2 

 41 birds and 7 reptiles that may consume bulbs (no listed terrestrial-phase amphibians 

were identified)1

 

 28 birds that consume seeds (no listed reptiles or terrestrial-phase amphibians were 

identified) 

 33 mammals that consume seeds 

 59 terrestrial invertebrates (non-subterranean species) 

The MA finding for direct effects to birds, reptiles, and mammals is based on exceedance of the 

listed species LOC for specific food items (treated seed and bulbs). The finding for potential 

direct effects to terrestrial invertebrates is based on insufficient data; exposure values are greater 

than the non-definitive endpoint (no effects observed) obtained in the available toxicity study. 

Thus, the magnitude of any potential effects to terrestrial invertebrates, if any, cannot be 

quantified. The MA finding is also based on the assumption that the geographic range of a listed 

species overlaps with the action area and that individuals may visit the treatment area. A NE 

determination is concluded for the 17 listed terrestrial invertebrate species categorized as 

subterraneous. In general, these species are unlikely to be exposed to labeled applications of 

denatonium saccharide because they are adapted to living in caves or similar underground 

habitats. 

The MA finding for indirect effects is limited to listed birds, reptiles, terrestrial-phase 

amphibians, mammals, terrestrial insects, and plants (terrestrial and aquatic vascular) that have 

an obligate relationship with a species that is directly affected by denatonium saccharide. A NE 

determination is concluded for indirect effects to listed species not having an obligate 

relationship with a species directly affected by denatonium saccharide due to the highly localized 

and short-term nature of any effects. 

A NLAA determination has been made for both direct and indirect effects because the 

probability of a listed species encountering areas of treatment is expected to be highly unlikely. 

This finding is based on the extremely limited use of denatonium saccharide (about 100 pounds 

is sold annually nationwide) and the nature of the use. There is no expectation that large 

contiguous areas of treatment will occur, rather use is expected to result in very small treatment 

areas (less than a typical residential yard) scattered across the landscape with few treatment areas 

occurring anywhere simultaneously. Given the combination of widely dispersed and limited use, 

it is highly unlikely that individuals of a listed species will encounter areas of treatment. 

Therefore, risk is expected to be discountable because potential exposure will be confined to 

small patches isolated in space and time. EPA asks for concurrence on this conclusion from the 

U.S. Fish and Wildlife Service (FWS). 

The EFED endangered species knowledge database was used to identify listed species included 

in the NLAA determination for direct effects (Tables 1-3). The NLAA determination was made 

only for species identified in the database as either seed consumers (birds, reptiles, terrestrialphase amphibians, and mammals) or bulb consumers (birds, reptiles, and terrestrial-phase 

amphibians). Bulbs are not specifically included as a food item in the database. This does not 

mean that there are no listed bird, reptile, or terrestrial-phase amphibian species that eat bulbs, 

rather that bulbs were not an identified food item. It was conservatively assumed that species 

 

1

 Includes 28 avian species that consume seeds 
3 

that eat plant material (nectar was excluded) may eat bulbs if available. Furthermore, some 

species may grub for bulbs while others may only opportunistically consume bulbs brought to 

the surface and discarded by another animal. Body weight was not used to exclude any animals 

included in the NLAA determination because the potential risk concern was for all assessed size 

classes of birds and mammals. The NLAA determination for terrestrial insects was for nonsubterraneous species. The database does not currently include attributes for terrestrial insects; 

therefore, this assessment relies on a recent preliminary risk assessment on silica (USEPA, 2013) 

for identifying and excluding listed species categorized as subterraneous dwellers. 

Table 1. Listed Mammals that Consume Seeds 

Common name Scientific name 

Alabama beach mouse Peromyscus polionotus ammobates 

Amargosa vole Microtus californicus scirpensis 

Anastasia Island beach mouse Peromyscus polionotus phasma 

Carolina northern flying squirrel Glaucomys sabrinus coloratus 

Choctawhatchee beach mouse Peromyscus polionotus allophrys 

Delmarva Peninsula fox squirrel Sciurus niger cinereus 

Florida salt marsh vole Microtus pennsylvanicus dukecampbelli 

Fresno kangaroo rat Dipodomys nitratoides exilis 

Giant kangaroo rat Dipodomys ingens 

Grizzly bear Ursus arctos horribilis 

Key Largo cotton mouse Peromyscus gossypinus allapaticola 

Key Largo woodrat Neotoma floridana smalli 

Little Mariana fruit Bat Pteropus tokudae 

Louisiana black bear Ursus americanus luteolus 

Mariana fruit Bat (=Mariana flying fox) Pteropus mariannus mariannus 

Morro Bay kangaroo rat Dipodomys heermanni morroensis 

Mount Graham red squirrel Tamiasciurus hudsonicus grahamensis 

Northern Idaho ground squirrel Spermophilus brunneus brunneus 

Pacific pocket mouse Perognathus longimembris pacificus 

Perdido Key beach mouse Peromyscus polionotus trissyllepsis 

Preble's meadow jumping mouse Zapus hudsonius preblei 

Riparian brush rabbit Sylvilagus bachmani riparius 

Salt marsh harvest mouse Reithrodontomys raviventris 

San Bernardino Merriam's kangaroo rat Dipodomys merriami parvus 

San Miguel Island Fox Urocyon littoralis littoralis 

Santa Catalina Island Fox Urocyon littoralis catalinae 

Santa Cruz Island Fox Urocyon littoralis santacruzae 

Santa Rosa Island Fox Urocyon littoralis santarosae 

Southeastern beach mouse Peromyscus polionotus niveiventris 

Stephens' kangaroo rat Dipodomys stephensi (incl. D. cascus) 

Tipton kangaroo rat Dipodomys nitratoides nitratoides 

Utah prairie dog Cynomys parvidens 

Virginia northern flying Squirrel Glaucomys sabrinus fuscus 
4 

Table 2. Listed Birds that Consume Seeds and Listed Birds and Reptiles that May 

Consume Bulbs 

Common name Scientific name Taxa Food Source 

Seeds Bulbs 

Akiapola`au (honeycreeper) Hemignathus munroi Birds X 

Attwater's greater prairiechicken 

Tympanuchus cupido 

attwateri 

Birds X 

Blunt-nosed leopard lizard Gambelia silus Reptiles X 

Bridled white-eye Zosterops conspicillatus 

conspicillatus 

Birds X 

Cape Sable seaside sparrow Ammodramus maritimus 

mirabilis 

Birds X 

Coachella Valley fringe-toed 

lizard 

Uma inornata Reptiles X 

Coastal California gnatcatcher Polioptila californica 

californica 

Birds X X 

Culebra Island giant anole Anolis roosevelti Reptiles X 

Desert tortoise Gopherus agassizii Reptiles X 

Florida grasshopper sparrow Ammodramus savannarum 

floridanus 

Birds X 

Florida scrub jay Aphelocoma coerulescens Birds X X 

Gopher tortoise Gopherus polyphemus Reptiles X 

Guam rail Rallus owstoni Birds X X 

Hawaii akepa (honeycreeper) Loxops coccineus coccineus Birds X 

Hawaiian (='alala) Crow Corvus hawaiiensis Birds X X 

Hawaiian (=koloa) Duck Anas wyvilliana Birds X X 

Hawaiian common moorhen Gallinula chloropus 

sandvicensis 

Birds X X 

Hawaiian goose Branta (=Nesochen) 

sandvicensis 

Birds X X 

Inyo California towhee Pipilo crissalis eremophilus Birds X X 

Island night lizard Xantusia riversiana Reptiles X 

Ivory-billed woodpecker Campephilus principalis Birds X X 

Kauai `o`o (honeyeater) Moho braccatus Birds X 

Kauai akialoa (honeycreeper) Hemignathus procerus Birds X 

Kauai creeper Oreomystis bairdi Birds X X 

Kirtland's warbler (=wood) Dendroica kirtlandii Birds X 

Large Kauai (=kamao) Thrush Myadestes myadestinus Birds X X 

Laysan duck Anas laysanensis Birds X X 

Laysan finch (honeycreeper) Telespyza cantans Birds X X 

Mariana common moorhen Gallinula chloropus guami Birds X 

Mariana (=aga) Crow Corvus kubaryi Birds X 

Masked bobwhite (quail) Colinus virginianus ridgwayi Birds X X 

Micronesian megapode Megapodius laperouse Birds X 
5 

Common name Scientific name Taxa Food Source 

Seeds Bulbs 

Mississippi sandhill crane Grus canadensis pulla Birds X X 

Molokai thrush Myadestes lanaiensis rutha Birds X 

Mona ground Iguana Cyclura cornuta stejnegeri Reptiles X 

Nihoa finch (honeycreeper) Telespyza ultima Birds X 

Nukupu`u (honeycreeper) Hemignathus lucidus Birds X 

Palila (honeycreeper) Loxioides bailleui Birds X X 

Puerto Rican parrot Amazona vittata Birds X X 

Puerto Rican plain Pigeon Columba inornata wetmorei Birds X X 

Red-cockaded woodpecker Picoides borealis Birds X X 

Rota bridled White-eye Zosterops rotensis Birds X X 

San Clemente sage sparrow Amphispiza belli clementeae Birds X X 

Small Kauai (=puaiohi) Thrush Myadestes palmeri Birds X 

Spectacled eider Somateria fischeri Birds X X 

Whooping crane Grus americana Birds X 

Yellow-shouldered blackbird Agelaius xanthomus Birds X 

`O`u (honeycreeper) Psittirostra psittacea Birds X 

Table 3. Listed Terrestrial Insects (non-subterraneous) 

Common name Scientific name Taxa 

American burying beetle Nicrophorus americanus Insect 

Ash Meadows naucorid Ambrysus amargosus Insect 

Bay checkerspot butterfly Euphydryas editha bayensis Insect 

Behren's silverspot butterfly Speyeria zerene behrensii Insect 

Blackburn's sphinx moth Manduca blackburni Insect 

Callippe silverspot butterfly Speyeria callippe callippe Insect 

Carson wandering skipper Pseudocopaeodes eunus obscurus Insect 

Caseys June Beetle Dinacoma caseyi Insect 

Comal Springs dryopid beetle Stygoparnus comalensis Insect 

Comal Springs riffle beetle Heterelmis comalensis Insect 

Delhi Sands flower-loving fly Rhaphiomidas terminatus abdominalis Insect 

Delta green ground beetle Elaphrus viridis Insect 

El Segundo blue butterfly Euphilotes battoides allyni Insect 

Fender's blue butterfly Icaricia icarioides fenderi Insect 

Florida leafwing butterfly Anaea troglodyta floridalis Insect 

Flying earwig Hawaiian damselfly Megalagrion nesiotes Insect 

Hawaiian picture-wing fly Drosophila digressa Insect 

Hine's emerald dragonfly Somatochlora hineana Insect 

Hungerford's crawling water Beetle Brychius hungerfordi Insect 

Karner blue butterfly Lycaeides melissa samuelis Insect 

Kauai cave wolf or pe'e pe'e maka 'ole 

spider 

Adelocosa anops Arachnid 

Kern primrose sphinx moth Euproserpinus euterpe Insect 
6 

Common name Scientific name Taxa 

Laguna Mountains skipper Pyrgus ruralis lagunae Insect 

Lange's metalmark butterfly Apodemia mormo langei Insect 

Lotis blue butterfly Lycaeides argyrognomon lotis Insect 

Mission blue butterfly Icaricia icarioides missionensis Insect 

Mitchell's satyr Butterfly Neonympha mitchellii mitchellii Insect 

Mount Charleston blue butterfly Plebejus shasta charestonensis Insect 

Myrtle's silverspot butterfly Speyeria zerene myrtleae Insect 

Northeastern beach tiger beetle Cicindela dorsalis dorsalis Insect 

Ohlone tiger beetle Cicindela ohlone Insect 

Oregon silverspot butterfly Speyeria zerene hippolyta Insect 

Pacific Hawaiian damselfly Megalagrion pacificum Insect 

Palos Verdes blue butterfly Glaucopsyche lygdamus 

palosverdesensis 

Insect 

Pawnee montane skipper Hesperia leonardus montana Insect 

Puritan tiger beetle Cicindela puritana Insect 

Quino checkerspot butterfly Euphydryas editha quino (=E. e. 

wrighti) 

Insect 

Saint Francis' satyr butterfly Neonympha mitchellii francisci Insect 

Salt Creek Tiger beetle Cicindela nevadica lincolniana Insect 

San Bruno elfin butterfly Callophrys mossii bayensis Insect 

Schaus swallowtail butterfly Heraclides aristodemus ponceanus Insect 

Smith's blue butterfly Euphilotes enoptes smithi Insect 

Spruce-fir moss spider Microhexura montivaga Arachnid 

Taylor's Checkerspot butterfly Euphydryas editha taylori Insect 

Uncompahgre fritillary butterfly Boloria acrocnema Insect 

Valley elderberry longhorn beetle Desmocerus californicus dimorphus Insect 

Zayante band-winged grasshopper Trimerotropis infantilis Insect 

[Unnamed] pomace fly Drosophila aglaia Insect 

[Unnamed] pomace fly Drosophila differens Insect 

[Unnamed] pomace fly Drosophila hemipeza Insect 

[Unnamed] pomace fly Drosophila heteroneura Insect 

[Unnamed] pomace fly Drosophila montgomeryi Insect 

[Unnamed] pomace fly Drosophila mulli Insect 

[Unnamed] pomace fly Drosophila musaphila Insect 

[Unnamed] pomace fly Drosophila neoclavisetae Insect 

[Unnamed] pomace fly Drosophila obatai Insect 

[Unnamed] pomace fly Drosophila ochrobasis Insect 

[Unnamed] pomace fly Drosophila substenoptera Insect 

[Unnamed] pomace fly Drosophila tarphytrichia Insect 

References 

USEPA, 2013. Environmental Fate and Ecological Effects Risk Assessment for the 

Registration Review of Silica (DP 393658) 

REGISTRATION REVIEW

ECOLOGICAL RISK ASSESSMENT AND 

EFFECTS DETERMINATION

Denatonium saccharide

CAS Number: 90823-38-4

PC Code: 129045

Prepared by:

Michael Lowit, Ph.D., Ecologist

He Zhong, Ph.D., Biologist

Environmental Risk Branch 1

Reviewed by:

Edward Odenkirchen, Ph.D., Senior Science Advisor

Stephen Wente, Ph.D., Biologist

Sujatha Sankula, Ph.D., Branch Chief

Environmental Risk Branch 1

Environmental Fate and Effects Division

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch I

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington, DC 20460
2

I. Executive Summary

Denatonium saccharide, registered by MCB, LLC, is a bittering agent used as a squirrel, vole, 

dog, and cat repellant on outdoor structures and surfaces applied with a coarse droplet sprayer or 

with a brush. It may also be applied to seeds and bulbs. There is currently one registered 

product, RO-PEL (Reg. No. 8117-1), a ready-to-use liquid formulation which contains 0.065% 

denatonium saccharide. Currently, only 50 Kg per annum or less of denatonium saccharide is 

imported (Email correspondence from the registrant, Roger Brown, MCB, LLC, to Andrea 

Mojica, EPA, 10/8/2012). There are no restrictions on application rates; however, 1 gallon of 

RO-PEL treats 1000 to 4000 ft2 and a second application may be made after 2-7 days. Although 

not explicitly stated on the product label, the Biological and Economic Analysis Division 

(BEAD) states that the product appears to be used only in residential settings, either by 

homeowners or commercial contractors (USEPA, 2013).

No previous risk assessments have been conducted for denatonium saccharide. Toxicity and fate 

data were recommended in the problem formulation (USEPA, 2008)

1 and a data call in (DCI) 

was issued for those studies on January 29, 2010. Subsequently, the registrant submitted a data 

waiver request for those studies and EFED recommended against waiving the studies (USEPA, 

2011); however, no new data have been submitted. Only one-registrant submitted toxicity study 

is available for denatonium saccharide. An acute oral toxicity study with the rat showed an LD50

> 5000 mg formulation/kg-bw (> 3.25 mg denatonium saccharide/kg-bw). No fate data are 

available. An open literature search did not yield any toxicity data for denatonium saccharide. 

Analog chemicals have not been identified (http://aim.epa.gov/); however, limited data are 

available for denatonium benzoate, which shares the same denatonium cation with denatonium 

saccharide. Denatonium benzoate is a bittering agent like denatonium saccharide. Denatonium 

benzoate toxicity was used as a surrogate for denatonium saccharide, given that it is the best 

available information. The denatonium cation was assumed to be primarily responsible for 

denatonium saccharide toxicity. Saccharine (sodium saccharide) is an artificial sweetener used 

in many foods. The risk characterization of denatonium saccharide was based on denatonium 

cation toxicity and exposure. The risk characterization is qualitative (i.e., RQs were not 

calculated) given the inherent uncertainty in using the denatonium benzoate data. 

There is an acute risk concern for listed and non-listed birds (surrogates for reptiles and 

terrestrial-phase amphibians) from use on seeds and bulbs. One possible option to reduce or 

eliminate the risk concern for birds is through label changes such as removal of seed and bulb 

use or changes to the application rates, although this may be impractical given reliance on the 

end-user to treat the seeds and bulbs. Risk could not be assessed for bees visiting flowers that 

develop from treated seeds, treated bulbs, and plants that take up denatonium saccharide used to 

treat structures (run-off or spray drift exposure ) because the only known toxicity study with bees

resulted in a non-definitive endpoint at a test concentration that is lower than exposure estimates. 

Risk to bees could be assessed with denatonium saccharide toxicity data (adult and larval oral) 

that tests at a higher test concentration or with a refined exposure estimate based on denatonium 

 1 (850.2100) Avian acute oral toxicity, (850.2200) Avian dietary toxicity, (850.1010) Aquatic invertebrate acute 

toxicity, (850.1075) Freshwater fish acute toxicity test, (835.2120) Hydrolysis, (835.4100) Aerobic soil 

metabolism, and (835.1230) Sediment and soil adsorption/desorption
3

saccharide residue data for labeled use rates. Furthermore, risk would be negligible if 

information is available that indicates that denatonium saccharide is not transported into plant 

material, although this seems unlikely given that it is water soluble and has a low Kow. There is 

a potential acute risk concern for listed mammals; however the concern is limited to use on some 

types of seeds (there is not a concern for six of the seven evaluated grass species). Possible 

options for reducing risk to listed mammals would be label changes such as requiring seed 

incorporation or lowering the maximum application rate for seed use in areas where listed 

mammals that eat seeds may be present. Although direct effects are expected for listed and nonlisted species, indirect risks to non-listed species are expected to be limited to obligate 

relationships given the small scale use of the product. Denatonium saccharide does not 

universally deter all organisms; thus, the risk concern for direct effects to birds and mammals is 

limited to animals that are not deterred by denatonium saccharide. The degree to which this may 

be the case for any given species cannot be addressed with the available information. Large 

margins of safety (low exposure compared to estimated toxicity) suggest little risk for aquatic 

organisms. Risk to terrestrial plants is assumed low given the low risk to aquatic plants. 

Chronic risk to aquatic and terrestrial organisms is assumed to be negligible given the limited 

and localized use of denatonium saccharide.

Table 1. Summary of Direct and Indirect Effects Associated with Registered Uses of 

Denatonium Saccharide

Taxon

Risk Concern for

Direct Effects?

Risk Concern for 

Indirect Effects to 

Non-Listed Listed Listed Species?

Birds, reptiles, and terrestrial-phase amphibians Yes Yes Yes1

Mammals No Yes Yes1

Terrestrial invertebrates (honeybees)2 Unknown3 NA Unknown3

Terrestrial (upland and semi-aquatic) plants – monocots No No Yes1

Terrestrial (upland and semi-aquatic) plants – dicots No No Yes1

Freshwater fish and aquatic-phase amphibians No No No

Marine/estuarine fish No No No

Freshwater crustaceans No No No

Mollusks No No No

Marine/estuarine crustaceans No No No

Aquatic vascular plants No No Yes1

Aquatic non-vascular plants No No No 1 Indirect effect as a result of direct effects to birds or mammals are expected only if a species has an 

obligate/dependent relationship with a bird or mammal given the limited use pattern. 2 Honeybee LOC = 0.4 and is used as a surrogate for terrestrial invertebrates. Currently there is not an LOC for 

listed terrestrial invertebrates. 3 The only known toxicity study with bees resulted in a non-definitive endpoint at a test concentration that is lower 

than exposure estimates.

NA = Not applicable 

II. Use Characterization and Methods of Application

The RO-PEL product label allows application to outdoor structures and surfaces such as trees, 

fences, poles, decks, planters, siding, garbage cans, and furniture. There are no restrictions on 

the application equipment except that the sprayer should be able to deliver a coarse droplet size 

and that mist blowers and ultra low volume sprayer are prohibited. It is assumed that hand-held 

sprayers are the most likely spray equipment given the assumed residential use. The application 
4

rate for this use is 1 gallon per 1,000 to 4,000 ft2

. A second application is permitted after 2-7 

days. There are no annual limits. This product is also labeled as a bulb and seed treatment to 

reduce consumption and damage by tree squirrels and voles. The directions for these uses only 

state "Using rubber gloves, place seeds or bulbs in a plastic or metal container and spray 

thoroughly until soaked." No further application information is provided on the label. Based on 

extension service and trade organization information, as well as best professional judgment, 

BEAD provided application rate estimates for seeds (ranging from 2.3 x 10-4 to 2.6 x 10-3 lb 

ai/1000 ft2 for seven common grass species) and bulbs (6 x 10-6 lb ai/bulb) (USEPA, 2013). 

III. Summary of Environmental Fate

Denatonium saccharide is formed by the ionic bond between two oppositely charged ions 

(denatonium and saccharide). The denatonium chemical and physical properties and the 

saccharide structure are summarized in Table 2.

No chemical or physical property data for denatonium saccharide have been submitted. The 

Agency recommended hydrolysis (835.2120), aerobic soil metabolism (835.4100), and 

adsorption/desorption (835.1230) studies in the problem formulation (USEPA, 2008). In 

addition, EFED did not concur with the data waiver requests for the above studies (USEPA, 

2011). Because of these unfulfilled data gaps, the Agency used EPI v4.10 to estimate the 

chemical and physical properties of the denatonium cation, which was assumed to be primarily 

responsible for denatonium saccharide toxicity. 

The Koc value estimated by the EPI v 4.10 may not be appropriate for the denatonium cation as 

its leaching behavior is regulated by soil pH and not by soil organic carbon content. Therefore, 

the Kd value is likely better suited to predict the leaching potential of the denatonium cation. 

The denatonium benzoate Kd value is used as a surrogate for denatonium saccharide because the 

Kd for the denatonium cation is not available. The denatonium benzoate Kd value was obtained

from a submitted study (MRID 47146904).

Table 2. Denatonium Physical and Chemical Properties and Saccharide Structure

Parameter Value Reference

Denatonium Saccharide

Chemical Name benzyl-[2-(2,6-dimethylanilino)-2-

oxoethyl]-diethylazanium;1,1-dioxo-1,

2-benzothiazol-3-olate

NCBI1

Empirical Formula C28H33N3O4S

Molecular mass 507.64

CAS Number 90823-38-4

Structure Sigma2

Denatonium

Empirical Formula C21H29N2O1

SMILES c1ccccc1CN(CC)(CC)CC(=O)Nc2c(C)ccc

c2C
5

Parameter Value Reference

Molecular mass 325.48 EPI V4.10

Solubility (25°C) 428.9 mg/L

Log Kow 0.18 IUPAC3

Vapor pressure (23.1°C) 3.57 x 10-13 mm Hg EPI V4.10

Henry’s law constant (20°C, pH 7) 1.21 x10-18 atm·m3/mol

Hydrolysis (t 1⁄2 ) Stable No Data

Aqueous Photolysis (t 1⁄2 ) Stable

Aerobic Aquatic Metabolism (t 1⁄2 ) 37.5 days EPI V4.10

Aerobic Soil Metabolism (t 1⁄2 ) 75 days

Anaerobic Soil Metabolism (t 1⁄2 ) 338 days

Kd (ml/g) Boghall sandy loam 7.9

Easter Howgate Sand 11.4

Kitleyknowe sandy loam 20.8

Balmanno silty clay loam 527.8

MRID 47146904

(a denatonium 

benzoate study)

Saccharide

SMILES (Saccharide) C1=CC=C2C(=C1)C(-NS2(=O)=O)=O

Empirical Formula (Saccharide) C7H4N1O3S1

Molecular mass 182.18

1 http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=64772

2 http://www.sigmaaldrich.com/catalog/product/sigma/d2418?lang=en®ion=US

3 The IUPAC website (http://sitem.herts.ac.uk/aeru/iupac/1398.htm) reports log Kow = 0.18 for denatonium 

benzoate. 

IV. Summary of Ecological Effects

One-registrant submitted toxicity study is available for denatonium saccharide. An acute oral 

toxicity study with the rat showed an LD50 > 5000 mg formulation/kg-bw (> 3.25 mg 

denatonium saccharide/kg-bw). An open literature search did not yield any toxicity data for 

denatonium saccharide. Analog chemicals have not been identified (http://aim.epa.gov/); 

however, limited data are available for denatonium benzoate, which shares the same denatonium 

cation with denatonium saccharide. Denatonium benzoate toxicity was used as a surrogate for 

denatonium saccharide given that it is the best available information (including acute oral 

toxicity to rats). Risk to mammals could not be assessed using the submitted denatonium 

saccharide data because of the lack of mortality at the limit test concentration which is lower 

than estimated environmental concentrations (EECs); therefore, submitted denatonium benzoate 

data was used to better characterize risk to mammals. Registrant submitted toxicity studies are 

summarized in Table 3; additional details about those studies and other toxicity information are 

discussed below. Denatonium benzoate toxicity is presented in this section, but the risk 

assessment considers toxicity of and exposure to the denatonium cation. This was calculated by 

multiplying the denatonium benzoate toxicity by the molecular weight ratio of denatonium 

(325.48) to denatonium benzoate (446.59). Exposure expressed as the denatonium cation was 

calculated by multiplying the denatonium saccharide exposure by the molecular weight ratio of 

denatonium (325.48) to denatonium saccharide (507.64).
6

Table 3. Summary of Toxicity of Denatonium Saccharide and Denatonium Benzoate 

(Registrant Submitted Studies)1

Test Species Exposure 

Scenario

Summary MRID

Denatonium Saccharide

Rat Single oral 

dose

LD50 > 5000 mg formulation/kg-bw 

LD50 > 3.25 mg denatonium saccharide/kg-bw 

No effects observed

43698107

Denatonium Benzoate

Northern 

bobwhite quail 

(Colinus 

virginianus)

Single oral 

dose

LD50 = 196 mg ai/kg-bw (95% C.I. 145-264)

NOAEL = 50 mg ai/kg-bw

LOAEL = 100 mg ai/kg-bw

Endpoints affected: mortality and clinical signs of 

toxicity

47146905

11 days 

(5 day 

exposure)

LC50 = 5080 mg ai/kg diet (95% C.I. 2520 to + infinity)

NOAEC < 657 mg ai/kg diet

LOAEC = 657 mg ai/kg diet

Endpoint affected: mortality

47146906

Mallard duck

(Anas 

platyrhynchos)

8 days 

(5 day 

exposure)

LC50 > 5150 mg ai/kg diet 

NOAEC = 5150 mg ai/kg diet

LOAEC > 5150 mg ai/kg diet

No effects observed

47146907

Rat Single oral 

dose

LD50 = 648 mg ai/kg-bw (95% C.I. 506-832) (female)

LD50 = 841 mg ai/kg-bw (95% C.I. 707-1000) (male)

NOAEL < 500 mg ai/kg-bw

Endpoints affected: mortality and clinical signs of 

toxicity

47135802

Rainbow trout

(Salmo 

gairdneri)

96 hr static

LC50 > 1000 mg/L* 

NOAEC = 500 mg/L

LOAEC = 1000 mg/L

Endpoints affected: mortality and sublethal effects

*There is uncertainty about the test material and test 

concentration in terms of active ingredient (denatonium 

benzoate).

47135816

Crangon sp. 96 hr static

LC50 = 554 mg/L (95% C.I. 412-755)*

NOAEC = 125 mg/L

LOAEC = 250 mg/L

Endpoint affected: mortality

*There is uncertainty about the test material and test 

concentration in terms of active ingredient (denatonium 

benzoate).

47135817
7

Test Species Exposure 

Scenario

Summary MRID

Water flea

(Daphnia 

magna)

48 hr

EC50 = 10.2 mg/L (95% C.I. 7.7-14.0) (nominal)

NOAEC = 5 mg/L

LOAEC = 10 mg/L

Endpoints affected: mortality

47135818

21 day

static renewal

Parental survival

NOAEC = 2 mg/L (nominal)

LOAEC = 5 mg/L

Reproduction (brood release and offspring per parent)

NOAEC ≥ 5 mg/L

LOAEC > 5 mg/L

Freshwater 

diatom

(Nitzschia 

palea)

70.5 hr

Yield (0-70.5 hr)

IC50 = 5.0 to 10.0 mg/L (nominal)

NOAEC = 5.0 mg/L

LOAEC = 10 mg/L

Growth Rate (0-45.5 hr)

IC50 = 6.1 mg/L (95% C.I. 5.8-6.5)

NOAEC = 5.0 mg/L

LOAEC = 10 mg/L

Area Under the Growth Curve (0-70.5 hr)

IC50 = 5.7 mg/L (95% C.I. 5.2-6.3)

NOAEC = 5.0 mg/L

LOAEC = 10 mg/L

47135819

1 The risk assessment considers toxicity of the denatonium cation. Toxicity in Table 3 is presented in terms of 

denatonium saccharide and denatonium benzoate. Toxicity as the denatonium cation is calculated by multiply 

toxicity of denatonium saccharide and denatonium benzoate by the molecular weight ratio of denatonium (325.48) 

to denatonium saccharide (507.64) and denatonium (325.48) to denatonium benzoate (446.59), respectively.

Denatonium Saccharide Data

Acute Oral Toxicity to Rats

The acute oral LD50 was > 5000 mg formulation/kg-bw (> 3.25 mg denatonium saccharide/kgbw) (MRID 43698107) for rats. No effects were observed.

Ancillary Information

A study report on the state of knowledge of several aversive agents (MRID 46760001) reports 

acute toxicity of denatonium saccharide to rats (LD50 = 1430 mg/kg), shrimp (species not 

specified, LC50 = 600 mg/L), and rainbow trout (LC50 = 1500 mg/L). Similar values were 

reported for denatonium benzoate; acute toxicity to rats (LD50 = 612 mg/kg), shrimp (species not 

specified, LC50 = 400 mg/L), and rainbow trout (LC50 = 1000 mg/L). No additional information 

was provided including the test material and the source of the data. Although the acute oral rat 

toxicity value reported for denatonium benzoate is similar to the value observed in the submitted 

study (LD50 = 648 mg ai/kg-bw; MRID 47135802; discussed below), collectively there is 

considerable uncertainty in what these values represent given that the test material is unknown. 
8

The test material may have been TGAI or a formulation of unknown percent active ingredient in 

some or all the studies. Therefore, a comparison cannot be made among the endpoints reported 

in MRID 46760001 (denatonium benzote vs. denatonium saccharide) and those endpoints cannot 

be compared to the submitted denatonium benzoate studies which are discussed below. These 

values are not considered in the risk assessment given the considerable uncertainty due to a lack 

of information about the studies.

Denatonium Benzoate Data

Acute Toxicity to Freshwater Fish

In a 96 hr acute toxicity study (MRID 47135816) with rainbow trout the LC50 was >1000 mg/L. 

The NOAEC = 500 mg/L and the LOAEC = 1000 mg/L based on 10% mortality. Increased 

pigmentation and loss of equilibrium were observed; however, it was not specified which 

treatment groups exhibited these sub-lethal effects. Therefore, a NOAEC value based on sublethal effects could not be determined. Information about the test material (purity and 

description) was not reported and test concentrations were based on nominal concentrations. For 

the risk assessment, it was assumed that the test was conducted using the formulation containing 

the lowest percent denatonium benzoate (0.0078%; source: all previously USEPA-registered 

denatonium benzoate products; see Table 4 for additional details). The study was classified as 

qualitative use because of uncertainty about test concentrations and the submission was a 

summary report which did not include critical information.

Acute Toxicity to Freshwater Invertebrates

A 48 hr acute toxicity study with Daphnia magna was conducted in conjunction with a 21day 

chronic toxicity study (MRID 47135818). The 48 hr EC50 = 10.2 (95% confidence interval; 7.7-

14.0) mg/L. The NOAEC = 5 mg/L and the LOAEC = 10 mg/L based on mortality. Sublethal 

effects were not reported. 

Acute Toxicity to Marine/Estuarine Invertebrates

In a 96 hr acute toxicity study with Crangon sp. the LC50 = 554 mg/L (95% confidence interval; 

412-755). The NOAEC = 125 mg/L and the LOAEC = 250 mg/L based on mortality (MRID 

47135817). Sublethal effects were not reported. Information about the test material (purity and 

description) was not reported and test concentrations were based on nominal concentrations. For 

the risk assessment, it was assumed that the test was conducted using the formulation containing 

the lowest percent denatonium benzoate (0.0078%; source: all previously USEPA-registered 

denatonium benzoate products; see Table 4 for additional details). The study was classified as 

qualitative use because of uncertainty about test concentrations and the submission was a toxicity 

summary report which did not include basic information about the experimental design and study 

conditions. 
9

Chronic Toxicity to Freshwater Invertebrates

Chronic toxicity to Daphnia magna was studied under static renewal conditions (MRID 

47135818). The 21 day EC50 = 4.6 (95% confidence interval; 3.2-6.4) mg/L. The NOAEC = 2 

mg/L and the LOAEC = 5 mg/L based on adult mortality. The NOAEC ≥ 5 mg/L (LOAEC > 5 

mg/L) for first brood release and mean number of offspring per parent. Terminal growth of the 

first-generation daphnia was not evaluated, thus the NOAEC was likely underestimated. 

Algal Toxicity

(Freshwater Diatom)

In a 70.5 hr toxicity study with Nitzschia palea, the NOAEC = 5 mg/L for yield, growth rate, and 

area under the growth curve (MRID 47135819). The IC50 was between 5 and 10 mg/L for all 

endpoints. The study was classified as qualitative use in part because the initial cell density was 

low and there was a lack of logarithmic growth after 45.5 hr. Toxicity values may have been 

lower if logarithmic growth had continued through 96 hr (minimum recommended test duration). 

(Freshwater Green Algae)

Portugal (2008) reports toxicity of ARBINOL B (formulation containing denatonium benzoate) 

to Scenedesmus subspicatus. The 72 h NOEC ≥ 100 mg/L ARBINOL B (LOEC > 100 mg/L 

ARBINOL B; no effects at the highest test concentration). ARBINOL B contains 0.11 g 

denatonium benzoate/L of product (EFSA, 2012); therefore, the NOEC ≥ 0.011 mg denatonium 

benzoate/L. EFED has not reviewed this study and takes the reported toxicity at face value for 

purposes of this assessment. 

Acute Oral Toxicity to Birds

The 14-day acute oral LD50 = 196 (95% confidence interval; 145-264) mg ai/kg-bw for northern 

bobwhite quail (MRID 47146905). The Probit slope = 5.52. The 14-day NOAEL = 50 mg 

ai/kg-bw and the LOAEL = 100 mg ai/kg-bw based on treatment-related mortality. Unsteady 

and/or subdued behavior was observed in birds that subsequently died and birds that survived 

until study termination at the ≥100 mg ai/kg-bw treatment levels. Recovery of affected birds was 

observed by day 6. No clinical signs of toxicity were observed in birds from the control, 25, or 

50 mg ai/kg-bw treatment levels. There were no apparent treatment-related effects on body 

weight or feed consumption at any dosage level. The beak and front of the head were observed 

to be wet in 9 of the 15 birds that died (100, 200, and 400 mg ai/kg-bw treatment levels); no 

further explanation was provided regarding this observation. No treatment-related abnormalities 

were detected in any other birds.

Acute Dietary Toxicity to Birds

The acute dietary toxicity of denatonium benzoate to northern bobwhite quail was assessed over 

11 days (5 day exposure) (MRID 47146906). The LC50 = 5080 (95% confidence interval; 2520 

to + infinity) mg ai/kg diet and the NOAEC < 657 mg ai/kg diet based on mortality. Observed 
10

mortality is assumed to be treatment related although pecking may have been a contributing 

factor. There were no clear clinical signs of toxicity, effects on body weight, or effects on feed 

consumption. In addition, no treatment-related gross pathological changes were observed. 

The acute dietary toxicity of denatonium benzoate to mallard duck was assessed over 8 days (5 

day exposure) (MRID 47146907). The LC50 > 5150 mg ai/kg diet. No treatment-related effects 

were observed on mortality, body weight, or feed consumption. No clinical signs of toxicity 

were observed and no treatment-related observations were made at necropsy. 

Acute Oral Toxicity to Rats 

The acute oral LD50 = 648 mg ai/kg-bw (95% confidence interval; 506-832 mg ai/kg-bw) for 

female rats and 841 mg ai/kg-bw (95% confidence interval; 707-1000 mg ai/kg-bw) for male rats 

(MRID 47135802). Complete mortality was observed at the highest treatment rate (of three) 

within 30 minutes of dosing. Sublethal effects (hunched posture, lethargy, decreased respiratory 

rate, ataxia, noisy respiration, or labored respiration) were observed at the lowest two doses (500 

and 707 mg ai/kg-bw) up to two days after dosing. All surviving animals appeared normal 

throughout the remainder of the study. Necropsy revealed abnormalities in the animals that died 

during the study (hemorrhagic lungs, dark liver, dark kidneys, pale gastric mucosa, and 

hemorrhage or slight hemorrhage of the gastric mucosa) but not in the animals that survived.

Acute Toxicity to Bees

EFED does not have data for bees; however, data were reported by the European Food Safety 

Authority (Portugal, 2008; EFSA, 2012). Testing was conducted with the formulation 

ARBINOL B which contains denatonium benzoate. The acute oral 48 hr LD50 > 362.5 μg 

ARBINOL B/bee and the acute contact 48 hr LD50 > 400 μg ARBINOL B/bee. ARBINOL B 

contains 0.11 g denatonium benzoate/L of product (EFSA, 2012). Therefore, the acute oral 48 hr 

LD50 > 0.039 μg denatonium benzoate/bee and the acute contact 48 hr LD50 > 0.044 μg 

denatonium benzoate/bee. Mortality was observed in the acute oral test at three of the five test 

concentrations (25, 200, and 400 μg ARBINOL B/bee). EFED assumes that it is more likely that 

the mortality was background noise and not treatment related given that the mortality was 3.3% 

at each of those test concentrations and there was no dose response. No mortality was observed 

in the acute contact test. EFED has not reviewed this study and takes the reported toxicity at 

face value for purposes of this assessment.

Incident Information

No incidents involving denatonium saccharide were reported in the Ecological Incident 

Information System (EIIS, v 2.1.1), the Aggregate Incident Data System, or the Avian Incident 

Monitoring System (AIMS)2 as of September 25, 2013.

 2 http://www.abcbirds.org/abcprograms/policy/toxins/aims/aims/index.cfm
11

V. Risk Characterization

Denatonium saccharide toxicity and fate data were recommended in the problem formulation; 

however, none are available at this time except for an acute oral toxicity to rat study. Analog 

chemicals have not been identified (http://aim.epa.gov/); however, limited data are available for 

denatonium benzoate, which shares the same denatonium cation with denatonium saccharide. 

Denatonium benzoate toxicity was used as a surrogate for denatonium saccharide given that it is 

the best available information. Risk to mammals could not be assessed using the submitted 

denatonium saccharide data because of the lack of mortality at the limit test concentration which 

is lower than estimated environmental concentrations; therefore, submitted denatonium benzoate 

data was used to better characterize risk to mammals. The denatonium cation was assumed to be 

primarily responsible for denatonium saccharide toxicity. This assumption is considered 

reasonable because saccharine (sodium saccharide) is a synthetic sugar found in many food 

products. Therefore, the risk characterization of denatonium saccharide was based on 

denatonium cation toxicity and exposure. The risk characterization is qualitative (i.e., RQs were 

not calculated) given the inherent uncertainty in using the denatonium benzoate data. 

Furthermore, there is uncertainty in the exposure estimates given the lack of fate data and general 

lack of use restrictions on the labels. Based on the characterization, lack of denatonium 

saccharide acute toxicity data for birds and honeybees has the greatest potential impact on the 

risk conclusions. Risk conclusions are not expected to change for other taxa. 

Chronic risk to aquatic and terrestrial organisms is assumed to be negligible given the limited 

and localized use of denatonium saccharide in addition to the limited annual use (less than 100 lb 

denatonium saccharide is sold per year according to the registrant; USEPA, 2011)

3

. 

Aquatic Organisms

Based on the qualitative assessment, it is unlikely that there would be a risk concern for aquatic 

organisms from the use of denatonium saccharide. Direct adverse effects to aquatic organisms 

are not expected.

For aquatic exposure, it was assumed that uses of denatonium saccharide may result in 

contamination of surface water, although exposure is expected to be low based on the nature of 

the labeled uses. It was assumed that exposure would occur primarily from runoff water 

transporting denatonium saccharide directly into surface waters or indirectly through storm 

sewers. Runoff could come from all labeled uses, although primarily from applications to 

impervious surfaces and unburied seeds. Spray drift is assumed negligible based on the 

application methods (most likely hand-held trigger sprayers4

). The labeled uses of denatonium 

saccharide will likely result in multiple, relatively small, spatially and temporally variable 

applications. Given the type of application and lack of fate data, standard aquatic exposure 

modeling (e.g. PRZM/EXAMS) was not used to directly calculate EECs for denatonium 

 3 An assessment of chronic risk to aquatic invertebrates is presented given that denatonium benzoate toxicity data 

were available.

4 There are no restrictions on the application equipment except that the sprayer should be able to deliver a coarse 

droplet size and that mist blowers and ultra low volume sprayer are prohibited. It is assumed that hand-held 

sprayers are the most likely spray equipment given the residential use.
12

saccharide. Instead, for illustrative purposes, aquatic exposure was estimated based on dilution 

of denatonium saccharide (i.e. denatonium equivalent) directly into the standard farm pond.

A concentration of 0.078 μg denatonium/L would result from spraying an entire bottle of ROPEL (1 gallon5

; 0.065% denatonium saccharide) directly into a 20 million liter pond (i.e., 

standard farm pond) assuming no degradation or sorption. Even with this conservative scenario, 

it would require the simultaneous discharge of many gallons of RO-PEL into the standard farm 

pond before there would be a risk concern for aquatic organisms (Table 4).

 5 The RO-PEL label is for a 1 gallon product. Each gallon contains 0.0054 lb denatonium saccharide (0.065% ai x 

8.34 lb/gallon = 0.0054 lb ai). 1 gallon of RO-PEL in the standard farm pond results in a concentration of 0.122 μg

denatonium saccharide/L (2,449,399 μg denatonium saccharide per gallon/20,000,000 L) or 0.078 μg denatonium/L 

(0.122 μg denatonium saccharide/L x 0.641). Molecular weight ratio of denatonium (325.48) to denatonium 

saccharide (507.64) = 0.641.
13

Table 4. Amount of RO-PEL Required to be a Risk Concern to Listed Aquatic Organisms1

Taxa Peak EEC from 

one gallon of 

RO-PEL (μg 

denatonium/L)

2

Toxicity 1 Number of 

gallons of 

RO-PEL to 

exceed LOC

(high estimate)3

Toxicity 24 Number of 

gallons of ROPEL to exceed 

LOC

(low estimate)3

Freshwater fish 

(acute)

0.078 LC50 > 

728,000 μg 

denatonium/L

>466667 LC50 > 56 μg 

denatonium/L

>365

Freshwater 

invertebrate 

(acute)6

0.078 EC50 = 7425

μg 

denatonium/L

4760 NA NA

Marine/estuarine 

invertebrate 

(acute)

0.078 LC50 = 403312

μg 

denatonium/L

258333 LC50 = 31 μg 

denatonium/L

205

Freshwater 

invertebrate 

(chronic)7

0.078 NOAEC = 

1456 μg 

denatonium/L

18667 NA NA

Marine/estuarine 

invertebrate 

(chronic)7,8

0.078 NOAEC = 

79080 μg 

denatonium/L

1013846 NOAEC = 6.2 μg 

denatonium/L

795

Algae (listed 

species)6

0.078 NOAEC = 

3640 μg 

denatonium/L

46667 NA NA

1 Exposure and toxicity of denatonium saccharide expressed in terms of denatonium cation.

2 EEC based on one gallon of RO-PEL added directly to the standard farm pond (20,000,000 L) and assuming complete 

mixing. 

3 Acute endpoints: Number of gallons RO-PEL = (toxicity as μg denatonium/L x 0.05)/0.078 μg denatonium/L. Chronic 

endpoints and algae (listed species): Number of gallons RO-PEL = (toxicity μg denatonium/L x 1)/0.078 μg 

denatonium/L. The acute listed species LOC = 0.05. The chronic LOC and the algae (listed species) LOC = 1. 4

In some studies, information about the test material including purity was not reported and test concentrations were based 

on nominal concentrations (“toxicity 1” assumes that the test material was TGAI and 100% purity). “Toxicity 2” 

assumes that the test was conducted using the formulation containing the lowest percent denatonium benzoate (0.0078%; 

source: all previously USEPA-registered denatonium benzoate products). See footnote 5 for more information. 5 The test material used in the studies was Bitrex®. According to the Bitrex website (www.bitrex.com), 0.256% is the 

lowest percentage of denatonium benzoate in currently available Bitrex® products. If toxicity is based on that percentage

then the low end number of gallons of RO-PEL to exceed the LOC would be much greater than shown in Table 4; > 1194 

for freshwater fish (acute), 662 for marine/estuarine invertebrates (acute), and 2590 for marine/estuarine invertebrates 

(chronic).

6 Portugal (2008) and EFSA (2012) report toxicity of ARBINOL B (formulation containing 0.11 g/L denatonium benzoate; 

EFSA, 2012) to Scenedesmus subspicatus (72 h NOEC ≥ 100 mg/L ARBINOL B or ≥ 0.011 mg denatonium benzoate/L)

and Daphnia magna (48 h EC50 > 500 mg/L). It is not clear if the Daphnia toxicity value is reported in terms of active 

ingredient or formulation (ARBINOL B); thus, the Daphnia toxicity value could be EC50 > 0.055 mg denatonium 

benzoate/L. The Scenedesmus subspicatus and potentially the Daphnia toxicity value are lower than those in the 

submitted Nitzschia palea (algae) and Daphnia studies; however, no mortality was observed in the acute toxicity of 

ARBINOL B to Daphnia magna study and no effects were observed in the ARBINOL B toxicity study with Scenedesmus 

subspicatus. These endpoints are not considered further because they would not change the risk conclusions (even 

assuming that the toxicity values represented a definitive endpoint, the gallons of RO-PEL to exceed the LOC is >102 for 

Scenedesmus and >26 for Daphnia). 7 Peak EEC was used as a conservative estimate of chronic exposure (21 days for invertebrates). 

8 Toxicity estimated with acute-to-chronic ratio using available data (ACR = acute toxicity to Daphnia (LC50 = 10.2 

mg/L)/chronic toxicity to Daphnia (NOAEC = 2 mg/L). ACR = 5.1 is applied to the acute toxicity to Crangon sp. (LC50 = 

554 mg/L). Chronic marine/estuarine toxicity = 108.6 mg denatonium benzoate/L or 79080 μg denatonium/L.

NA = Not applicable 
14

Twenty gallons of RO-PEL is the minimum volume required to trigger a risk concern for listed 

species.6

 Actual exposure is expected to be much lower. The following conservative 

hypothetical scenario was considered:

1. The standard agricultural field simulated using PRZM has an area of 24.7 acres (10 ha). 

Within that 10 ha it is assumed that there are 58 residential lots (see EFED’s 2010 

California red-legged frog assessment for carbaryl7)

2. Every home in that neighborhood has a deck with an area of 200 ft2. The total area of 

deck surface is 11600 ft2 /10 ha.

3. Each homeowner purchases a gallon of RO-PEL and all decks in the neighborhood are 

treated at the rate of 1000 ft2 (one gallon treats 1000 to 4000 ft2). This would result in 

11.6 gallons of RO-PEL applied /10 ha

4. This volume is slightly more than half that needed to trigger a risk concern (20 gallons 

applied directly to the waterbody). Concentrations would still not be high enough to 

trigger a risk concern even if the 11.6 gallons was applied to all the decks, was 

simultaneously washed off all the decks, and all the denatonium entered the standard 

farm pond without loss of material.

Although there are uncertainties about application rates and the toxicity data (i.e., use of 

denatonium equivalent toxicity to represent denatonium saccharide toxicity, lack of data on all 

required test species, and questions about the quality of some of the denatonium benzoate 

studies), that uncertainty is likely offset by the conservative assumptions about the fate of 

denatonium saccharide (all applied material ends up in the water body) and the large volumes of 

RO-PEL needed to trigger a risk concern. 

Terrestrial Organisms

Based on the qualitative assessment, it is unlikely that there would be a risk concern for 

terrestrial plants from the use of denatonium saccharide if data were available for quantitative 

assessment (i.e., if RQs were calculated); however, there is a risk concern for birds (listed and 

non-listed species)8 and mammals (listed species). Risk could not be assessed for bees visiting 

flowers that develop from treated seeds, treated bulbs, and plants that take up denatonium 

saccharide used to treat structures (run-off or spray drift exposure) because the only known 

toxicity study with bees resulted in a non-definitive endpoint at a test concentration that is lower 

than exposure estimates.

RO-PEL is specifically labeled as a bittering agent used as a deterrent to mammals (squirrels, 

voles, dogs, and cats) but not birds or terrestrial invertebrates. It was assumed that some 

 6 This is the most conservative estimate for marine/estuarine invertebrates. It was assumed that the product used for 

testing was the one containing the lowest percentage denatonium benzoate. Previously USEPA-registered products 

contained 0.0078% (1 product), 0.065% (2), 0.025% (1), and 0.2% (15) denatonium benzoate. The test material, 

Bitrex®, may have had a greater percentage of denatonium benzoate than those products (see Table 4 footnotes). 7 http://www.epa.gov/oppfead1/endanger/litstatus/effects/redleg-frog/2010/carbaryl-deltasmelt/appendix-g.pdf 8 Toxicity data are not available for a passerine species which are potentially more sensitive to denatonium 

saccharide than larger birds due to their smaller size and higher metabolic rate.
15

mammals, birds, and terrestrial invertebrates may not be deterred from consuming denatonium 

saccharide in the absence of evidence to the contrary. It is a reasonable assumption given that 

studies indicate that denatonium (typically as denatonium benzoate) does not deter all animals 

and deterrence is often dependent on concentration. For instance, German wasps were not 

deterred from denatonium benzoate treated bait at a concentration of 10 ppm in one study 

(Sackmann et al. 2010) and diamondback moth were not deterred from consuming denatonium 

benzoate treated leaves up to a concentration of 50 mg/L in another study (Perera et al., 2000). 

Although counter-intuitive given the labeled use of RO-PEL, some mammals may not be 

deterred by denatonium saccharide. For example, two studies showed that bank vols (Kleinkauf 

et al., 1999) and pocket gophers (Witmer et al., 1998) were not deterred from consuming food 

items treated with denatonium benzoate. Furthermore, denatonium benzoate is used in baits for 

commensal rodents (e.g., Talon® and RIDAKTM) to prevent accidental ingestion of the bait by 

children. Studies submitted to the USEPA indicate that at least some birds may not be deterred. 

A submitted efficacy study indicates that geese showed dislike for the taste of RO-PEL treated 

turf (observed shaking their heads) but they continuously tasted the treated turf until moving into 

a patch of untreated turf and returned to the treated area at a later time (MRID 44589301). Two 

avian dietary studies with denatonium benzoate showed no evidence of food avoidance by 

mallard duck or bobwhite quail (MRID 47146906 and 47146907). Nonetheless, some animals 

may be deterred, thus reducing risk. The degree to which this may be the case for any given 

species cannot be addressed with the available information.

Risk to terrestrial plants (all uses)

Any risk to terrestrial plants would be from runoff given the small scale use and type of 

application equipment. Risk from spray drift is negligible. There are no terrestrial plant toxicity 

data for denatonium saccharide or denatonium benzoate. There are no incidents with 

denatonium saccharide involving plants, however, there is one minor plant incident involving 

denatonium benzoate (no details are available; source: Aggregate Incident Data System). 

Although the label states that RO-PEL should not cause damage to plants (it is labeled for use on 

seeds, bulbs, and woody parts of plants), it also states that treated seeds should be tested for 

germination. It is reasonable to expect that denatonium saccharide may be toxic to terrestrial 

plants at some concentration given that algae growth (freshwater diatom) was inhibited from 

exposure to denatonium benzoate. Although toxicity was observed, exposure to nearly 50,000 

gallons of RO-PEL (i.e., applied directly to the standard farm pond) would be required to cause a 

risk concern for algae (listed species). For perspective, less than 100 lb denatonium saccharide 

(ca. 18,500 gallons of RO-PEL) is sold on an annual basis according to the registrant (USEPA, 

2011). There is no expectation that terrestrial plants and algae are equally sensitive to 

denatonium saccharide and aquatic exposure cannot be directly compared to potential terrestrial 

exposure; however, adverse effects on listed terrestrial plants from use of RO-PEL seems 

unlikely given the large volumes of RO-PEL required to affect algae and the limited use of the 

product. 

Risk from use on seed

There is a risk concern for birds (listed and non-listed) and mammals (listed) consuming treated 

seeds. Risk could not be assessed for bees visiting flowers that develop from treated seeds 
16

because the only known toxicity study with bees resulted in a non-definitive endpoint at a test 

concentration that is lower than exposure estimates. 

There is potential exposure to birds, mammals, and terrestrial invertebrates (pollinators) from use 

of denatonium saccharide on seeds. Two exposure pathways were considered. First, birds and 

mammals may consume treated seeds which are not required to be incorporated. Second, it is 

assumed that denatonium saccharide is transported into the above ground parts of the plant; thus, 

pollinators could be exposed to residues in pollen and nectar (USEPA, 2012). Available 

information indicating that denatonium saccharide is soluble and has a low Kow is consistent 

with this assumption.

The RO-PEL label does not specify the type of seeds (i.e., plant species) that may be treated or 

the application rate (“spray until thoroughly soaked”). Grasses were used as representative seeds 

because grass seed is the most commonly planted seed around residences and the aggregate 

surface area of grass seeds is expected to be greater than any other seed planted in residential 

areas (USEPA, 2013). Therefore, grass seed use should result in greater exposure than any other 

seed planted around residences. The types of seed (seven common species), number of seeds/lb, 

seeding rates, and estimated application rates were provided by BEAD (USEPA, 2013). 

According to BEAD (communication with Jonathon Becker) the recommended seeding rates are 

applicable to establishing a new lawn and overseeding applications would require lower seeding 

rates. EECs and RQs were calculated in T-REX (v 1.5.2). A representative output is presented 

in Appendix A.

At the highest application rate (1 gallon RO-PEL/1000 ft2

), the EECs (avian Nagy dose) from the 

seed treatment (all seven grass seeds evaluated) are more than 1/10th (ca. 11%9 to 282%) of the 

LD50 for 20 g birds10. EECs for two type of grass seed, bluegrass and colonial bentgrass, are 

more than 1/10th (ca. 12% and 40%, respectively) of the LD50 for 1000 g birds. Even at the 

lowest application rate (1 gallon RO-PEL/4000 ft2), colonial bentgrass EECs are equal to or 

more than 1/10th of the LD50 for the 20 g (72%), 100 g (32%), and 1000 g (10%) birds size 

classes. This risk evaluation assumes that there is enough seed on the field to meet the daily 

dietary needs of a bird based on typical seeding rates and recognizes that the label does not 

require soil incorporation. A bird may not consume 100% of its daily diet as treated seeds; 

however, as little as 4% of the diet of a listed bird would need to be RO-PEL treated seed to be a 

risk concern (Table 5). Table 5 presents a characterization of consumption of seed treated at the 

maximum application rate resulting in an exposure that is 1/10th the LD50 (i.e., RQ = 0.1). For 

example, a 20-g bird would not exceed the listed species LOC if it consumed less than 4% (2419 

or less seeds) of its daily diet as treated colonial bentgrass seed but that same bird would exceed 

the listed species LOC if it consumed 4% or greater (2420 or more seeds) of its daily diet as 

treated colonial bentgrass seed. A small foraging area would be required to consume enough 

seeds to be a risk concern; in some cases the area is less than 1 ft2 (e.g., the seeding rate for 

colonial bentgrass is 9195 seeds/ft2 and a 20 g listed bird would only need to eat 2420 seeds to be 

of concern).

 9 For example, an EEC that is 11% of the LD50 represents an RQ = 0.11. 10 A risk concern is triggered for listed species for EECs ≥ 1/10th the LD50 (i.e., RQ = 0.1). There is also a risk 

concern for non-listed birds (EECs ≥ 1/2 the LD50) consuming colonial bentgrass treated at both 1 gallon ROPEL/1000 ft2 and 4000 ft2 rates (20g and 100g birds) and bluegrass treated at the 1000 ft2 rate (20 g birds).
17

For this assessment it is assumed that a bird can consume seed up to 100% of its daily dietary 

needs and that the seed is readily available for foraging. It is unknown what rate a bird will 

consume seeds distributed on the ground; however, it is expected to vary with species and 

availability of the seed. The likelihood that a bird will consume enough seed to exceed the LOC 

will depend on factors including the individual bird, the availability of the seed, and how many 

treated seeds must be consumed to attain a dose of concern. Therefore, the likelihood of an 

individual attaining that dose decreases with increasing number of seeds and increasing foraging 

area. For example, all things being equal, a larger bird will need to forage for a greater number 

of seeds over a greater area than a smaller bird, likely increasing the amount of time spent 

foraging exclusively on treated seeds and perhaps the likelihood that enough seeds would be 

consumed. The LOC exceedences are based on seeding rates for establishment of a new lawn. 

Overseeding a lawn (lower seeding rates) is a more typical application of seed according to 

BEAD. In that case, foraging areas will be greater than presented. Any incorporation or 

covering of the seed (not required by the label) would further decrease availability, requiring 

greater effort and time to obtain enough seeds to attain a dose of concern. Of the three size 

classes, the likelihood of a risk concern is greatest for the smallest birds (20 g) given that the 

LOC is exceeded for all seven grass seed species, the relatively small number of seeds needed to 

be consumed, and the small foraging area required to exceed the LOC (ca. 0.3-3 ft2

).

Risk to birds can also be characterized in terms of dietary toxicity data. At the highest 

application rate (1 gallon RO-PEL/1000 ft2), the EEC (1126.32 mg denatonium/kg seed) is more 

than 1/10th (30%) of the LC50 for bobwhite quail (3698 mg denatonium/kg-bw) for one of the 

seven grass seeds (colonial bentgrass)11. This indicates a risk concern for listed birds at the 

highest application rate based on one of the seven evaluated grass seeds. At all labeled 

application rates, the EECs (≤ 323 mg denatonium/kg-bw) are less than 1/10th the LC50 for the 

other six evaluated grass seeds (no risk concern for listed or non-listed species).

 11 See Appendix A for representative calculation of seed application rate. 
18

Table 5. Characterization of Risk to Listed Birds from Consuming RO-PEL Treated 

Seeds1

Seed Minimum # of treated seeds 

consumed to exceed listed 

LOC (EEC ≥ 1/10th LD50)

2

% daily diet of listed birds 

consuming treated seeds to 

exceed listed LOC (EEC ≥ 1/10th

LD50)

3

Minimum foraging area (ft2) to 

obtain enough seeds to exceed 

the listed LOC (EEC ≥ 1/10th

LD50)

4

20 g 

bird

100 g 

bird

1000 g 

bird

20 g 

bird

100 g 

bird

1000 g 

bird

20 g 

bird

100 g 

bird

1000 g 

bird

red 

fescue

2420 NA5 NA 59 NA NA 2.2 NA NA

bluegrass 2420 15405 217599 12 28 87 1.4 8.8 123.8

tall 

fescue

2420 NA NA 93 NA NA 1.7 NA NA

perennial 

ryegrass

2420 NA NA 80 NA NA 3.0 NA NA

chewings 

fescue

2420 NA NA 48 NA NA 1.8 NA NA

hard 

fescue

2420 15405 NA 37 82 NA 1.4 8.7 NA

colonial 

bentgrass

2420 15405 217599 4 8 25 0.3 1.7 23.7

1 Based on the maximum labeled application rate, maximum number of seeds/lb from BEAD report (USEPA 2013), 

and the dietary needs for birds (20 g bird – 5.06 g/day; 100 g bird – 14.4 g/day; 1000 g bird – 64.6 g/day) from TREX (v 1.5.2). 2 Minimum # treated seeds = (EEC (mg denatonium/kg-bw/day) x bird weight (kg)) / 0.0000833 mg denatonium per 

seed. EEC = 1/10th of the size adjusted LD50. Size adjusted LD50 reported in Appendix A. 3 % daily diet to exceed LOC = minimum # seeds to exceed LOC/# seeds equaling 100% daily diet. # seeds 

equaling 100% daily diet = # seeds per g x daily dietary need for birds (g) 4 Number of seeds consumed/number of seeds per ft2 = minimum foraging area (ft2

). For example (colonial 

bentgrass, 1000 g bird): 217599 seeds consumed/9195 seeds per ft2 = 23.7 ft2 foraging area. Number of seeds/ft2 is

based on seeding rates provided in BEAD report. It is assumed that the lowest seeding rate is used for seed batches 

with the maximum number of seeds/lb when a range is provided for both rate and seeds/lb (tall fescue, perennial 

ryegrass, and Kentucky bluegrass) as advised in the BEAD report.

5 NA – not applicable, EEC is less than 1/10th of the LD50.

For mammals, there is a risk concern for listed species at the highest application rate (1 gallon 

RO-PEL/1000 ft2) but not at the lowest application rate (1 gallon RO-PEL/4000 ft2). At the 

highest application rate, the EECs (mammal Nagy dose) are more than 1/10th (ca. 12% to 26%) 

of the LD50 for all size classes (15 g, 35 g, and 100g based on denatonium benzoate toxicity

data) of mammals for use of denatonium saccharide on colonial bentgrass (one of the seven 

evaluated grass species)12. Even at the highest application there is not a risk concern based on 

the six other grass species evaluated (EECs are less than 1/10th of the size-class adjusted LD50s). 

This risk evaluation assumes that there is enough seed on the field to meet the daily dietary needs 

of a mammal based on typical seeding rates and recognizes that the label does not require soil 

incorporation. A mammal may not consume 100% of its daily diet as treated seeds; however, as 

little as 38% of a listed mammals diet would need to be RO-PEL treated seed to exceed the listed 

species LOC (EEC is greater than 1/10th of the LD50) (Table 6). A relatively small foraging area 

(less than 2 ft2) would be required for a 15 g mammal to consume enough seeds to exceed the 

 12 A risk concern is triggered for listed species for EECs greater than 1/10th the LD50. 
19

listed species LOC (e.g., the seeding rate for colonial bentgrass is 9195 seeds/ft2 and a 15 g listed 

mammal would need to eat 16450 seeds to be of concern).

Table 6. Characterization of Risk to Listed Mammals from Consuming RO-PEL Treated 

Seeds1

Seed Minimum # of treated seeds 

consumed to exceed listed 

LOC (EEC ≥ 1/10th LD50)

2

% daily diet of listed mammals 

consuming treated seeds to 

exceed listed LOC (EEC ≥ 1/10th

LD50)

3

Minimum foraging area (ft2) to 

obtain enough seeds to exceed 

the listed LOC (EEC ≥ 1/10th

LD50)

4

15 g 

mammal

35 g 

mammal

1000 g 

mammal

15 g 

mammal

35 g 

mammal

1000 g 

mammal

15 g 

mammal

35 g 

mammal

1000 g 

mammal

colonial 

bentgrass5

16450 31056 383794 38 45 84 1.8 3.4 41.7

1 Based on the maximum labeled application rate, maximum number of seeds/lb from BEAD report (USEPA 2013), 

and the dietary needs for mammals (15 g mammal – 3.17 g/day; 35 g mammal – 5.12 g/day; 1000 g mammal –

33.95 g/day) from T-REX (v 1.5.2). 2 Minimum # treated seeds = (EEC (mg denatonium/kg-bw/day) x mammal weight (kg)) / 0.0000833 mg 

denatonium per seed. EEC = 1/10th of the size adjusted LD50. Size adjusted LD50 reported in Appendix A. 3 % daily diet to exceed LOC = minimum # seeds to exceed LOC/# seeds equaling 100% daily diet. # seeds 

equaling 100% daily diet = # seeds per g x daily dietary need for mammals (g) 4 Number of seeds consumed/number of seeds per ft2 = minimum foraging area (ft2

). For example (colonial 

bentgrass, 1000 g mammal): 383794 seeds consumed/9195 seeds per ft2 = 41.7 ft2 foraging area. Number of 

seeds/ft2 is based on seeding rates provided in BEAD report. 5 Colonial bentgrass is the only seed considered in this assessment with EECs greater than 1/10th of the LD50. 

For this assessment it is assumed that a mammal can consume seed up to 100% of its daily 

dietary needs and that the seed is readily available for foraging. It is unknown what rate a 

mammal will consume seeds distributed on the ground; however, it is expected to vary with 

species and availability of the seed. The likelihood that a mammal will consume enough seed to 

exceed the LOC will depend on factors including the individual mammal, the availability of the 

seed, and how many treated seeds must be consumed to attain a dose of concern. Therefore, the 

likelihood of an individual attaining that dose decreases with increasing number of seeds and 

increasing foraging area. For example, all things being equal, a larger mammal will need to 

forage for a greater number of seeds over a greater area than a smaller mammal, increasing the 

amount of time spent foraging exclusively on treated seeds and perhaps the likelihood that 

enough seeds would be consumed. The LOC exceedences are based on seeding rates for

establishment of a new lawn. Overseeding a lawn (lower seeding rates) is a more typical 

application of seed according to BEAD. In that case, foraging areas will be greater. Any 

incorporation or covering of the seed (not required by the label) would further decrease 

availability, requiring greater effort and time to obtain enough seeds to attain a dose of concern. 

The likelihood of a risk concern is about the same for small (15 g) and medium (35 g) sized 

mammals given the relatively small number of seeds needed to be consumed and the relatively 

small foraging area required to exceed the LOC (ca. 1.8-3.4 ft2

) in comparison to 1000 g 

mammals.

Although the listed species LOC is exceeded, the potential risk to mammals is anticipated to be 

low all things considered. The risk concern is based on only one of the seven evaluated grass 

species and although a small mammal may only need to consume 38% of its diet in treated seeds, 

it would only be in a situation where colonial bentgrass (or similar seed) is planted. Possible 
20

options for reducing risk to listed mammals from use on colonial bentgrass (or similar seed) 

would be label changes such as requiring seed incorporation or lowering the maximum 

application rate for seed use in areas where listed mammals that eat seeds may be present. 

BEAD assumed a hypothetical seed size (3 mm x 1 mm) for estimating application rates to all 

seven grass species. Grass seeds such as fescue tend to be a bit larger and thus there would be 

more denatonium saccharide per seed. Therefore, risk would be greater than presented for some 

seeds as a bird or mammal would need to eat a fewer number of seeds to consume the same 

amount of denatonium saccharide. Grass seeds are often sold as blends so the examples 

presented represent a range of potential outcomes in terms of risk. 

Given the Tier 1 approach that assumes that denatonium saccharide is transported into above 

ground plant material, bees and other pollinators could be exposed to denatonium saccharide in 

the pollen and nectar of plants. Although grass seed is assumed the most typical use in a 

residential setting, other uses such as wildflower seeds are possible. A Tier 1 estimate of 1 μg

ai/g in pollen and nectar was proposed in the White Paper in Support of the Proposed Risk 

Assessment Process for Bees (USEPA, 2012). The dietary exposure estimate for adult worker 

bees is 0.29 μg denatonium/bee based on food consumption rates and the assumed concentration 

in pollen and nectar.13 Given the available information, risk could not be assessed for bees 

visiting flowers that develop from denatonium saccharide treated seeds because the only known 

toxicity study with bees resulted in a non-definitive endpoint at a test concentration (LD50 > 

0.029 μg denatonium/bee) that is lower than the exposure estimate (0.29 μg denatonium/bee). 

The screening concentration in pollen and nectar does not account for differences in application 

rate. Available data for seeding rates ranging from 0.7‐1.25 mg ai/seed were used to evaluate 

the screening value which is expected to apply to pesticide treatments that are at a similar rate 

(the highest measured residues were 28x lower than the screening estimate). Denatonium 

saccharide rates are expected to be lower (0.0000833 mg denatonium/seed at the 1000 ft2 rate14) 

than those rates. Based on rate alone it may seem that nectar and pollen concentrations of 

denatonium may be lower than residues observed for other pesticides; however, the plants used 

to assess the screening value were relatively large (e.g., corn and soybean) compared to plants 

most likely treated with denatonium at a residence (e.g., grass and wildflowers). Therefore, all 

things being equal, nectar and pollen concentration would be expected to be greater in smaller 

plants treated at the same application rate. Given the uncertainty, it is assumed that the 1 μg ai/g

screening value is applicable to the denatonium saccharide seed treatment despite the lower 

application rates. In conclusion, risk to bees could be assessed with denatonium saccharide 

toxicity data (adult and larval oral) that tests at a higher concentration or with a refined exposure 

estimate based on denatonium saccharide residue data for labeled use rates (EFED is not aware 

of any information at this time). Furthermore, risk would be negligible if information is 

available that indicates that denatonium saccharide is not transported into plant material, 

although this seems unlikely given that it is water soluble and has a low Kow.

 13 EEC (0.29 μg ai/bee) = 1 μg denatonium/g x 0.29 g/day (USEPA, 2012) 14 Equations from BEAD report (USEPA, 2013): (0.2 mm3

/1 seed) x (1 gallon/3785441.78 mm3) x (8.345 lb/1 

gallon) x (0.065%) x (453592.4 mg/1 lb) = 0.000129 mg denatonium saccharide/seed or 0.0000833 mg 

denatonium/seed.
21

Risk from run-off from outdoor structures and surfaces

Given the Tier 1 approach that assumes denatonium saccharide is transported into above ground 

plant material, bees and other pollinators could be exposed to RO-PEL applied to outdoor 

structures and surfaces that then runs off into vegetative areas and is taken up by plants. Two 

Tier 1 estimates of pesticides in pollen and nectar were proposed in the White Paper in Support 

of the Proposed Risk Assessment Process for Bees (USEPA 2012): a 1 μg ai/g default value that 

is a reasonably conservative estimate for pollen and nectar of crops treated with pesticides via

soil applications and estimates using the Briggs model which accounts for application rate and 

physiochemical properties of the chemical. Use of the Briggs model is preferred over the default 

value given that it accounts for application rate and pesticide specific characteristics; however, 

use of this model is inappropriate in this case because it is based on data from non-ionic organic

chemicals and denatonium saccharide is an ionic compound. Therefore, 1 μg ai/g plant material 

was assumed a conservative estimate of denatonium saccharide in pollen and nectar. The dietary 

exposure estimate for adult worker bees is 0.29 μg denatonium/bee based on food consumption 

rates and the assumed concentration in pollen and nectar.13 Given the available information, risk 

could not be assessed for bees visiting flowers that develop from plants that take up denatonium 

saccharide because the only known toxicity study with bees resulted in a non-definitive endpoint 

at a test concentration (LD50 > 0.029 μg denatonium/bee) that is lower than the exposure 

estimate (0.29 μg denatonium/bee). The 1 μg ai/g screening estimate is based on the maximum 

value from data including pesticide residues measured in different plant parts (leaves, fruit, green 

part, inflorescence, whole plant, and grain) following applications to soil or seed treatments; the 

value is considered conservative for soil treatments because available data from residue studies 

following soil application showed pollen and nectar concentrations lower than 1 μg ai/g with a 

single exception. Measured pollen and nectar concentrations were typically two orders of 

magnitude lower than 1 μg ai/g which would put the exposure estimate slightly less than 1/2.5th

(i.e., LOC = 0.4) the honeybee LD50 (> 0.029 μg denatonium/bee). The cited residue in pollen 

and nectar data from other chemicals suggests a possibility that exposure to denatonium may not 

be high enough to be a risk concern; however, those estimates only represent a subset of 

chemicals and application rates. In conclusion, risk to bees could be assessed with denatonium 

saccharide toxicity data (adult and larval oral) that tests at a higher concentration or with a 

refined exposure estimate based on denatonium saccharide residue data for labeled use rates. 

Furthermore, risk would be negligible if information is available that indicates that denatonium 

saccharide is not transported into plant material, although this seems unlikely given that it is 

water soluble and has a low Kow.

Risk from treated bulbs

There is not a risk concern for mammals but there is a risk concern for listed and non-listed birds 

consuming treated bulbs. Risk could not be assessed for bees visiting flowers that develop from 

treated bulbs because the only known toxicity study with bees resulted in a non-definitive 

endpoint at a test concentration that is lower than exposure estimates.

Although it is assumed that treated bulbs will be buried, birds may grub below ground for plant 

material (Gauthier et al., 2006 and DeGraaf et. al, 1985) and mammals (e.g. squirrels) are known 

to dig up and consume bulbs. Birds and mammals that are not repelled by concentrations of 
22

denatonium saccharide on the treated bulbs may consume all or part of the treated bulbs. 

Exposed bulbs may be available to non-grubbing birds given that denatonium saccharide works 

as a bittering agent targeting mammals such as squirrels (i.e., mammals may dig up the bulbs but 

be deterred from completely consuming them). Birds including galliformes and cranes consume 

bulbs (Harris, 2009); however, EFED does not currently have information about bulb 

consumption by listed species. 

EFED does not have information on bulb size but a web search of bulbs typically planted in 

residential settings (e.g., crocus, daffodils, tulips, hyacinth, etc.) suggests that 20-100 g/bulb is a 

reasonable approximation. At the test concentration proposed by BEAD (1.74 mg 

denatonium/bulb was proposed as a conservative estimate), bulbs less than 197 g would be a risk 

concern for small listed birds (i.e., the EEC of a 197 g bulb would be 1/10th the LD50 of a 20 g 

bird) 15 and bulbs less than 28 g would be a risk concern for large birds (1000 g). Smaller bulbs 

would pose a greater risk because the concentration of denatonium in smaller bulbs would be 

greater than larger bulbs given the “per bulb” application rate provided by BEAD. For example, 

the EEC for a 20 g bulb would be 98% of the LD50 of a 20 g bird (i.e., RQ = 0.98) and 14% of 

the LD50 of a 1000 g bird. Therefore, the listed species LOC is exceeded for all size classes of 

birds (20 g to 1000 g) consuming treated bulbs within the size range of 20-100 g/bulb

(approximate range assumed for residential setting). For non-listed birds, bulbs less than 40 g 

would be a risk concern for small birds (i.e., the EEC of a 40 g bulb would be 1/2 the LD50 of a 

20 g bird) and bulbs less than 6 g would be a risk concern for large birds (1000 g). This analysis 

assumes that a bird consumes up to 100% of its daily diet as treated bulbs (20 g bird = 22.8 g/day 

and 1000 g bird = 291 g/day; T-REX v1.5.2); for example, to have a risk concern, a 20 g listed 

bird would need to consume either 22.8 g (100% daily diet) of a treated 197 g bulb or 2 g (9% 

daily diet) of a treated 20 g bulb16. There is a high degree of uncertainty in the potential 

exposure because in reality the actual concentration will be a function of the bulb size, porosity, 

amount of product applied, duration of spraying, amount of time the bulb is resident in soil, etc. 

The BEAD estimate is a conservative peak estimate because it is based on soaking a 50 g bulb in 

a solution of RO-PEL for 10 minutes. BEAD indicated that this method would likely result in 

higher concentrations on the bulb than could be achieved from spraying bulbs as directed on the 

label. Nonetheless, it appears possible that bulb concentrations could be high enough to be a 

concern for listed and non-listed birds assuming that the bulbs are available for foraging. The 

foraging area required to result in a risk concern would likely be small given that bulbs are 

typically clustered together in beds; however, the density of flower beds with treated bulbs may 

be relatively low in a given residential area. Exposure from consuming above ground plant 

material is also possible assuming that denatonium is transported into the above ground plant 

material; however, concentrations in the above ground plant material are assumed to be lower 

 15 For example, the dose-based EEC (20 g bird) = 1.74 mg denatonium/196.7 g bulb = 8.845 mg denatonium/kg bulb 

x 1.14 kg diet/kg-bw = 10.08 mg denatonium/kg-bw. Dose-based toxicity for a 20 g bird (LD50) = 100.8 mg 

denatonium/kg-bw. Calculations for larger size classes are similar. An EEC that is 1/10th of the LD50 represents an 

RQ = 0.10. See T-REX (v.1.5.2) for equations and dietary needs of different size classes. Size class adjusted 

toxicity (LD50) is based on bobwhite quail and rat data (Appendix A). 16 The amount of bulb consumed to be a concern assumes that denatonium is evenly distributed throughout the bulb. 

For example, the dose-based EEC (20 g bird) = 1.74 mg denatonium/20 g bulb = 87 mg denatonium/kg bulb x 1.14 

kg diet/kg-bw = 99.18 mg denatonium/kg-bw. Dose-based EEC to exceed listed species LOC (0.1) for a 20 g bird = 

10.08 mg denatonium/kg-bw. Mass of a treated 20 g bulb that would exceed the LOC = (10.08 mg denatonium/kgbw)/( 99.18 mg denatonium/kg-bw) = 0.101 x 20 g bulb = 2 g of a treated 20 g bulb. 
23

than that on the bulb due to plant growth. There is a high degree of uncertainty in the potential 

exposure and risk to birds; however, it is within reason that there could be a risk concern at least 

in some situations; therefore, risk to birds cannot be precluded. There is not a risk concern for 

mammals assuming that an application of 1.74 mg denatonium/bulb is conservative and that 

treated bulbs are at least 18 g (below EFEDs low end estimate on bulb weight).

Given the Tier 1 approach that assumes that denatonium saccharide is transported into above 

ground plant material; bees could be exposed to denatonium saccharide in the pollen and nectar 

of plants (USEPA, 2012). EFED does not currently have methodology for assessing systemic 

risk from bulbs. As a screen, it was assumed that the application rate on the bulb is translocated 

throughout the plant without loss of material. EFED does not have information about the mass 

of the non-bulb parts (i.e., stems, leaves, pollen, nectar, fruit and/or seeds) for species that would 

typically be planted in a residential setting. As a first approximation, it was assumed that the 

non-bulb parts have at least the same mass as the bulb. That results in a total plant mass in the 

range of 40-200 g/plant for bulbs that are initially 20-100 g. Risk could not be assessed for bees 

visiting flowers that develop from treated bulbs because the only known toxicity study with bees 

resulted in a non-definitive endpoint at a test concentration that is lower than exposure estimates

for plants of a mass reasonably expected to develop from bulbs planted in a residential setting. 

Assuming an application rate of 1.74 mg denatonium/bulb, the exposure estimate is greater than 

1/2.5th (i.e., LOC = 0.4) of the bee oral LD50 (> 0.029 μg denatonium/bee) for plants weighing 

less than 96 lb17. In conclusion, risk to bees could be assessed with denatonium saccharide 

toxicity data (adult and larval oral) that tests at a higher test concentration or with a refined 

exposure estimate based on denatonium saccharide residue data for labeled use rates. 

Furthermore, risk would be negligible if information is available that indicates that denatonium 

saccharide is not transported into plant material, although this seems unlikely given that it is 

water soluble and has a low Kow.

VI. Risk to Listed Species

Based on this screening-level assessment, there are potential risks of direct effects to listed 

mammals and birds from the use of denatonium saccharide on some of its registered use sites

(uncertainties are discussed in Section V). Listed mammal, bird, terrestrial plant, and aquatic 

vascular plant taxa may also be affected through indirect effects because of the potential direct 

effects on listed and non-listed species (expected only if a species has an obligate/dependent 

relationship with a bird or mammal given the limited use pattern). Potential direct effects on 

listed mammals and birds from the use of denatonium saccharide may be associated with 

modification of Primary Constituent Elements (PCEs) of designated critical habitats, where such 

designations have been made. However, at this current stage of the Registration Review process, 

it is premature to make effects determinations for listed species until further scientific analysis 

and refinements are conducted, based on recommendations received from the National Academy 

of Sciences’ (NAS) National Research Council (NRC) April 2013 report, available at

http://www.nap.edu/catalog.php?record_id=18344. The NAS report outlines recommendations 

 17 1744 μg denatonium/43600 g plant = 0.04 μg denatonium/g plant. EEC (0.0116 μg denatonium/bee) = 0.04 μg 

denatonium/g plant x 0.29 g/day. The exposure estimate is based on the assumed concentration in pollen and nectar 

(0.04 μg denatonium/g plant) and the food consumption rate of adult worker bees (0.29 g/day) (USEPA, 2012). The 

acute oral LD50 > 0.029 μg denatonium/bee. 43600 g = 96.1 lb
24

on specific scientific and technical issues related to the development of pesticide risk 

assessments that are compliant with the Endangered Species Act (ESA) and the Federal 

Insecticide, Fungicide, and Rodenticide Act (FIFRA).

The EPA along with the U.S. Fish & Wildlife Service (USFWS), the National Marine Fisheries 

Service (NMFS) (collectively, the Services), and the U.S. Department of Agriculture (USDA), 

released a summary of their implementation plan for assessing risks of pesticides to listed species 

ahead of the stakeholder workshop held on November 15, 2013. This plan was developed in 

response to the NAS’ recommendations, including a common approach to risk assessment as a 

way of addressing scientific differences between EPA and the Services. During the workshop, 

the agencies received feedback from the public on the interim scientific approaches that were 

developed as part of the initial implementation of the NAS recommendations. These approaches 

will be jointly implemented and vetted as part of a phased iterative process. Once fully vetted, 

EPA will further refine the listed species effects determination portion of this risk assessment. 

To make effects determinations for individual listed species, useful refinements may include, but 

are not limited to, analyses of: 1) more detailed, species-specific ecological and biological data; 

2) more detailed and accurate information on residential use patterns; and 3) sub-county level 

spatial proximity data for the co-occurrence of potential effects areas and listed species and any 

designated critical habitat. Examples of such refinements are described below.

EFED is currently developing tools that are expected to further refine the assessment and are 

designed to support effects determinations for individual federally listed species and their 

designated critical habitats (where applicable). Scientific information obtained from the 

Services, and other reliable sources is being collated by EFED to address all currently listed 

species. This information is being stored in an Office of Pesticide Programs (OPP) Pesticide 

Registration Information SysteM (PRISM) listed species knowledgebase. The listed species 

knowledgebase consists of an information repository that houses biological and behavioral 

information relevant to individual species (e.g., habitat, diet, and life history, including specific 

temporal and spatial associations) and a document repository that contains supporting documents 

(e.g., USFWS recovery plans) and electronic information (e.g., GIS data files). For terrestrial 

taxa, biological information relevant to the assessment (e.g., diet and body weight) will be used 

to parameterize exposure estimates using a method consistent with currently used methods in the 

T-REX and T-HERPS models. 

Refinements will also include more detailed analyses of the registered uses and specific use 

patterns that result in either “Likely to Adversely Affect” (LAA) or “Not Likely to Adversely 

Affect” (NLAA) determinations for federally listed species. The analyses may include more 

information on where, when, and how denatonium saccharide is used on all use sites. Actual 

usage data (when available) and national land-cover datasets that indicate potential use sites 

[e.g., National Land Cover Dataset (NLCD), Cropland Data Layer (CDL)] may be used to 

support a more refined analysis of where denatonium saccharide is reasonably expected to be 

used. Similarly, refinements on the timing of applications and a more in-depth exploration of 

use practices for denatonium saccharide may be included as part of the refinement.
25

The refinements based on individual species data; additional, detailed usage information, when 

available; and recommendations from the NRC report are expected to help to more accurately 

identify potential areas of effect and to better inform effects and habitat determinations for listed 

species and any designated critical habitats.

VII. Endocrine Disruptor Screening Program

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse 

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and 

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental, 

reproductive, and general or systemic toxicity. These studies include endpoints which may be 

susceptible to endocrine influence, including effects on endocrine target organ histopathology, 

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss, 

and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and 

chronic studies that assess growth, developmental and reproductive effects in different 

taxonomic groups. As part of registration review of denatonium saccharide, EPA reviewed these 

data (denatonium benzoate data was used as surrogate data) and selected the most sensitive 

endpoints for relevant risk assessment scenarios from the existing hazard database. However, as 

required by FFDCA section 408(p), denatonium saccharide is subject to the endocrine screening 

part of the Endocrine Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the Administrator 

may designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine-related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between 

October 2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list 

of chemicals identified for EDSP screening was published on June 14, 201318 and includes some 

pesticides scheduled for registration review and chemicals found in water. Neither of these lists 

should be construed as a list of known or likely endocrine disruptors. 

Denatonium saccharide is on List 2. List 2 represents the next set of chemicals for which EPA 

intends to issue test orders/data call-ins in the near future. For further information on the status 

 18 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals.
26

of the EDSP, the policies and procedures, the lists of chemicals, future lists, the test guidelines 

and the Tier 1 screening battery, please visit our website.19

VIII. References

DeGraff, R.M., Tilghman, N.G., and Anderson, S.H. 1985. Foraging Guilds of North American 

Birds. Environmental Management. 9(6):493-536.

EFSA, 2012. Conclusion on the Peer Review of the Pesticide Risk Assessment of the Active 

Substance Denatonium Benzoate (http://www.efsa.europa.eu/en/efsajournal/pub/2483.htm)

Gauthier, G., Giroux, J.-F., and Rochefort, L. 2006. The Impact of Goose Grazing on Arctic and 

Temperate Wetlands. Acta Zoologica Sinica. 52(supplement):108-111.

Harris, T. (ed.) 2009. National Geographic Complete Birds of the World. Retrieved from 

http://books.google.com

Kleinkauf, A., Macdonald, D.W., and Tattersall, F.H. 1999. A Bitter Attempt to Prevent NonTarget Poisoning of Small Mammals. Mammal Review. 29(3):201-204.

Perera, D.R., Armstrong, G., and Senanayake, N. 2000. Effect of Antifeedants on the 

Diamondback Moth (Plutella xylostella) and its Parasitoid Cotesia plutellae. Pest Management 

Science. 56:486-490.

Portugal, 2008. Draft Assessment Report (DAR) - public version - Initial Risk Assessment 

Provided by the Rapporteur Member State Portugal for the Existing Active Substance 

Denatonium Benzoate of the Fourth Stage of the Review Programme Referred to in Article 8(2) 

of Council Directive 91/414/EEC

Sackmann, P., Corley, J.C., Masciocchi, M., and Novas, G. 2010. Effects of the Bittering Agent 

Denatonium Benzoate on the Success of Toxic Baiting of Pestiferous German Wasps (Vespula 

germanica). International Journal of Pest Management. 56(1):69-74. 

USEPA. 2008. Problem Formulation for Ecological Risk Assessment for Denatonium 

Saccharide (Amended). D352197

USEPA. 2011. Response to Data Waiver Request for the Denatonium Saccharide (PC 129045) 

Registration Review Problem Formulation Data Call In. D352551

USEPA. 2012. White Paper in Support of the Proposed Risk Assessment Process for Bees, 

September 11 – 14, 2012

USEPA. 2013 [Revised] Application Rate Information for Uses of RO-PEL, a Ready-To-Use 

Liquid Bittering Agent Containing Denatonium Saccharide (PC Code 129045), April 24, 2013

 19 http://www.epa.gov/endo/
27

Witmer, G.W., Pipas, M.J., and Bucher, J.C. 1998. Field Tests of Denatonium Benzoate to 

Reduce Seedling Damage by Pocket Gophers (Thomomys talpoides Rich.). Crop Protection. 

17:35-39.
28

Appendix A

Representative Input and Output from T-REX (v 1.5.2) for Seed Treatment (Kentucky 

Bluegrass, 1 Gallon RO-PEL/1000 ft2

)

seed

lb seed/

1000ft21 Seeds/lb1 Seeds/A2

lb denatonium 

saccharide/1000 ft21

lb 

denatonium/A3

mg 

denatonium/

seed4

mg 

denatonium/

kg seed5

Bluegrass 1 1758000 76768559 0.000503 0.0141 0.0000833 323.0 1 Values and calculations as in USEPA 2013 2

 Seeds/A = (lb seed/1000 ft2) x (seeds/lb) x (1000 ft2/0.0229 A) 3 lb denatonium/A = (lb denatonium saccharide/1000 ft2

) x (325.48 g denatonium/507.64 g denatonium saccharide) x (1000 

ft2

/0.0229 A) 4 mg denatonium/seed = ((lb denatonium/A) / (seeds/A)) x (453592.4 mg/lb) 5 mg denatonium/kg seed = ((seeds/lb) x (lb/0.4535924 kg)) x mg denatonium/seed

Endpoints Reported Tested Body Adjusted 

LD50

Size class for adjusted 

LD50 Weight (g)

Avian LD50: 142.70 203 100.80 Small (20g)

128.32 Medium (100g)

181.26 Large (1000g)

Mammalian LD50: 471.70 211 913.51 Small (15g)

739.13 Medium (35g)

319.70 Large (1000g)

Animal Size

Crop

Maximum 

Application 

Rate

Maximum Seed 

Application Rate Avian Nagy Dose Mammalian Nagy Dose Available AI

(lbs ai/A) (mg ai/kg seed) (mg ai/kgbw/day)

(mg ai/kgbw/day) (mg ai ft-2)

Small

Kentucky 

bluegrass 0.0141 323.00

81.75 68.43

Medium 46.62 47.30 0.15

Large 20.87 10.97

Crop

Risk Quotients†

Avian (20 g) Mammalian (15 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

Kentucky bluegrass 0.81 0.07 NA 0.07 0.01 NA

Avian (100 g) Mammalian (35 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

Kentucky bluegrass 0.36 0.01 NA 0.06 0.01 NA

Avian (1000 g) Mammalian (1000 g)

Acute (# 1) Acute (# 2) Chronic Acute (# 1) Acute (# 2) Chronic

Kentucky bluegrass 0.12 0.00 NA 0.03 0.00 NA

Acute RQ #1 = (mg ai /kg-bw/day) / LD50

Acute RQ #2 = mg ai ft-2 /(LD50*bw)