Human Health and Ecological Risk Assessment for the Use 

of Wildlife Damage Management Methods by APHIS-Wildlife 

Services

Chapter XXX

USE OF DIPHACINONE IN WILDLIFE 

DAMAGE MANAGEMENT

Draft 

September 2023
2

Table of Contents

1 INTRODUCTION ................................................................................................................ 5

1.1 Use Pattern of Diphacinone ......................................................................................... 5

2 PROBLEM FORMULATION...............................................................................................11

2.1 Physical and Chemical Properties...............................................................................11

2.2 Environmental Fate.....................................................................................................12

2.3 Hazard Identification ...................................................................................................12

2.3.1 Mechanism of Action and Metabolism..................................................................12

2.3.2 Human Incidents..................................................................................................13

2.3.3 Toxicity ................................................................................................................13

2.3.4 Acute Toxicity ......................................................................................................14

2.3.5 Sub-chronic/Chronic Toxicity ...............................................................................15

2.3.6 Developmental and Reproductive Effects ............................................................16

2.3.7 Neurotoxicity and Immunotoxicity Effects.............................................................16

2.3.8 Carcinogenicity and Mutagenicity.........................................................................16

2.3.9 Endocrine System Effects....................................................................................16

2.3.10 Toxicity of Other Ingredients ................................................................................16

3 DOSE-RESPONSE ASSESSMENT...................................................................................17

3.1 Human Health Dose-Response Assessment ..............................................................17

3.2 Ecological Effects Analysis .........................................................................................18

3.2.1 Aquatic Effects Analysis.......................................................................................18

3.2.2 Terrestrial Effects Analysis...................................................................................18

4 EXPOSURE ASSESSMENT..............................................................................................20

4.1 Human Health Exposure Assessment.........................................................................20

4.1.1 Identification of Potentially Exposed Human Populations and Complete Exposure 

Pathways ...........................................................................................................................20

4.1.2 Occupational Exposure Evaluation ......................................................................21

4.2 Ecological Exposure Assessment ...............................................................................21

4.2.1 Aquatic Exposure Assessment ............................................................................22

4.2.2 Terrestrial Exposure Assessment ........................................................................22

5 RISK CHARACTERIZATION .............................................................................................26

5.1 Human Health.............................................................................................................27

5.2 Ecological Risks..........................................................................................................28

5.2.1 Aquatic.................................................................................................................28

5.2.2 Terrestrial ............................................................................................................28

6 UNCERTAINTIES AND CUMULATIVE IMPACTS .............................................................30

7 LITERATURE CITED.........................................................................................................31
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8 PREPARERS: WRITERS, EDITORS, AND REVIEWERS .................................................34

8.1 APHIS WS Methods Risk Assessment Committee......................................................34

8.2 Internal Reviewers ......................................................................................................36

APPENDIX A. Risk Characterization for Accidental Occupational Exposure* to Diphacinone ...38
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EXECUTIVE SUMMARY

Diphacinone is a rodenticide used by APHIS Wildlife Services (WS) to control various rodents

and invasive mongooses in terrestrial environments. The registered uses of diphacinone include 

control of rodents and mongooses at agricultural sites (orchards, cropland, rangeland, and forest), 

in and around animal burrows and buildings (industrial, commercial, agricultural, and public), as 

well as non-crop areas (such as sewers, dumps, irrigation ditches, along fences, and gullies), and 

for conservation purposes (e.g., islands, grounded vessels). WS uses diphacinone baits (98 

pound annual average from FY11-FY20) to reduce damage from pocket gophers, rats, mice, and 

mongooses.

As a first-generation anticoagulant rodenticide (FGAR), diphacinone usually requires multiple 

feedings by rodents to deliver a lethal dose. FGARs inhibit the production of active vitamin K,

which decreases the synthesis of blood coagulation factors. This disturbance of clotting factors in 

the blood interferes with blood coagulation and leads to bleeding at capillaries, widespread 

internal hemorrhage, and death from excessive bleeding generally within 5 to 7 days.

APHIS evaluated the human health and ecological risk of diphacinone under WS proposed use 

patterns. The risk to human health, including workers applying diphacinone baits, is expected to 

be low based on the use pattern. The low risk to workers was determined based on label 

requirements regarding use of appropriate personnel protective equipment designed to reduce 

exposure. Risk to the public is not expected because WS uses tamper-resistant bait stations in 

areas accessible to the public, reducing potential exposure to the public, including children. 

Current broadcast applications for conservation use occur primarily on uninhabited islands or 

remote portions of inhabited islands. In cases where the public has access to applications on 

islands, tamper-proof bait stations may be used, as well as signage to reduce risk.

Diphacinone risk to terrestrial plants and aquatic environments is negligible based on the WS use 

pattern. WS has no records of nontarget species taken through its use of diphacinone bait. 

However, APHIS recognizes it is not possible to catalog all nontarget take because exposed 

animals may die belowground, be difficult to locate in certain habitats, or are wide-ranging and 

dispersed outside the treatment area. WS may monitor treatment sites to collect carcasses found 

aboveground to minimize secondary exposure to nontarget animals. WS avoids treating areas 

with threatened or endangered species that could be affected (many conservation treatments are 

to protect or are in close proximity to sensitive species) as well as burrows where nontarget

species are detected. Despite this monitoring, APHIS recognizes nontarget species can be 

exposed to diphacinone bait. APHIS expects most of the target animals it baits belowground with 

diphacinone will die underground, reducing exposure potential to nontarget species; however, 

some will die aboveground. 

The product labels instruct applicators to collect and dispose of dead or exposed animals during 

and after treatment, and some labels instruct to remove leftover bait but do not provide the time 

interval after treatment for doing this. Despite these mitigations, poisoned animals that are 

aboveground between searches or are missed during searches remain available to predators and 

scavengers. Birds are not as sensitive to diphacinone as mammals, and the risk of mortality to 

birds feeding on bait or animals with diphacinone residues is low. However, birds may experience 

sublethal effects. APHIS finds its use of diphacinone will have the greatest risk to mammals that 

access treated burrows or are in an area that receives an aboveground broadcast treatment

where they may ingest bait or animals with diphacinone residues.
5

1 INTRODUCTION

The United States Department of Agriculture (USDA) Animal Plant Health Protection Service 

(APHIS) Wildlife Services Program (WS) controls populations of several rodent species and 

mongooses with the first-generation anticoagulant rodenticide (FGAR) diphacinone. 

Anticoagulants interfere with blood clotting, lead to bleeding at capillaries and internal bleeding, 

and cause death from excessive bleeding, generally within 5 to 7 days. Specifically, 

anticoagulants inhibit the formulation of prothrombin, which is a critical component in blood 

clotting (USEPA 2020b). As a first-generation anticoagulant, diphacinone usually requires 

multiple feedings by the target animal to deliver a lethal dose.

Diphacinone is a rodenticide first registered as a vertebrate control agent in the U.S. in 

1962(USEPA 2015a). The registered uses of diphacinone include rodent control at agricultural 

sites (orchards, cropland, rangeland, and forest), in and around buildings (industrial, commercial, 

agricultural, and public), non-crop areas (such as sewers, dumps, irrigation ditches, along fences, 

and gullies, landscaping, and lawns), and for conservation purposes in rodent eradication efforts 

on islands (USEPA 2015a).

WS controls populations of several rodent species, including black rats1

, brown rats, Pacific rats, 

house mice, meadow voles, woodrats, cotton rats, pocket gophers, and small Indian mongooses

(Urva auropunctata)2 with diphacinone to prevent or minimize damage to property, crops, trees, 

shrubs, levees, and vulnerable native species and ecosystems. WS manages rodent populations 

and the damage they cause per label instructions by placing diphacinone baits where these 

species feed. However, WS very rarely conducts commensal rodent control in urban areas.

This human health risk assessment and ecological risk assessment evaluates potential risks and 

hazards to human health, nontarget fish, and wildlife from exposure to diphacinone from WS

proposed use. The methods used for the human health risk assessment to assess potential 

human health effects follow standard regulatory guidance and methodologies and generally 

conform to other Federal agencies such as U.S. Environmental Protection Agency (USEPA)

(National Research Council 1983, USEPA 2016b). The methods used for the ecological risk 

assessment to assess potential ecological risk to nontarget fish and wildlife generally follow 

USEPA methodologies (USEPA 2016b).

This risk assessment uses a standard approach of first identifying the hazard during problem 

formulation. Next, the toxicity of the hazard is evaluated in the dose-response assessment, 

followed by the determination of potential exposure populations and pathways. Lastly, the toxicity 

and exposure assessment information are integrated into the risk characterization (determining 

whether there is adverse human health and ecological risk). This risk assessment also includes 

a discussion of the uncertainties associated with the risk assessment and cumulative effects.

1.1 Use Pattern of Diphacinone

Diphacinone is commercially available in multiple pesticide products registered with the USEPA 

under the Federal Insecticide, Fungicide, and Rodenticide Act; formulations contain 0.005% 

diphacinone. Two of the diphacinone products used by WS are classified as restricted use 

pesticides (RUP), which may only be applied by certified pesticide applicators. The others used 

1 Scientific names are given in Chapter I: Introduction to the Risk Assessments.

2 The small Indian mongoose has had changes in its common (small Asian mongoose) and scientific (Herpestes 

javanicus) names since the Chapter I: Introduction was written.
6

by WS are general use and can be used by WS and the public. Diphacinone comes in prepared 

bait blocks, pelletized and granular (e.g., hulled oat groats) baits, or tracking powder formulations. 

From FY113

–FY20, WS used four diphacinone products under three Section 3 labels and one

Special Local Needs (SLN) label under an additional Section 3 parent label. WS proposes to 

continue to use these formulations in wildlife damage management (Table 1). These formulations 

are pellets or bait blocks. It should be noted that WS used a fifth label in FY21, Apple Bait Block®

Rodenticide (EPA Reg. No. 56-74), that was included in Table 1 (this label has the same use 

restrictions as Bait Block® Rodenticide with Peanut Butter FlavorizerTM; EPA Reg. No. 56-42).

Each diphacinone formulation has specific label instructions that applicators must follow when 

applying the baits (Table 1). Depending on the formulation’s label, diphacinone products are

applied via aircraft, bait boxes, cloth bags, hand bulb dusters, hand probes, spoons, spreaders, 

and bait stations (USEPA 2015a). 

WS applicators primarily apply diphacinone pellet and block baits using tamper-resistant bait 

boxes/stations (commensal rodents), hand spot baiting directly into subterranean burrow systems 

of specific burrowing rodents (e.g., pocket gophers) following label instructions to ensure that 

people and pets cannot access the baits. Mechanical broadcast bait spreaders, aerial 

applications, or spot treatments could occur on islands for invasive commensal rodents (under 

the Dipacinone-50 Conservation label; EPA Reg. No. 56228-35). Application rates vary with the 

density of rodent populations, the target species being controlled, and the potential nontarget 

species present. 

Hand spot baiting involves placing the bait within an active burrow opening or scattering it on 

trails, runways, or bare ground adjacent to burrow entrances and covering each placement with 

grass or shingle to avoid exposing nontarget organisms (restricted to Diphacinone-50 

Conservation). 

For open burrow systems, applicators may directly insert bait into burrows. For pocket gophers, 

specifically, a probe is often used to find the burrow; applicators find the burrow by pressing a 

probe into the ground about a foot from gopher mounds to find the main runway (the probe will 

easily drop when it hits a burrow). A hole is opened, and a four-ounce block is placed as far into 

the runway as the applicator can reach (USEPA 2019b). The hole created by the probe is then 

covered with debris and dirt while being careful not to cover the bait with dirt. Covering the hole 

discourages pocket gophers from plugging it and covering the bait with dirt. The number of 

applications per pocket gopher burrow system often depends on the number of burrow mounds 

because baits are placed a foot to a yard away from each mound in the main underground 

runways.

The labeled uses for Diphacinone-50 Conservation are for the control or eradication of brown, 

black, and Pacific rats, house mice, and other types of invasive rodents on islands for 

conservation purposes or on grounded vessels or vessels in peril of grounding (USEPA 2019a). 

The Diphacinone-50 Conservation label allows manual applications with tamper-resistant bait 

stations, burrow baiting, bait bolas (sachets), or ground and aerial broadcast methods (USEPA 

2015a;2019a). The rate of application and frequency of treatment depends on the label-specified 

application method and target species. Bait stations must be tamper-resistant and prevent access 

by children, pets, and nontarget wildlife larger in body size than the target species (USEPA 

2019a).

3 FY11 equals the federal Fiscal Year 2011 which is October 1, 2010–September 30, 2011 (the year is 

denoted by FY11, FY12, and so on and is the federal Fiscal Year for 2011, 2012, and so on.
Table 1. Diphacinone products WS used in its animal damage management program (FY11-20) and proposes to use in the future.

Product Brand Name 

(registrant)

USEPA 

Registration 

Number

(label date)

Formulation 

(0.005% 

Diphacinone)

Application Rate Labeled Target Animals

Diphacinone-50 

Conservation (Alternate 

Brand Names: 

Diphacinone-50

[Conservation]: Pelleted 

Rodenticide Bait for 

Conservation Purposes) 

56228-35 

(11/12/2019)

Restricted Use 

Pesticide

Pellets

Several rates are provided on the 

label and are adjusted based on 

the application method, location, 

and target species

Norway [brown] rats, roof [black] rats, Polynesian [Pacific] rats, 

house mice, other types of invasive rodents on islands for 

conservation purposes, or on grounded vessels or vessels in 

peril of grounding

Ramik® Mini Bars AllWeather Bait

61282-26* 

(9/23/2021) Bait blocks

Rates are variable depending on 

the target species. Can only be 

used within 100 feet of structures 

in bait stations (no burrow 

baiting)

House mouse, Norway [brown] rat, roof [black] rat, cotton rats 

(Sigmodon hispidus), eastern harvest mouse 

(Reithrodontomys humuli), golden mouse (Ochrotomys 

nuttalli), Polynesian [Pacific] rat, meadow vole, white-throated 

woodrat (Neotoma albigula), southern plains woodrat (N. 

micropus), Mexican woodrat, bushy-tailed woodrat 

Ramik® Mini Bars AllWeather Bait

SLN No. HI-980005 

(expires 8/13/2024)

Restricted Use 

Pesticide

Bait blocks

Bait stations with 16 oz. of bait 

(number stations and distance 

variable)

Small Indian mongoose, house mouse, brown rat, black rat, 

Pacific rat, and other invasive rodents in Hawaii for 

conservation purposes

J.T. EatonTM Answer ® for 

the Control of Pocket 

Gophers (referred to as 

Eaton's Gopher Bait Block 

in this risk assessment)

56-57 (9/5/2019) Bait blocks 

Manual belowground 

applications. One 4-oz. bait block 

into each opened hole into

runway, 2–3 locations in each 

main runway system

Botta’s [Valley] pocket gophers, northern pocket gophers, 

Mazama [western] pocket gophers, Townsend's pocket 

gophers (Thomomys townsendii), Sierra or mountain pocket 

gophers (T. monticola), and plains pocket gophers in

rangeland, cropland, forests, and noncrop areas

Bait Block® Rodenticide 

with Peanut Butter 

FlavorizerTM

56-42 (9/23/2015) Bait blocks

Rats: 4 to 16 1-oz blocks per 

placement at 15-30 ft. intervals 

Mice: 1 to 2 1-oz blocks per 

placement at 8–10-ft intervals

Use in tamper-resistant bait 

boxes aboveground

Norway [brown] rats, roof [black] rats and house mice in and 

within 100 feet of man-made structures constructed in a 

manner so as to be vulnerable to commensal rodent invasions 

and/or to harboring or attracting rodent infestations

Apple Bait Block®

Rodenticide

(used minimally in FY21)

56-74

(9/23/2015)

Bait blocks

Rats: 4 to 16 1-oz blocks per 

placement at 15-30 ft. intervals 

Mice: 1 to 2 1-oz blocks per 

placement at 8–10-ft intervals

Use in tamper-resistant bait 

boxes aboveground

Norway [brown] rats, roof [black] rats and house mice in and 

within 100 feet of man-made structures constructed in a 

manner so as to be vulnerable to commensal rodent invasions 

and/or to harboring or attracting rodent infestations

*WS did not use this bait from FY11–FY20 but it is the parent label for the SLN bait
Bait stations are allowed for all use sites specified on the label, all of which are non-crop areas. 

Burrow baiting is only permitted in uninhabited non-crop areas, and bait bolas are allowed for use 

on uninhabited grounded vessels or vessels in peril of grounding. Bait bolas may also be used in 

tree and shrub canopies in non-crop areas. Aerial and ground broadcast applications are not 

permitted on vessels or in areas of human habitation. Aerial broadcast applications are also not 

permitted when sustained winds exceed 35 miles per hour (mph) (USEPA 2019a). Warning signs 

must be posted in treatment areas that are accessible to the public (USEPA 2019a). The label 

instructs applicators to periodically monitor the baited area and remove dead carcasses during 

and after treatment operations. WS makes diphacinone applications under the Diphacinone-50 

Conservation label for invasive rodent eradication or control projects on select islands to protect 

the islands’ native animals and plants.

WS uses Ramik® Mini Bars All-Weather Bait (EPA Reg. No. 61282-26) under a Special Local 

Needs (SLN) label to control house mice, Pacific rats, and black rats in Hawaii. WS may also use 

this SLN product to control small Indian mongooses in Hawaii and, if labeled, in the Caribbean 

Islands. The parent label and Hawaii SLN label require the use of tamper-resistant bait stations 

for all outdoor aboveground applications and when applications are made in areas accessible to 

children, pets, and nontarget mammals and birds (USEPA 2019c;2021b). The parent label allows 

use in and within 100 feet of man-made structures, including homes, food processing facilities, 

industrial and commercial buildings, trash receptacles, agricultural and public buildings, transport 

vehicles (ships, trains, aircraft), docks, and port or terminal buildings. The parent label does not 

allow fence and perimeter baiting beyond 100 feet from a structure, direct application to food or 

feed crops, or burrow baiting. The parent label restricts use directly to water, areas where surface 

water is present, or intertidal areas below the mean high-water mark. The Hawaii SLN label

specifies the use of tamper-resistant bait boxes with bait secured on rods within the bait stations 

preventing bait removal "in native ecosystems, such as forests, wetlands, coastal areas, and 

offshore islands, and other non-crop areas to protect native Hawaiian plants and animals"

(USEPA 2019c). The Hawaii SLN label requires bait stations to be anchored to the ground or 

trees to prevent access to bait in areas prone to vandalism or where feral pigs are present.

WS also uses J.T. EatonTM Answer® for the Control of Pocket Gophers (EPA Reg. No. 56-57; 

referred to as Eaton's Gopher Bait Block in this risk assessment) to control pocket gophers, 

primarily in New Mexico. The product provides a homeowner use label in addition to a professional 

use label. WS applies the professional use label for the control of pocket gophers in rangeland, 

orchards, vineyards, cropland, forest, and non-crop areas, including golf courses, parks, 

nurseries, and around homes (USEPA 2019b). Applicators place the bait blocks into burrows 

underground, using two to three bait blocks per burrow (USEPA 2019b). The label does not allow 

for aboveground use. The label does not allow direct application to the water where surface water 

is present or to intertidal areas below the mean high-water mark. The label requires checking the 

treatment area 10–15 days after treatment to remove any visible dead or dying pocket gophers.

WS lastly may use Bait Block® Rodenticide with Peanut Butter FlavorizerTM (EPA Reg. No. 56-42) 

and the similar Apple Bait Block® Rodenticide (EPA Reg. No. 56-74) to control brown rats, black

rats, and house mice in and within 100 feet of man-made structures constructed in a manner to 

be vulnerable to commensal rodent invasions and/or to harboring or attracting rodent infestations 

(USEPA 2015b) on islands with these structures to protect bird nesting sites. The label defines 

structures to include homes, food processing facilities, industrial and commercial buildings, trash 

receptacles, agricultural and public buildings, transport vehicles (ships, trains, aircraft), and docks 

and port of terminal buildings. Applications per the label are placed by hand in areas rodents 

occur or placed in tamper-resistant bait stations. The label instructs the applicator to provide an 

uninterrupted supply of fresh bait for at least 10 days for rats, 15 days for mice, or until animal 
9

activity ceases. The label does not allow use in food or feed crops. Broadcast bait and burrow 

baiting are prohibited. The label does not allow applications to water, areas where surface water 

is present, or intertidal areas below the mean high-water mark. The label requires collecting and 

disposing of all dead animals and leftover bait.

In the Management Information System (MIS)4

, WS personnel generally record the number of 

acres or burrows treated or estimate the number of target species taken with treatments. The 

quantity of bait applied is always recorded in the MIS. Most burrowing rodents taken with 

rodenticides die belowground (Nolte and Wagner 2001, Lee and Hyngstrom 2007, Ramey et al. 

2007). Thus, counting the exact number of burrowing rodents taken with treatments is not possible

because WS does not dig up burrows to determine the actual take of target or nontarget species. 

If WS specialists estimated take in the MIS, that number was used for the average annual take 

numbers. If the take was not estimated in the MIS, take numbers were estimated based on the 

average occupancy of burrows by adults and young (number per burrow) or were estimated based 

on the expected number of target animals taken per pound of bait applied (number per pound) or 

expected density (number per acre). In general, these methods likely overestimate actual take 

numbers because it assumes 100% exposure and take of all target animals estimated to be 

present, which rarely occurs under actual field conditions. The following parameters and 

assumptions were used to estimate WS take with diphacinone baits for each target rodent 

species: 

• Northern and Botta’s Pocket Gophers: These pocket gopher species are very territorial 

and live solitary lives except during breeding and when mothers are with young. They 

generally have about 4 young per litter and 1 litter per year in the north and 2 litters in the 

south part of their range, where they may not hibernate in the winter. WS take occurred in 

New Mexico. Young are weaned in 3–4 weeks and quickly leave the burrow afterward. 

Average weights equal 3.8 oz. (108 g) for northern pocket gophers and 5.5 oz (156 g) for 

Botta’s pocket gophers. Pocket gophers can consume 50% or more of their body weight 

per day. If they consume 20% of a bait block over two days, five northern pocket gophers

and four Botta’s pocket gophers would be taken per one 4-ounce bait block. Both species

occur at densities up to 8 per acre in high-density situations, but this risk assessment 

assumed that 4 pocket gophers were taken per acre treated on average. 

• Old World Invasive Rodents: Old World rodents, invasive throughout the United States, 

are common, especially in urban areas and ports as well as islands. Old World rodents 

include the house mouse and brown, black, and Pacific rats. These species are highly 

commensal and live in loose to dense colonies in burrows, structures, and tree canopies 

(especially black and Pacific rats). Rats eat more grams of food per day, but for estimating 

take, it was assumed that rats consume 6% and house mice 12% of their body weight per 

day from treated baits. Body weights on average are 340 g, 153 g, 71 g, and 23 g for 

brown, black, and Pacific rats and house mice, respectively. Therefore, this risk 

assessment assumed that 20 brown rats, 45 black rats, 96 Pacific rats, and 148 house 

mice were taken per pound of bait applied (including 10% wastage, i.e., bait destroyed or 

not consumed).

4 MIS - Computer-based Management Information System used for nationally tracking WS wildlife damage 

management methods used and activities. Methods and activities are tracked such as diphacinone projects 

but take is not always estimated.
10

• Small Indian Mongoose: This small invasive mongoose species is native to Iraq and 

northern South Asia but has been introduced to many regions of the world, including the 

Caribbean and Pacific Islands. They were introduced to Caribbean Islands to kill rats in 

sugar cane and later to Hawaii. Without predators, they colonized islands in high densities

and caused problems for native wildlife. Mongooses are weasel-like in body shape, agile, 

adept at climbing, and hunt any time, day or night. They can weigh up to 900 g (2 lbs). 

Gestation is 49 days, with litter sizes ranging from 1–4 pups. They have two to three litters 

per year. Young mongooses are weaned and begin hunting at six weeks. The pups stay 

with their mother until they are sexually mature at 4 to 6 months. Life expectancy is 4–5 

years. It should be noted that baits designed for rodents are generally not preferred by 

mongooses and result in lower mortality rates (Sugihara et al. 2018). Thus, we estimated 

comparatively low bait consumption rates (10% of bait applied) by mongooses during 

rodent and mongoose baiting projects that used Ramik® Mini Bars All-Weather Bait. Given 

their sensitivity to diphacinone (acute oral LD50 5 = 0.18 milligrams diphacinone per 

kilogram bodyweight (mg/kg-bw) (Keith et al. 1989), this risk assessment assumed that

one mongoose was taken for every two 1-oz. baits that were consumed by mongooses 

(rather than rodents). In the future, take may be higher with a new soft diphacinone meat 

bait that WS is developing with higher palatability for mongooses (Antaky et al. 2023).

Overall, WS applied four different diphacinone baits (under five labels) in four states and one U.S. 

territory (Table 2a) from FY11 to FY20 to control Old World rodents and mongooses to protect 

sensitive species and pocket gophers to protect turf, alfalfa, and rangeland. An additional bait 

label was used by WS in FY21, but minimally. In addition to the species taken, other species that 

can be controlled with diphacinone are listed in Table 1.

Estimates of the annual average number of target rodents and mongooses killed with diphacinone 

baits by WS in wildlife damage management activities and the annual average amounts of 

diphacinone baits used from FY11 to FY15 and from FY16 to FY20 throughout the United States 

are provided in Table 2a. Between FY11 and FY15, WS used an annual average of 177 pounds 

of diphacinone baits to take an annual estimated average of 72 pocket gophers, 10,990 invasive 

Old World rodents, and 120 invasive mongooses. Between FY16 and FY20, WS used an annual 

average of 18 pounds of diphacinone baits, a much-reduced usage, to take an annual estimated 

average of 23 pocket gophers and 378 brown rats. All applications for gophers were belowground 

directly into the burrow, for Old World rodents in terrestrial bait stations. These products were 

applied in four states and one U.S. territory, including Hawaii, Massachusetts, New Mexico, 

Virginia, and the U.S. Virgin Islands (Table 2a). While it is likely that some nontarget animals were

taken, no known nontarget animals were found. For WS projects, the most likely nontarget

animals that would be taken would be nontarget rodents in gopher burrows following their 

removal, as gophers are very aggressive towards other animals in their burrows while they are 

active. The other projects were on islands where nontarget mammals would not be present, and 

baits were not broadcast, minimizing potential bird take.

WS also sold Eaton’s Gopher Blocks to certified pesticide applicators, but minimally at 4.5 lbs.

per year from FY11–FY20 (Table 2a,b). This diphacinone product was only sold in New Mexico. 

5 LD50, or lethal dose, is the amount of a chemical given all at once that kills 50% of a population of test animals
11

Table 2a. Annual Average Target Animal Take and Diphacinone Products Applied by WS from 

FY11 to FY15 and FY16 to FY20. 

Species FY11-15 

TakeA

FY11-15 

Lbs Used 

FY16-20 

TakeA

FY 16-20 

Lbs Used 

USEPA 

Registration

Number

State 

Northern Pocket Gopher - - 5 0.2 56-57 NM

Botta’s Pocket Gopher 72 4.5 18 1.3 56-57 NM

House Mouse* 4,440 30.0 - - HI-980005 HI

Black Rat* 76 2.4 - - 56228-35 VI

4,056 90.5 - - HI-980005 HI

Brown Rat*

98 4.9 258 12.9 56228-35 MA 

150 7.5 - - HI-980005 HI

- - 120 3.8 56-42 VA

Pacific Rat* 2,160 22.5 - - HI-980005 HI

Small Indian Mongoose* 120 15.0 - - HI-980005 HI

Total (8 spp.) 11,172 177.3 401 18.2 4 States, 

1 Terr. ATarget animals estimated to have been taken. If the WS applicator did not estimate take, the number of target animals 

taken was estimated. Only black rats and Botta’s pocket gophers had a few projects with take estimated by WS 

specialists for FY11-FY15, but most take was for brown rats and Botta’s pocket gophers for FY16-FY20.

*Introduced species SLN = Special Local Need registration 

Table 2b. Annual Diphacinone Products Sold by WS from FY11 to FY15 and FY16 to FY20.

Product Name FY11-15 Lbs Sold FY 16-20 Lbs Sold

USEPA 

Registration 

Number

State 

Eaton's Gopher Bait Block 6 2.9 56-57 NM

2 PROBLEM FORMULATION

The following sections discuss physical and chemical properties, environmental fate, and hazard 

identification for diphacinone. 

2.1 Physical and Chemical Properties 

Diphacinone (synonym: 2-(diphenylacetyl)indan-1,3-dione; chemical formula: C23H16O3; CAS No.: 

82-66-6) is an organic compound with a molecular weight of 340.48 and a molecular structure 

shown in Figure 1 (USEPA 2015e, NIH 2023). 

Figure 1. Chemical structure for diphacinone
12

Diphacinone is a yellowish-to-whiteish powder at room temperature (NIH 2023). It has a low vapor 

pressure of 1.03x10-10 mm Hg at 25°C and Henry’s Law constant of 2x10-10 atm-m3/mol (USEPA 

2015e, NIH 2023). It has a low water solubility of 0.3 milligrams (mg)/Liter (L) (USEPA 2015e). 

Diphacinone has an octanol-water partition coefficient (Kow) of 20,000 and an estimated 

adsorption coefficient (Koc) between 1,700 milliliters (ml)/gram (g) and 2,100 ml/g (USEPA 2015e). 

2.2 Environmental Fate 

Environmental fate describes the processes by which chemicals move and degrade in the 

environment. The environmental fate processes include 1) persistence, degradation, and mobility 

in soil; 2) movement to air; 3) migration potential to groundwater and surface water; 4) degradation 

in water; and 5) plant uptake.

Due to the label-mandated use, where applied, diphacinone will be present in environmental 

media. Due to low vapor pressure, diphacinone released in the air will remain in the particulate 

phase at 25°C and be removed from the atmosphere by wet and dry deposition (NIH 2023). 

Diphacinone has a low water solubility with a low potential for migration to groundwater via 

leaching or to surface water via runoff (USEPA 2015e). The Kow (20,000) indicates that there is a 

potential for bioconcentration in aquatic biota (USEPA 2015e). Diphacinone was stable against 

hydrolysis at pH 7 and 9 but underwent hydrolysis at pH 5 with an extrapolated half-life of 44 days

(USEPA 2015e). Although diphacinone is stable between pH 7 and 9 and will adsorb to 

suspended solids and sediment in the water, it has been observed in water to undergo 

degradative photolysis. 

Diphacinone is only slightly mobile in soil based on its Koc (USEPA 2015e). Calculated Koc values 

range from 1,700 to 2,100 (Log Koc from 3.24 to 3.32). Diphacinone is immobile in tightly packed 

soil in laboratory settings and will reside in the top 3 inches. Diphacinone is unlikely to volatilize 

from moist soils or water due to a Henry's Law constant of 2x10-10 atm-m3

/mol (USEPA 2015e). 

The biodegradation half-life for diphacinone was 28.3 to 31.7 days in sandy loam soil. 

2.3 Hazard Identification 

The following section summarizes available acute and chronic toxicity and the experimental 

exposure data used to evaluate the hazards of diphacinone to human health and nontarget 

mammals. On an acute basis, technical diphacinone is a hazard to human health and can be 

lethal due to acute oral, inhalation, and dermal toxicity. The end-use bait formulations containing 

0.005% w/w diphacinone have low acute oral and dermal toxicity. USEPA waived the data 

requirement for acute inhalation toxicity for the waxy pelleted and block registered products due 

to the exposure route not being relevant (Bell Laboratories 2020). However, a recent EPA review 

concluded that users of “loose" pellet products containing diphacinone may have inhalation 

exposures (USEPA 2020a). 

2.3.1 Mechanism of Action and Metabolism

Diphacinone is a first-generation anticoagulant rodenticide (USEPA 2015a;2020c). Diphacinone 

toxicity is manifested as an increase in prothrombin and partial thromboplastin time (USEPA 

2015c). Signs of toxicity include dyspnea, lethargy, hemorrhage from the nose, and uterine 

bleeding (USEPA 2015c). Death follows excessive external or internal bleeding (USEPA 2015c). 
13

Diphacinone absorption peaked in rats at 4 hours after administration. Distribution data following 

ingestion shows that the liver (14–25% of the dose), muscle (0.18–4.4% of the dose), fat (0.55–

1.16% of the dose), and lung (0.04–0.5% of the dose) were the tissues with the most significant 

retention of diphacinone. Elimination of diphacinone was primarily in feces (47–77% of the dose 

in rats and 69–73% of the dose in mice) (USEPA 1998).

2.3.2 Human Incidents

USEPA (2008) published the Risk Mitigation Decision (RMD) for ten rodenticides to minimize 

exposure to children and ecological systems, including wildlife. To minimize children’s exposure 

to rodenticide products used in homes, USEPA (2008) required that all rodenticide bait products 

marketed to general and residential consumers (packages containing ≤1 lb. of bait) be sold only 

with bait stations, with loose bait (e.g., pellets and meal) prohibited as a bait form. All 

“professional” products containing FGARs, which includes all products containing the active 

ingredients warfarin, chlorophacinone, and diphacinone, had to be sold in containers of ≥4 pounds

net contents to discourage non-professional (general residential consumer) use.

In 2015, USEPA (2015c) reviewed the Incident Data System (IDS), American Association of 

Poison Control Centers (AAPCC), and the Annual Report for rodenticide incident trends. From 

2011, the first year of mandatory compliance with USEPA (2008) RMD, to 2014, there was a 17% 

reduction in overall rodenticide exposure to children under six years old. The USEPA Health 

Effects Division (HED) completed an updated analysis of exposure incidents for eleven 

anticoagulant and non-anticoagulant rodenticides (USEPA 2020a). As per the USEPA HED

(USEPA 2020a), IDS and AAPCC data suggest that the overall frequency of rodenticide incidents 

appears to be decreasing over time, and the total number of reported child exposures was

reduced by 46% between 2011 and 2017 (USEPA 2020a). While the IDS reported a 60% increase 

in FGAR incidents between 2009 and 2018, the total number of incidents was low (10 FGAR 

incidents in 2009 and 52 in 2018) (USEPA 2020a;c). Also, the number of AAPCC-reported FGAR

incidents declined in the same period. USEPA (2020a) concluded that the IDS and AAPCC data 

suggest that overall FGAR exposure trended downward after the publication of the 2008 RMD. In 

addition, data from the IDS suggests that reported diphacinone incidents to humans, domestic 

animals and wildlife have decreased since 2014 (USEPA 2023d)6

.

A retrospective occupational exposure analysis of the National Institute for Occupational Safety 

and Health Sentinel Event Notification System for Occupational Risk (SENSOR)-pesticides 

(2011-2015), the California Pesticide Illness Surveillance Program (PISP) (2012-2016), and IDS 

(2015-2019), conducted by the HED reported 21 occupational exposure incidents to rodenticides

(USEPA 2020a). Further review indicated that most exposures reported to SENSOR and PISP 

databases involved the manual application of zinc phosphide-containing products and were low 

in severity. However, one severe occupational exposure case involved diphacinone and a 

pregnant employee driving a company vehicle with diphacinone bait glued to the vehicle’s 

interior, which was an illegal product use. The woman experienced headaches and nausea, and 

the neonate was born with hemolytic anemia.

2.3.3 Toxicity

Technical-grade diphacinone has high acute toxicity via oral, dermal, and inhalation routes of 

exposure (Toxicity Category I). Technical diphacinone is a moderate irritant to the eye (Category 

6 USEPA does not guarantee the completeness or adequacy of the contents of the IDS USEPA. 2023d. 

Pesticide Registration Incident Data System. in US Environmental Protection Agency..
14

III) but is not irritating to the skin (Category IV) and is not a dermal sensitizer (USEPA 1998;2015c)

(Table 3a). 

Table 3a. Acute technical grade diphacinone toxicity data for mammals. 

Test/Species Toxicity Value Toxicity 

Category Reference

Oral LD501 (rat) 2.5 mg/kg-bw (male) I (USEPA 2015c)

Oral LD50 (rat) 2.1 mg/kg-bw (female) I (USEPA 2015c)

Oral LD50 (rat) 2.3 mg/kg-bw (combined) I (USEPA 2015c)

Dermal LD50 (rabbit) 3.6 mg/kg-bw (male) I (USEPA 2015c)

Inhalation LC501 (rat) <6 μg/L (male/female) I (USEPA 2015c)

Eye irritation (rabbit) Moderate irritation clearing by day 4 III (USEPA 2015c)

Dermal irritation (rabbit) 

Slight erythema clearing within 48 

hours, but 4/6 rabbits died between

8 and 10 days

IV (USEPA 2015c)

Dermal sensitization 

(guinea pig, Cavia 

porcellus)

Neither a dermal irritant nor a 

sensitizer at a non-lethal dose level 

(2.5 mg/day)

na (USEPA 2015c)

1LD50 (median lethal dose) and LC50 (median lethal concentration) is a toxicity measurement for the dose or 

concentration (airborne) of a substance necessary to kill 50% of a population

Table 3b. Acute toxicity data for end-use bait product containing 0.005% w/w (50 ppm) 

diphacinone in mammals. 

Test/Species Toxicity Value Toxicity Category Reference

Oral LD50 (rat) >5,000 mg/kg-bw IV (Bell Laboratories 2020)

Dermal LD50 >5,000 mg/kg-bw IV (Bell Laboratories 2020)

Inhalation LC50 Waived Na (Bell Laboratories 2020)

Eye irritation Not an irritant IV (Bell Laboratories 2020)

Dermal irritation Not an irritant IV (Bell Laboratories 2020)

2.3.4 Acute Toxicity 

Oral Toxicity 

An acute toxicity study (5/sex/dose level) to determine the lethal dose for 50% (LD50) of 

diphacinone to Spartan rats was conducted at doses 0, 0.79, 1.25, 1.98, 3.15, 5.00, 7.94, 12.60, 

20.01, 31.76, 50.40, or 201.7 mg/kg-bw/day (USEPA 1998). Symptoms of exposure were clear 

or colored nasal discharge, soft stool, diarrhea, decreased motor activity, occasional dry cornea 

at lower doses, lacrimation (excessive tearing), ataxia (impaired muscle movement function), 

cyanosis (bluing of the skin), and orifice/body cavity hemorrhage at higher doses. Mortalities 

occurred at 1.25 mg/kg-bw/day (1/5 female), and 100% mortality for both sexes occurred at 5 

mg/kg-bw/day. The LD50 for male rats was 2.50 mg/kg-bw/day, and for female rats, the LD50 of 

2.31 mg/kg-bw/day was established for this study. In conjunction with others, these representative 

exposure studies place diphacinone into oral Toxicity Category I (USEPA 1998;2015c). An 

additional study exposed male and female Sprague-Dawley rats 5/sex/dose level) to technical

grade diphacinone (99.0%) at 0.13, 0.20, 1.0, or 2.5 mg/kg-bw (USEPA 1998). The lowest 

observed adverse effect level (LOAEL) from the single-dose administration was 0.20 mg/kg-bw 

based on increased activated partial thromboplastin time in female rats. The no observed adverse 

effect level (NOAEL) was 0.13 mg/kg-bw. 
15

Dermal Toxicity

New Zealand white rabbits were dermally exposed to 0, 1, 5, 10, 25, or 50 mg/kg-bw for 24 hours. 

Male rabbits (n=1) at all doses died (USEPA 1998). Female rabbits at the three highest doses 

died. Mortalities occurred days 7-13 post-exposure, and symptoms of exposure were 

hemorrhage, discoloration of organs, somnolence (sleepiness), loss of fluids, absence of feces, 

and vasoconstriction. The estimated LD50 for dermal exposure to diphacinone is 3.6 mg/kg-bw. 

These data place dermal exposure to diphacinone into the Toxicity Category I (USEPA 2015c).

Inhalation

In a range-finding inhalation study (1/sex/dose level), male Sprague-Dawley rats exposed for 1 

hour to 0, 0.01, 0.11, or 1.1 mg per liter (mg/L) of diphacinone died (USEPA 1998). Two female 

rats survived to the end of the 7-day observation period. Symptoms observed were hemorrhage 

in the thoracic cavity, cranial cavity, and various organs. To determine the lethal concentration of 

airborne diphacinone, a follow-up study was conducted in Sprague-Dawley rats (5/sex/dose level) 

exposed to 4 hours of a time-weighted average aerosol of 6 micrograms per liter (μg/L) and 

observed for 14 days (USEPA 1998). Mortality occurred on day 4–8 in 5/5 males and 4/5 female 

rats. Based on the observed LC50 of <6 μg/L in rats, inhalation exposure to diphacinone is in a 

Toxicity Category of I (USEPA 2015c).

Eye and Dermal Irritation and Sensitization

New Zealand white rabbits (n=3) were exposed to 0.1 g diphacinone (98%) via eye irrigation for 

20–30 seconds (USEPA 1998;2015c). No corneal opacity was observed. Technical grade 

diphacinone is in the Toxicity Category III for eye irritation. The acute systemic toxicity of 

diphacinone complicated dermal irritation and sensitivity studies. However, it was determined that 

non-lethal concentrations of diphacinone are neither a dermal sensitizer nor an irritant (USEPA 

1998;2015c).

2.3.5 Sub-chronic/Chronic Toxicity

Oral: In a 14-day oral toxicity study, Sprague-Dawley rats (5/sex/dose level) were orally gavaged 

technical grade diphacinone (99.0%) at 0, 0.025, 0.040, 0.085, or 0.175 mg/kg-bw/day (USEPA 

1998;2015c). No clinical signs of toxicity were observed at doses of 0.025, 0.040, or 0.085 mg/kgbw/day. At 0.175 mg/kg-bw/day, dyspnea, lethargy, hemorrhage from the nose, and ptyalism 

(overproduction of saliva) increased. At the 0.175 mg/kg-bw/day exposure, all female (5/5) and 3

of the 5 male rats died by treatment day 11. The LOAEL and NOAEL for the 14-day study were

0.085 and 0.040 mg/kg/day, respectively. 

Dermal Toxicity

In a 21-day dermal toxicity study, rabbits were dermally exposed to diphacinone at dosage levels 

of 0, 0.1, 1.0, or 10.0 mg/kg-bw/day (USEPA 1998). Two males and females in each group had 

doses applied to abraded skin. The subchronic NOAEL was 0.1 mg/kg-bw/day, and the LOAEL 

was 1.0 mg/kg-bw/day, based on mortality accompanied by clinical signs of anticoagulant activity

(USEPA 1998). No chronic studies testing have been conducted due to the non-food uses for 

diphacinone.
16

2.3.6 Developmental and Reproductive Effects

Pregnant Sprague-Dawley rats (25/group) were exposed to 0, 0.010, 0.025, or 0.075 mg/kgbw/day for 9 days (gestational days 6–15) (USEPA 2015c). Adverse effects were observed in all 

treatment groups. Based on clinical signs consistent with anticoagulant activity (reddish urogenital 

staining and reddish fluid in cage), a maternal NOAEL could not be established (NOAEL <0.010 

mg/kg-bw/day). No effects were reported on body weight, food consumption, or histopathology. 

Although compound-related altered growth and/or developmental abnormalities were not 

observed, there were increased early fetal resorptions and increased fetal resorptions per animal 

at 0.075 mg/kg-bw/day. The developmental LOAEL was determined to be 0.075 mg/kg-bw/day, 

and the developmental NOAEL was 0.025 mg/kg-bw/day. The prenatal developmental toxicity 

study in rabbits and a reproduction and fertility toxicity study in rats was not required by the Hazard 

and Science Policy Council (USEPA 2015d). 

2.3.7 Neurotoxicity and Immunotoxicity Effects

Although diphacinone has been observed to exert overt acute anticoagulant activity, potentially

leading to severe injury or death, neurotoxicology and immunology sensitivity studies do not 

indicate these are a concern (USEPA 1998;2020a). As a result, the Hazard and Science Policy 

Council has not required neurotoxicity studies and waived immunotoxicity studies for diphacinone 

registration, and determined that available studies adequately characterize toxicity for any 

quantitative risk assessments (USEPA 2015c). A literature review resulted in similar exposure 

sensitivity conclusions to other FGARs with similar modes of action (Kataranovski et al. 2003). 

2.3.8 Carcinogenicity and Mutagenicity

Diphacinone is not considered to be a carcinogen, although USEPA did not require 

carcinogenicity studies for diphacinone due to its non-food uses (USEPA 2020a). Available 

studies do not indicate mutagenicity for diphacinone (USEPA 2015c;2020a). 

2.3.9 Endocrine System Effects

An Endocrine Disruptor Screening Program (EDSP) was developed to characterize endocrine 

activity in commercial products, pesticides, and environmental contaminants (USEPA 2023b). 

EDSP uses a two-tier risk characterization approach consisting of screening candidate 

compounds for estrogen, androgenic, and thyroid receptor activity and quantifying their impact on 

environmental and human health (USEPA 2023b). Before 2012, Tier 1 screening involved five in 

vitro and six in vivo assays (Browne et al. 2015). To address the growing need for a more rapid 

but equally comprehensive review of thousands of candidate compounds, the EDSP revised Tier 

1 screening to include computational endocrine activity models and high-throughput assays. Tier 

2 testing data characterizes the endocrine-related health effects, dose response, and health risks 

of candidate compounds and substances. Although Diphacinone is listed in the EDSP Universe 

of Chemicals (USEPA 2012;2023a). Diphacinone was not screened for estrogen receptor 

bioactivity. 

2.3.10 Toxicity of Other Ingredients

The other 99.995 percent of all diphacinone product formulations are other ingredients (e.g., 

(USDA APHIS 2009, HACCO 2021). The identity and safety profile of non-active ingredient 

contents of diphacinone formulations is not presented on labels or safety data sheets (HACCO 

2021). The other or “inert” ingredients listed on pesticide labels are considered confidential 
17

business information and not generally available to the public. However, they still must be 

approved inert ingredients for non-food use pesticides by USEPA (2023c).

3 DOSE-RESPONSE ASSESSMENT

3.1 Human Health Dose-Response Assessment

A dose-response assessment evaluates the dose levels (toxicity criteria) for potential human 

health effects. A 14-day diphacinone oral exposure study in rats reported a NOAEL of 0.085 

mg/kg-bw/day and LOAEL of 0.175 mg/kg-bw/day based on a critical effect of increased 

prothrombin time and activated partial thromboplastin time for repeated doses (USEPA 2015c). 

A 21-day oral study in mice reported a NOAEL of 0.5 mg/kg-bw/day and a LOAEL of 1 mg/kgbw/day based on hemorrhages and external bleeding (USEPA 2015c). A 21/28-day dermal study 

in rabbits using diphacinone technical grade reported a NOAEL of 0.1 mg/kg-bw/day and LOAEL 

of 1 mg/kg/day based on hemorrhagic lesions in tissues/organs (USEPA 2015c). 

Maternal diphacinone toxicity studies in pregnant Sprague-Dawley rats were inconclusive. A 

NOAEL for maternal effects could not be established due to adverse effects at every exposure 

level (NOAEL <0.010 mg/kg-bw/day) (USEPA 1998). Based on the increased incidence of fetal 

resorptions, the developmental NOAEL and LOAEL were observed to be 0.025 and 0.075 mg/kgbw/day, respectively (USEPA 1998). 

USEPA did not include toxicology studies with mice as an experimental model in determining the 

point of departure in their risk assessment for diphacinone (USEPA 2015c). The USEPA reported 

that "the mouse is considerably less susceptible to the toxic effects of diphacinone than other 

mammalian species” (USEPA 1998). Studies involving rabbit models have been evaluated but 

were not selected in determining the point of departure due to sample size limitations and

increased tolerance to diphacinone (Correll et al. 1952). Therefore, for this risk assessment and 

to protect the most vulnerable of the population that is at the highest risk of injury from exposure, 

the prenatal rat developmental evaluation (USEPA 1998) was chosen as the critical study with 

the critical effects of anticoagulant activity and increased fetal resorptions determining the point 

of departure (NOAEL of 0.025 mg/kg-bw/day). 

An uncertainty factor is a human health risk assessment conceptual tool predicated on the 

assumption that a biological response increases or decreases in relation to increasing or 

decreasing exposure to a biologically active compound. If a sufficient reduction in exposure from 

the first known boundary of deleterious effects can be accomplished, then a level of exposure 

protective of populations, including the vulnerable, can be determined (Dankovic et al. 2015).

Uncertainty factors are used to address the differences between experimental data and actual 

human health effects. These include uncertainty factors for interspecies and intraspecies 

differences, differences in exposure duration, and data quality issues. For this risk assessment, 

uncertainty factors of 10 and √10 were applied to the NOAEL to account for genetic variability 

between humans and the variability between animals and humans. To accommodate for the 

absence of male rodents in the reference study, an additional factor of √10 was applied to the 

uncertainty extrapolation for a total uncertainty factor of 100. The application of uncertainty factors 

to the point of departure results in a reference dose of 0.25 μg/kg-bw/day.

Diphacinone has only non-food uses. Thus, USEPA (2015c) did not establish food tolerances or 

conduct a dietary exposure assessment.
18

3.2 Ecological Effects Analysis 

This section summarizes available diphacinone toxicity data for aquatic and terrestrial species. 

3.2.1 Aquatic Effects Analysis

Diphacinone has moderate acute oral toxicity to freshwater fish and invertebrates (USEPA 2015e)

(Table 4). Toxicity data is lacking for estuarine and marine fish, invertebrates, and aquatic plants 

(USEPA 2015e). 

Table 4. Diphacinone toxicity in aquatic species.

Test Species Scientific Name LC50/EC50 (mg a.i./L) Reference

Rainbow Trout Oncorhynchus 

mykiss

96-hr LC50 = 2.6 (moderately toxic) (USEPA 2015e;2020b)

Bluegill Lepomis macrochirus 96-hr LC50 = 7.5 (moderately toxic) (USEPA 2015e)

Water Flea Daphnia magna 48-hr EC50 = 1.8 (moderately toxic) (USEPA 2015e;2020b)

3.2.2 Terrestrial Effects Analysis

3.2.2.1 Mammals

Technical diphacinone has very high toxicity to mammals on an acute oral and dietary basis

(Table 5) (USEPA 2015e;2020b). In laboratory brown rats, the acute oral LD50 was 1.9 mg/kg-bw, 

and the acute dietary LC50 was 2.08 mg/kg-diet. Small Indian mongoose are highly susceptible to 

diphacinone at 0.18 mg/kg-bw (Keith et al. 1989). Marsh (1985) (cited in (USEPA 2004a)) 

calculated an acute oral LD50 of 85 g of diphacinone bait for a 10-lb dog. No chronic toxicity studies 

are available. 

Target and nontarget mammals that ingest sufficient amounts of diphacinone-treated bait to be 

fatal may take several days to die from toxicity. Due to this delay in symptoms, animals may 

continue to eat available bait, ingesting more than a sufficient dose to cause toxicity. The 

elimination half-life for diphacinone in mammals, based on elimination rates from the liver, is 12.4 

days (USEPA 2020c). 

Table 5. Diphacinone toxicity in mammals.

Test species Scientific Name Test Reference

Laboratory (Brown) 

Rat 

Rattus norvegicus Acute oral LD50 = 1.9 

mg/kg-bw (very highly toxic)

(USEPA 2015e;2020b)

Laboratory (Brown) 

Rat 

Rattus norvegicus Acute dietary LC50 = 2.08 

mg/kg-diet, 5-day 

exposure (very highly 

toxic)

(USEPA 2015e)

Domestic Cat Felis catus LD50 = 15 mg/kg-bw (Eisemann and Swift 2006)

Domestic Dog Canis domesticus LD50 = 2 to 3 mg/kg-bw (Eisemann and Swift 2006)

Woodland [Pine] Vole Microtus pinetorum LD50 = 67.7 mg/kg-bw (Byers 1978)

Meadow Vole Microtus 

pennsylvanicus

LD50 = 11.7 mg/kg-bw (Byers 1978)

Common Vampire Bat Desmodus rotundus LD50 <0.91 mg/kg-bw (Eisemann and Swift 2006)

Small Indian 

Mongoose

Herpestes javanicus 

[Synonym: Urva 

auropunctata]

LD50 = 0.18 mg/kg-bw (Keith et al. 1989)
19

3.2.2.2 Birds

Birds are more tolerant of diphacinone than mammals. In birds, diphacinone is moderately toxic 

to practically nontoxic on an acute oral basis (Table 6) (USEPA 2020b). Diphacinone is 

moderately toxic (LD50 of 96.8 mg/kg-bw) to the American kestrel, which is more sensitive than 

the standard test species of northern bobwhite and mallards (Rattner et al. 2011). USEPA used 

the acute oral data for northern bobwhite as an endpoint in their risk assessment (referenced

below under the exposure assessment and risk characterization) (USEPA 2020c). 

On an acute dietary basis, diphacinone is moderately toxic with an LC50 of 906 mg/kg-diet in 

mallards and slightly toxic in the northern bobwhite with an LC50 of 5,000 mg/kg-diet (USEPA 

2020b). No chronic avian studies are available for diphacinone; however, to be conservative, 

USEPA used a chronic toxicity study for chlorophacinone to represent chronic toxicity for 

diphacinone, since chlorophacinone is more toxic to birds than diphacinone (USEPA 2020c). 

Chlorophacinone reduced the mean 14-day survivor weights in mallards, with a NOAEC of 0.046 

mg/kg-diet and LOAEC of 0.096 mg/kg-diet (USEPA 2020c). 

Table 6. Diphacinone toxicity in birds.

Test species Scientific Name Test Reference

Northern 

Bobwhite 

Colinus 

virginianus

Acute oral LD50 = 1,630 mg/kg-bw

(slightly toxic)

Acute dietary LC50 = 5,000 mg/kgdiet (slightly toxic)

(Eisemann and Swift 

2006, USEPA 

2015e;2020b) 

American Kestrel Falco sparverius Acute oral LD50 = 96.8 mg/kg-bw, 

7-day test (moderately toxic)

(USEPA 2015e)

Mallard Anas 

platyrhynchos

Acute oral LD50 = 3,160 mg/kg-bw

(practically nontoxic)

Acute dietary LC50 = 906 mg/kgdiet, 25-day test (moderately toxic)

(USEPA 2004b;2015e)

3.2.2.3 Reptiles and Amphibians (terrestrial phase)

In the brown tree snake, diphacinone was highly toxic with an acute oral (single dose) LD50 of 

20.75 mg/kg-bw (USEPA 2015e). 

3.2.2.4 Terrestrial Invertebrates and Microorganisms

Limited information exists on the toxicity of diphacinone to invertebrates. In one study, 

diphacinone residues were quantified in snails and slugs fed diphacinone baits for 7 days in the 

laboratory or collected on or near diphacinone baits in the field (Johnston et al. 2005). Although 

diphacinone residues were detected in laboratory and field-collected glass snails (Oxychilus spp.), 

great gray slugs (Limax maximus), and marsh slugs (Deroceras laeve), no mortality occurred, 

indicating acute toxicity of diphacinone to snails and slugs is likely minimal.

3.2.2.5 Terrestrial Plants

Toxicity data is unavailable for terrestrial plants (USEPA 2015e). 
20

4 EXPOSURE ASSESSMENT

4.1 Human Health Exposure Assessment

Diphacinone-50 Conservation (a pellet product) is a restricted use pesticide, and only certified 

applicators or persons under their direct supervision who are employees or under the direct 

supervision of federal agencies responsible for wildlife management may use the product (USEPA 

2019a). The other four diphacinone block products used by WS are general use pesticides that 

the public and WS can use without a certified pesticide applicator’s license.

Exposure assessments estimate the potential exposure of humans to diphacinone. An identified 

exposure pathway for diphacinone includes (1) a release from a source, (2) an exposure point 

where contact can occur, and (3) an exposure route such as ingestion, inhalation, or dermal 

contact by which contact can occur (USEPA 1989). Exposures for the identified human 

populations are qualitatively evaluated for each identified exposure pathway.

4.1.1 Identification of Potentially Exposed Human Populations and Complete Exposure 

Pathways

Based on the expected WS use pattern and label restrictions for diphacinone applications, 

workers applying the rodenticide baits in the field are the most likely subgroup of the human 

population to be exposed to diphacinone. Exposure during transportation is not anticipated 

because the material is sealed. The baits are ready to use with no mixing required. Diphacinone50 Conservation is a pelleted bait; the other four are bait blocks. The label application methods

include bait stations, burrow baiting, aerial and ground broadcast baiting, and canopy baiting, with 

broadcast baiting having the greatest potential for exposure. Broadcast aerial and ground baiting 

of Diphacinone-50 Conservation pellets are usually permitted only on islands uninhabited by 

humans. Although the potential for exposure from the proposed WS use pattern is low, accidental 

exposure may occur during application. The risk of exposure via ingestion or inhalation is deemed 

to be minimal due to outdoor use and label-mandated application restrictions and personal 

protective equipment (PPE). Block products cannot be applied using broadcast application 

methods.

Applicators and other handlers must wear long pants, shoes plus socks, and barrier laminate 

gloves. Any person who retrieves carcasses or unused bait following the application of 

diphacinone must wear barrier laminate gloves. Following label directions, including the use of 

proper PPE, will minimize worker exposure to diphacinone via inhalation and dermal contact 

routes. 

A significant direct exposure pathway to Diphacinone-50 Conservation bait is not identified for the 

public because this product is a restricted use pesticide and can only be sold to certified 

applicators that are employees of federal agencies responsible for wildlife management for 

conservation uses on islands. Diphacinone-50 Conservation can only be used by a certified 

applicator or persons under their direct supervision. Belowground burrow baiting is only permitted 

in uninhabited non-crop areas, and bait bolas or sachets must only be applied to tree canopies or 

uninhabited grounded vessels or vessels in jeopardy of being grounded. Manual or aerial 

broadcast applications of Diphacinone-50 Conservation are prohibited by the label in areas of 

human habitation unless allowed by USEPA for specific island projects under supplemental 

labels. 
21

General use block formulations of diphacinone and Diphacinone-50 Conservation pellets used 

aboveground in areas that are accessible to children and pets must be applied in tamper-resistant 

bait stations. Tamper-resistant bait stations also minimize post-application exposure in adults. As 

a result of these use restrictions for diphacinone products used by WS, the public is not 

considered a vulnerable population for direct exposure to diphacinone used by WS (see Section 

2.1).

Although oral exposure to all formulations of diphacinone is acutely hazardous, label restrictions 

render accidental dietary exposure an incomplete exposure pathway. Diphacinone has a low 

potential for volatilization due to its low vapor pressure. It will adsorb readily to organic material in 

the soil, minimizing inhalation risks and mobility into nearby water bodies or aquifers. In addition, 

label restrictions state that no applications are allowed directly to water, areas where surface 

water is present, or intertidal areas below the mean high-water mark (USEPA 2019a). As a result, 

surface and groundwater exposure pathways are also incomplete. 

4.1.2 Occupational Exposure Evaluation

This section quantitatively evaluates worker exposures from an incidental direct contact pathway 

while applying baits. Under the accidental exposure scenario, it assumes that chemical-resistant 

gloves are broken. 

The estimates of exposure to certified applicators using diphacinone are based on surrogate study 

data available in the Pesticide Handlers Exposure Database (PHED) (USEPA 2021a). The 

"trap/bait station refillable" or "applicator granules by hand" exposure scenario in the PHED table 

is the closest scenario representing potential exposure for the application method on the label for

pellets and sachets containing diphacinone at 0.005%. Under this exposure scenario, the 

exposure unit for dermal contact under the single layer (long-sleeve shirt, long pants, shoes plus 

socks) and single and double pair of gloves is 71 and 40.3 mg/lb ai, respectively. The exposure 

unit for a scenario where a WS applicator lacks or dons inadequate hand protection, such as

broken gloves, is 104 mg/lb ai. Lower and upper application use rates were estimated based on 

4 to 16 ounces (113 to 454 g) of bait per treatment location with placements at an interval of 16 

to 82 feet (5 to 25 meters) per label directions to bait rats (USEPA 2016a). 

Other conservative assumptions used for this exposure evaluation include an accidental dermal 

contact of 1% pelleted baits when PPE integrity is compromised without replacement for 8 hours. 

A worker spends a half-hour at each placement. The following equations are used to estimate the 

exposure dose of direct contact for workers:

Exposure Dose = Daily Dose Rate/Body Weight 

Daily Dose Rate = Unit Exposure (mg/lb ai) x Application Rate (lb ai/acre) x Area Treated 

(acre/day) x Dermal Absorption Rate (%)

Exposure doses were estimated for the application rates specified on the labels and summarized 

in Appendix A.

4.2 Ecological Exposure Assessment

WS has specific use patterns for diphacinone. WS uses diphacinone to eradicate or manage 

invasive rodents on islands for conservation purposes; house mice, Pacific rats, and black rats in 
22

Hawaii; Norway rats on islands in Massachusetts; and pocket gophers in New Mexico to protect

lawns, landscaping, and pecan trees. 

The application method will affect the primary and secondary exposure potential of nontarget 

species. Depending on the formulation used for treatment, the label may allow for applications 

belowground by hand, bait bolas, in tamper-resistant bait stations, or through aerial and ground 

broadcast applications. WS applied baits in bait stations or underground in gopher burrows from 

FY11–FY20.

The USEPA's ecological risk assessments (USEPA 2004a;2020c) calculate the risk quotient (RQ) 

for different species groups and compares this to the regulatory levels of concern (LOC) to 

understand the potential exposure risk associated with the use of a pesticide. The RQ is 

calculated by dividing an estimated environmental concentration by a toxicity endpoint and 

determines if the estimated environmental concentration is above or below the concentration with 

an effects endpoint. The vertebrate LOC for acute and chronic risks are 0.5 and 1.0, respectively, 

and for plants, the LOC is 1.0; the LOCs are meant to be protective of community-level effects.

The LOCs for aquatic and terrestrial species are summarized in their respective sections below.

4.2.1 Aquatic Exposure Assessment

The labels for diphacinone formulations do not allow applications to water resources. The 

Diphacinone-50 Conservation label allows for ground and aerial broadcast applications, with 

aerial applications allowed only when wind speeds are below 35 mph. Diphacinone is slightly 

mobile in soil and has a low water solubility and low potential for migration to groundwater via 

leaching or surface water via runoff, indicating a localized effect. The USEPA (2015e) found the 

exposure of surface water or groundwater to diphacinone is negligible based on the application 

amounts and methods and the environmental fate properties of diphacinone.

USEPA's (2020c) aquatic exposure assessment for diphacinone broadcast applications relied on 

its estimate for chlorophacinone because USEPA found its estimate for chlorophacinone was 

inclusive and protective of the potential risks to aquatic organisms from diphacinone; 

chlorophacinone is more toxic to the test organisms rainbow trout and Daphnia magna than is 

diphacinone. USEPA used the highest modeled exposure in their estimate, which was the 

broadcast application of chlorophacinone in an orchard. USEPA's analysis included chronic 

exposure to freshwater fish and Daphnia magna, acute exposure to green algae, and acute and 

chronic exposure to saltwater fish and Mysid shrimp (USEPA 2020b). USEPA found aquatic taxa 

are not at risk (USEPA 2020b). WS finds its use patterns would also result in negligible exposure 

to water resources, based on label requirements, diphacinone’s environmental fate properties, 

WS use patterns, and USEPA exposure estimates.

4.2.2 Terrestrial Exposure Assessment

Between 1986 and 2019, there were 122 wildlife incident reports of diphacinone exposure in the 

Incident Data System maintained by USEPA. Of these incidents, 24 had detectable diphacinone 

residue in live animals, 29 were highly probable, 15 as probable, and 54 as possible diphacinone 

exposure (USEPA 2020c). Incidents involving avian species exposure to diphacinone included 

American kestrel, barn owl, Canada goose, rock dove, wild turkey, and turkey vulture. Mammal

species involved in these incidents included badger, coyote, dog, squirrel, fox, kangaroo rat, 

opossum, fisher, pig, rabbit, raccoon, skunk, and deer (USEPA 2020c). Federally listed and 

protected species included bald eagles (2 incidents) and kangaroo rats (1 incident). The 

suspected exposure scenario was not provided with the incident reports; it is not possible to 
23

determine those incidents that resulted from exposure through broadcast applications, 

belowground applications, or another scenario. These exposures were not from WS applications

of diphacinone baits.

Primary Exposure

Direct exposure of nontarget species to diphacinone from belowground applications and 

belowground caching is mainly limited to animals that can access the burrow. Diphacinone bait 

may dislodge from the burrow and become accessible to nontarget species aboveground as 

animals enter and exit the burrow (USEPA 2009), though this would be very minimal. WS expects 

small mammals that use a treated burrow or colonize it after the target animal has died are likely 

to ingest a toxic amount of diphacinone bait if it is still present and palatable. 

Tamper-resistant bait boxes/stations are required for aboveground applications accessible to the 

public. They are also used in areas open to hoofed livestock, raccoons, bears, or potentially 

destructive animals. The use of tamper-resistant bait stations in aboveground applications 

reduces primary exposure to nontarget species; however, some researchers speculate the 

constant bait supply may result in the target animal consuming more bait than other application 

methods, allowing for a higher concentration of diphacinone within the target rodent, which would 

be a concern for secondary exposure (as summarized in (Baldwin et al. 2021)). However, this 

was not observed in a study on California ground squirrels, suggesting no increase in diphacinone 

residues when using bait stations (Baldwin et al. 2021). The authors observed the ground 

squirrels removed a substantial amount of bait from the bait stations and suggested that the 

squirrels may have gathered and stored the grain underground for consumption later.

Exposure of nontarget species occurs with aboveground broadcast applications. In a field study 

to test the efficacy of 0.005% and 0.01% diphacinone bait applied through broadcast or spot 

application to control California ground squirrels on rangeland, the researchers conducted 

carcass surveys on study plots to determine the nontarget poisoning rate (Salmon et al. 2007). In 

the study, bait was applied twice, three days apart, and the search for carcasses occurred for at 

least 10 days after the final bait application. Sixteen nontarget animals (14 kangaroo rats and 2 

deer mice) died from anticoagulant poisoning, representing a nontarget poisoning rate of 0.37 

carcasses/hectare. No bird carcasses were collected.

In Washington state, orchards treated with an average of 12.9 kg/ha (11.5 lb/A) of 0.005% 

diphacinone bait mechanically broadcast to control voles were evaluated for exposure to game 

birds (Hegdal 1985 unpublished study cited in (USEPA 2004a)) (Eisemann and Swift 2006). Most 

of the orchards were treated twice, with 20 to 30 days between treatments. The researchers 

monitored the fate of ring-necked pheasants, California quail, and chukar with telemetry. They 

collected dead game birds, some of which were shot by hunters. Diphacinone residues were 

detected in the liver of 10 of 44 ring-necked pheasants (average concentration of 0.23 ppm), 11 

of 19 California quail (average concentration of 0.21 ppm), and 1 of 10 chukar. Five pheasants 

and 10 quail contained bait pellets in the crop. Although exposure to game birds occurred in 

treated orchards and the birds ate the bait, the risk appeared to be low. During the study, adjoining 

landowners applied bait at the same time, which may have increased the birds’ exposure to 

treated pellets. Also, the researchers used a combination of wild and pen-raised birds, and the 

pen-raised birds may have been more accustomed to foraging on a pelleted diet. The study did 

not look at long-term effects, such as susceptibility to predation.

Exposure of terrestrial and soil-dwelling invertebrates could occur should they come into direct 

contact with diphacinone bait aboveground or in burrows. Belowground applications would not 
24

result in widespread exposure of invertebrates. Information on the toxicity of diphacinone to 

invertebrates is limited. In one study, diphacinone residues were quantified in snails and slugs

fed baits containing diphacinone for 7 days in the laboratory or collected on or near diphacinone 

baits in the field (Johnston et al. 2005). Although diphacinone residues were detected in laboratory 

and field-collected glass snails, and great gray and marsh slugs, no mortality occurred, indicating 

acute toxicity of diphacinone to snails and slugs is minimal. In another field study, diphacinone

residues in target and non-target species were measured pre- and post-application of diphacinone 

bait hand broadcasted at a rate of 12.82 kg/ha, about 1 kg/ha under the maximum bait application 

rate on the Diphacinone-50 label (Shiels, 2017). No diphacinone was detected in species sampled

pre-application. However, detectable levels of diphacinone were found in non-native mammals

(residues of 0.3-32.2 ppm in target rodent species), birds (0-1.2 ppm), gastropods (0-9.8 ppm), 

and arthropods (0-0.4 ppm) with only one non-target mortality occurring in a single mongoose

(diphacinone residue of 0.809 ppm). Skinks did not have detectable diphacinone levels, although 

the sample size was small (n=3).

USEPA (2004b) estimated the amount of diphacinone mammals and birds of different weight 

sizes would need to ingest to receive an LD50 dose. A 25-g, 100-g, and 1000-g mammal would 

need to ingest 1.2 g, 4.6 g, and 46 g, respectively, of diphacinone at a rate of 50 mg a.i./kg-bait 

to receive an LD50 dose. For mammals, the acute LOC was exceeded for all three weight classes 

after a single day of exposure (50 mg ai/kg-diphacinone bait) and six consecutive days of 

exposure (USEPA 2020c). 

Birds appear to be less sensitive to diphacinone than mammals. At a rate of 50 mg a.i./kgdiphacinone bait, a 25-g passerine would need to eat 200 g of bait (1,000 pellets), a 100-g nonpasserine would need to eat 800 g of bait (4,000 pellets), and a 100-g non-passerine would need 

to eat 8,000 g of bait (>1,000 pellets) to reach an LD50 dose. Passerine birds did not exceed the 

LOC for single-day primary exposure or six consecutive days of primary bait exposure (50 mg 

ai/kg-diphacinone bait) (USEPA 2020c). Birds that are mainly herbivorous or insectivorous are 

not expected to be at risk from primary exposure to diphacinone bait. However, USEPA (2020c)

found a potential chronic risk from diphacinone secondary exposure in birds; USEPA used the 

avian reproductive data for chlorophacinone to characterize diphacinone’s chronic risk to birds. 

Exposure of birds to underground applications of bait is expected to be minimal. Exposure of 

nontarget species during spot bait applications or bait station applications of baits containing 

0.01% w/w and 0.005% w/w diphacinone to control ground squirrels in rangeland did not result in 

take of nontarget birds on treatment plots, but 30 small mammal carcasses were found (Baroch 

1996). This represents a nontarget mammal poisoning rate was 0.5 carcasses/ha. However, this 

may be an underestimate since carcass searches occurred once each day during the baiting 

period and not beyond the last day of the baiting period. Predators were observed in the study 

area, including turkey vultures consuming squirrel carcasses, but no secondary poisoning cases 

were recorded. 

Exposure of terrestrial plants is negligible with belowground applications and aboveground 

applications in bait boxes.

Secondary Exposure

Secondary exposure could occur for predators and scavengers that can access burrows with 

poisoned animals or if they kill or scavenge poisoned animals aboveground. Generally, more than 

one feeding is needed to receive a toxic dose for both the prey animal and predator or scavenger. 

It can take four to ten or more days for the animal to die after ingesting a toxic dose of diphacinone
25

bait (USEPA 2015e;2020b), and during this time, the animal continues to feed, potentially 

ingesting a dose well above the toxic dose. Animals that have ingested diphacinone may exhibit 

behavior (e.g., lethargy) that makes them more susceptible to predators (Cox and Smith 1992 

cited in (USEPA 2015e, Vyas et al. 2017)).

The diphacinone residue level in an animal depends on the amount of bait the animal eats, how 

close together its feedings are, and its metabolism of diphacinone. Diphacinone is metabolized in 

and disposed of from an animal’s body, which may reduce the toxicity to a predator or scavenger.

In a laboratory study on the metabolism and disposition of diphacinone, rats were given a single 

oral dose of radiolabeled diphacinone at 0.2 or 1.5 mg/kg-bw, and mice were given a single oral 

dose at 0.6 mg/kg-bw (Yu et al. 1982). In rats, about 70% of the oral dose was eliminated in feces 

and 10% in urine within 8 days. In mice, most of the diphacinone was eliminated within 4 days; 

only 7% was retained in body tissues. USEPA (2004b) summarized an unpublished report by 

Diaz and Whitacre (1976) in which they orally dosed rats with diphacinone (0.32 mg/kg-bw/day) 

for 1 or 2 days. In the rats dosed for two days and sacrificed 72 hours after the second dose, 

about 45% of the diphacinone was excreted (86% in feces, 14% in urine), 25% remained in the 

body tissues, and 30% was not recovered. In rats dosed once and sacrificed 48 hours later, about 

5% of the dose was excreted, 61% retained, and the remaining not recovered. Diphacinone has 

a short half-life in ground squirrels (67–120 hours in the liver) (as cited in (Baldwin et al. 2021). 

USEPA (2004b) summarized field studies that evaluated diphacinone residue levels in ground 

squirrels. The number of days the squirrels were exposed to bait was unknown. Whole carcass 

residue in squirrels exposed to 50–100 mg a.i./kg-bait had 0.9–1.4 ppm diphacinone wholecarcass residue.

In a study on California ground squirrels, there were no statistically significant differences in 

diphacinone residue levels in the livers regardless of broadcast, spot treatment, or bait station 

application method, despite less bait applied by broadcast application (Baldwin et al. 2021). In 

the study, a 0.01% w/w diphacinone bait was used for broadcast applications, not the 0.005%

w/w diphacinone bait used for spot treatments and bait stations (Baldwin et al. 2021). The authors 

suggest this higher concentration of diphacinone was the reason for the observed (but not 

significantly different) high residues despite less bait used in the broadcast applications compared 

to spot treatment and bait station applications. The researchers noted a general trend toward 

lower residue concentrations when using lower bait concentrations. The formulations WS uses 

are all 0.005% w/w diphacinone.

USEPA (2004b) summarized diphacinone studies looking at toxicity to mammalian predators and 

scavengers, including mink, mongoose, short-tailed weasel, and dogs. The mammals were given 

rodents that fed on 0.01% w/w diphacinone bait for 10 days, except for the mongoose that were 

given rodents that fed on 0.005% w/w diphacinone bait for 5 days. All 3 mink and dogs, 7 of 8 

mongoose, and 1 of 2 ermine died.

In the laboratory, secondary exposure of predator and scavenger birds to diphacinone resulted in 

the death of some exposed birds, and some survivors displayed signs of diphacinone toxicity. 

Mendenhall and Pank (1980 cited in (USEPA 2004b)) exposed two owls for 10 days to carcasses 

of rats fed 50 ppm 0.005% w/w diphacinone bait; the owls did not show signs of toxicity. In this 

study, the diphacinone residue levels in the rats were not provided. Great horned owls and 

northern saw-whet owls (Aegolius acadicus) given two mice per day that fed on 0.01% w/w

diphacinone bait for 10 days caused death in 2 of 3 great horned owls and 1 saw-whet owl (only 

one bird was exposed) (USEPA 2004b;2015e). One great horned owl survived but had an 

increase in blood coagulation time. Neither of 2 barn owls exposed ad libitum to mice fed 0.01% 
26

w/w diphacinone bait for 10 days showed signs of toxicity (USEPA 2004b;2015e). No American 

crows or golden eagles given rats fed 0.005% w/w diphacinone bait or meat laced with 2.7 mg/kgdiet, respectively, died from exposure (USEPA 2004b;2015e). However, 5 of 11 crows and all

eagles showed signs of external bleeding and increased blood coagulation time (USEPA 

2004b;2015e). USEPA (2015e) summarized that nine percent mortality occurred across these 

secondary bird exposure studies, which involved 5 bird species and 34 individual birds.

Researchers have observed predators and scavengers hunting anticoagulant-treated areas more 

than nearby untreated areas (Vyas et al. 2012, Vyas et al. 2017), likely because poisoned target 

animals become lethargic prior to death and easier to capture (USEPA 2020b). One field study 

involving chlorophacinone, not diphacinone, demonstrated this. In the field study, Vyas et al. 

(2017) compared the number of visits by ferruginous hawks to black-tailed prairie dog colonies 

that were treated with Rozol Prairie Dog Bait (0.005% w/w chlorophacinone, another firstgeneration anticoagulant that is more toxic to birds than diphacinone (USEPA 2020c) or remained 

untreated. They found the hawks hunted in only the treated colonies, likely because the poisoned

prairie dogs were lethargic and not as aware of their surroundings, making them easier to capture.

Target animals treated with belowground applications usually die belowground; however, a 

percentage die aboveground. Field studies indicate that 82%–91% of ground squirrel mortalities 

from diphacinone exposure occur within burrow systems (Baldwin et al. 2021). In a field study

across 10 treated prairie dog towns treated with chlorophacinone, not diphacinone, bait, 10 animal 

carcasses, nine of which were prairie dogs, were found aboveground (USEPA 2009) summarizes 

(Lee and Hyngstrom 2007). Eight of the carcasses were intact, but two had been scavenged. The 

rate of prairie dog carcasses was 1 per 14 acres searched. The researchers also noted they saw 

5 impaired prairie dogs 10 days or more after bait applications.

Birds that feed on insects, snails, and slugs that have been exposed to diphacinone bait could 

potentially receive a toxic dose themselves. Johnston et al. (2005) summarized several studies 

indicating a link between fatalities in birds with the consumption of rodenticide baits (in this case, 

brodifacoum). Johnston et al. (2005) analyzed diphacinone residues in snails and slugs exposed

to bait in the laboratory as well as in the field. As expected, diphacinone residues in snails and 

slugs that fed on 0.005% w/w diphacinone bait for 7 days in the laboratory were higher than 

residues in field-exposed snails and slugs, likely due to the variety of food sources available in 

the field. Mean residue concentrations in glass snail species for laboratory-exposed was 1.77 

ppm and field-exposed was 0.69 ppm. Mean residue concentrations in marsh slugs was 2.64 ppm 

in laboratory-exposed and 0.23 ppm for field-exposed. An estimate of the exposure potential of 

birds to diphacinone in baiting areas via consumption of gastropods containing diphacinone 

residues (based on the upper 95% confidence limit for laboratory-exposed gastropod diphacinone 

residues or 4.93 ppm diphacinone) had risk quotients that were less than the 0.5 level of concern, 

indicating the mortality risks to nonthreatened or nonendangered birds consuming gastropods 

containing diphacinone appear to be acceptable (Johnston et al. 2005). However, a conservative 

estimate exceeded the level of concern for threatened and endangered birds. The authors

indicated several assumptions might overestimate this risk, including birds only consumed 

gastropods with diphacinone residues and residue levels in gastropods in the field being the same 

as gastropods that were fed a strict diphacinone bait diet in the laboratory.

5 RISK CHARACTERIZATION

This section discusses the quantitative and qualitative risks associated with the proposed use of 

diphacinone. The evaluation of documented diphacinone health exposure data and relevant 

animal exposure studies applied to exposure assumption scenarios can quantify the risk of impact 
27

to human health and nontarget fish and wildlife if accidentally exposed. Deterministic methods 

are used, where appropriate, to determine if expected environmental residues exceed toxicity 

data suggesting possible risk. In other cases, a more qualitative discussion regarding risk may 

rely on literature and additional information to further elaborate on the potential for injury or harm.

5.1 Human Health 

Risks associated with adverse human health are characterized quantitatively for this section's 

potential accidental occupational exposure scenario. Due to the limited volume applied annually 

and WS's adherence to label requirements, diphacinone applications to control rodent populations 

are expected to pose minimal risks to the public. 

To quantify the human health risks associated with an accidental occupational dermal exposure 

to diphacinone during WS pest-control activities, a hazard quotient (HQ) was calculated using the 

following USEPA risk estimation equation for non-carcinogens: 

 

HQ = Exposure (Dose) / Reference Dose

A hazard quotient of 1 was used to calculate the number of acres a certified applicator could apply 

0.005% w/w diphacinone bait. The label-restricted use of diphacinone rodenticides combined with 

the certified applicator PPE dictates the number of acres a person could work daily without 

incurring acute or chronic toxicity. The calculated acres a WS employee could safely work per 

day is summarized in Table 7. The reference dose of 0.00025 mg/kg-bw/day was based on a 

NOAEL of 0.025 mg/kg-bw/day established from a developmental study which resulted in fetal 

resorption in pregnant rats after 9 days of exposure to 0, 0.010, 0.025, or 0.075 mg/kg-bw/day

diphacinone (gestational days 6–15). Adverse effects were observed in all treatment groups 

(USEPA 1998). A total uncertainty factor of 100, i.e., factors of 10, √10, and √10, was applied to 

the NOAEL to account for genetic variability between humans and variability between animal and 

human species exposure-response. This observed critical effect is a sub-lethal manifestation of 

the well-established anticoagulant activity of first-generation anticoagulant rodenticides such as 

diphacinone. Detailed calculations are provided in Appendix A, Table 1. 

Table 7. Maximum number of acres a WS employee could safely treat with diphacinone baits in 

the field per day given different parameters.

Personal 

protective

equipment

Lowconcentration 

ground 

broadcast 

(12.5 lb./acre) 

Highconcentration 

ground 

broadcast (20

lb./acre) 

†Low 

application 

density-Low 

treatment 

(7/acre; 4 

oz. per 

application) 

†Low 

application 

densityHigh 

treatment 

(7/acre; 16

oz. per 

application) 

†High 

application 

density-Low 

treatment 

(194/acre; 4 

oz. per 

application) 

†High 

application 

densityHigh 

treatment 

(194/acre; 

16 oz. per 

application) 

No gloves/ 

broken 

gloves

3 2 19 5 <1 <1

Singlelayer PPE26 19 184 46 7 1.5

Double 

layer PPE 

36 22 255 64 9.2 2
28

†Application density and treatment levels are based on label restrictions.

WS uses first-generation anticoagulants in addition to diphacinone in its program to control 

rodents. The effects and environmental impacts of diphacinone in combination with other 

chemicals, such as pesticides, are poorly understood. As a result, WS personnel could potentially 

be exposed to multiple compounds of similar metabolic and environmental fate. However, when 

applied according to label requirements, acute and cumulative exposure risks should be minimal 

for workers. WS personnel strictly follow product label use requirements, including the proper use 

of PPE when loading, administering, or handling products containing diphacinone. The risk of 

public exposure to diphacinone should also be low for the restricted-use formulations 

(Diphacinone-50 Conservation and the Hawaii SLN). It should not be applied in areas where 

public exposure could occur. Public exposure would be greater for the general-use formulations 

that the public can get over the counter. WS applies the general use formulations with the public 

in mind to minimize potential exposure to the public and uses bait stations where required. 

Diphacinone baits applied in bait stations that are not consumed by target species would be 

expected to be retrieved from the field by WS applicators. Any unrecoverable baits left in the 

environment from WS use will begin to biodegrade within 45 days.

Although these health protective recommendations are derived from no observed adverse effect 

exposures, the critical reference study did not determine a "safe" level of in-utero exposure to 

diphacinone. As a result, pregnant WS applicators should not work with or around diphacinone 

and avoid areas treated with diphacinone.

5.2 Ecological Risks

Risk characterization combines information from the dose-response assessment with the 

exposure assessment to determine the potential adverse effects on aquatic and terrestrial 

species. In this risk assessment, WS uses USEPA's risk evaluations (USEPA 2011;2020b), peerreviewed scientific literature, product labels, and WS use patterns and take data (Tables 2 and 3

a, b) to characterize the risks associated with WS applications of diphacinone bait. 

5.2.1 Aquatic

The exposure assessment (section 4.2) found aquatic exposure negligible based on WS use 

pattern, label restrictions, and findings from USEPA's ecological risk assessment (USEPA 

2020b). Eisemann and Swift (2006) calculated risk quotients for the aerial application of 

diphacinone into Hanawi Stream, Hawaii. They found the risk quotients (the comparison of 

diphacinone’s predicted concentration in the water column to the toxicity of diphacinone to 

aquatic organisms) for fish, crustaceans, or other aquatic invertebrates did not exceed the level 

of concern. No risk was predicted for any aquatic organism. 

Based on the negligible aquatic exposure, label restrictions, and WS use pattern, APHIS 

considers the risk to aquatic species as negligible.

5.2.2 Terrestrial

The exposure assessment (section 4.2) found that WS applications of diphacinone would result 

in primary and secondary dietary exposure to terrestrial vertebrates and invertebrates. It can take 

several days of consecutive feedings to deliver a lethal dose of diphacinone to the target animal

and nontarget species (USEPA 2020c). Similarly, secondary exposure to nontarget species would 

require eating multiple prey items with diphacinone residues over a time period relevant to the 
29

metabolic half-life of diphacinone to receive a lethal dose of diphacinone. Additionally, 

diphacinone residues may contribute to the secondary toxicity of other first-generation and 

second-generation anticoagulant active ingredient residues in the prey items consumed by 

nontarget species (USEPA 2020c), but only in areas where additional anticoagulant rodenticides

may be in use by non-WS applicators.

WS makes belowground applications directly to burrows to control pocket gophers. Belowground 

applications reduce but do not eliminate the primary exposure of nontarget terrestrial species. WS 

monitors burrows for pocket gopher activity, generally fresh mounds, and gophers being very 

territorial keep other species out of their tunnel systems, which thereby reduces nontarget species 

primary exposure. WS recognizes nontarget species, mostly small mammals and snakes, would

still likely access treated burrows, especially following treatment after pocket gophers succumb 

to baiting and could be directly exposed to diphacinone bait. WS expects mammals that access 

burrows and eat diphacinone bait could receive a lethal dose of diphacinone if sufficient amounts 

of palatable bait remain to allow multiple feedings. Snakes that access burrows are not expected 

to receive a lethal dose of diphacinone since they will not consume baits but could secondarily by 

consuming treated pocket gophers. Although some birds may access burrows (e.g., burrowing 

owls (Vaughan 1961)), WS does not anticipate that burrowing owls will access pocket gopher

burrows because they are smaller in diameter than necessary for the owls.

Studies indicate that most target animals that live belowground will die belowground (as cited in 

(Primus et al. 2001, USEPA 2009) summarizes (Lee and Hyngstrom 2007), reducing secondary 

exposure. APHIS expects most of the target animals that WS baits belowground with diphacinone 

will die underground, reducing exposure potential to nontarget species; however, some will die 

aboveground. In a study on a different anticoagulant (brodifacoum), 90% of plains pocket gophers 

died belowground (as cited in (Primus et al. 2001)). Baldwin et al. (2021) found a vast majority of 

California ground squirrels died belowground (82-91%) following spot treatments, broadcast

applications, and bait station applications of diphacinone in rangelands. 

Between FY11–20, WS primarily used diphacinone bait to control commensal rodents (Table 2a).

However, WS past and expected future use patterns include aboveground spot, mechanical 

spreader, and aerial applications of Diphacinone-50 Conservation for island conservation 

projects. During these types of applications, APHIS expects that aboveground treatments of 

diphacinone bait will result in sublethal effects on birds that consume bait or prey or scavenge on 

animals with diphacinone residues. Therefore, APHIS expects mortality in birds to be minimal, 

based on their relatively low sensitivity to diphacinone, the requirement for multiple feedings for 

lethality, and their foraging preferences (they are not attracted to the bait, their diet is diverse, and 

so on), as well as, with the exception of some island conservation applications, their foraging area 

is often larger than the treatment area. USEPA (2004b) conducted a comparative risk evaluation 

of several rodenticides and concluded the primary risks of diphacinone to birds were low and 

secondary risks were moderate. Using birds as surrogate species, consuming diphacinone bait 

by terrestrial amphibians would have the same primary and secondary risks as birds. Snakes are 

sensitive to diphacinone but are unlikely to be attracted to the diphacinone baits themselves, so

they must be exposed to diphacinone residues over multiple feedings on exposed prey to have

secondary risks. Aboveground spot, mechanical spreader, and aerial applications would impact 

nontarget mammals through primary and secondary exposure; USEPA (2004b) estimated 

mammalian primary and secondary risks to diphacinone were high.

Insects that feed on bait may subsequently be consumed by terrestrial vertebrates; however, the 

terrestrial vertebrates are not likely to ingest a toxic dose of diphacinone given the amount of 

diphacinone carried inside the insect, and the number of poisoned insects the terrestrial 
30

vertebrates would need to eat within a toxicity-relevant timespan to receive a lethal dose.

Johnston et al. (2005) estimated risks to juvenile and adult bobwhites and mallards consuming 

snails and slugs with diphacinone residues in Hawaii. In their estimate, they assumed the birds 

solely ate gastropods containing 4.93 ppm diphacinone (the highest 95% percentile residue 

concentration in gastropods fed only 0.005% w/w diphacinone bait in the laboratory). They 

concluded the broadcast application of diphacinone baits in Hawaii would have an acceptable 

level of risk for non-threatened birds. However, their estimate did find risks to endangered bird 

species consuming gastropods with diphacinone residues, indicating the use of diphacinone bait

in the range of threatened or endangered species should be avoided in most circumstances. In 

some situations, diphacinone baits targeting invasive rodents or mongooses are used to benefit

threatened or endangered species in a conservation area or on islands.

WS has no records of nontarget take through its use of diphacinone baits. However, APHIS 

recognizes it is impossible to detect all nontarget take because animals may die belowground, be 

difficult to find in certain habitats, or are wide-ranging and dispersed outside of the treatment area. 

WS does monitor proposed treatment sites and does not treat areas with threatened and 

endangered species outside of conservation uses. 

The Diphacinone-50 Conservation and Ramik® Mini Bars All-Weather Bait labels provide 

instructions to collect and dispose of dead, exposed animals during and after treatment but do 

not provide the time interval after treatment for doing this. The Ramik® Mini Bars All-Weather Bait 

labels also instruct applicators to remove unused bait. Similarly, the Eaton’s Gopher Bait Block 

label and Bait Block® Rodenticide with Peanut Butter FlavorizerTM (and Apple Bait Block®

Rodenticide) require applicators to remove dead animals and unused bait; the Eaton’s Gopher 

Bait Block label indicates to check the treatment area for 10 to 15 days after treatment. Despite 

these mitigations, poisoned animals that are aboveground between searches or are missed 

during searches remain available to predators and scavengers. 

APHIS expects minimal risk to terrestrial plants based on lack of exposure through WS use pattern 

and diphacinone's mode of action. The reduction in available prey in an area from diphacinone 

treatment could impact food availability for predators; however, APHIS anticipates predators 

would extend their hunting range into new areas. This document uses the risk assessments for 

diphacinone (USEPA 2004b;2020c), as well as assessments published in the scientific literature

(e.g., (Eisemann and Swift 2006)) to assist in estimating risks associated with WS use patterns; 

these risk assessments are conservative and assume nontarget species feed solely on 

diphacinone bait or only on animals with diphacinone residues. Although this assumption is 

unlikely to occur in nature, APHIS still finds WS use of diphacinone could negatively impact 

terrestrial mammals that ingest bait or feed on animals that contain diphacinone residues. To 

reduce the exposure potential to nontarget animals, WS complies with label instructions to reduce 

nontarget animal exposure (e.g., removal of unused bait and carcasses), monitors treatment 

areas for nontarget species activity, and evaluates each treatment site and selects a labelapproved application method that would minimize exposure potential.

6 UNCERTAINTIES AND CUMULATIVE IMPACTS

The uncertainties associated with this risk evaluation arise primarily from the limited toxicity 

information available for the various diphacinone FGAR formulations. Unpublished acute 

toxicology studies submitted to USEPA by registrants and individual formulation safety data 

sheets provide limited acute mammalian toxicity values, which can be used to make general 

conclusions concerning the impact of diphacinone on humans and the non-target environment.
31

Other uncertainties related to chronic and sublethal effects data for some fish and wildlife and 

surrogacy of test organisms are typical for most pesticides; however, there is a considerable 

amount of field data related to poisoning of nontarget wildlife with anticoagulant rodenticides. This 

information provides a weight-of-evidence approach to conservatively evaluate the risk of 

diphacinone to nontarget organisms. The conservative assumptions regarding the potential for 

exposure to human health, nontarget species, and the environment address the uncertainties to 

some extent. A lack of risk using these conservative assumptions supports the reasonable 

certainty that impacts on human health and the environment will be negligible. However, primary 

and secondary risks to nontarget mammals are not negligible, particularly for broadcast 

applications.

Diphacinone use patterns vary for different sites and application methods. This risk assessment 

used conservative assumptions regarding human exposure to account for variations in use 

patterns and ensure that risks are not underestimated for workers. Individuals with impaired liver 

function may be at greater risk than other population members because first-generation 

anticoagulants such as diphacinone inhibit the synthesis of vitamin K and are metabolized by liver 

mixed-function oxidases (i.e., the cytochrome P450 enzyme system). 

Another area of potential uncertainty in this risk assessment is the potential for cumulative impacts 

on human health, nontarget species, and the environment from the proposed use of diphacinone 

in the WS program. Areas, where cumulative impacts could occur are: 1) repeated worker and 

environmental exposures to diphacinone from program application; 2) co-exposure to other 

chemicals with a similar mode of action; and 3) exposures to other chemicals affecting the toxicity 

of diphacinone.

Repeated exposures that could lead to increased risks of injury from accidental diphacinone 

exposure by WS applications are expected to be minimal due to strict WS applicator adherence 

to label-required PPE. To minimize the risk of cumulative impacts of repeated exposures, WS 

applicators could consider a pesticide application rotation schedule that rolls applicators off and 

on application assignments weekly. Diphacinone baits can only be administered using tamperresistant applications in areas where people reside or can access the product. These measures 

reduce the potential for cumulative impacts related to exposures to diphacinone and other 

stressors. 

7 LITERATURE CITED

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Environmental Hygiene 12:S55-68.

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diphacinone rodenticide baits in native Hawaiian ecosystems. 2006.

HACCO, I. 2021. Ramik® Green Label.

Johnston, J. J., W. C. Pitt, R. T. Sugihara, J. D. Eisemann, T. M. Primus, M. J. Holmes, J. Crocker, and A. Hart. 

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contact hypersensitivity to DNCB in rats. Toxicology 188:83-100.

Keith, J. O., D. N. Hirata, D. I. Espy, S. Greiner, and D. G. Griffin. 1989. Field evaluation of 0.00025% 

diphacinone bait for mongoose control in Hawaii, DWRC Job Completion Report. Denver Wildlife 

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Lee, C. D., and S. E. Hyngstrom. 2007. Field efficacy and hazards of Rozol bait for controlling black-tailed 

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Primus, T. M., J. D. Eisemann, G. H. Matschke, C. Ramey, and J. J. Johnston. 2001. Chlorophacinone 

Residues in Rangeland Rodents: An Assessment of the Potential Risk of Secondary Toxicity to 

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American Chemical Society, Washington D.C., USA.

Ramey, C. A., G. H. Matschke, and R. M. Engeman. 2007. Chlorophacinone baiting for Belding’s ground 

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and coagulopathy in American kestrels (Falco sparverius) following administration of the 

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Salmon, T. P., D. A. Whisson, A. R. Berentsen, and W. P. Gorenzel. 2007. Comparison of 0.005% and 0.01% 

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Sugihara, R. T., W. C. Pitt, A. R. Berentsen, and C. G. Payne. 2018. Evaluation of the palatability and toxicity 

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Distinct Population Segment, And the Federally Endangered California Tiger Salamander 

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8 PREPARERS: WRITERS, EDITORS, AND REVIEWERS 

8.1 APHIS WS Methods Risk Assessment Committee

Writers for “Use of Diphacinone in Wildlife Damage Management Risk 

Assessment”:

Primary Writer: Andrea Lemay 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Biological Scientist, Raleigh, NC

Education: BS Plant and Soil Science (Biotechnology) - University of Massachusetts; MS Plant 

Pathology - North Carolina State University 

Experience: Thirteen years of service in APHIS conducting risk analysis. Nine years of 

experience in preparing environmental analyses in compliance with the National 

Environmental Policy Act.
35

Primary Writer: Michael McCaskill 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Environmental Health Specialist, Riverdale, MD

Education: B.S. Environmental Science – University of Florida; MPH Industrial HygieneUniversity of South Carolina, Ph.D. Toxicology-Florida Agriculture and Mechanical University

Experience: Ten years of experience conducting human toxicological research at Florida 

Agriculture and Mechanical University, University of Nebraska Medical Center, and Tulane 

University. Four years of experience conducting human health and environmental 

toxicological risk assessments and assisting environmental compliance programs at the 

Florida Department of Health, Commonwealth of Pennsylvania, and USDA.

Writer: Thomas C. Hall

Position: USDA-APHIS-WS, Operational Support Staff, Staff Wildlife Biologist, Fort Collins, CO

Education: BS Biology (Natural History) and BA Psychology – Fort Lewis College; MS Wildlife 

Ecology – Oklahoma State University

Experience: Special expertise in wildlife biology, identification, ecology, and damage 

management. Thirty-seven years of service in APHIS Wildlife Services including operations 

and research in CO for research and OR, GU, CA, OK, and NV for operations conducting a 

wide variety of programs including bird damage research and management, livestock 

protection, invasive species management, wildlife hazard management at airports, property 

and natural resource protection including waterfowl, brown tree snake, feral swine, rodent, 

and beaver damage management. Applied and supervised chlorophacinone use.

Writer: Jim Warren 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Environmental Toxicologist, Little Rock, AR 

Education: B.S. Forest Ecology and M.S. Entomology – University of Missouri; Ph.D. 

Environmental Toxicology – Clemson University

Experience: Thirteen years of experience working for APHIS preparing ecological risk 

assessments and providing assistance on environmental compliance. Prior experience 

before joining APHIS includes other government and private sector work regarding 

ecological risk assessments related to various environmental regulations.

Editors/Contributors for “Use of Diphacinone in Wildlife Damage Management Risk 

Assessment”:

Editor: Shelagh DeLiberto 

Position: USDA-APHIS-Wildlife Services (WS), National Wildlife Research Center (NWRC), 

Wildlife Biologist, Fort Collins, CO

Education: BA Biology and Environmental Science – Ithaca College; MS Wildlife Biology –

Colorado State University 

Experience: Sixteen years of service in APHIS conducting wildlife research. Two years of 

experience in preparing categorical exclusions and environmental analyses in compliance 

with the National Environmental Policy Act.

Editor: Emily Ruell 

Position: USDA-APHIS-WS, NWRC, Registration Manager, Fort Collins, CO

Education: B.S. Zoology and Biological Aspects of Conservation – University of Wisconsin -

Madison; M.S. Ecology – Colorado State University (CSU); M.A. Political Science – CSU

Experience: Nine years of experience with APHIS WS NWRC preparing and reviewing 

vertebrate pesticide registration data submissions and other registration materials, and 
36

providing pesticide regulatory guidance to WS, WS NWRC, and collaborators. Prior 

experience before joining APHIS includes seven years of conducting field and laboratory 

wildlife research at CSU, and environmental policy research for the U.S. Geological Survey.

8.2 Internal Reviewers

USDA APHIS Wildlife Services

Reviewer: Gary Witmer

Position: Supervisory Wildlife Biologist and Research Project Leader (Retired)

Education: BS and MS in Biology, University of Michigan; MS in Wildlife Ecology, Purdue 

University; Ph.D. in Wildlife Science with minors in Statistics and Forest Management, 

Oregon State University.

Experience: Thirty years researching wildlife populations, damage, and damage management 

with an emphasis on rodents and about 300 scientific publications.

Reviewer: Aaron Shiels

Position: USDA-APHIS-WS, National Wildlife Research Center: Research Biologist and 

Rodents Project Leader, Fort Collins, CO

Education: BS Environmental Sciences, University of Denver; MS Biology, University of 

Nevada, PhD Botany/Ecology University of Hawaii

Experience: Fifteen years researching rodent damage management techniques including the 

use of rodenticides.

Reviewer: Tyler Bogardus 

Position: USDA-APHIS-WS, Hawaii Operations, District Supervisor, Kapolei, HI.

Education: BS Wildlife Biology – Colorado State University

Experience: 15 years conducting predator control and management specializing in small 

vertebrates.

Reviewer: Jon Grant 

Position: USDA-APHIS-WS, State Director, Albuquerque, NM 

Education: BS Wildlife - Univ. Wisconsin, Stevens Point 

Experience: Expertise in wildlife biology and wildlife damage management operations and 

research. Thirty years of service in APHIS Wildlife Services operational programs in WI and 

NM. Experience in a wide variety of damage management to include feral swine, livestock 

protection, bird damage, wildlife hazard management at airports. Have extensive experience 

applying and supervising the use of zinc phosphide.

Reviewer: Ken Gruver

Position: USDA-APHIS-WS AL/PR/USVI State Director

Education: BS Wildlife Management – Sul Ross State University, MS Wildlife Management –

Sul Ross State University

Experience: Thirty-two years service with APHIS WS conducting and supervising Wildlife 

Damage Management Research and Operations 

Reviewer: Donald J. Wilda

Position: USDA-APHIS-WS, State Director MA, CT & RI, Amherst, MA

Education: BS Wildlife Biology-University of Massachusetts
37

Experience: Thirty-one years of service in APHIS conducting and supervising wildlife damage 

management
APPENDIX A. Risk Characterization for Accidental Occupational Exposure* to Diphacinone

Parameters Values Values Values Sources

PPE used (gloves) ‡No gloves or 

damaged PPE

‡Single layer gloves ‡Double layer PPE Reference

PDR exposure; PDR=UE*AR*A*ABS

UE applicator granules by hand 104 71 40.3 (USEPA 2021a)1

Application AR (A/day) AR (A/day) AR (A/day) Calculated From or 

Reference

Low Conc. GB 12.5 lb./acre 0.00063 (3) 0.00063 (26) 0.00063 (36)

High Conc. GB 20 lb./acre 0.001 (2) 0.001 (18.5) 0.001 (22)

†Low Application Density-Low Treatment 

(7/acre; 4 oz. per application)

0.00009 (19) 0.00009 (184) 0.00009 (255) Calculated from 

(USEPA 2019a) 1

Low Application Density-High Treatment 

(7/acre; 16 oz. per application)

0.00035 (5) 0.00035 (46) 0.00035 (64) Calculated from 

(USEPA 2021a) 1

High Application Density-Low Treatment

(194/acre; 4 oz. per application)

0.0024 (<1) 0.0024 (6.6) 0.0024 (9.2) Calculated from 

(USEPA 2021a) 1

High Application Density-High Treatment 

(194/acre; 16 oz. per application)

0.0097 (<1) 0.0097 (1.5) 0.0097 (2) Calculated from 

(USEPA 2021a) 1

ABS 0.1 0.01 0.01 (USEPA 2007) 1

PDR (mg/day) 0.0175 0.0175 0.0175 Calculated

BW 70 70 70 (USEPA 2007) 1

Dose (mg/kg-bw/day) 0.00025 0.00025 0.00025 Calculated

RfD (mg/kg-bw/day) 0.00025 0.00025 0.00025

Hazard Quotient 1 1 1 **

PPE = Personal protective equipment PDR = Potential Daily Rate UE = Unit Exposure or mg/lb a.i. AR = Application Rate (lb a.i./acre) A = Acres GB

= Ground Broadcast Conc. = Concentrate ABS = Fraction absorbed dermally BW = Body weight (kg) RfD = Reference Dose 

†Application density and treatment levels are based on label restrictions. 

‡ Based on hand application estimates of exposure by Pesticide handler exposure database

*Exposure risks are calculated from dermal and inhalation exposures.

** Based on NOAEL of 0.025 mg/kg-bw/day, 9-day developmental study (see Section 3)

1Appendix References in Section 7. Literature Cited.