Parameters for Use in Modeling the Environmental Fate and Transport of Pesticides” dated February 28, 

2002. 

2

 Spray drift fraction assumed a buffer of 150 feet. The fraction was calculated using AgDrift and default 

Tier 1 parameters. 

3.2.3. Results 

The aquatic EECs for the various scenarios and application methods are listed in Table3-

3 based on the first application date indicated in column three of this table. The highest 

EECs (113 μg/L) are for rice/ornamental pond (direct application to water). Because 

rice/ornamental pond EECs were calculated using the rice model which assumes no 

degradation, the highest peak, 1-in-10-year 21-day average, and 1-in-10-year 60-day 

average EECs are all the same. Excluding the rice/ornamental pond EECs: peak EECs 

varied by a factor of 5400 between uses from 0.01 to 54 μg/L; 1-in-10-year 21-day 

average EECs varied by a factor of 6900 between uses from 0.006 to 42 μg/L; and 1-in10-year 60-day average EECs varied by a factor of 7500 between uses from 0.004 to 27 

μg/L. 

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

Scenario 

(Ground, 

Airblast, or Air) 

Appl. 

Rate

1st – last 

Appl. Date1 Crops Represented 

Peak 

EEC 

21-day 

average 

EEC 

60-day 

average 

EEC 

1. Barnyard/ Spray on manure 54.20 42.00 27.30 

Mushroom G 8.508 Year-round17 appl. Treated manure applied to field 7.29 5.63 3.65 

2. Beech G 0.0408 NP (1/12) 0.11 0.06 0.03 

Nut A 0.0408 NP (1/12) Beech Nut 0.21 0.13 0.08 

3. Brassica G 0.0313 NP (12/30) Brassica (head and stem) vegetables 0.07 0.04 0.02 

G 0.3125 4/23 – 5/14 Citrus hybrids, grapefruit, orange, 0.24 0.10 0.05 

pummelo (shaddock), and tangerines 4. Citrus A 0.125 4/23 – 5/14 0.50 0.30 0.18 

Broccoli raab, cabbage - Chinese, 

collards, and kale 5. Cole Crop G 0.25 9/11 – 12/4 0.36 0.20 0.14 

G 0.3125 5/14 – 6/4 0.15 0.11 0.06 6. Cotton Cotton (unspecified) A 0.3125 3/14 – 12/9 0.82 0.47 0.30 

G 0.22048 6/14 – 7/5 0.58 0.31 0.19 7. Forest A 0.25 3/10 

Christmas tree plantations and forest 

trees (softwoods - conifers) 0.80 0.45 0.26 

G 0.3125 2/20 – 3/12 0.26 0.14 0.08 

8. Fruit AB 0.3125 2/20 – 3/12 0.66 0.43 0.26 

A 0.3125 3/24 – 4/14

Apricot, nectarine, peach, pear, 

plum, prune, and stone fruits 1.36 0.86 0.54 

G 0.3125 1/13 – 4/6 0.90 0.46 0.25 

AB 0.3125 1/13 – 4/6 1.26 0.70 0.42 9. General 

Nuts 

A 0.3125 1/26 – 3/29

Almond, Brazil nut, cashew, 

chestnut, chinquapin, filbert 

(hazelnut), hickory nut, macadamia 

nut (bushnut), pecan, tree nuts, and 

walnut (English/black) 0.85 0.49 0.28 

G 0.25 NP (1/19) 1.34 0.78 0.56 10. Grains A 0.125 NP (1/19) 

Barley, canola\rape, oats, triticale, 

and wheat 0.93 0.52 0.41 

G 0.25 7/4 0.20 0.11 0.08 

11. Nursery 

A 0.25 12/5 

Forest nursery plantings (for 

transplant purposes), ornamental 

and/or shade trees, ornamental 

woody shrubs and vines, and 

shelterbelt plantings 

0.65 0.38 0.30 

G 0.25 2/28 0.12 0.08 0.04 

12. Pasture AB 0.25 2/28 0.29 0.17 0.09 

A 0.25 6/6 

Grass forage/fodder/hay, pastures, 

and rangeland 0.55 0.25 0.13 

13. Pistachio G 0.75 3/30 Pistachio 0.15 0.08 0.06 

14. G 0.25 NP (1/10) 0.08 0.05 0.03 

Residential A 0.25 NP (1/10) 

Household/domestic dwellings 

outdoor premises 0.40 0.30 0.25 

15. Rice/ 

Ornamental 

Pond 

A 0.625 5/14 – 6/13 Rice/Ornamental Pond 112.92 112.92 112.92 

G 0.25 NP (1/12) 5.55 3.04 1.61 

AB 0.25 NP (1/12) 8.13 4.39 2.32 16. Rightsof-way 

A 0.25 NP (1/12) 

Agricultural rights-ofway/fencerows/hedgerows, 

fencerows/hedgerows, and 

nonagricultural rights-ofway/fencerows/hedgerows 5.59 3.07 1.63 

17. Row 

Crop 

G 0.3125 8/14 - 10/16 Artichoke, mustard, peanuts 

(unspecified), and pepper 

0.02 0.01 0.01 

54
Scenario 

(Ground, 

Airblast, or Air) 

Appl. 

Rate

1st – last 

Appl. Date1 Crops Represented 

Peak 

EEC 

21-day 

average 

EEC 

60-day 

average 

EEC 

A 0.3125 2/1 – 3/14 1.15 0.77 0.58 

18. Urban A 0.25 NP (1/10) 

Commercial/institutional/industrial 

premises/equipment (outdoor), 

nonagricultural outdoor 

buildings/structures, paved areas 

(private roads/sidewalks), 

refuse/solid waste sites (outdoor), 

and wide area/general outdoor 

treatment (quarantine/eradication 

use) 

34.13 21.98 15.28 

G 0.25 NP (1/10) 1.15 0.65 0.38 19. Squash A 0.25 NP (1/10) Butternut 1.87 1.18 0.79 

G 0.25 7/5 – 8/2 0.01 0.01 0.00 

AB 0.25 7/5 – 8/2 0.21 0.13 0.08 

20. Turf 

(High Appl. 

Rate) A 0.25 5/26 – 6/5 

Agricultural fallow/idleland, golf 

course turf, and nonagricultural 

uncultivated areas/soils 0.61 0.37 0.20 

G 0.0313 7/5 – 8/2 0.02 0.01 0.01 

AB 0.0313 7/5 – 8/2 0.07 0.04 0.02 

21. Turf 

(Low Appl. 

Rate) A 0.0313 5/26 – 6/5 

Ornamental sod farm (turf) and 

recreational areas 0.13 0.08 0.05 

22. Turnip G 0.25 NP (1/18) Turnip (greens) 0.72 0.35 0.19 1

NP = Not in CDPR Pesticide Use Reporting data set. Typically, the date of first application that yielded 

the highest peak concentration (indicated in parentheses) was used as the first application date in the 

scenario modeled (see text for date selection for residential and right-of-way uses). 

The EECs for several of the uses are based on application dates that were selected 

specifically because that application date would yield the highest EECs. These highest 

dates were used to produce a conservative estimate EECs because there was no PUR data 

available to better indicate when diflubenzuron would be applied for that use (Section 

3.2.1). Figure 3-2 depicts the variation in PRZM/EXAMS EECs based on the day of the 

year chosen as the first application date. As shown in Figure 3.2a, using the date that 

yields the highest EECs (typically in the winter) could be extremely conservative (high 

EECs) if the actual application of diflubenzuron would only occur in the summer. This is 

not always the case though as is shown in Figure 3.2b. Typically, ground applications 

(Figure 3.2a) will show large variations between winter and summer diflubenzuron EECs 

because ground application EECs are dominated by runoff contributions which vary with 

rainfall. Aerial applications (Figure 3.2b) result in a much larger spray drift contribution, 

which does not vary with application date and therefore, results in less variation in EECs 

between summer and winter. Appendix D, Figure D4, provides similar graphs for all of 

the diflubenzuron uses for which PUR data was not available. 

55
Ground Applications Aerial Applications 

Diflubenzuron EECs (μg/L) 

0

0.02

0.04

0.06

0.08

0.1

0.12

J FM AM J J A S O N D J

peak

21day

60day

Beech Nut

0

0.05

0.1

0.15

0.2

0.25

J FM AM J J A S O N D J

peak

21day

60day

Beech Nut

Calendar Date Calendar Date 

a b 

Figure 3-2. Variation in diflubenzuron EECs (peak, 21-day, and 60-day) as function of application 

date (first application date for scenarios with multiple applications). 

However, even for those uses for which the PUR data suggest a specific date is typically 

used for diflubenzuron applications (as in Figure 3-1), it is important to consider that 

diflubenzuron is applied over a range of dates. In Figure 3-3, the distribution of PUR data 

application dates is compared to the diflubenzuron EECs for each first application date. 

According to the PUR data, March 2nd is the most typical application date, while January 

25th generates the highest EECs. It is important to consider that diflubenzuron is applied 

on the date that yields the highest EECs, but because of the procedure used to select 

application dates, application dates surrounding March 2nd were used in this assessment. 

Appendix D, Figure D5, provides similar graphs for all of the diflubenzuron uses for 

which PUR data were available. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

J FM AM J J A S O N D J

Calendar Date

Diflubenzuron EECs (μg/L)

0

1

2

3

4

5

6

7

Pounds Applied (lbs. ai/day)

peak

21day

60day

PUR

Figure 3-3. Variation in diflubenzuron EECs (peak, 21-day, and 60-day) as function of application 

date (first application date for scenarios with multiple applications) compared to variation in the 

amount of diflubenzuron applied (lbs. ai/day) per day in California for fruit use. 

56
3.2.4. Existing Monitoring Data 

A critical step in the process of characterizing EECs is comparing the modeled estimates 

with available surface water monitoring data. EFED was unable to locate diflubenzuron 

monitoring for surface and groundwater and atmospheric data (Fellers et al 2004, LeNoir 

et al 1999, and McConnell et al 1998). The sources checked included the USGS National 

Water-Quality Assessment (NAWQA) Program and California Department of Pesticide 

Regulation (CDPR) data sets. 

3.3. Terrestrial Animal Exposure Assessment 

T-REX (Version 1.4.1) is used to calculate dietary and dose-based EECs of diflubenzuron 

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. 

Terrestrial EECs for foliar formulations of diflubenzuron were derived for the uses 

summarized in Table 3-4. Foliar dissipation half-lives were reported in Willis and 

McDowell (1987) for diflubenzuron of 12, 27, and 35 days. Therefore, a 35-day half-life 

was used in this assessment. Use specific input values, including number of applications, 

application rate and application interval are provided in Table 3-4. 

57
Table 3-4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for 

Diflubenzuron with T-REX 

Maximum Application Rate (lbs/A) 

Number of Applications 

(Re-treatment Interval in Days) 

Scenario Namea Aerial Ground Other Aerial Ground Other 

Barnyard/Mushroom 8.508 17 (21) 

Beech nut 0.0408 0.0408 2.3 (7) 2.3 (7) 

Brassica 0.0313 1 (NA) 

Citrus 0.125 0.3125 3 (7) 3 (21) 

Cole Crop 0.25 4 (21) 

Cotton 0.3125 0.3125 3 (90) 3 (7) 

Forest 0.25 0.22048 1 (NA) NS1 (14) 

Fruit 0.3125 0.3125 3 (7) 3 (7) 

General Nuts 0.3125 0.3125 3 (90) 3 (7) 

Grains 0.125 0.25 3 (NS2) 4 (21) 

Nursery 0.25 0.25 1 (NA) 1 (NA) 

Pasture 0.25 0.25 1 (NA) 1 (NA) 

Pistachio 0.75 1.333 (14) 

Residential 0.25 0.25 1 (NA) 6 (10) 

Rice/Ornamental Pond 0.0625 6 (5) 

Rights-of-way 0.25 0.25 1.5 (5) 1.5 (5) 

Row Crop 0.3125 0.3125 3 (14) 3 (21) 

Squash 0.25 0.25 NS3 (15) 4 (15) 

Turf (High App. Rate) 0.25 0.25 1.5 (5) 1.5 (14) 

Turf (Low App. Rate) 0.0313 0.0313 3 (7) 3 (7) 

Turnip 0.25 2 (21) 

NA = Not applicable (there is no re-treatment interval if only 1 application is allowed). 

NS = Not specified on any label. 

a

No terrestrial assessment was performed for bait, pond, and soybean exposure scenarios (see text). 1

The number of applications was assumed to be 4 as a conservative assumption because 4 applications were 

the most applications allowed on any non-residential label. 

2

A re-treatment interval of 21 days is assumed because 21 days is the smallest re-treatment interval allowed 

on the ground application to grains label. 

3

The number of applications was assumed to be 4 as a conservative assumption because 4 applications were 

the most applications allowed on any non-residential label. 

T-REX is also used to calculate EECs for terrestrial insects exposed to diflubenzuron. 

Dietary-based EECs calculated by T-REX for small and large insects (units of a.i./g) are 

used to bound an estimate of exposure to terrestrial insects. Available acute contact 

toxicity data for bees exposed to diflubenzuron (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 diflubenzuron through contaminated 

food are estimated using the EECs for the small bird (20 g) which consumes small 

insects. Dietary-based and dose-based exposures of potential prey are assessed using the 

small mammal (15 g) which consumes short grass. Upper-bound Kenega nomogram 

values reported by T-REX for these two organism types are used for derivation of EECs 

for the CRLF and its potential prey (Table 3-5). Dietary-based EECs for small and large 

58
insects reported by T-REX as well as the resulting adjusted EECs are available in Table 

3-6. 

Table 3-5. Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based Exposures of the 

CRLF and its Prey to Diflubenzuron. 

EECs for CRLF (small insect food EECs for Prey (short grass food item) 

item) (small mammals) 

Dose-based EEC Dose-based EEC 

Use 

Dietary-based 

EEC (ppm) (mg/kg-bw) 

Dietary-based 

EEC (ppm) (mg/kg-bw) 

Barnyard/Mushroom 3369 3840.66 5990 5690.5 

Beech nut 10.3 11.742 18.32 17.404 

Brassica 4.2 4.788 7.5 7.125 

Citrus 88 100.32 157 149.15 

Cole Crop 80 91.2 143 135.85 

Cotton 111 126.54 197 187.15 

Forest 34 38.76 60 57 

Fruit 111 126.54 197 187.15 

General Nuts 111 126.54 197 187.15 

Grains 80 91.2 143 135.85 

Nursery 34 38.76 60 57 

Pasture 34 38.76 60 57 

Pistachio 101 115.14 180 171 

Residential 131 149.34 232 220.4 

Rice/Ornamental Pond 40 45.6 71 67.45 

Rights-of-way 34 38.76 60 57 

Row Crop 88 100.32 157 149.15 

Squash 91 103.74 162 153.9 

Turf (High App. Rate) 34 38.76 60 57 

Turf (Low App. Rate) 11 12.54 20 19 

Turnip 56 63.84 100 95 

59
Table 3-6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to Terrestrial 

Invertebrate Prey Items 

Use Small Insect Large Insect 

Barnyard/Mushroom 3369 374 

Beech nut 10.3 1.1 

Brassica 4.2 0.47 

Citrus 88 9.8 

Cole Crop 80 8.9 

Cotton, fruit, nuts 111 12 

Forest 34 3.8 

Grains 80 8.9 

Nursery 34 3.8 

Pasture 34 3.8 

Pistachio 101 11 

Residential 131 15 

Rice/Ornamental Pond 40 4.4 

Rights-of-way 34 3.8 

Row Crop 88 9. 8 

Squash 91 10 

Turf (High App. Rate) 34 3.8 

Turf (Low App. Rate) 11 1.2 

Turnip 56 6.2 

3.5 Terrestrial Plant Exposure Assessment 

Exposures to terrestrial plants were not quantified because no guideline toxicity data are 

available for diflubenzuron. Therefore, RQs were not calculated for terrestrial plants. 

See Section 5.2 for discussion of potential risks to terrestrial plants in the context of this 

effects determination. 

4.0 Effects Assessment 

This assessment evaluates the potential for diflubenzuron 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 because no aquatic phase amphibian data have been submitted or were 

located in the open literature. Terrestrial-phase effects assessments are based on avian 

toxicity data because no open literature or registrant-submitted studies on terrestrial phase 

amphibians were located. Because the frog’s prey items and habitat requirements are 

dependent on the availability of freshwater fish and invertebrates, small mammals, 

terrestrial invertebrates, and aquatic and terrestrial plants, toxicity information for these 

taxa are also discussed. Acute (short-term) and chronic (long-term) toxicity information 

60
is characterized based on registrant-submitted studies and a comprehensive review of the 

open literature on diflubenzuron. 

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). A complete list of all available registrantsubmitted studies is in Appendix H. Open literature data presented in this assessment 

were obtained from the ECOTOX database searched on April 28, 2009. 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;5

(2) the toxic effects are on an aquatic or terrestrial plant or animal species; 

(3) there is a biological effect on live, whole organisms; 

(4) a concurrent environmental chemical concentration/dose or application 

rate is reported; and 

(5) there is an explicit duration of exposure. 

Data that pass the ECOTOX screen are evaluated along with the registrant-submitted 

data, and may be incorporated qualitatively or quantitatively into this endangered species 

assessment. In general, effects data in the open literature that are more conservative than 

the registrant-submitted data are considered. The degree to which open literature data are 

quantitatively or qualitatively characterized for the effects determination is dependent on 

whether the information is relevant to the assessment endpoints (i.e., maintenance of 

CRLF survival, reproduction, and growth) identified in Section 2.8. For example, 

endpoints such as behavior modifications are likely to be qualitatively evaluated, because 

quantitative relationships between modifications and reduction in species survival, 

reproduction, and/or growth are not available. Although the effects determination relies 

on endpoints that are relevant to the assessment endpoints of survival, growth, or 

reproduction, it is important to note that the full suite of sublethal endpoints potentially 

available in the effects literature (regardless of their significance to the assessment 

endpoints) are considered to define the action area for diflubenzuron. 

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 G1 to G4. Appendix G also 

includes a rationale for rejection of those studies that did not pass the ECOTOX screen 

 

5

 The studies that have information on mixtures are listed in the bibliography as rejected due to the 

presence of mixtures. These studies are evaluated by EFED when applicable to the assessment; however, 

the data is not used quantitatively in the assessment. 

61
and those that were not evaluated as part of this endangered species risk assessment. 

Rejection Codes are described in Appendix I. 

A detailed spreadsheet of the available ECOTOX open literature data, including the full 

suite of lethal and sublethal endpoints is presented in Appendix G1. Appendix E includes 

a summary of the human health effects data for diflubenzuron. 

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 diflubenzuron. A summary of the available aquatic 

and terrestrial ecotoxicity information, use of the probit dose response relationship, and 

the incident information for diflubenzuron are provided in Sections 4.3 through 4.4 

respectively. A detailed summary of the available registrant-submitted ecotoxicity 

information is presented in Appendix A, and open literature information is presented in 

Appendix G. 

4.1. Evaluation of Aquatic Ecotoxicity Studies 

Table 4-1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based 

on an evaluation of both the submitted studies and the open literature, as previously 

discussed. A brief summary of submitted and open literature data considered relevant to 

this ecological risk assessment for the CRLF is presented below. Additional information 

is provided in Appendix A and Appendix G.

62
Table 4-1 Freshwater Aquatic Toxicity Profile for Diflubenzuron 

Freshwater Aquatic Toxicity Profile for Diflubenzuron (TGAI) 

Assessment 

Endpoint 

Surrogate 

Species Toxicity Value Used Source Citation Study Classification

Acute Toxicity to 

Fish and Aquaticphase Amphibians 

Bluegill sunfish 

(Lepomis 

macrochirus) 

129 mg/L 

(CI: 116 to 142) 

Slope: 4.7 (3.5 to 5.9) 

MRID 

00056150 Supplemental Note 1

Chronic Toxicity 

Fish and Aquaticphase Amphibians Fathead minnow NOAEC = 0.100 mg/L a.i.

LOAEC = >0.100 mg/L a.i.

MRID 

00099755 Acceptable Note 2 

Acute Toxicity to 

Aquatic 

Invertebrates 

2nd instar Aedes 

albopictus

(mosquito larvae)

EC50 was 0.0028 μg/L 

Slope: Not available 

. Ho et al. (1987) 

(Open literature 

study:ECOTOX # 

16591) 

Quantitative Note 3

Chronic Toxicity to 

Aquatic 

Invertebrates 

Water flea

(Daphnia magna) NOAEC = 0.00025 μg/L Kashian et al 

(2002) (Open 

literature study; 

ECOTOX 93397) 

Quantitative Note 4

Acute Toxicity to 

non-vascular 

Aquatic Plants 

Green algae 

(Selanustrum 

capricornutum) 

EC50 = > 0.20 mg/L a.i. 

NOAEL = 0.20 mg/L a.i. 45252205 Supplemental Note 5

Acute Toxicity to 

vascular Aquatic 

Plants 

Duckweed EC50 = >0.190 mg/L a.i. 

NOAEC = 0.190 mg/L 

MRID 

42940103 Supplemental Note 6

Note 1: The available data evaluation record does not specify whether the LC50 was based on the mean measured or 

nominal concentration. It is important to note that given the persistence of the compound, nominal concentrations 

could potentially underestimate the toxicity. Note 2: Although the data evaluation record (DER) deems this study as acceptable, the DER does not specify whether 

the LC50 was based on the mean measured or nominal concentration. It is important to note that given the persistence 

of the compound, nominal concentrations could potentially underestimate the toxicity. Note 3: The study Authors do not specify whether the EC50 was based on the mean measured or nominal concentration. 

It is important to note that given the persistence of the compound, nominal concentrations could potentially 

underestimate the toxicity. 

Note 4: The NOAEC is based on the nominal concentration. It is important to note that given the persistence of the 

compound, nominal concentrations could potentially underestimate the toxicity. 

Note 5: The study was deemed supplemental because the test procedures deviated from guidelines in the 

following manner: The study was not continued after three days because of an unexpected high growth of 

the algae, which caused the pH to increase to a value of 9 after 3 days. Additionally the NOAEC is based on 

the nominal concentration. It is important to note that given the limited persistence of the compound, nominal 

concentrations could potentially underestimate the toxicity. 

Note 6: The study was deemed supplemental because 20% of the initial measured concentration. Based on the mean 

measured concentration of 190 μg/l, growth of L. gibba exposed to diflubenzuron was not reduced in comparison to the 

solvent control over the 14-day study period. 

Toxicity to fish and aquatic invertebrates is categorized using the system shown in Table 

4-2 (U.S. EPA 2004). Toxicity categories for aquatic plants have not been defined. 

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

LC50 (ppm) Toxicity Category 

< 0.1 Very highly toxic 

> 0.1 - 1 Highly toxic 

> 1 - 10 Moderately toxic 

63
LC50 (ppm) Toxicity Category 

> 10 - 100 Slightly toxic 

> 100 Practically nontoxic 

4.1.1. Toxicity to Freshwater Fish 

Given that no diflubenzuron 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 diflubenzuron to the CRLF from reduction in available food. As discussed in Section 

2.5.3, over 50% of the prey mass of the CRLF may consist of vertebrates such as mice, 

frogs, and fish (Hayes and Tennant 1985). 

A summary of acute and chronic freshwater fish data, including data from the open 

literature, is provided below in Sections 4.1.1.1 through 4.1.1.3. 

4.1.1.1. Freshwater Fish: Acute Exposure (Mortality) Studies 

There are numerous registrant submitted freshwater fish acute toxicity studies testing the 

technical grade active ingredient of diflubenzuron. These studies tested 7 different 

species of freshwater fish and demonstrate that the technical grade active ingredient of 

diflubenzuron is slightly to practically non-toxic to freshwater fish. The LC50 from these 

studies range from 129 ppm to >500 ppm. Further details about these studies are 

provided in Appendix A. The most sensitive, acceptable, freshwater fish LC50 of 129 

ppm (MRID 00056150) will be use to assess the acute risk of the technical grade active 

ingredient to the aquatic-phase CRLF and to freshwater fish prey of the CRLF. 

Appendix A provides further details regarding all the fish acute toxicity studies of the 

TGAI. 

There are 16 registrant submitted studies testing several formulated products of 

diflubenzuron. The toxicity results of these studies range from slightly to practically 

nontoxic (LC50= 57 ppm to >1000 ppm), which suggests that the toxicity of formulated 

products are not dramatically greater than the toxicity of the technical grade material. 

Further details about these studies are also provided in Appendix A. Potential risks from 

exposure to formulated products were not quantified as discussed in Section 2. No 

studies were located in the open literature that reported LC50s that were lower than those 

reported in the registrant-submitted studies (see Appendix G). 

Degradate toxicity data are summarized in Appendix A. These data suggest that PCA is 

more toxic than diflubenzuron to fish with LC50 values ranging from 2 mg/L to 23 mg/L. 

DFBA and PCPU appear to have similar toxicity relative to parent diflubenzuron. As 

discussed in Section 2, potential risks from degradates were not quantified, and were 

considered to have negligible impact on conclusions of this assessment. 

64
4.1.1.2. Freshwater Fish: Chronic Exposure (Early Life Stage and 

Reproduction) Studies 

There is one available registrant submitted freshwater fish chronic toxicity study 

(Appendix A). Based on the results of this study, there were no effects to survival, 

reproduction, or growth at concentrations up to 0.1 mg/L in a full life-cycle study (egg to 

egg exposure). The LOAEC was > 0.1 mg a.i./L and the NOAEC was 0.1 mg a.i./L 

(MRID 00099755). The NOAEC of 0.1 mg/L will be used to assess the chronic risk of 

diflubenzuron to the aquatic-phase CRLF and to freshwater fish prey of the CRLF since 

this endpoint is the most sensitive chronic toxic endpoint available for freshwater fish. 

However, because a LOAEC was not established (no effects occurred at any test 

concentration), use of 0.1 mg a.i./L as a chronic endpoint is likely conservative. 

4.1.1.3. Freshwater Fish: Sublethal Effects and Additional Open 

Literature Information 

The available freshwater fish toxicity data do not indicate that diflubenzuron caused 

sublethal effects to freshwater fish at levels below those used to calculate RQs in this 

assessment. 

Currently, there are no available guideline acute or chronic aquatic-phase amphibian 

toxicity studies testing diflubenzuron. However, one field study was located in ECOTOX 

in Pacific chorus frogs that did not observe any effects at 0.28 kg a.i./hectare (0.25 lbs 

a.i./Acre) (Ecotox No. 111232). No other test levels were evaluated, and a LOAEC was 

not established. Therefore, fish are used in this assessment as a surrogate for aquatic 

phase amphibians. 

4.1.2. Toxicity to Freshwater Invertebrates 

Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects 

of diflubenzuron to the CRLF via prey reduction. As discussed in Section 2.5.3, the main 

food source for juvenile aquatic- and terrestrial-phase CRLFs is thought to be aquatic 

invertebrates found along the shoreline and on the water surface, including aquatic 

sowbugs, larval alderflies, and water striders. 

A summary of acute and chronic freshwater invertebrate data, including data published in 

the open literature, is provided below in Sections 4.1.2.1 through 4.1.2.3. Diflubenzuron 

is a chitinase inhibitor, impairing the ability of insects to synthesize a critical component 

of their exoskeleton (U.S. EPA, 1997). Chitin synthesis is particularly important in the 

early life stages of insects, as they molt and form a new exoskeleton in various growth 

stages. Thus, aquatic guideline tests, which typically run for 48 hours for the aquatic 

invertebrate (Daphnia magna), may not capture a molting stage, and are not an 

appropriate “most sensitive” acute endpoint for assessments. Endpoints derived from 

studies that test the toxic effects of diflubenzuron on the larval/molting stages of 

freshwater invertebrates and that test the chronic exposure of freshwater invertebrates to 

diflubenzuron more appropriately assess the toxicity of this type of chemical. 

65
Diflubenzuron is fairly persistent in both aquatic and terrestrial environments. For this 

reason, data that evaluated the toxic effects of diflubenzuron at life stages (larval/molting 

stages) most vulnerable to this chemical were used as both acute and chronic endpoints 

for aquatic invertebrate RQs. Results from acute studies are also presented. Likewise, 

guideline tests on honeybees frequently run for only 48 hours and are conducted on adult 

bees, and, therefore, may not reflect the toxicity of diflubenzuron. 

4.1.2.1. Freshwater Invertebrates: Acute Exposure (Mortality) Studies 

from Registrant-Submitted Studies 

There are 8 registrant submitted freshwater invertebrate aquatic toxicity studies testing 

the technical grade diflubenzuron. Based on the registrant submitted freshwater 

invertebrate toxicity studies diflubenzuron is very highly toxic to slightly toxic to 

freshwater invertebrates (EC50 ranges from 0.0026 ppm to > 100 ppm). Further details 

about these studies are provided in Appendix A. 

There are also several diflubenzuron formulated product studies (Appendix A). The 

toxicity values of these studies range from 1300 μg/L to >1,000,000 μg/L. These data 

suggest that the toxicity of the formulated products is not greater than toxicity of the 

technical grade material. 

Degradate toxicity data are also summarized in Appendix A. The degradates were orders 

of magnitude less toxic to invertebrates than parent diflubenzuron. For example, LC50s in 

midge for PCA and DFBA are 43 mg/L and >100 mg/L, respectively, compared with an 

LC50 of 0.07 mg/L for parent chemical in midge (See Appendix A for additional details). 

4.1.2.2. Freshwater Invertebrates: Chronic Exposure (Reproduction) 

Studies from Registrant-Submitted Studies 

Registrant-submitted chronic studies in aquatic invertebrates are summarized in 

Appendix A. The most sensitive endpoint produced in these studies was a NOAEL of 

0.05 μg/L and a LOAEL of 0.075 μg/L. In registrant submitted studies, no sublethal 

effects were observed at levels lower than the NOAEC used to calculate RQs. There is 

uncertainty in the most sensitive NOAEC because diflubenzuron was not detected in 4 of 

the 7 test samples analyzed. Therefore, the mean measured values assumed a test 

concentration of the detection limit when measured values were below the detection 

limit. Appendix A contains more detail on registrant-submitted studies. Registrantsubmitted toxicity studies were not used to calculate RQs because an open literature 

study was located that produced a lower toxicity value. See Section 4.1.2.3. 

66
4.1.2.3. Freshwater Invertebrates: Sublethal Effects and Open 

Literature Data 

Several open literature studies have been identified that produced more sensitive 

endpoints than any of the registrant submitted studies. Ho et al. (1987) reported 30% and 

44% mortality in 2nd instar Aedes albopictus (mosquito larvae) at diflubenzuron 

concentrations of 0.00000025 mg/L (0.00025 μg/L) and 0.0000025 mg/L (0.0025 μg/L), 

respectively, in a 1 day study. Pairwise comparisons between control and treated groups 

were not made; however, no larval mortality occurred in controls. The EC50 was 

0.0000028 mg/L (0.0028 μg/L) in this study as determined by probit analysis. A 

concentration dependent reduction in normal pupation and adult emergence was observed 

in this study in 2nd, 3rd, and 4th instar animals, with younger animals being more sensitive. 

A number of sublethal effects were observed in this study including physical 

abnormalities and biochemical changes. However, these sublethal effects were only 

observed at levels that were also associated with increased mortality. The EC50 of 

0.0000028 mg/L will be used to calculate the acute risk of diflubenzuron to aquatic 

invertebrates because this value is based on a larval/molting stage of mosquito and 

because it is the most sensitive freshwater invertebrate acute toxicity value. 

. 

In addition, Kashian et al (2002) observed significant (p<0.05) reduction in survival at 

diflubenzuron concentrations of 0.01 μg/L (NOAEC = 0.00001 mg/L) in a 6-day semistatic study in daphnia magna (40 per test level, replicated test). Test concentrations 

were not measured; however, the study was replicated. Mortality rate was approximately 

30% at 0.01 μg/L (estimated from Figure 2). 

The toxicity of 0.00025 μg/L from Ho et al. (1987) will be used to calculate the chronic 

risk of diflubenzuron to aquatic invertebrates because it is the most sensitive toxicity 

value available for freshwater invertebrates impacting vulnerable life stage that this 

chemical is expected to impact. 

4.1.3. Toxicity to Aquatic Plants 

Aquatic plant toxicity studies were used as one of the measures of effect to evaluate 

whether diflubenzuron 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 diflubenzuron to affect 

aquatic plants. Laboratory and field studies were used to determine whether 

diflubenzuron 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 

4.1.3.2 

67
4.1.3.1. Aquatic Plants: Laboratory Data 

There are four available aquatic plant toxicity studies including three nonvascular aquatic 

plant studies and 1 vascular plant study. In all of these studies the EC50 values were 

greater than the highest diflubenzuron concentrations tested. The most sensitive 

endpoints are summarized in the following table and produced an EC50 and NOAEL of 

>0.20 ppm and 0.20, respectively, in algae and duckweed (MRID 45252205 and 

42940103). Since these are the most sensitive endpoints, these values will be used to 

assess the risk of diflubenzuron to aquatic plants. Appendix A provides further details 

regarding these studies. 

Freshwater Aquatic Toxicity Profile for Diflubenzuron (TGAI) 

Assessment 

Endpoint 

Surrogate 

Species Toxicity Value Used Source Citation Study Classification

Acute Toxicity to 

non-vascular 

Aquatic Plants 

Green algae 

(Selanustrum 

capricornutum) 

EC50 = > 0.20 mg/L a.i. 

NOAEL = 0.20 mg/L a.i. 45252205 Supplemental Note 1

Acute Toxicity to 

vascular Aquatic 

Plants 

Duckweed EC50 = >0.190 mg/L a.i. 

NOAEC = 0.190 mg/L 42940103 Supplemental Note 2

Note 1: The study was deemed supplemental because the test procedures deviated from guidelines in the 

following manner: The study was not continued after three days because of an unexpected high growth of 

the algae, which caused the pH to increase to a value of 9 after 3 days. Additionally the NOAEC is based on 

the nominal concentration. It is important to note that given the limited persistence of the compound, nominal 

concentrations could potentially underestimate the toxicity. 

Note 2: The study was deemed supplemental because 20% of the initial measured concentration. Based on the mean 

measured concentration of 190 ug/l, growth of L. gibba exposed to diflubenzuron was not reduced in comparison to the 

solvent control over the 14-day study period. 

4.1.4. Freshwater Field Studies 

The USEPA Mid-Continent Ecology Division, Duluth, MN conducted a study of the 

biological effects, persistence, and distribution of diflubenzuron in littoral enclosures 

(MRID 44386201). The Duluth study investigated the distribution of diflubenzuron 

(water, sediments and biota) and effects (plants, invertebrates, and fish) of diflubenzuron 

applied to oligotrophic littoral enclosures (2 applications 30 days apart). The following 

table (4-3) describes the observed effects in this study. 

68
Table 4-3. Diflubenzuron Effects Reported in a Littoral Enclosure Study (MRID 443862-01). 

Endpoint LOEC NOEC Comments 

Aquatic plant Effects 

phytoplankton 

(chlorophyll a: 

numbers, and 

biovolume) 

Not Applicable 

(highest dose tested 

produced no statistical 

effects) 

30 μg/L The littoral enclosures were 

determined by the study authors to be 

nutrient limited with respect to 

phytoplanktonic growth. Therefore, the 

experiment was suitable for measuring 

direct effects on phytoplankton by 

diflubenzuron only. It could not 

determine any indirect effects related 

to adverse impacts to herbivorous 

zooplankton, macroinvertebrates, 

insects, or fish. 

periphyton 

(reduced biomass 

and chlorophyll a) 

7 μg/L 2.5 μg/L -- 

macrophytes 

(standing crop of 

submerged and 

floating plants, 

species 

composition) 

Not Applicable 

(highest dose tested 

produced no statistical 

effects) 

30 μg/L -- 

decompos~t~ornat e 

of plant l~tter 

Not Applicable 

(highest dose tested 

produced no statistical 

effects) 

30 μg/L -- 

Zooplankton Effects 

Cladocera (total 

numbers) 

0.7 μg/L 0.6 μg/L Though the effects observed at 0.7 

μg/L were not statistically significant, 

they were very severe, with a reduction 

in total cladoceran population of 

>89%. 

0 6 μg/L is the study authors' 

extrapolated MATC value 

Copepoda (adults, 

juveniles, and total 

number) 

0.7 μg/L 0.3 μg/L At the lowest does tested, reductions in 

the numbers of adult, juvenile, and 

total copepod numbers reached 71%, 

83%, and 81%, respectively. 

0 3 μg/L is the study authors' 

extrapolated MATC value 

Rotifera (total number) Not Applicable 

(highest dose tested 

produced no statistical 

effects) 

-- 

Ostracoda (total number) 7 μg/L 2.5 μg/L The control variability was very large, 

thereby severely limiting the sensitivity 

of the test. Reductions in ostracods at 

the 7.0 and 30 μg/L doses were 59 to 

98% lower than controls 

69
Endpoint LOEC NOEC Comments 

Macroinvertebrate Effects 

Chironomidae 

abundance) 

7 μg/L 2.5 μg/L The high CV among treatment blocks 

severely limits the sensitivity of this 

endpoint. 

The LOEC for this endpoint equates to 

an approximate 50% reduction In 

Abundance. 

Epherneroptera 

(abundance)

2.5 μg/L 0.7 μg/L The high CV among treatment blocks 

severely limits the sensitivity of this 

endpoint. 

The LOEC for this endpoint equates to 

an approximate 50% reduction in 

Abundance 

total adjusted 

macroinvertebrate 

community biomass 

2.5 μg/L 0.7 μg/L -- 

Mean macroinvertebrate 

taxa richness 

0.7 μg/L Not Applicable

(lowest dose 

tested produced 

adverse effects)

-- 

Insect Emergence Effects 

Insect emergence (all 

insects) 

0.7 μg/L Not Applicable

(lowest dose 

tested produced 

adverse effects)

The power of the test was severely 

limited by the number of replicates 

within treatments. Power analysis 

conducted by the study authors 

concluded that mean differences 

between treatments and control for 

insect emergence would have to be 

greater than 93% before the test would 

show statistically significant results. 

Emergence reductions of 17.7 to 29.9% 

were observed at the lowest 

concentration tested. 

Chironomidae 

(emergence) 

0.7 μg/L Not Applicable

(lowest dose 

tested produced 

adverse effects)

The power of the test was severely 

limited by the number of replicates 

within treatments. Power analysis 

conducted by the study authors 

concluded that mean differences 

between treatments and control for 

insect emergence would have to be 

greater than 93% before the test would 

show statistically significant 

Emergence reductions of 17. 4 to 

31.1% were observed at the lowest 

concentration tested 

70
Endpoint LOEC NOEC Comments 

Fish 

Bluegill sunfish 

young or the year 

(weight, length, 

growth rate) 

2.5 0.7 μg/L The authors concluded that young of 

the year bluegill growth rates were 

directly correlated to the density of 

several invertebrate taxa (notably 

Cladocera and Copepoda) that are 

preferred prey items for the species 

The authors concluded that 

the study clearly demonstrated the 

indirect effect of the pesticide on fish 

population without direct toxic effects 

on the fish 

Indigenous fish (mean 

size, population 

numbers, biomass) 

Not Applicable 

(highest dose tested 

produced no statistical 

effects) 

30 μg/L There is considerable uncertainty with 

respect to endpoints for indigenous 

fish. No measurements of effects 

variables were made immediately 

following treatments. Instead, 

measurements were made 70 days after 

application of the first treatment of 

pesticide and 37 days after the second 

application. 

The results indicate that reductions in the abundance of zooplankton (particularly 

cladocerans and copepods) can be expected to occur at initial nominal concentrations 

lower than 0.7 μg/L. Macroinvertebrate biomass and abundance of selected insect 

families are reduced at initial nominal diflubenzuron concentrations greater than 0.7 

μg/L. Aquatic insect emergence and the richness of macroinvertebrate taxa are reduced at 

concentrations of diflubenzuron at or below an initial nominal concentration 0.7 μg/L 

diflubenzuron. These impacts to the aquatic community were also observed to be 

correlated to reductions in the growth rate of introduced bluegill sunfish, with reductions 

in length, weight, and growth rate occurring at initial nominal concentrations of 2.5 μg/L 

and above. It is important to note that MRID 44386201 fails to empirically demonstrate a 

NOEC for a number of endpoints, namely effects on cladocerans, copepods, aquatic 

insect emergence and richness of macroinvertebrate taxa. Therefore, the study does not 

allow EFED to evaluate risks to these organisms for diflubenzuron concentrations below 

0.7 μg/L with any certainty. 

Results of several field studies and mesocosm studies were presented in MRID 

44460702. The results of these other studies do not materially differ from results 

reported for the Duluth Study discussed above. Ali et al.(1 988) reported no observable 

effects on benthos and zooplankton in a pond adjacent to citrus treated with 

diflubenzuron. Concentrations were reported to range from a high of 0.197 pg/L 

immediately after diflubenzuron application to the orange groves to <0.027 pg/L by 14 

days after pesticide application. Booth and Ferrell (1977) reported significant (up to 97%) 

reductions in aquatic invertebrates for up to 30 days in a pond with diflubenzuron at 36 

μg/L immediately after treatment to trace (4 μg/L) after 23 days. Boyle et al. (1996) 

exposed mesocosms to 5 monthly or 9 biweekly treatments of diflubenzuron at a targeted 

rate of10 pg/L. The authors observed significant reductions in zooplankton and insect 

numbers, insect diversity, and growth of bass and bluegill recruit. 

71
4.2. Toxicity of diflubenzuron to Terrestrial Organisms 

Table 4-4 summarizes the most sensitive terrestrial toxicity endpoints used to evaluate 

toxicity of diflubenzuron to terrestrial phase CRLFs, 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. Additional information can be found in Appendix A. 

Table 4-4 Terrestrial Toxicity Profile for Diflubenzuron 

Assessment 

Endpoint 

Acute/ 

Chronic Species 

Toxicity Value 

Used in Risk 

Assessment MRID Classification

Acute oral 

LD50

Red-winged blackbird 

LD50= 3763 mg/kg 

a.i. bwt MRID 

00038614 Supplemental 

Acute Toxicity to 

Birds Acute 

dietary 

LC50

Bobwhite quail LC50 >4640 mg/kg 

a.i. bwt 

MRID 

00039080 Acceptable 

Reproduction 

Toxicity to Birds 

NOAEC: Bobwhite quail 

NOAEC= 500 

mg/kg a.i. diet 

LOAEC= 1000 

mg/kg a.i. diet 

MRID 

41668001 Supplemental 

Acute Toxicity to 

Mammals LD50: Rat LD50>5,000 mg/kg a.i. bwt 

MRID 

00157103 Acceptable 

Reproduction 

Toxicity to 

Mammals 

NOAEC: Rat NOAEC = 250 

mg/kg-bw 

MRID 

43578301 Acceptable 

Acute toxicity to 

Terrestrial 

Invertebrates 

LD50: Honeybee LD50 > 30 μg 

a.i./bee a.i. 

MRID 

05001991 Acceptable 

Risk to Terrestrial 

Plants 

No data 

submitted N/A N/A N/A N/A 

Acute toxicity to terrestrial animals is categorized using the classification system shown 

in Table 4-5 (U.S. EPA 2004). Toxicity categories for terrestrial plants have not been 

defined. 

Table 4-5 Categories of Acute Toxicity for Avian and Mammalian Studies 

Toxicity Category Oral LD50 Dietary LC50

Very highly toxic < 10 mg/kg < 50 ppm 

Highly toxic 10 – 50 mg/kg 50 - 500 ppm 

Moderately toxic 51 - 500 mg/kg 501 - 1000 ppm 

Slightly toxic 501 - 2000 mg/kg 1001 - 5000 ppm 

Practically non-toxic > 2000 mg/kg > 5000 ppm 

4.2.1. Toxicity to Birds 

As specified in the Overview Document, the Agency uses birds as a surrogate for 

terrestrial-phase amphibians when amphibian toxicity data are not available (U.S. EPA 

72
2004). No terrestrial-phase amphibian data have been submitted or were located in the 

open literature for diflubenzuron; therefore, acute and chronic avian toxicity data are used 

to assess the potential direct effects of diflubenzuron to terrestrial-phase CRLFs. 

4.2.1.1. Birds: Acute Exposure (Mortality) Studies 

Diflubenzuron is categorized as practically non-toxic to avian species on an acute oral 

toxicity basis. Studies in bobwhite quail and mallard duck did not produce toxic effects 

at any dose tested (MRID 00073935). The most sensitive study available produced an 

LD50 value of 3763 mg/kg-bw in the Red-Winged Blackbird (MRID 00038614). All three 

studies used technical grade material (Appendix A). 

Using the TGAI, diflubnezuron is categorized as practically non-toxic to avian species on 

a subacute dietary toxicity basis based on an LC50 value of >4640 ppm for the bobwhite 

quail and mallard duck (MRID 00039080). A 1 % Granular formulation was also 

categorized as practically non-toxic to bobwhite quail and mallard ducks based on an 

LC50 value of >20,000 ppm (MRID 00060381). These studies were classified as 

"Acceptable". Appendix A provides further details regarding these studies. 

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

The most sensitive NOAEC from a reliable/verifiable study was 500 mg/kg-diet (MRID 

4166800102). Other studies are available that produced lower NOAECs. However, 

these studies either did not produce a LOAEC (no effects were observed at any test 

concentration) or the NOAECs were considered unreliable. In particular, a NOAEC of 

<10 mg/kg-diet has been reported in a non-guideline study in mallard ducks (MRID 

99862, Reinert ett al, 1975) based on a reduction in the number of eggs embryonated at 

10 mg/kg-diet. However, this effect was not observed at higher test concentrations in the 

same study and has not been observed in more recent guideline studies. Therefore, a 

NOAEC of <10 mg/kg-diet was not considered reliable and was not chosen for use in risk 

estimation. 

The NOAEC of 500 mg/kg-food was based on effects on eggshell thickness in mallard 

ducks and reduced egg production in bobwhite quail at 1000 mg/kg-diet. Further details 

about the avian reproduction studies are summarized in Appendix A. 

4.2.2. Terrestrial-phase Amphibian Acute and Chronic Studies 

No terrestrial-phase amphibian acute or chronic studies have been submitted by 

registrants or were located in the open literature. 

4.2.3. Toxicity to Mammals 

73
Mammalian toxicity data are used to assess potential indirect effects of diflubenzuron to 

the terrestrial-phase CRLF. Effects to small mammals resulting from exposure to 

diflubenzuron may 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). 

4.2.3.1. Mammals: Acute Exposure (Mortality) Studies 

The available mammalian acute toxicity data demonstrate that diflubenzuron is 

practically nontoxic to mammals (LD50 > 5000 mg/kg; MRID 00157103). Appendix A 

provides further details regarding this data. 

4.2.3.2. Mammals: Chronic Exposure (Growth, Reproduction) Studies 

In a 2-generation reproduction study technical grade diflubenzuron was administered in 

the diet to rats at dose levels of 0 (control), 500, 5000 or 50000 ppm (equivalent to about 

0, 25, 250 or 2500 mg/kg/day). 

No effects on reproductive performance were observed at any dose level in F0 or F1 

males or females. Litter and mean pup weights decreased slightly from birth to 21 days 

postpartum in F1 offspring at 2500 mg/kg/day. The NOEL for reproductive performance 

in parental adults is 2500 mg/kg/day. The NOEL for developmental toxicity in progeny is 

250 mg/kg/day and the LEL is 2500 mg/kg/day, based on decreased body weights in F1 

pups from birth to 21 days postpartum. (MRID 43578301). A NOAEL of 250 mg/kg-bw 

is used in this assessment. 

4.2.4. Toxicity to Terrestrial Invertebrates 

Terrestrial invertebrate toxicity data are used to assess potential indirect effects of 

diflubenzuron to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting 

from exposure to diflubenzuron may indirectly affect the CRLF via reduction in available 

food. 

4.2.4.1. Terrestrial Invertebrates: Acute Exposure (Mortality) Studies 

Guideline studies in honeybees have been submitted and accepted by the Agency. These 

tests produced acute oral and contact LD50s of >30 μg/bee (MRID 05001991), resulting 

in a practically non-toxic classification. However, given the short duration of these tests 

(typically 48-72 hours), and the typical age of organisms used in the tests, it is unlikely 

that the contact / ingestion coincided with a molting stage, and, thus, the results may not 

accurately reflect the toxicity of diflubenzuron to terrestrial invertebrates. 

74
A number of field studies regarding effects on terrestrial invertebrates were submitted to 

the Agency and reviewed in 1984 to 1985 (MRIDs 00099743, 000171816, 00071207, 

00095416, 00071215, 00070179, 00071212, 00095407, and 00071210). The following 

information is based on the reviews of these studies, which evaluated a number of 

different organisms and exposure routes. 

Three honey bee studies resulted in no effects on colonies near fields treated with 

application rates of up to 0.5 oz Dimilin W-25/A or 0.06 lbs a.i./A (MRIDs 0071816, 

00099743, 00071212). Another study that exposed honey bees to 100 ppm ai in their 

water source determined that the exposure "almost eliminated production of a sealed 

brood" (MRID 0095407). A study on a stream receiving runoff from a treated field 

indicated that amphipod and aquatic beetle larvae populations were reduced and 

copepods and ostracods may also have been impacted" at an application rate of 0.08 lb 

ai/A (MRID 00071210). A study evaluating the effects of diflubenzuron (2.24 kg ai/ha) 

on soil arthropods when the chemical was incorporated into the soil showed some 

decrease in numbers of springtails at 2 months post-treatment and on soil mites at 6 

weeks, but little effect on other soil organisms (MRID 0007121 5). Differences between 

control and treatment plots were not distinguishable at 4 months post-treatment. The 

Agency reviewer concluded diflubenzuron had "little adverse effect on the soil fauna." A 

study on beneficial arthropods showed some species were severely affected, while other 

species were not affected at all (MRID 00070179). Aerial application of granular Dimilin 

to salt marsh mosquito habitat significantly reduced numbers and diversity of aquatic 

invertebrates (MRID 00095416). Measured water concentrations of Dimilin 

(diflubenzuron) ranged from 0.24-1.8 μg/L. Based on a study on soil arthropods, Dimilin 

appears to have "little or no effect on soil microarthropods, but may adversely affect 

insect parasites" (MRID 007 1207). 

Overall, studies on terrestrial invertebrates illustrate that the effect of diflubenzuron is 

highly dependent on the life stage of the organism when it is exposed. Diflubenzuron is 

very highly toxic to invertebrates that rely on chitin as an exoskeleton at the critical life 

stage (i.e., a molting event). Available data are not sufficiently robust to estimate an 

assessment endpoint (e.g, LC50 or NOAEC) for this taxon, but adverse effects on nontarget arthropods should be anticipated following use of diflubenzuron given its mode of 

action as an insecticide. 

4.2.4.2. Terrestrial Invertebrates: Open Literature Studies 

A number of open literature studies that evaluated effects to terrestrial invertebrates were 

identified in the ECOTOX database (Appendix G). These studies support the 

conclusions presented in Section 4.2.4.1 and do not add additional insight into the 

potential effects of diflubenzuron to terrestrial invertebrates (Ecotox Nos. 65290, 110971, 

and 111061). 

75
4.2.5. Toxicity to Terrestrial Plants 

Terrestrial plant toxicity data are used to evaluate the potential for diflubenzuron to affect 

riparian zone and upland vegetation within the action area for the CRLF. Impacts to 

riparian and upland (i.e., grassland, woodland) vegetation may result in indirect effects to 

both aquatic- and terrestrial-phase CRLFs, as well as modification to designated critical 

habitat PCEs via increased sedimentation, alteration in water quality, and reduction in 

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 are typically reviewed for this type of 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 diflubenzuron, 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. 

Currently, there are no available guideline terrestrial plant toxicity data for use in this 

assessment. However, Cooper et al. (1990) did not observe differences in characteristics 

of vegetation when comparing plots treated with diflubenzuron to untreated controls. 

The study evaluated 10 circular 0.04 ha plots (5 treated and 5 untreated). Vegetative 

characteristics evaluated included percent litter cover, litter depth, percent herbaceous 

cover, percent shrub cover, percent canopy cover in each of 5 canopy height classes, 

percent deciduous canopy cover, percent coniferous canopy cover, percent total canopy 

cover, and number of trees and number of snags in 6 dbh classes. Diflubenzuron was 

applied at 0.06 lbs a.i./Acre. 

Similarly, Seidel and Whitmore did not observe differences in vegetative characteristics 

including litter depth, number of saplings, number of 2 classes of tree snags, and presence 

of herbaceous growth, shrubs. Diflubenzuron was applied to test plots at 140 g/ha by air 

(0.12 lbs a.i./Acre). 

In addition, a number of efficacy studies have been submitted to the Agency that also 

evaluated phytotoxic effects to target species. These studies have typically noted no or 

minimal phytotoxicity (See Appendix H for citations). Last, diflubenzuron is labeled for 

76
use on numerous commodoties without reports of incidents that have been clearly 

associated with difblubenzuron toxicity. 

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 diflubenzuron on par with the acute toxicity endpoint selected for RQ 

calculation. To accomplish this interpretation, the Agency uses the slope of the dose 

response relationship available from the toxicity study used to establish the acute toxicity 

measures of effect for each taxonomic group that is relevant to this assessment. The 

individual effects probability associated with the acute RQ is based on the mean estimate 

of the slope and an assumption of a probit dose response relationship. In addition to a 

single effects probability estimate based on the mean, upper and lower estimates of the 

effects probability are also provided to account for variance in the slope, if available. 

Individual effect probabilities are calculated based on an Excel spreadsheet tool IECV1.1 

(Individual Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP, 

Environmental Fate and Effects Division (June 22, 2004). The model allows for such 

calculations by entering the mean slope estimate (and the 95% confidence bounds of that 

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

as the desired threshold. 

Probit slopes used in this analysis are summarized in Table 4-6. 

Table 4-6. Probit slopes used for taxonomic groups evaluated in this assessment 

Taxonomic Group Probit Slope 95% CI MRID Species 

Fish 4.7 3.5 to 5.9 00056150 Bluegill 

Aquatic 

Invertebrates 

0.41 0.3 to 0.5 Ho et al., 1987 

(Ecotox No. 16591) 

Mosquito larvae 

Birds Default of 4.5: Data 

do not allow for 

calculation of 

reliable slope 

2 to 9 00038614 Red-winged 

blackbird 

Mammals Default of 4.5: No 

mortality occurred 

in acute mammal 

studies 

2 to 9 00157103 Rat 

77
4.4. Incident Database Review 

One incident (I009846-002) had been reported for diflubenzuron when the EIIS database 

was queried (10/2/2009). Damage to oranges in Polk County, Florida was reported 

following direct application. Legality of the application was classified as registered use, 

and certainty that diflubenzuron caused the damage was rated as possible. However, the 

incident was also associated with petroleum distillates, which have a number of other 

terrestrial plant incidents associates with its use. Given that diflubenzuron has a long 

history of use on citrus trees without other incidents and that petroleum distillates has 

been associated with several similar incidents on terrestrial plants, it is more likely that 

the damage to oranges was caused by use of petroleum distillates and not by 

diflubenzuron. 

A lack of reported incidents should not be taken as an indicator of little or no ecological 

effects, especially in the case of a chemical like diflubenzuron, which acts primarily to 

inhibit growth of insects. Insects and other invertebrates that are vulnerable to chitinase 

inhibitors are generally not a highly visible component of the ecosystem, and even largescale die-offs may go unreported. 

78
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 

diflubenzuron 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 

concern (LOCs) for each category evaluated (Appendix C). For acute exposures to the 

CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC 

is 0.05. For acute exposures to the CRLF and mammals, the LOC is 0.1. The LOC for 

chronic exposures to CRLF and its prey, as well as 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 diflubenzuron usage 

scenarios summarized in Table 3-3 and the appropriate aquatic toxicity endpoint from 

Section 4. 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 diflubenzuron (Section 3) and the appropriate toxicity endpoint from 

Section 4. 

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 EECs in the standard pond and the 

lowest acute toxicity value for freshwater fish. In order to assess direct chronic risks to 

the CRLF, 60-day EECs and the lowest chronic toxicity value for freshwater fish are 

used. Based on the RQ calculations none of the LOCs were exceeded for risk to 

freshwater water fish or aquatic phase amphibians except for the aerial use of 

diflubenzuron on rice crops (See Table 5-1). The LOC was exceeded for chronic risk to 

freshwater fish or aquatic phase amphibians for this use. Based on these results only the 

diflubenzuron aerial use on rice “may affect” aquatic-phase CRLFs. No LOCs were 

exceeded for other uses. Therefore, all other registered uses are expected to have “no 

effect” on the aquatic-phase CRLF. RQs presented in Table 5-1 are further discussed in 

the context of an effects determination in Section 5.2. 

79
Table 5-1 Summary of Direct Effect RQ LOCs for the Aquatic-phase CRLF 

Direct 

Effects to 

CRLFa

Use Surrogate 

Species 

Toxicity 

Value 

(mg/L) 

EEC 

(mg/L)b 

RQ Probability 

of Individual 

Effect at 

ES LOC 

Probability of 

Individual 

Effect at RQ 

Direct 

Application to 

rice

60-day: 

0.112 

1.12c

Manure 0.027 0.27 

Chronic 

Direct 

Toxicity 

All other uses 

Fathead 

minnow 

NOAEC = 

0.100 

<0.015 <0.15 

Not calculated for chronic 

endpoints

Acute direct 

toxicity 

All uses All 

species 

tested 

129 <0.11 <0.01 1 in 3.7E20 

(1 in 7.8E11 to 1 in 5E30) 

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

 The highest EEC based on aerial use of diflubenzuron on rice paddies (see Table 3-3). c

 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 diflubenzuron to the aquatic-phase CRLF (tadpoles) via reduction in 

non-vascular aquatic plants in its diet or via habitat modification are based on peak EECs 

and the lowest toxicity value (EC50) for aquatic non-vascular plants. Based on the RQ 

calculations there were no LOC exceedances of diflubenzuron to aquatic plants. 

Therefore, diflubenzuron is expected to have “no effect” to CRLFs by affecting nonvascular aquatic plants. These RQs are further discussed in the context of an effects 

determination in Section 5.2. 

Table 5-2 Summary of RQs Used to Estimate Indirect Effects to the CRLF via Effects to NonVascular Aquatic Plants (diet of CRLF in tadpole life stage and habitat of aquatic-phase CRLF) 

Uses 

Toxicity Value 

(μg/L) Peak EEC (μg/L)

Indirect effects RQ 

(food and habitat) 

All Uses 200 

(MRID 45252205) <113 <0.6 

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. Due to the mode of action of diflubenzuron, 

results from chronic toxicity studies are also considered to be acute effects. Therefore, 

80
chronic RQs are calculated using peak EECs. However, chronic RQs are also presented 

using 21-day EECs. 

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 RQ calculations 

diflubenzuron exceeds either the acute or chronic LOC for risk to freshwater 

invertebrates for all the modeled uses of diflubenzuron. The acute RQs ranged from 4 to 

40,000 and the chronic LOC exceedances range from 40 to 450,000. Based on these 

results, diflubenzuron “may affect” CRLFs via reduction in freshwater invertebrates prey 

items. These RQs are further discussed in the context of an effects determination in 

Section 5.2. 

81
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) 

Scenario Group 

Application 

Type 

Water 

Column 

Peak EEC 

(μg/L) 

Acute RQ 

Based on a 6-

day LC50 of 

0.0028 μg/L) 

Chronic RQ 

Based on a 6-Day 

NOAEC of 

0.00025 μg/L and 

a 21-Day EEC 

Chronic RQ Based 

on a 6-Day NOAEC 

of 0.00025 μg/L and 

a peak EEC 

Ground 0.11 39.07 227.21 437.56* Beech Nut Air 0.21 73.94 521.28 828.16* 

Brassica Ground 0.07 23.49 156.96 263.12* 

Ground 0.24 87.48 393.05 979.76* Citrus Air 0.50 178.06 1204.76 1994.32* 

Cole Crop Ground 0.36 128.46 817.76 1438.72* 

Ground 0.15 54.34 429.96 608.60* Cotton Air 0.82 293.71 1874.28 3289.56* 

Ground 0.58 205.99 1221.32 2307.08* Forest Air 0.80 285.44 1786.60 3196.96* 

Ground 0.26 94.15 556.48 1054.48* 

Fruit Airblast 0.66 236.88 1702.68 2653.00* 

Air 1.36 485.86 3424.92 5441.60* 

Ground 0.90 322.81 1859.68 3615.44* 

General Nuts Airblast 1.26 450.56 2794.92 5046.24* 

Air 0.85 303.31 1965.88 3397.04* 

Ground 1.34 477.32 3117.32 5346.00* Grains Air 0.93 333.08 2084.80 3730.48* 

Ground 0.20 71.60 449.60 801.96* Nursery Air 0.65 231.78 1504.92 2595.88* 

Ground 0.12 41.64 302.73 466.40* 

Pasture Airblast 0.29 102.95 665.00 1153.04* 

Air 0.55 195.64 1003.56 2191.20* 

Pistachio Ground 0.15 52.45 331.80 587.40* 

Ground 0.08 29.37 184.23 328.96* Residential Air 0.40 143.71 1207.47 1609.50* 

Rice/Ornamental 

Pond Air 112.92 40327.13 451663.90 451663.90* 

Ground 5.55 1981.66 12178.53 22194.60* 

Rights-of-way Airblast 8.13 2903.57 17541.57 32520.00* 

Air 5.59 1997.41 12268.73 22371.00* 

Ground 0.02 6.86 59.72 76.82* Row Crop Air 1.15 409.57 3098.08 4587.20* 

Urban Air 34 12187.86 87924.00 136504.00* 

Ground 1.15 409.11 2610.32 4582.00* Squash Air 1.87 666.75 4736.00 7467.60* 

Ground 0.01 3.60 24.39 40.26* 

Airblast 0.21 76.57 506.32 857.60* 

Turf (High 

Application 

Rate) Air 0.61 216.57 1489.40 2425.56* 

Ground 0.02 7.67 32.30 85.88* 

Airblast 0.07 23.89 146.85 267.52* 

Turf (Low 

Application 

Rate) Air 0.13 46.49 326.01 520.68* 

Turnip Ground 0.72 256.26 1402.12 2870.12* 

* Exceeds the chronic LOC of 1 for risk to aquatic invertebrates

82
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 the RQ calculations none of the LOCs were exceeded for 

risk to freshwater fish or aquatic phase amphibians except for the aerial use of 

diflubenzuron on rice (See Table 5-1). The LOC was exceeded for chronic risk to 

freshwater fish for this use. These RQs are further discussed in the context of an effects 

determination in Section 5.2. 

5.1.1.3. Indirect Effects to CRLF via Reduction in Habitat 

and/or Primary Productivity (Freshwater Aquatic 

Plants) 

Indirect effects to the CRLF via direct toxicity to aquatic plants are estimated using the 

most sensitive non-vascular and vascular plant toxicity endpoints. Because there are no 

obligate relationships between the CRLF and any aquatic plant species, the most sensitive 

EC50 values, rather than NOAEC values, were used to derive RQs. Based on the RQ 

calculations there were no LOC exceedances for risk of diflubenzuron to aquatic plants. 

Therefore, the effects determination is “no effect” for all uses. These RQs are further 

discussed in the context of an effects determination in Section 5.2. 

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

Aquatic Plants (habitat of aquatic-phase CRLF)a

 

Uses 

Peak EEC 

(μg/L) 

EC50 

(μg/L) 

Indirect effects RQ 

(habitat) 

All Uses <113 190 <0.6 a

 RQs used to estimate indirect effects to the CRLF via toxicity to non-vascular aquatic plants are 

summarized in Table 5-2 

5.1.2. Exposures in the Terrestrial Habitat 

5.1.2.1. Direct Effects to Terrestrial-phase CRLF 

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 5-5) and acute oral and subacute dietary toxicity endpoints for 

avian species. 

Potential direct chronic effects of diflubenzuron to the terrestrial-phase CRLF are derived 

by considering dietary-based exposures modeled in T-REX for a small bird (20g) 

consuming small invertebrates. Chronic effects are estimated using the lowest available 

toxicity data for birds. EECs are divided by toxicity values to estimate chronic dietarybased RQs. Subacute dietary based RQs were not calculated because the dietary toxicity 

data indicated that no toxicological effects were observed at dietary concentrations that 

were equivalent to the limit concentration for acute dietary toxicity. 

83
Based on the acute and chronic RQ calculations there are only LOC exceedances for risks 

to the terrestrial-phase of the CRLF for use of diflubenzuron on barnyard manure. Thus, 

the preliminary determination is a “may effect” only for the use on barnyard manure and 

“no effect” for all other modeled uses. Table 5-5 lists only dose based acute RQs because 

no dietary LC50 study produced a definitive toxicity value (all LC50s were above the limit 

dose). 

Table 5-5 Summary of RQs Used to Estimate Direct Effects to the Terrestrial-phase CRLF 

Chronic RQ Acute RQ 

Model Use EEC RQ 

Dose-based EEC 

(based on 20-

gram bird) RQ 

Barnyard (Manure) 3369 6.738* 3840.66 1.2** 

Beech nut 10.3 0.0206 11.742 <0.01 

Brassica 4.2 0.0084 4.788 0.00 

Citrus 88 0.176 100.32 0.03 

Cole Crop 80 0.16 91.2 0.03 

Cotton 111 0.222 126.54 0.04 

Forest 34 0.068 38.76 0.01 

Fruit 111 0.222 126.54 0.04 

General Nuts 111 0.222 126.54 0.04 

Grains 80 0.16 91.2 0.03 

Nursery 34 0.068 38.76 0.01 

Pasture 34 0.068 38.76 0.01 

Pistachio 101 0.202 115.14 0.04 

Residential 131 0.262 149.34 0.05 

Rice 40 0.08 45.6 0.01 

Rights-of-way 34 0.068 38.76 0.01 

Row Crop 88 0.176 100.32 0.03 

Squash 91 0.182 103.74 0.03 

Turf (High App. Rate) 34 0.068 38.76 0.01 

Turf (Low App. Rate) 11 0.022 12.54 <0.01 

Turnip 56 0.112 63.84 0.02 

* Exceeds the chronic of LOC of 1. 

** Exceeds the acute LOC of 0.5. 

5.1.2.2. Indirect Effects to Terrestrial-Phase CRLF via 

Reduction in Prey (terrestrial invertebrates, mammals, 

and frogs) 

a) Terrestrial Invertebrates 

In order to assess the potential risks of diflubenzuron to terrestrial invertebrates, which 

are considered prey of CRLF in terrestrial habitats, the honey bee is used as a surrogate 

for terrestrial invertebrates. The toxicity value for terrestrial invertebrates is calculated by 

multiplying the lowest available acute contact LD50 of 30 μg a.i./bee by 1 bee/0.128g, 

which is based on the weight of an adult honey bee. EECs (μg a.i./g of bee) calculated by 

T-REX for small and large insects are divided by the calculated toxicity value for 

84
terrestrial invertebrates, which is 30 μg a.i./g of bee. Based on the RQ calculations, the 

modeled diflubenzuron uses except beechnut and brassica exceed the LOC for risks to 

terrestrial invertebrate (Table 5-6). Based on these results, the preliminary effects 

determination is a “may effect” for the potential of diflubenzuron to indirectly affect the 

CRLF by reducing terrestrial invertebrate prey. Section 5.2 further discusses these RQs 

and additional terrestrial invertebrate toxicity information in the context of this effects 

determination. 

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

Direct Effects on Terrestrial Invertebrates as Dietary Food Items based on an LC50 of 234 ppm (30 

μg per bee / 0.128 grams per bee) 

Uses 

Small Insects 

EEC Small Insect RQ

Large Insects 

EEC Large Insect RQ

Barnyard/Mushroom 3369 14.37 374 1.60 

Beechnut 10.3 0.04 1.14 0.00 

Brassica 4.2 0.02 0.47 0.00 

Citrus 88 0.38 9.78 0.04 

Cole 80 0.34 8.89 0.04 

Cotton 111 0.47 12.33 0.05 

Forest 34 0.15 3.78 0.02 

Fruit 111 0.47 12.33 0.05 

General Nut Crop 111 0.47 12.33 0.05 

Grain 80 0.34 8.89 0.04 

Impervious 34 0.15 3.78 0.02 

Nursery 34 0.15 3.78 0.02 

Pistachio 101 0.43 11.22 0.05 

Residential 131 0.56 14.56 0.06 

Rice/Ornamental Pond 40 0.17 4.44 0.02 

Rights away 34 0.15 3.78 0.02 

Row crops 88 0.38 9.78 0.04 

Squash 91 0.39 10.11 0.04 

Turf (high application rate) 34 0.15 3.78 0.02 

Turf (low application rate) 11 0.05 1.22 0.01 

Turnip 56 0.24 6.22 0.03 

b) Mammals 

Risks associated with reduction of small mammals as prey by large terrestrial-phase 

CRLFs are derived for dietary-based and dose-based exposures modeled in T-REX for a 

small mammal (15g) herbivore and insectivore. 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. 

There were no mortalities in the mammalian acute toxicity studies at the limit dose of 

5000 mg/kg-bw; therefore, acute RQs were not calculated and risk was presumed to be 

below levels that could result in indirect effects to CRLFs. Based on the chronic RQ 

calculations, there are no chronic LOC exceedances for risk to small mammal prey items 

except for the barnyard/mushroom use (Table 5-7). Based on these results, the 

preliminary effects determination is “no effect” regarding potential risks to the CRLF 

85
from reduction in mammalian prey except for the barnyard/mushroom use. The chronic 

RQ is 10 for the barnyard/mushroom use. Therefore, the preliminary effects 

determination for this use is “may effect.” These RQs are further discussed in the context 

of an effects determination in Section 5.2. 

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

via Direct Effects on Small Mammals as Dietary Food Items 

Chronic RQ 

Model Use EEC RQ 

Barnyard/Mushroom 5690.5 10.37 (exceeds LOC of 1.0) 

Beech nut 17.404 0.03 

Brassica 7.125 0.01 

Citrus 149.15 0.27 

Cole Crop 135.85 0.25 

Cotton 187.15 0.34 

Forest 57 0.10 

Fruit 187.15 0.34 

General Nuts 187.15 0.34 

Grains 135.85 0.25 

Nursery 57 0.10 

Pasture 57 0.10 

Pistachio 171 0.31 

Residential 220.4 0.40 

Rice/Ornamental Pond 67.45 0.12 

Rights-of-way 57 0.10 

Row Crop 149.15 0.27 

Squash 153.9 0.28 

Turf (High App. Rate) 57 0.10 

Turf (Low App. Rate) 19 0.03 

Turnip 95 0.17 

c) Amphibians

An additional prey item of the adult terrestrial-phase CRLF is other amphibians. In order 

to assess risks to these organisms, dietary-based and dose-based exposures modeled in TREX for a small bird (20g) consuming small invertebrates are used. See Section 5.1.2.1 

and associated table (Table 5-5) for results. Based on LOC exceedances for the 

barnyard/mushroom use, this labeled use of diflubenzuron may affect the CRLF. Avian 

LOCs were not exceeded for any other use; therefore, the preliminary effects 

determination for diflubenzuron for all other uses is “no effect.” These RQs are further 

discussed in the context of an effects determination in Section 5.2. 

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. Currently there are no available guideline 

86
terrestrial plant toxicity data. Therefore, RQs were not calculated. Additional lines of 

evidence including open literature studies and non-guideline registrant-submitted studies 

regarding potential risks to terrestrial plants in the context of an effects determination for 

the CRLF are described in Section 5.2. 

5.1.3. Primary Constituent Elements of Designated Critical Habitat 

For diflubenzuron use, the assessment endpoints for designated critical habitat PCEs 

involve a reduction and/or modification of food sources necessary for normal growth and 

viability of aquatic-phase CRLFs, and/or a reduction and/or modification of food sources 

for terrestrial-phase juveniles and adults. Because these endpoints are also being 

assessed relative to the potential for indirect effects to aquatic- and terrestrial-phase 

CRLF, the effects determinations for indirect effects from the potential loss of food items 

are used as the basis of the effects determination for potential modification to designated 

critical habitat. As discussed in Sections 5.1.3 and 5.2.4, effects to aquatic and terrestrial 

plants are not expected to occur. 

However, one PCE is “alteration of other chemical characteristics necessary for normal 

growth and viability of CRLFs and their food source.” To assess the impact of 

diflubenzuron on this PCE (i.e., alteration of food sources), 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 LOC exceedances for aquatic invertebrates, diflubenzuron 

may modify aquatic-phase 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. 

Two terrestrial phase PCEs are not evaluated using solely terrestrial plant toxicity data. 

The PCE of “reduction and/or modification of food sources for terrestrial phase juveniles 

and adults” is evaluated using acute and chronic toxicity endpoints for birds, mammals, 

and terrestrial invertebrates as measures of effects. RQs for these endpoints were 

calculated in Section 5.1.2.2. Diflubenzuron has the potential for modifying critical 

habitat via this PCE because as an insecticide, terrestrial invertebrates are likely to be 

affected if diflubenzuron is used near critical habitat. 

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. Based on potential impacts to terrestrial invertebrates for all assessed uses, 

diflubenzuron may impact critical habitat by altering chemical characteristics necessary 

for normal growth and viability of juvenile and adult CRLFs for all uses assessed. 

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 

87
to adversely affect,” or “likely to adversely affect”) for the CRLF and its designated 

critical habitat. 

Based on the RQs presented in the Risk Estimation (Section 5.1) a preliminary effects 

determination is “may affect” for the CRLF and modification of critical habitat. 

A summary of the results of the risk estimation results are 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.

Table 5-8 Risk Estimation Summary for Diflubenzuron - 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 

Direct application to 

rice only 

The chronic fish RQ was 1.12 for direct 

application to rice. All other chronic RQs were 

lower than the LOC of 1.0. Acute RQs were 

<0.01. The probability of an individual mortality 

at the acute RQs of 0.01 is <1 in 1,000,000 based 

on a slope of 4.7 (95% CI of 3.5 to 5.9 (MRID 

56150). 

Indirect Effects 

Survival, growth, and 

reproduction of CRLF 

individuals via effects to food 

supply (i.e., freshwater 

invertebrates, non-vascular 

plants) 

Yes 

All uses 

Acute RQs ranged from approximately 4 to 

40,000. The probability of an individual effect at 

these RQs approaches 100% for reasonable lower 

and upper bound slopes of 2 to 9. Chronic RQs 

also exceeded LOCs and were 25 to 450,000. 

Indirect Effects 

Survival, growth, and 

reproduction of CRLF 

individuals via effects on 

habitat, cover, and/or primary 

productivity (i.e., aquatic plant 

community) 

No No LOCs were exceeded for vascular or nonvascular aquatic plants. RQs were less than 0.6. 

Indirect Effects 

Survival, growth, and 

reproduction of CRLF 

individuals via effects to 

riparian vegetation, required to 

maintain acceptable water 

quality and habitat in ponds 

and streams comprising the 

species’ current range. 

No 

RQs were not calculated due to the lack of 

guideline terrestrial plant toxicity data; further 

discussion of potential risks to CRLFs resulting 

from impacts to terrestrial plants is presented in 

Section 5.2. 

Terrestrial Phase 

(Juveniles and adults)

88
LOC Exceedances 

Assessment Endpoint (Y/N) Description of Results of Risk Estimation 

Direct Effects 

Survival, growth, and 

reproduction of CRLF 

individuals via direct effects on 

terrestrial phase adults and 

juveniles 

Yes 

Barnyard/mushroom 

use only 

Acute and chronic RQs exceeded LOCs. The 

acute RQ was 1.2 and the chronic RQ was 6.7. At 

an RQ of 1.2, the probability of an individual 

effect is 1 in 1.6 based on the default probit slope 

of 4.5. The range of probabilities of an individual 

effect is 1 in 1.4 to 1 in 1.8 based on lower and 

upper bound probit slopes of 2 to 8. 

LOCs were not exceeded for any other use. 

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 

All uses 

Diflubenzuron is an insecticide. Therefore, there 

is potential for terrestrial invertebrate abundance 

to be impacted. In addition, LOCs were exceeded 

for all uses based on submitted data in the honey 

bee. 

Indirect Effects 

Survival, growth, and 

reproduction of CRLF 

individuals via effects on 

habitat (i.e., riparian 

vegetation) 

No 

RQs were not calculated for terrestrial plants 

based on the lack of terrestrial plants toxicity data 

available for diflubenzuron. 

Table 5-9 Risk Estimation Summary for Diflubenzuron – PCEs of Designated Critical Habitat for 

the CRLF 

Assessment Endpoint 

Habitat 

Modification (Y/N) Description of Results of Risk Estimation 

Aquatic Phase PCEs 

(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)

Alteration of channel/pond 

morphology or geometry and/or 

increase in sediment deposition within 

the stream channel or pond: aquatic 

habitat (including riparian vegetation) 

provides for shelter, foraging, predator 

avoidance, and aquatic dispersal for 

juvenile and adult CRLFs. 

No 

There are no LOC exceedances for risk to 

aquatic plants. 

Alteration in water chemistry/quality 

including temperature, turbidity, and 

oxygen content necessary for normal 

growth and viability of juvenile and 

adult CRLFs and their food source. 

No -- 

Alteration of other chemical 

characteristics necessary for normal 

growth and viability of CRLFs and 

their food source. 

Yes 

There are acute and chronic LOC exceedances 

for risk of diflubenzuron to aquatic 

invertebrate prey of the CRLF for all uses. 

Reduction and/or modification of 

aquatic-based food sources for premetamorphs (e.g., algae) 

No There were no LOC exceedances for risk to 

Algae for any use evaluated. 

89
Habitat 

Assessment Endpoint Modification (Y/N) Description of Results of Risk Estimation 

Terrestrial Phase PCEs 

(Upland Habitat and Dispersal Habitat)

Elimination and/or disturbance of 

upland habitat; ability of habitat to 

support food source of CRLFs: 

Upland areas within 200 ft of the edge 

of the riparian vegetation or dripline 

surrounding aquatic and riparian 

habitat that are comprised of 

grasslands, woodlands, and/or 

wetland/riparian plant species that 

provides the CRLF shelter, forage, and 

predator avoidance 

No 

Currently, there are no guideline terrestrial 

plant toxicity data available to assess this 

risk. See Section 5.2 for additional 

description of potential risks to CRLFs 

resulting from impacts to terrestrial 

vegetation. 

Elimination and/or disturbance of 

dispersal habitat: Upland or riparian 

dispersal habitat within designated 

units and between occupied locations 

within 0.7 mi of each other that allow 

for movement between sites including 

both natural and altered sites which do 

not contain barriers to dispersal 

No 

Currently, there are no guideline terrestrial 

plant toxicity data available to assess this 

risk. See Section 5.2 for additional 

description of potential risks to CRLFs 

resulting from impacts to terrestrial 

vegetation. 

Reduction and/or modification of food 

sources for terrestrial phase juveniles 

and adults 

Yes 

There are LOC exceedances for risk terrestrial 

invertebrate prey for all uses, and potential 

magnitude of effect could be substantial such 

that CRLFs could be impacted. 

Alteration of chemical characteristics 

necessary for normal growth and 

viability of juvenile and adult CRLFs 

and their food source. 

Yes 

There are LOC exceedances for risk terrestrial 

invertebrate prey, and potential magnitude of 

effect could be substantial such that CRLFs 

could be impacted. 

Following a “may affect” determination, additional information is considered to refine 

the potential for exposure at the predicted levels based on the life history characteristics 

(i.e., habitat range, feeding preferences, etc.) of the CRLF. Based on the best available 

information, the Agency uses the refined evaluation to distinguish those actions that 

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

adversely affect” the CRLF and its designated critical habitat. 

The criteria used to make determinations that the effects of an action are “not likely to 

adversely affect” the CRLF and its designated critical habitat include the following: 

• 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 

90
patterns which include, but are not limited to, breeding, feeding, or 

sheltering. 

• Likelihood of the Effect Occurring: Discountable effects are those that are 

extremely unlikely to occur. 

• Adverse Nature of Effect: Effects that are wholly beneficial without any adverse 

effects are not considered adverse. 

A description of the risk and effects determination for each of the established assessment 

endpoints for the CRLF and its designated critical habitat is provided in Sections 5.2.1 

through 5.2.3. 

5.2.1. Direct Effects 

5.2.1.1. Aquatic-Phase CRLF 

The aquatic-phase considers life stages of the frog that are obligatory aquatic organisms, 

including eggs and larvae. It also considers submerged terrestrial-phase juveniles and 

adults, which spend a portion of their time in water bodies that may receive runoff and 

spray drift containing diflubenzuron. 

No acute LOCs were exceeded for fish based on the most sensitive species tested. No 

open literature studies were located that reported lower toxicity values in either fish or 

amphibians than those used to calculate risk quotients. No incidents have been reported 

for diflubenzuron that associate its use with effects to aquatic animals. The probability of 

an individual mortality at the acute RQs of <0.01 is < 1 in 3.7E20 (<1 in 7.8E11 to 

<5.1E31) based on a slope of 4.7 (95% CI of 3.5 to 5.9, MRID 56150). 

Therefore, the effect determination for potential acute direct effects to the CRLF is “no 

effect”. 

The chronic LOC was exceeded only for direct application to rice. The RQ for this use 

was 1.12. RQs for other uses were <0.3. However, the RQ was based on results from a 

full life-cycle study that did not achieve a LOAEC (no effects were observed in this study 

at any concentration tested). In addition, the 60-day EEC used to calculate the rice 

exposure values did not allow for any degradation over time. Because diflubenzuron 

degradates somewhat rapidly in the standard pond, similar rates of degradation would be 

expected to occur in rice paddies; therefore, the EEC is likely an over-estimation of 

exposure. For these reasons (the close proximity of the RQ to the LOC, the lack of a 

LOAEC in the available study, and the lack of degradation in the 60-day EEC 

calculation), the effects determination for direct application to rice is “not likely to 

adversely affect” because the potential for a direct effect is discountable (highly unlikely 

to occur). 

The effects determination for all other uses is “no effect” based on the lack of LOC 

exceedances and the low probability of an individual mortality. 

91
5.2.1.2. Terrestrial-Phase CRLF 

Barnyard/Mushroom Use 

Acute and chronic RQs were exceeded for CRLFs based on the avian assessment used as 

a surrogate species. The acute RQ was 1.2. Assuming a probit slope of 4.5, the 

probability of an individual effect is approximately 1 in 2. The reproduction RQ was 6.7. 

The reproduction RQ was based on reduced egg production at 1000 mg/kg-food 

(NOAEC was 500 mg/kg-food). At an RQ of 6.7, exposure values were estimated to be 

approximately 3 times higher than levels associated with reduced egg production in birds 

(surrogate for CRLF). This assessment also assumed applications at the highest labeled 

application rate for 17 applications per year (21 days between applications). However, 

even assuming a single application results in LOC exceedances (acute RQ = 0.74; chronic 

RQ = 2). 

Potential effects to CRLFs depend on the proportion of its diet that may occur from 

contaminated food located at or near the use site. The barnyard/mushroom use is a spot 

treatment; therefore, the likelihood that a CRLF will consume contaminated food depends 

on the number and size of “spots” applied to any given field, which may vary 

considerably from field to field. In addition, manure use is typically associated with feed 

lots, which are not attractive habitats for CRLFs due to high density of animals and 

resulting lack of habitat such as clean water and vegetation. 

This assessment was based on food intake equations for birds. Birds are currently used as 

surrogates for reptiles and terrestrial-phase amphibians. However, reptiles and 

amphibians are poikilotherms (body temperature varies with environmental temperature) 

while birds are homeotherms (temperature is regulated, constant, and largely independent 

of environmental temperatures). Therefore, reptiles and amphibians (collectively referred 

to as herptiles) 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 or reptiles on a daily dietary intake basis, assuming similar 

caloric content of the food items. This can be seen when comparing the estimated caloric 

requirements for free living iguanid lizards (Iguanidae) (EQ 1) to passerines (song birds) 

(EQ 2) (U.S. EPA, 1993): 

 iguanid FMR (kcal/day)= 0.0535 *(bw in g)0.799 (EQ 1) 

 

 passerine FMR (kcal/day) = 2.123 *(bw in g)0.749 (EQ 2) 

 

With relatively comparable exponents (slopes) to the allometric functions, one can see 

that, given a comparable body weight, the free living metabolic rate of birds can be 40 

times higher than reptiles, though the requirement differences narrow with high body 

weights. Consequently, birds are conservative surrogates for terrestrial phase CRLFs 

because birds are expected to have higher potential exposures relative to terrestrial phase 

amphibians. 

92
The potential differences in exposure of CRLFs and birds were quantified using a 

modified version of T-REX, called T-HERPS (version 1.0). T-HERPs incorporates food 

intake equations that are specific for terrestrial phase amphibians and includes two 

additional food items of the CRLF that are not included in T-REX (CRLFs that consume 

small mammals and other amphibians). 

The results of this analysis for the barnyard/mushroom use are in Table 5-10. RQs 

remained above the endangered species LOC for the small mammal food item (CRLFs 

that consume small mammals). However, LOCs were not exceeded for other food items. 

Table 5-10. Results from T-HERPS for the Barnyard/Mushroom Use. 

Amphibian/Reptile Acute RQs for Small, Medium, and 

Dose-based RQs (Dose-based Large Species (grams) 

EEC/adjusted LD50) 1.4 37 238 

Short Grass 0.06 0.06 0.04 

Tall Grass 0.03 0.03 0.02 

Broadleaf plants/sm insects 0.03 0.03 0.02 

Fruits/pods/seeds/lg insects 0.00 0.00 0.00 

Small herbivore mammals N/A 0.97 0.15 

Small insectivore mammals N/A 0.06 0.01 

Small terrestrial phase amphibian N/A 0.00 0.00 

Although barnyards and feedlots are not likely to typically provide suitable habitat for 

CRLFs, the presence of CRLFs in such habitats cannot be precluded. Therefore, based 

on acute and chronic LOC exceedances and the likelihood of potential effects at the 

estimated exposure levels, the effects determination for direct effects to the CRLF is 

“likely to adversely affect” for the barnyard/mushroom use. 

All Other Uses 

No LOCs were exceeded. Acute RQs ranged from <0.01 to 0.04. At these RQs, the 

probability of an individual effect is 1 in 6E9 based on the default probit slope of 4.5 (see 

Section 4). Based on reasonable lower and upper bounds on the slope of 2 to 8, the 

probability of an individual effect ranged from 1 in 4E2 to 1 in 7E35. 

No incidents associating diflubenzuron with effects to terrestrial animals have been 

reported to the Agency. 

Also, as previously discussed, birds were used as a surrogate for the CRLF, which results 

in a conservative estimate of exposure and risk due to the greater energetic requirements 

and resulting greater food intake (and resulting potential exposures) of birds relative to 

terrestrial phase CRLFs. 

Based on the lack of LOC exceedances in birds, the low probability of an individual 

mortality, and the lack of reported incidents, diflubenzuron is expected to have “no 

effect” via direct toxic effects on terrestrial phase CRLFs. 

93
5.2.2. Indirect Effects (via Reductions in Prey Base) 

5.2.2.1. Algae (non-vascular plants) 

 

As discussed in Section 2.5.3, the diet of CRLF tadpoles is composed primarily of 

unicellular aquatic plants (i.e., algae and diatoms) and detritus. LOCs were not exceeded 

for aquatic plants. No effects were observed in any of the available toxicity studies in 

aquatic plants, and all RQs were less than 1 (the highest RQ was 0.6). Therefore, 

estimated diflubenzuron concentrations were lower than NOAEC values in all aquatic 

plant species tested. 

Therefore, the effects determination for CRLFs based on potential indirect effects from 

impacts to aquatic plants is “no effect.” 

5.2.2.2. Aquatic Invertebrates 

The potential for diflubenzuron 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. 

Based on the most sensitive species tested (mosquito), RQs exceeded LOCs and ranged 

from 4 to 40,000 (acute) and 25 to 450,000 (chronic). At these acute RQs, the probability 

of an individual mortality approaches 100%, and potential impacts to abundance of 

aquatic invertebrates could be sufficient to reduce available food of the CRLF. 

As shown in Figure 5-1 below, based on open literature studies (Appendix G), the lowest 

EEC across all uses of diflubenzuron is above the average acute toxicity value for a 

majority of species tested (RQ >1). These values represent short-term toxicity studies, 

and may, therefore, underestimate potential impacts to aquatic invertebrates as discussed 

in Section 4. 

The life stage of the exposed invertebrate is likely to influence potential effects. Adults 

are less sensitive to diflubenzuron than larvae and juveniles. The results of littoral 

enclosure studies discussed in Section 4 suggested that some taxonomic groups of aquatic 

invertebrates could be impacted at diflubenzuron levels lower than 0.7 μg/L (NOEC not 

achieved). These impacts to the aquatic community were also observed to be correlated 

to reductions in the growth rate of introduced bluegill sunfish, with reductions in length, 

weight, and growth rate occurring at initial nominal concentrations of 2.5 μg/L and 

above. Growth reductions in fish were attributed to reductions in food source (indirect 

94
effect). It is important to note that MRID 44386201 fails to empirically demonstrate a 

NOEC for a number of endpoints, namely effects on cladocerans, copepods, aquatic 

insect emergence and richness of macroinvertebrate taxa.

Based on the number of potential species that may be affected and the potential 

magnitude of effect, diflubenzuron is likely to adversely affect the CRLF by reducing 

available aquatic prey. This effects determination applied to all labeled uses included in 

this assessment. 

Based on AgDRIFT analysis (version 2.1.0.3, Tier 3 analysis using aerial and ground 

spray), spray drift could be sufficient to result in indirect effects at applications greater 

than 5000 feet from treated sites as evidenced by LOC exceedances for the most sensitive 

species tested. 

Mean Acute EC50 Values from Open Literature Data (Ecotox)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Blackfly Cyclopoid

copepod

Fairy Shrimp Insect class Midge Mosquito Water flea

Species

EC50 (ug/L)

Solid line represents minimum EEC

Solid Line represents approx. 20th Percentile EEC

Figure 5-1. Mean acute EC50 values from open literature data according to the Ecotox data base. 

Figure note: Values for fairy shrimp and water flea are low such that they are not visible on the graph. 

EC50s for two species were not included in this figure due to the scale of the graph. A single EC50 for 

parasitic nematode (EC50: 4.4 μg/L) and a single EC50 for backswimmer (EC50: 1.9 μg/L). 

5.2.2.3. Fish and Aquatic-phase Frogs 

As described in Section 5.2.1.1, the effects determination for potential direct effects to 

the CRLF is “no effect” for acute effects and chronic effects for all uses except rice and is 

“not likely to adversely affect” for chronic effects for the rice use. These conclusions are 

based on potential effects to the most sensitive fish species tested. The same conclusions 

are reached for the potential for diflubenzuron to adversely affect the CRLF based on 

reduction in fish or frogs as food. See Section 5.2.1.1 for additional detail. 

95
5.2.2.4. Terrestrial Invertebrates 

When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly 

composed of terrestrial invertebrates. RQs were based on acute studies in adult bees. 

Although RQs suggest that there is a potential for impacts to terrestrial invertebrates, 

these RQs were based on a study that did not produce mortality at any concentration 

tested (30 μg/bee). However, as discussed in Section 4, acute studies in adult bees are 

not considered to represent sensitive invertebrate life stages due to the mode of action of 

diflubenzuron as an insect growth regulator, which affects insects at molting. Results of 

field studies have been inconsistent as discussed in Section 4; however, several field 

studies have shown reduced abundance in invertebrates at currently approved application 

rates. Also, given the use of diflubenzuron as an insecticide, many insect species are 

presumably at risk if exposed at a sensitive life stage. Therefore, the effects 

determination from labeled use of diflubenzuron is “likely to adversely affect.” The 

potential impact to the diet of the CRLF will depend on the timing of application and 

predominance of invertebrates in sensitive life stages. 

The distance from treated fields that effects may occur has not been defined because the 

available data do not allow for quantitative measures of effects. 

5.2.2.5. Mammals 

Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume 

terrestrial vertebrates, including mice. No acute LOCs were exceeded for mammals. No 

effects were observed in the available acute oral toxicity study at doses up to 5000 

mg/kg-bw. No reproduction RQs were exceeded for any use except the 

barnyard/mushroom use. Therefore, the effects determination is “no effect” for all uses 

except barnyard/mushroom use based on the lack of LOC exceedances and the resulting 

low magnitude of any potential effects to mammals. 

Reproduction LOCs were exceeded for barnyard/mushroom use (RQ = 10). There is a 

10-fold dose spacing in the available 2-generation reproduction toxicity study; therefore, 

at an RQ of 10, estimated exposures are equivalent to the LOAEC in mammals. At the 

LOAEC, there was a reduction in body weight in F1 pups. Frank reproductive effects 

were not observed. The EECs were based on the short grass food item and assumed that 

small mammal prey consume only contaminated short grass at high-end exposure levels. 

However, the barnyard/mushroom use is a spot treatment (directed spray at manure). 

Therefore, it is unlikely that a small mammal will obtain 100% of its diet as contaminated 

short grass in environments consistent with the barnyard/mushroom use. Nonetheless, 

such effects could not be precluded. Therefore, the effects determination is “likely to 

adversely affect” based on potential reductions in mammalian prey. These conclusions 

are highly uncertain. 

96
5.2.2.6. Terrestrial-phase Amphibians 

Terrestrial-phase adult CRLFs also consume other amphibians. RQ values representing 

direct exposures of diflubenzuron to terrestrial-phase CRLFs are used to represent 

exposures of diflubenzuron to frogs in terrestrial habitats. The direct effects assessment 

was “no effect” for all uses except the barnyard/mushroom use. Therefore, the effects 

determination is also “no effect” via reduction in amphibian prey. 

For the barnyard/mushroom use, the effects determination was “likely to adversely 

affect” based on potential direct effects to the CRLF. However, the potential magnitude 

of effect is expected to be relatively low when considering dietary factors of terrestrial 

phase amphibians relative to the surrogate avian species. 

When incorporating food intake equations specific for amphibians and reptiles using the 

T-HERPS model, LOCs were not exceeded for terrestrial phase amphibians that consume 

plants or insects. See Section 5.2.1.2 for additional information. Therefore, the effects 

determination for potential effects to the CRLF via reductions in available amphibian 

prey is “not likely to adversely affect.” 

5.2.3. Indirect Effects (via Habitat Effects) 

5.2.3.1. Aquatic Plants (Vascular and Non-vascular) 

Aquatic plants serve several important functions in aquatic ecosystems. Non-vascular 

aquatic plants are primary producers and provide the autochthonous energy base for 

aquatic ecosystems. Vascular plants provide structure as attachment sites and refugia for 

many aquatic invertebrates, fish, and juvenile organisms, such as fish and frogs. In 

addition, vascular plants also provide primary productivity and oxygen to the aquatic 

ecosystem. Rooted plants help reduce sediment loading and provide stability to 

nearshore areas and lower streambanks. In addition, vascular aquatic plants are important 

as attachment sites for egg masses of CRLFs. 

Potential indirect effects to the CRLF based on impacts to habitat and/or primary 

production were assessed using RQs from freshwater aquatic vascular and non-vascular 

plant data. No LOCs were exceeded for vascular aquatic plants. No effects were 

observed in the available toxicity studies in aquatic plants at any test level. EECs were 

lower than all vascular plant NOAECs. Therefore, effects to vascular plants are not 

expected to occur. For this reason, the effects determination for the CRLF based on 

indirect effects resulting from impacts to vascular plants is “no effect.” 

5.2.3.2. Terrestrial Plants 

Terrestrial plants serve several important habitat-related functions for the CRLF. In 

addition to providing habitat and cover for invertebrate and vertebrate prey items of the 

97
CRLF, terrestrial vegetation also provides shelter for the CRLF and cover from predators 

while foraging. Terrestrial plants also provide energy to the terrestrial ecosystem through 

primary production. Upland vegetation including grassland and woodlands provides 

cover during dispersal. Riparian vegetation helps to maintain the integrity of aquatic 

systems by providing bank and thermal stability, serving as a buffer to filter out sediment, 

nutrients, and contaminants before they reach the watershed, and serving as an energy 

source. 

Impacts to terrestrial plants are not expected to occur at levels that may result in indirect 

effects to CRLFs. Although no suitable toxicity data are available in terrestrial plants, 

diflubenzuron is an insecticide that has been used to treat numerous crops and non-crop 

plants for many years typically without incident. However, one incident has associated 

diflubenzuron with damage to oranges. This incident may or may not have been caused 

by diflubenzuron. It was assigned a certainty index of “possible”. 

Although the data do not allow for quantification of potential risks to terrestrial plants, 

the weight of evidence suggests that effects to plants from labeled uses of diflubenzuron 

are minimal. 

No guideline studies have been submitted that evaluated effects to terrestrial plants. 

Therefore, additional lines of evidence were used to determine if diflubenzuron may 

affect terrestrial plants at a magnitude that may result in indirect effects to the CRLF. 

Diflubenzuron has a long history of use on numerous types of crops including grasses, 

trees, vines, and numerous fruit and vegetable crops without clear evidence of any 

adverse effects to terrestrial plants. Two field studies were located in the open literature 

study that evaluated impacts to vegetation from use of diflubenzuron that did not observe 

impacts to vegetation (discussed in Section 4). No incidents have been reported that 

definitively linked use of diflubenzuron use to damage to terrestrial plants. One incident 

was associated with damage to orange trees; however, that incident is more likely 

associated with use of petroleum distillates as discussed in Section 4. In addition, 

numerous studies have been submitted to the Agency that evaluated efficacy of 

diflubenzuron, and no evidence of plant damage was reported in these studies. 

For these reasons, use of diflubenzuron is not likely to affect the CRLF by impacting 

terrestrial vegetation. 

The effects determination for the CRLF resulting from indirect effects from impacts to 

terrestrial plants is “no effect.” 

5.2.4. Modification to Designated Critical Habitat 

Risk conclusions for designated critical habitat of the CRLF are the same as those for 

indirect effects. Critical habitat may be modified by impacting availability of prey 

(terrestrial and aquatic invertebrates) as follows: 

98
• PCE: “reduction and/or modification of food sources for terrestrial phase 

juveniles and adults.” This PCE may be modified by diflubenzuron by potentially 

impacting invertebrate food sources. See Section 5.2.2.4. 

• PCE: “alteration of chemical characteristics necessary for normal growth and 

viability of juvenile and adult CRLFs and their food source.” This PCE may be 

impacted by effects to food sources such as terrestrial invertebrates. See Section 

5.2.2.4. 

5.2.5. Spatial Extent of Potential Effects 

Direct effects to the CRLF are limited to the barnyard/mushroom use. Locations of cattle 

feed lots (locations where diflubenzuron may be used on cattle manure) are depicted in 

Figure 5.2. For this use, diflubenzuron is applied as a directed spray to manure. Based 

on the low magnitude of the RQ for this use and the low potential for spray drift, it is 

assumed that potential risks are limited to treated sites. 

99
Figure 5-2. Comparison of location of sites likely to use diflubenzuron for control of 

flies at livestock facilities (see text) and CRLF occupied core areas. 

100
RQs used to estimate potential for indirect effects to aquatic phase CRLFs were as high 

as approximately 500,000. The distance needed to reduce potential exposures to levels 

that do not exceed LOCs is greater than distances that can be predicted with any 

reliability. Potential indirect effects could occur at distances >5000 feet from the treated 

sites as described in Section 5.2.5.1 

Indirect effects to the CRLF may also occur via impacts to terrestrial invertebrates. 

However, the data do not allow for quantification of the potential distance from treated 

sites to which risks extend primarily because the toxicity data used to estimate potential 

impacts to terrestrial invertebrates are based on adult honey bee studies. Sensitivity of 

adult honey bees to diflubenzuron is expected to be considerably lower than sensitivity of 

other insects, particularly invertebrates in early life stages. Therefore, potential impacts 

to terrestrial invertebrates are assumed to extend to distances greater than 1000 feet from 

treated fields. 

An LAA effects determination applies to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat. 

To determine this area, the footprint of diflubenzuron’s use pattern is identified, using 

land cover data that correspond to diflubenzuron’s use pattern. Areas of potential 

exposure also include areas beyond the initial area of concern (e.g., use footprint) that 

may be impacted by runoff and/or spray drift. The identified direct and indirect effects 

and modification to critical habitat are anticipated to occur only for those currently 

occupied core habitat areas, CNDDB occurrence sections, and designated critical habitat 

for the CRLF that overlap with the initial area of concern plus >5000 feet from its 

boundary. It is assumed that non-flowing waterbodies (or potential CRLF habitat) are 

included within this area. Direct effects are only expected to occur for the 

barnyard/mushroom use. For this use, risks are expected to be predominantly restricted 

to the treated site because of the mode of application (directed spray to manure). 

The determination of the buffer distance and downstream dilution for spatial extent of the 

effects determination is described below. 

5.2.5.1. Spray Drift 

In order to determine terrestrial and aquatic habitats of concern due to diflubenzuron 

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 (version 2.01). AgDRIFT was run using default parameters. Tier 3 was used 

to expand the distance evaluated to 5000 feet. 

101
Table 5-11. Summary of AgDRIFT* Predicted Terrestrial Spray Drift Distances 

Landcover Uses Represented Highest RQ

Distance to reduce 

spray drift to level that 

is <LOC 

Forest Forestry 3200 >5000 feet 

Non-cropland 

agriculture 

Turf, nursery 2600 >5000 feet 

Residential Residential, urban, rights of 

ways 

137,000 >5000 feet 

Orchard Fruit, nuts, pistachio, Beech 

nut 

5400 >5000 feet 

Pasture/Rangeland Barnyards, pastures+ 217,000 (barnyard) 

2200 (pasture) 

>5000 feet 

Cropland Turnip, squash, row crops, 

rice, grains, nuts, cotton, 

cole crops, brassica, 

452,000 >5000 feet 

* AgDRIFT was run in Tier 3, with a maximum distance of 5000 feet and RQs for aquatic invertebrates. 

AgDRIFT has not been validated at distances >1000 feet; therefore, there is uncertainty in the 

estimations presented in Table 5-11. 

+.Minimal drift is expected from the barnyard uses because of the mode of application (downward spray 

directed at manure); therefore, the drift distance was based on the pastureland RQ of 2200. 

5.2.6.2. Downstream Dilution Analysis 

The downstream extent of exposure in streams and rivers where the EEC could 

potentially be above levels that would exceed the most sensitive LOC was not completed 

because of the large number of uses and large corresponding landcover area. The large 

geographic extent of potential use sites precludes quantification of dilution potential 

based on landcover data representing non-use sites. 

5.2.6.3. Overlap between CRLF habitat and Spatial Extent of 

Potential Effects 

An LAA effects determination is made to those areas where it is expected that the 

pesticide’s use will directly or indirectly affect the CRLF or its designated critical habitat 

and the area overlaps with the core areas, critical habitat and available occurrence data 

for CRLF. 

For diflubenzuron, the use pattern encompasses all land cover classes. Therefore, the 

initial area of concern is the entire state of CA. As described in Section 5, diflubenzuron 

could potentially impact the CRLF via indirect effects to aquatic and terrestrial 

invertebrates. Effects could occur at distances >5000 feet from the site of application. 

Potential overlap between the CRLF and diflubenzuron uses is graphically represented in 

Figure 5.3. 

102
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 5-3 Recovery Unit, Core Area, Critical Habitat, and Occurrence 

Designations for CRLF 

103
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. 

Several fast-maturing crops may have multiple crop cycles within a single year. For these 

crops, it is often the case that labels do not specify a maximum annual application rate, 

but rather a maximum crop cycle rate (maximum pounds per acre per crop cycle). 

Because diflubenzuron dissipates rapidly in the environment (aerobic soil metabolism 

rate is ~2 days), additional applications of diflubenzuron during following crop cycles are 

not expected to greatly increase terrestrial EECs within the field where it is applied or in 

the terrestrial spray drift zone. However, aquatic EECs can be affected because the 

multiple crop cycles extend the growing season into those parts of the year when runoff is 

more likely (fall through early spring). To address this uncertainty, aquatic EECs were 

modeled across a large range of application dates (Section 3.2.3 and Appendix tables D4 

and D5). 

6.1.2 Crops Not Grown in California (include if you had crops that are not 

grown in CA) 

Soybeans are the only crop listed on diflubenzuron labels that is not grown in California 

according to National Agricultural Statistical Service data. Because these crops are not 

currently grown in California and are not expected to be grown in California in the future, 

use of diflubenzuron was not assessed for these crops. If use patterns indicate soybeans 

are grown in California in the future, the conclusions of this assessment may need to be 

revisited. 

6.1.3 Aquatic Exposure Modeling of Diflubenzuron 

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 

104
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 (U.S. FWS/NMFS 2004). 

In general, the linked PRZM/EXAMS model produces estimated aquatic concentrations 

that are expected to be exceeded once within a ten-year period. The Pesticide Root Zone 

Model is a process or “simulation” model that calculates what happens to a pesticide in 

an agricultural field on a day-to-day basis. It considers factors such as rainfall and plant 

transpiration of water, as well as how and when the pesticide is applied. It has two major 

components: hydrology and chemical transport. Water movement is simulated by the use 

of generalized soil parameters, including field capacity, wilting point, and saturation 

water content. The chemical transport component can simulate pesticide application on 

the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in 

the soil are estimated by simultaneously considering the processes of pesticide uptake by 

plants, surface runoff, erosion, decay, volatilization, foliar wash-off, advection, 

dispersion, and retardation. 

Uncertainties associated with each of these individual components add to the overall 

uncertainty of the modeled concentrations. Additionally, model inputs from the 

environmental fate degradation studies are chosen to represent the upper confidence 

bound on the mean values that are not expected to be exceeded in the environment 

approximately 90 percent of the time. Mobility input values are chosen to be 

representative of conditions in the environment. The natural variation in soils adds to the 

105
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. 

6.1.4 Usage Uncertainties 

County-level usage data were obtained from California’s Department of Pesticide 

Regulation Pesticide Use Reporting (CDPR PUR) database. Eight years of data (1999 – 

2006) were included in this analysis 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 1998 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 Diflubenzuron 

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. 

106
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. EPA 1993). If it is assumed that laboratory chow is formulated to maximize 

assimilative efficiency (e.g., a value of 85%), a potential for underestimation of exposure 

may exist by assuming that consumption of food in the wild is comparable with 

consumption during laboratory testing. In the screening process, exposure may be 

underestimated because metabolic rates are not related to food consumption. 

For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

6.1.6 Spray Drift Modeling 

Although there may be multiple diflubenzuron applications at a single site, it is unlikely 

that the same organism would be exposed to the maximum amount of spray drift from 

every application made. In order for an organism to receive the maximum concentration 

of diflubenzuron from multiple applications, each application of diflubenzuron would 

have to occur under identical atmospheric conditions (e.g., same wind speed and – for 

plants – same wind direction) and (if it is an animal) the animal being exposed would 

have to be present directly downwind at the same distance after each application. 

Although there may be sites where the dominant wind direction is fairly consistent (at 

least during the relatively quiescent conditions that are most favorable for aerial spray 

applications), it is nevertheless highly unlikely that plants in any specific area would 

receive the maximum amount of spray drift repeatedly. It appears that in most areas 

(based upon available meteorological data) wind direction is temporally very changeable, 

even within the same day. Additionally, other factors, including variations in 

topography, cover, and meteorological conditions over the transport distance are not 

accounted for by the AgDRIFT model (i.e., it models spray drift from aerial and ground 

applications in a flat area with little to no ground cover and a steady, constant wind speed 

and direction). Therefore, in most cases, the drift estimates from AgDRIFT may 

107
overestimate exposure even from single applications, especially as the distance increases 

from the site of application, since the model does not account for potential obstructions 

(e.g., large hills, berms, buildings, trees, etc.). Furthermore, conservative assumptions 

are often made regarding the droplet size distributions being modeled (‘ASAE Very Fine 

to Fine’ for orchard uses and ‘ASAE Very Fine’ for agricultural uses), the application 

method (e.g., aerial), release heights and wind speeds. Alterations in any of these inputs 

would change the area of potential effect. 

6.2 Effects Assessment Uncertainties 

6.2.1 Age Class and Sensitivity of Effects Thresholds 

It is generally recognized that test organism age may have a significant impact on the 

observed sensitivity to a toxicant. The acute toxicity data for fish are collected on 

juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on 

recommended immature age classes (e.g., first instar for daphnids, second instar for 

amphipods, stoneflies, mayflies, and third instar for midges). 

Testing of juveniles may overestimate toxicity at older age classes for pesticide active 

ingredients that act directly without metabolic transformation because younger age 

classes may not have the enzymatic systems associated with detoxifying xenobiotics. In 

so far as the available toxicity data may provide ranges of sensitivity information with 

respect to age class, this assessment uses the most sensitive life-stage information as 

measures of effect for surrogate aquatic animals, and is therefore, considered as 

protective of the CRLF. 

For diflubenzuron, the most sensitive endpoints are for immature invertebrates that are 

molting. However, acute toxicity test procedures typically select organisms that are not 

molting during the test. This may result in an underestimation of risk to molting 

individuals of the same species. 

6.2.2 Use of Surrogate Species Effects Data 

Guideline toxicity tests and open literature data on diflubenzuron 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. 

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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. 

No open literature or registrant studies on sublethal effects associated with exposure to 

diflubenzuron were identified. To the extent to which sublethal effects are not considered 

in this assessment, the potential direct and indirect effects of diflubenzuron on CRLF may 

be underestimated. 

6.2.4 Location of Wildlife Species 

For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal 

was assumed to occupy either the treated field or adjacent areas receiving a treatment rate 

on the field. Actual habitat requirements of any particular terrestrial species were not 

considered, and it was assumed that species occupy, exclusively and permanently, the 

modeled treatment area. Spray drift model predictions suggest that this assumption leads 

to an overestimation of exposure to species that do not occupy the treated field 

exclusively and permanently. 

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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 diflubenzuron to the CRLF and its 

designated critical habitat. 

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

determination for the CRLF from the use of diflubenzuron. The Agency has determined 

that there is no potential for modification of CRLF designated critical habitat from the 

use of the chemical. All the uses of diflubenzuron are likely to adversely affect the 

CRLF primarily because of a there is a significant risk to terrestrial and aquatic 

invertebrate prey of the CRLF. There is a LAA for risk of direct effect to the aquatic 

phase CRLF only for the diflubenzuron use on rice and there is a LAA for risk of direct 

effect to the terrestrial phase CRLF for diflubenzuron use in barnyard/mushroom sites.

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 diflubenzuron’s use pattern is identified, using 

land cover data that correspond to diflubenzuron’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 indirect effects 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 + 

5000 feet from its boundary (refer to analysis in Section 5.1.4). It is assumed that nonflowing waterbodies (or potential CRLF habitat) are included within this area. 

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

critical habitat, given the uncertainties discussed in Section 6, is presented in Table 7-1 

and 7-2. 

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

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Potential for Direct Effects

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

Direct effects are not expected for any labeled use. All effects 

determinations are either NE or NLAA. 

Terrestrial-phase (Juveniles and Adults): 

Direct effects are only expected for diflubenzuron’s use on barn 

yards based on acute and chronic LOC’s for birds. 

Potential for Indirect Effects

Survival, growth, 

and/or 

reproduction of 

CRLF individuals 

LAA, all uses 

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

productivity

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Assessment 

Endpoint 

Effects Basis for Determination 

Determination 1

Aquatic prey of the CRLF are expected to be impacted to an extent 

that could adversely affect CRLFs. 

Terrestrial prey items, riparian habitat 

Terrestrial prey items of the CRLF are expected to be impacted 

based on LOC exceedances for risk to terrestrial invertebrates. 1

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

affect (LAA)

Table 7-2. Effects Determination Summary for Diflubenzuron Use and CRLF Critical Habitat 

Impact Analysis 

Assessment 

Endpoint 

Effects 

Determination 1

Basis for Determination 

Modification of 

aquatic-phase PCE There are LOC exceedances for risk to terrestrial and aquatic 

invertebrate prey of the CRLF. 

Modification of 

terrestrial-phase 

PCE 

Habitat 

Modification 

There are LOC exceedances for risk to terrestrial and aquatic 

invertebrate prey of the CRLF. 

1

 Habitat Modification or No effect (NE)

Based on the conclusions of this assessment, a formal consultation with the U. S. Fish 

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

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 aquatic- and 

terrestrial-phase frogs. While existing information provides a preliminary picture 

of the types of food sources utilized by the frog, it does not establish minimal 

111
requirements to sustain healthy individuals at varying life stages. Such 

information could be used to establish biologically relevant thresholds of effects 

on the prey base, and ultimately establish geographical limits to those effects. 

This information could be used together with the density data discussed above to 

characterize the likelihood of adverse effects to individuals. 

• Information on population responses of prey base organisms to the pesticide. 

Currently, methodologies are limited to predicting exposures and likely levels of 

direct mortality, growth or reproductive impairment immediately following 

exposure to the pesticide. The degree to which repeated exposure events and the 

inherent demographic characteristics of the prey population play into the extent to 

which prey resources may recover is not predictable. An enhanced understanding 

of long-term prey responses to pesticide exposure would allow for a more refined 

determination of the magnitude and duration of resource impairment, and together 

with the information described above, a more complete prediction of effects to 

individual frogs and potential modification to critical habitat. 

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Previous Assessments 

U.S. EPA 2002a. Environmental Fate and Ecological Effects Assessment and 

Characterization for a Section 3 for Aerial and Ground Spray Treatment to Pears, 

Stone Fruits, Almonds, Peppers, and Pasture Grass (DP 321153, DP 321278) 

Ecological Fate and Effects Division, Office of Pesticide Programs, U.S. 

Environmental Protection Agency, Washington D.C. May 2002. 

U.S. EPA 2006. Environmental Fate and Ecological Risk Assessment for the Registration 

of Diflubenzuron (Dimilin-2L) Use on Peanuts, Okra, Small Grains-(Winter 

wheat, Spring wheat, Durum, Barley, and Oats) , Pummelo, Mustard Greens, 

Broccoli, Raab, Cabbage, Chinese (bok choy); Collards; Kale; Mizuna; Mustard 

spinach; Rape, Greens and Turnip greens. (DP 321153, DP 321278). Ecological 

Fate and Effects Division, Office of Pesticide Programs, U.S. Environmental 

Protection Agency, Washington D.C., September 2006.

U.S. EPA 2008. New Use of Dimilin®2L (active ingredient 22% Diflubenzuron) on Fly 

Breeding Areas in Livestock Operations. (DP 348755). Ecological Fate and 

Effects Division, Office of Pesticide Programs, U.S. Environmental Protection 

Agency, Washington D.C., September 2008.

U.S. EPA 2009. County-Level Usage for Strychnidin; Strychnine; Triclopyr, butoxyethyl 

ester; Triclopyr, triethylamine salt; Diflubenzuron; Trifluralin ; Thiobencarb; 

Chlorpyrifos; Vinclozolin; Iprodione in California in Support of a Red Legged Frog 

Endangered Species Assessment, DP # TBD. Biological and Economic Analysis 

Divis ion, Office of Pesticide Programs, U.S. Environmental Protection Agency, 

Washington D.C., June 8th, 2009. 

116