Human Health and Ecological Risk Assessment 

for the Use of Wildlife Damage Management Methods 

by USDA-APHIS-Wildlife Services 

Chapter XIX 

The Use of Strychnine in 

Wildlife Damage Management 

September 2021 

Peer Reviewed Final 

January 2023 
i 

THE USE OF STRYCHNINE IN WILDLIFE DAMAGE MANAGEMENT 

EXECUTIVE SUMMARY 

Strychnine is a rodenticide used by USDA-APHIS-Wildlife Services (WS) to control pocket 

gophers belowground. APHIS evaluated the potential human health and ecological risks from the 

proposed use of strychnine to control pocket gopher damage. Pocket gophers are small, 

burrowing rodents that feed on a variety of vegetation, including crops, shrubs, and trees, and 

can cause serious damage in a short time. WS applicators used strychnine to take an annual 

average of 3,535 pocket gophers of four species in ten states between FY16 and FY20 using 99 

pounds of strychnine baits (0.5 pounds of the active ingredient strychnine). 

Strychnine is highly toxic to humans and other vertebrates. Strychnine is a neurotoxin and inhibits 

glycine, a neurotransmitter essential to the nervous system in vertebrates. Although the hazard 

potentially could be high, the anticipated minimal exposure to this restricted-use pesticide results 

in low risk. Exposure is greatest for workers who apply the product but using the required 

personnel protective equipment results in a low potential for exposure. The potential exposure 

and risk to the public is negligible given the use pattern and label restrictions, as well as lack of 

dietary exposure through food or drinking water. 

Rodents and other small mammals that share burrows with pocket gophers and eat strychnine 

bait may die, and any animals that find spillage aboveground. Secondary exposure of predators 

and scavengers feeding on pocket gophers poisoned by strychnine is possible, but several 

studies indicate most pocket gophers and any nontarget rodents die belowground and are 

inaccessible. However, secondary hazards could occur if animals that eat bait die above ground. 

Several studies did not find long-term population impacts to nontarget species from primary and 

secondary exposure. Direct access to treated bait inside the burrow is negligible for most bird 

species. Insectivorous bird species and terrestrial reptiles and amphibians are unlikely to receive 

lethal tertiary exposure by eating insects exposed to strychnine. Exposure of surface and 

groundwater is unlikely; therefore, the risk to aquatic species is negligible. Exposure of terrestrial 

vertebrates and invertebrates through the intake of strychnine-contaminated surface water is also 

negligible. WS's use of strychnine to control pocket gophers was determined to be a minimal risk 

to human health and safety and the environment. 

The USEPA Office of Pesticide Programs is completing registration review for all registered 

pesticides. This process includes the publication of a Proposed Interim Registration Review 

Decision (PID) that provides information about the use and risks of each pesticide and new 

measures to reduce risks to human health and the environment. USEPA published in the Federal 

Register PIDs for public comment for all registered rodenticides in November 2022. USEPA is 

proposing pesticide label language changes for rodenticides in the PIDs. The proposed new label 

language is not discussed in this risk assessment since the changes have not been finalized by 

USEPA. APHIS-WS will update this risk assessment to reflect the human health and ecological 

risks related to the new label language once approved by USEPA. USEPA proposed label 

language changes in the PIDs are generally more restrictive than the current label language for 

rodenticides. Therefore, the following risk assessment likely overestimates the risks that would 

be associated with use under any future label language changes required by USEPA. 
ii 

Table of Contents 

1 INTRODUCTION .................................................................................................................... 1

1.1 Strychnine Formulations ................................................................................................... 2

1.2 Use Pattern ...................................................................................................................... 2

2.0 PROBLEM FORMULATION ................................................................................................. 5

2.1 Chemical Description and Product Use ............................................................................ 5

2.2 Physical and Chemical Properties .................................................................................... 6

2.3 Environmental Fate .......................................................................................................... 6

2.4 Hazard Identification ......................................................................................................... 7

2.4.1 Toxicokinetics ............................................................................................................ 8

2.4.2 Acute Toxicity ............................................................................................................. 8

2.4.3 Subchronic and Chronic Toxicities ............................................................................. 9

2.4.4 Effects on Nervous, Immune, and Endocrine Systems and Reproduction and 

Development ....................................................................................................................... 9

2.4.5. Human Incidents ......................................................................................................10

3.0 DOSE-RESPONSE ASSESSMENT ....................................................................................10

3.1 Human Health .................................................................................................................10

3.2 Ecological ........................................................................................................................11

3.2.1 Terrestrial Dose-Response Assessment ...................................................................11

3.2.2 Aquatic Dose-Response Assessment .......................................................................14

4 EXPOSURE ASSESSMENT ..................................................................................................15

4.1 Human Health .................................................................................................................15

4.1.1 Potentially Exposed Human Populations and Complete Exposure Pathways ............15

4.1.2 Exposure Evaluation .................................................................................................16

4.2 Exposure Assessment for Nontarget Species ..................................................................16

4.2.1 Direct Exposure ........................................................................................................17

4.2.2 Secondary and Tertiary Exposure .............................................................................18

5 RISK CHARACTERIZATION .................................................................................................19

5.1 Human Health .................................................................................................................19

5.2 Ecological ........................................................................................................................20

5.2.1 Terrestrial Risk Characterization ...............................................................................20

5.2.2 Aquatic Risk Characterization ...................................................................................21

6 UNCERTAINTIES AND CUMULATIVE EFFECTS .................................................................21
iii 

7 SUMMARY ............................................................................................................................22

8 LITERATURE CITED .............................................................................................................23

9 PREPARERS .........................................................................................................................27

9.1 APHIS-WS Methods Risk Assessment Committee ..........................................................27

9.2 Internal Reviewers ...........................................................................................................28

9.3 Peer Review ....................................................................................................................29

9.3.1 Peer Reviewers Selected by the Association of Fish and Wildlife Agencies ..............29

9.3.2 Comments.................................................................................................................29

Appendix A. WS Strychnine Use for Pocket Gophers for FY11-FY15. ......................................33
1 

1 INTRODUCTION 

Pocket gophers (Cratogeomys sp., Geomys spp. and Thomomys spp.)1 are small, burrowing 

rodents that feed on a variety of vegetation, including crops, trees, and shrubs, and can cause 

serious damage in a short time. The United States Department of Agriculture’s (USDA) Animal 

and Plant Health Inspection Service’s (APHIS) Wildlife Services (WS) may manage pocket gopher 

populations and their damage by placing strychnine bait inside pocket gopher burrows. 

Strychnine baits are applied underground in pocket gopher burrows either by hand with a probe 

or mechanically with a burrow builder. Typically, WS uses hand baiting with a probe for small 

areas, and if a burrow builder is not available, for larger areas as well. Hand applicators find 

burrow runways by pressing the probe into the ground about a foot from gopher mounds to find 

the main runway (the probe easily drops when it hits a burrow). The probes are hollow tubes with 

a bait dispenser or side funnel that allows a teaspoon of bait to be inserted into the burrow through 

the tube. Solid probes can be used, and the hole into the burrow from the probe is widened so a 

teaspoon of bait can be deposited into the burrow system. The holes created by probes are then 

covered with dirt and debris, being careful not to cover the bait with dirt to discourage pocket 

gophers from plugging the area with dirt. Burrow builders are tractor-drawn implements that create 

tunnels in the soil and drop a measured amount of strychnine baits in the burrow runs it creates. 

They are used for larger areas with high densities of gophers (e.g., pastures, alfalfa (Medicago 

sativa) fields, parks, playing fields, golf courses, orchards, roadsides, and windbreaks where a 

burrow builder can be dragged without hitting obstructions). In rough terrain or areas with 

obstructions such as rocks, a free-floating hitch is used, and shorter burrow runs are made, 

especially in orchards where root systems could be damaged. Where obstructions are expected, 

such as underground boulders, short runs of tunnels can be made to avoid damaging the burrow 

builder. Burrow builder tunnels are run through fields and expected to intersect gopher burrows. 

Gophers find the tunnels and will use them where they find the bait. 

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

human health and the environment, including nontarget fish and wildlife, as a result of exposure 

to strychnine from its proposed use by WS. The methods used to assess potential human health 

effects follow standard regulatory guidance and methodologies (National Research Council 1983) 

and generally conform to other Federal agencies such as the U.S. Environmental Protection 

Agency (USEPA 2019). The methods used to assess potential ecological risk to nontarget fish 

and wildlife generally follow USEPA (2019) methodologies. 

This risk assessment starts with problem formulation (identifying hazard) and then evaluates 

toxicity (dose-response assessment) and exposure (identifying potentially exposed populations 

and determining potential exposure pathways for these populations). Lastly, the integration of the 

toxicity and exposure assessments provides a characterization of risks (determining if adverse 

human health or ecological risks are present and their significance). A discussion of the 

uncertainties associated with the risk assessment and cumulative effects is also included in this 

risk assessment. 

 

1

 Scientific names are given in Chapter 1: Introduction to Risk Assessments for Methods Used in Wildlife Damage Management and 

not in this document except for species not in that chapter. 
2

1.1 Strychnine Formulations 

Strychnine baits come in several APHIS and commercial formulations. Each formulation with its 

label and instructions must be followed when applying the baits. The primary formulations that 

WS uses are the APHIS registrations for milo (Sorghum bicolor) and oat (Avena sativa) baits 

delivered by burrow builders or hand baiting. Table 1 gives the formulations used by WS 

personnel during FY162-FY20, the USEPA registration number, the amount of active ingredient 

(a.i.), treatment rates, and target species. 

Table 1. Strychnine formulations available for USDA-APHIS WS use. 

Other commercial strychnine formulations are available with the percentage of active ingredients 

that WS could potentially use. Other products are typically used for small projects because they 

are available in local areas at some distributors in small quantity buckets. However, some 

previously available gopher formulations have replaced strychnine with diphacinone or other 

toxicants in their products. In general, the APHIS formulations (Table 1) will be used, with minimal 

use of other products. 

1.2 Use Pattern 

WS used strychnine in ten states to take an estimated annual average of 3,535 pocket gophers 

of four species during FY16-FY20 (Table 2). All of these species are common and abundant. 

Plains pocket gophers (36%) and northern pocket gophers (32%) were the primary target species. 

Burrow builder formulations with milo (37%) and oat (34%) baits were used the most, followed by 

hand baiting formulations with oat (16%) and milo (12%) baits. WS applied strychnine under four 

labels; three APHIS labels and one commercial label (Table 2). Additionally, WS distributed or 

sold 736 pounds of bait annually to certified pesticide applicators, as necessary, for use on their 

properties, which included an additional label and species (Table 2). WS primarily used APHIS 

2 FY16 equals the federal Fiscal Year 2016, which is October 1 2015-September 30 2016 (the year is denoted by FY17, FY18, ...).

Trade Name 

(Registrant) 

EPA Reg. No. 

(label date) 

Formulation 

(% a.i.) 

Application Rate 

lb. formulation per 

acre (lb. a.i./acre)

Target Pocket Gophers 

Strychnine milo - 

burrow builders 

(APHIS) 

56228-11 

(30 Aug. 2019) 

Strychnine 

(0.5%) 

Inert ingredients: 

(99.5%) 

1.0 (0.45 kg) to 2.5 

lb (1.13 kg) of bait 

per acre (0.405 ha)

(0.005 to 0.0125 lb. 

a.i./ac) 

Cratogeomys spp., Geomys

spp. and Thomomys spp., 

excluding Mazama pocket 

gopher (T. mazama) in 

Thurston and Pierce 

Counties, WA 

Strychnine oat -

burrow builders 

(APHIS) 

56228-12 

(30 Aug. 2019) 

Strychnine milo - 

hand-baiting 

(APHIS) 

56228-19 

(3 Jan. 2020) 

1.0 lb (0.45 kg) bait 

will treat 1 to 8 

acres (0.41 to 3.24 

Strychnine oats - ha) 

hand-baiting 

(APHIS) 

56228-20 

(3 Jan. 2020) 

RCO Omega 

Gopher Grain Bait, 

0.5% strychnine 

grain bait for pocket 

gophers (RCO 

International, Inc.) 

5042-32 

(3 Jan. 2020) 

1.0 lb. (0.45 kg) 

bait/ 1 to 8 acres 

Northern (T. talpiodes), 

Southern (T. umbrinus), 

Camas (T. bulbivorus), Botta’s 

(T. bottae), Mazama (T. 

mazama), plains (G. 

bursarius), mountain (T. 

monticola), and Townsend’s 

pocket gophers (T. townsendii)
3

products (99.8%) (Table 2). Use decreased from a 266-pound annual average for FY11-FY15 

(Appendix 1) to a 99-pound average from FY16-FY20. 

Because applications of strychnine are belowground and most pocket gophers that ingest bait die 

underground (Nolte and Wagner 2001, Ramey et al. 2002), it is not possible to count the exact 

number of pocket gophers taken with strychnine treatments nor any nontarget species taken. 

Since WS personnel did not dig up burrows to determine the actual take of target or nontarget 

species, numbers were estimated from the expected number of target animals taken per ounce 

of bait applied based on consumption. In the MIS3, WS personnel record the number of target 

species taken with treatments or the acreage or burrows treated. With that said, they often record 

bait placements rather than burrows treated. Therefore, take based on consumption makes a 

better estimate of take. Additionally, it is often difficult to determine where one burrow system 

ends and the next one starts or the amount of bait put into a burrow (4 tsp of oats = 0.30 oz 

whereas milo = 0.54 oz), making estimates more difficult in the field. 

Table 2. The annual average number of target pocket gophers killed with strychnine by APHIS-WS in WDM 

activities for FY16 to FY20 throughout the United States and the pounds of strychnine used. No known 

nontarget take occurred during this time. 

1

If the take was not estimated, the number of target animals taken was estimated at 8 per pound of bait for burrow builders or 1 per 

ounce for hand baiting. 

If take numbers were estimated by field personnel, the number of pocket gophers taken was used. 

If no take was estimated for an application, the take was estimated based on the acres treated or 

the pounds of bait used. Consumption was based on an estimate that pocket gophers likely 

consume about 8% of body weight (bw) per day. The amount of feed an adult gopher would 

consume can be calculated given the average weight of a pocket gopher by species (males tend 

to be larger than females, and average weights vary by species (Table 3). Given that amount, 

and an assumption that they consume all the bait in their burrow system except 10% wastage 

(e.g., baits inaccessible, covered by dirt, or taken by the gopher back to the food chamber in its 

cheek pouches, “pockets”), and that all pocket gophers die from eating a lethal dose of bait, 

provide the parameters for an estimate of take for the pounds used. If acreage is recorded, typical 

pocket gopher density/ha or acre can be used. Finally, if the number of burrows treated is 

3 MIS - Computer-based Management Information System used for tracking WDM activities. Throughout the text, data for a year (e.g., 

FY16 to FY20) will be given and is from the MIS. MIS reports will not be referenced in the text or Literature Cited Section because 

MIS reports are not kept on file. A database is kept that allows queries to be made to retrieve the information needed.

ANNUAL AVERAGE SPECIES KILLED WITH STRYCHNINE AND FORMULATIONS USED

SPECIES TARGETED

Species States Used Take1 Lb. Bait States Sold Lb. Sold

Northern Pocket Gopher ND OR WA 1,130 12.2 ND NM 33

Botta’s Pocket Gopher NM UT 560 14.0 NM UT 593 

Desert Pocket Gopher NM 574 17.5 NM 64 

Plains Pocket Gopher CO KS MN NE TX 1,272 55.1 NE 10 

Yellow-faced Pocket Gopher - - - NM 37 

FORMULATION USE

Formulation States Used Take1 Lb. Bait States Sold Lb. Sold

Omega Gopher in Bait OR 6 0.2 - -

Strychnine Milo – Burrow Builder NE NM 297 42.0 NM UT 325 

Strychnine Milo – Hand Bait CO KS MN ND NE 

NM TX UT WA 

2,148 44.7 ND NE NM 311 

Strychnine Oats – Burrow Builder - - - UT 100 

Strychnine Oats – Hand Bait MN OR WA 1,084 12.0 - - 

Annual Average Take and Use 10 States 3,535 98.9 4 States 736
4

recorded, an estimate can be made for how many gophers will be in a burrow system. The male 

is solitary except for breeding, and the female weans the pups in relatively few weeks. Within two 

months, they are usually in their own burrow. Most pocket gophers have two litters per year, so 

being conservative, an estimate of the number per burrow would be for 6 months. Table 3 gives 

the parameters used to make estimates. 

Table 3. The estimated annual average number of target pocket gophers killed with an ounce of strychnine 

bait for burrows treated or per acre treated when it was not estimated. It is assumed that pocket gophers 

consume 8% of their body weight per day, 4 tsp. are used per burrow, and that 10% of the strychnine baits 

are waste (e.g., bait lost to being unconsumed or buried and unavailable). 

1

 10% of the strychnine baits applied are lost to going unconsumed, being buried by gophers, or being unavailable. 

Pocket gophers live solitary lives except for breeding and mothers with young. Most female gophers have young in 

burrow systems about 50% of the year. It is also expected that four teaspoons treat each burrow system (2-7 tsp/burrow 

system is the label suggestion). Also, 4 tsp weighs for milo 0.54 oz. and for oats 0.30 oz. Thus, for 1 gopher some could 

be left over if milo used whereas it is likely to be consumed by a gopher if oats are used. For the yellow-faced pocket 

gopher (e.g., larger pocket gophers), generally more bait is applied. Larger gophers tend to build longer burrow 

systems, and one gopher can maintain 100 m or more of burrow; it is often hard to tell if treating one burrow or another 

in the field, especially from mounds, since one burrow system can have many. The average density of burrows in 

alfalfa, a desirable gopher habitat, was 14/ha or about 6/acre (Smallwood et al. 2001). 

Table 4. The annual average number of target pocket gophers killed1 with strychnine and ounces used in 

each State by APHIS-WS in WDM activities for FY16 to FY20. No known nontarget take was recorded. 

ANNUAL AVERAGE STRYCHNINE USED WITH TARGET TAKE1 BY STATE FOR FY16-FY20

Burrow Builder 

Baits 

Hand Baits Omega Baits 

State Milo Milo Oats Grain

Take Oz. Take Oz. Take Oz. Take Oz. 

Colorado - - 520 172.8 - - - -

Kansas - - 397 32.9 - - - - 

Minnesota - - 1 6.4 13 12.8 - - 

Nebraska 270 640.0 14 2.4 - - - - 

New Mexico 27 32.0 714 360.4 - - - - 

North Dakota - - 8 0.7 - - - - 

Oregon - - - - 2 2.4 6 2.8 

Texas - - 68 14.4 - - - - 

Utah - - 393 111.1 - - - - 

Washington - - 33 13.6 1,081 176.3 - - 

Total 297 672.0 2,148 714.7 1,096 191.5 6 2.8

States 2 9 3 1

1

If take was not estimated, the number of pocket gophers taken was estimated based on parameters from Table 3. 

WS mostly used the strychnine formulations registered for use by USDA-APHIS 99.7% of the time 

(Table 4). WS applicators applied an annual average of 1,581 ounces (98.9 pounds) of strychnine 

baits. Utah (70%), Washington (12%), Texas (6%), and Oregon (5%) used the most strychnine 

ANNUAL AVERAGE NUMBER OF POCKET GOPHERS KILLED PER OUNCE OF BAIT

Species Weight 

(oz.) 

Feed 

(oz./day) 

#/oz. 

Bait1

#/Litter #/Burrow #/Acre 

Northern Pocket Gopher 11.6 0.3 2.9 5.5 3 14

Botta’s Pocket Gopher 5.5 0.4 2.0 5.5 3 9 

Desert Pocket Gopher 6.8 0.6 1.6 4 2 6 

Plains Pocket Gopher 6.5 0.5 1.7 4 2 7 

Yellow-faced Pocket Gopher 9.5 0.8 1.2 2 2 5 
5

baits. The primary use of strychnine baits was at airports to reduce the attractiveness of airfields 

to raptors hunting pocket gophers. WS also used baits to minimize damage to pastures, alfalfa, 

orchards, property, and parks. Some areas are retreated in subsequent years, especially airfields, 

because pocket gophers reproduce annually and may recolonize areas. 

2.0 PROBLEM FORMULATION 

Strychnine is a nonselective rodenticide first registered in the U.S. in 1947 (USEPA 1996b). 

Strychnine was registered to control rodents such as ground squirrels and pocket gophers, birds 

such as feral pigeons, and predators such as skunks and coyotes in both aboveground and 

belowground applications. In 1988, aboveground uses were suspended and, since, can only be 

used belowground for pocket gophers. The following sections discuss the chemical description, 

product use, physical and chemical properties, environmental fate, and hazard identification for 

strychnine. 

2.1 Chemical Description and Product Use 

Strychnine (CAS Number 57-24-9) is a toxic alkaloid extracted from seeds of the strychnine tree 

(Strychnos nux-vomica), Ignatius tree (S. ignatii (S. sancta ingnatius)), and upas tree (S. tiente), 

all native to the Asia-Pacific region including India, Sri Lanka, Vietnam, Indonesia, the Philippines, 

and northern Australia with some being introduced in the U.S. (USEPA 1996b, Centers for 

Disease Control 2018). It is a competitive antagonist at glycine receptors, making it a convulsant. 

Strychnine has been used as an analeptic in the treatment of hyperglycinemia and sleep apnea, 

as well as a rodenticide. 

WS applicators apply strychnine bait directly into subterranean burrow systems of pocket gophers 

with a machine-drawn mechanical burrow builder or by hand-baiting. The mechanical burrow 

builder is a device that digs an artificial burrow and places the bait into the burrow. During handbaiting, applicators use a bait dispensing probe, a hollow tube, or a metal rod to locate a burrow 

and release bait into the burrow. The applicator typically applies bait at up to seven locations per 

burrow system; the number of applications per burrow system often depends on the number of 

mounds because baits are placed a foot to a yard away from each mound. 

APHIS is the registrant for four USEPA-registered strychnine formulations that WS uses to control 

pocket gopher populations (Table 1). Two formulations are burrow builder products, and two are 

hand-bait products. WS has also used Omega Gopher Grain Bait, a hand-bait product. These 

products all contain 0.5% strychnine pre-mixed with oats or milo. The products are restricted to 

underground use to control pocket gophers in rangelands, pastures, croplands, forests, and nonagricultural areas. The labels for the two burrow builder products specify a higher application rate 

than the hand-bait products. Application rates vary with the density of gopher populations. 

The use of this product may pose a hazard to federally threatened and endangered species. The 

label identifies several species that must be considered prior to use. Strychnine baits cannot be 

used within the ranges of threatened and endangered (T&E) species that would eat the baits 

(belowground) or prey or scavenge on dead or dying pocket gophers including the Olympia 

(Thomomys mazama pugetensis), Roy Prairie (T. m. glacialis), Tenino (T. m. tumuli), and Yelm 

(T. m. yelmensis) pocket gophers; Point Arena mountain beaver (Aplodontia rufa nigra); Utah 

prairie dog (Cynomys parvidens); northern Idaho ground squirrel (Urocitellus brunneus 

brunneus); giant (Dipodomys ingens), Morro Bay (D. heermanni morroensis), Fresno (D. 

nitratoides exilis), Tipton (D. n. nitratoides), and Stephen’s (D. stephensi) kangaroo rats; Pacific 
6

little pocket mouse (Perognathus longimembris pacificus); Preble’s (Zapus husonius preblei) and 

New Mexico (Z. h. luteus) meadow jumping mice; Amargos vole (Microtus californicus scirpensis); 

riparian woodrat (Neotoma fuscipes riparia); salt marsh harvest mouse (Reithrodontomys 

raviventris); Columbia Basin pygmy rabbit (Brachylagus idahoensis distinct population segment); 

riparian brush rabbit (Sylvilagus bachmani riparia); San Joaquin kit fox (Vulpes macrotis mutica); 

gray wolf; grizzly bear (Ursus horribilis horribilis Continental U.S. distinct population segment); 

black-footed ferret (Mustela nigripes); California condor (Gymnogyps californianus); and northern 

(Strix occidentalis caurina) and Mexican (S. o. lucida) spotted owls. It is a federal offense to use 

any pesticide in a manner that results in the death of a T&E species. The label requires that the 

U.S. Fish and Wildlife Service be consulted on local species that could be impacted by use of 

strychnine. WS conducts Section 7 consultations with USFWS to ensure that T&E species are 

not impacted. 

2.2 Physical and Chemical Properties 

Strychnine (C21H22N2O2, Figure 1) is a white, odorless, 

and stable solid crystalline powder with a bitter taste 

(USEPA 1996b; Centers for Disease Control 2018). It 

has a high melting point (273°C) and very low vapor 

pressure (2.93X10-9 mm Hg at 25 °C (est)) (NIH 2022). 

It's Henry’s Law Constant is 7.56 x 10-14 atm m3/mol 

(NIH, 2018), suggesting it does not volatilize into the 

atmosphere from water. 

Strychnine is not soluble in water (0.0115 g/100ml) 

(USEPA 1996b). It has a dissociation constant of 8.26 at 

25°C (NIH, 2022). Strychnine’s log of the octanol-water 

partition coefficient, Kow is 1.93 (NIH 2022). 

2.3 Environmental Fate 

Environmental fate describes the processes by which strychnine moves and transforms in the 

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

soil, movement to air, migration potential to groundwater and surface water, and plant uptake. 

Strychnine in the soil is stable to photolysis (Timm et al. 1993). Batch adsorption/desorption data 

show that strychnine binds strongly to various soils with Freundlich Kd-adsorption/desorption 

values of 39-168 mL/g (USEPA 2015a). The mean sorption (Kf), desorption (Kfdes), and sorption 

coefficient (Kfoc) for strychnine technical are 105.9, 101.3, and 13823, respectively, which 

indicates strychnine is strongly sorbed to soil and likely immobile in the environment (Timm et al. 

1992); particularly in soils with moderate to high cation exchange capacities (Starr et al. 1996, 

USEPA 1996b; Kookana et al. 1997). In one laboratory study, strychnine sorption to soil particles 

was rapid, with 90% sorption occurring within 15 minutes to a couple of hours in experiments 

combining 1 gram of soil mixed with a strychnine solution of 10 mg/L (Kookana et al. 1997). 

Strychnine is not likely to accumulate in soil because it is biodegradable (Starr et al. 1996, Rogers 

et al. 1998). Approximately 50% of the 10 ppm (ug/g) of strychnine in soil was lost through soil 

sorption and degradation by microorganisms within 24 days for sandy loam soil and 27 days for 

sandy-clay loam soil (Starr et al. 1995, Starr et al. 1996). Similar degradation rates occurred in 

another study on two alkaline soils despite the higher starting concentration of 50 mg/kg 

strychnine (Rogers et al. 1998). In sandy loam and sandy-clay soils, the time for 50% of the 

Figure 1. Chemical structure of 

strychnine (C21-H22-N2-O2) (NIH 

2020).
7

strychnine to dissipate (DT50) under aerobic conditions was 24 and 27 days, respectively. About 

90% of the strychnine dissipated within 33 to 40 days in both soil types (Starr et al. 1996). In one 

laboratory study, when strychnine strongly adsorbed to soil with a pH of 5.5, degradation did not 

occur during the 98-day incubation period (Rogers et al. 1998). The authors attribute this to the 

low bioavailability of strychnine due to the strong adsorption (Rogers et al. 1998). This may 

indicate that in some environments, strychnine may persist in soil; however, this observation was 

made in a laboratory study, and field conditions may be different. 

In a study looking at the release of strychnine from treated grain to soil, strychnine took 

approximately 2 months to move from treated grains placed on the soil surface to the soil; 

however, 90% of strychnine transferred within 7 days when treated grain was buried (Kookana et 

al. 1997). Strychnine is not expected to leach based on its high soil-water partition coefficient for 

organic compounds (Koc), sorption to soil, particularly with higher clay content, and its insolubility 

in water (Starr et al. 1996, Kookana et al. 1997, Ghadiri et al. 1999). In laboratory studies, clay 

soil retained higher amounts of strychnine. It resisted releasing strychnine during desorption 

studies with strychnine concentrations higher than those used in mouse bait or treated grains in 

Australia (Ghadiri et al. 1999). The binding capacity of strychnine to soils decreased as clay 

content decreased (Ghadiri et al. 1999). 

Off-site movement of strychnine formulations is unlikely to occur in surface and ground water 

runoff because strychnine application is within the gopher burrows, and strychnine is essentially 

immobile. In one atypical finding, the U.S. Forest Service (USFS 2010) detected strychnine in 

stream water following strychnine bait applications. The USFS applied 0.5% strychnine bait 

belowground at 42 sites between 1996 and 1997. USFS (2010) detected concentrations of 

strychnine (13-23ppb) in stream water at two sites; both streams were located below treatment 

areas that received heavy rains following strychnine application. In subsequent years, there were 

no reports of detectable concentrations of strychnine in surface water, indicating the off-site 

movement of strychnine was atypical. The estimated half-life for strychnine is seven to 28 days in 

surface water based on unacclimated aqueous aerobic biodegradation half-life (Howard 1991). 

Strychnine does not hydrolyze at pH 5, 7, or 9 (Mishalanie et al. 1989). 

Strychnine is not likely to be present in the air, except as a particulate, because of its low vapor 

pressure. The photolysis process does not appear to degrade strychnine significantly (USEPA 

1996b). 

Laboratory studies indicate plants can uptake strychnine from the soil, but plants in the field do 

not appear to uptake measurable amounts of strychnine. In a greenhouse study, fava bean (Vicia 

faba), pea (Pisum sativum), and lupine plants (Lupinus angustifolius) took up strychnine from bait 

applied approximately 10 to 30 times the recommended application rate (Oliver et al. 2000), 

indicating that plants can take up strychnine. However, when field tested, crops did not have 

measurable uptake of strychnine from surface-applied strychnine bait applied at 8 times the 

recommended rate (Oliver et al. 2000). The authors speculated that the recommended field 

application rates would result in no detectible limits of strychnine in plants. A study on the 

strychnine uptake from subsurface bait (up to 5.26% strychnine-treated grain bait) did not show 

evidence that alfalfa and apple (Malus domestica) trees took up strychnine from the soil (Smith 

1982). 

2.4 Hazard Identification 

Strychnine is highly toxic to humans and other vertebrates. Strychnine is a neurotoxin and inhibits 

glycine, a neurotransmitter essential to the nervous system in vertebrates. The inhibition of glycine 
8

in the spinal cord causes excessive motor neuron activity and muscle spasms (convulsions) 

(Borges et al. 1989, USEPA 1996b, USFS 2010). Muscular hyperactivity and convulsions can 

quickly lead to respiratory failure and death (Borges et al. 1989). Symptoms may appear within 

15 to 60 minutes, with respiratory failure and brain death occurring within 30 minutes of exposure 

to high doses (Borges et al. 1989, Centers for Disease Control 2018). 

The formulations used by WS contain 0.5% strychnine. In these products, the other ingredients 

are primarily grains (oats or milo), which are not hazardous to humans or nontarget animals. The 

other additives used in these formulations occur in minor amounts. They are typically dyed to 

mark toxic bait and other materials intended to make the formulation more palatable to pocket 

gophers. The following section summarizes available acute and chronic toxicity data for humans 

and mammals that are used to evaluate the hazards of strychnine to human health and nontarget 

animals. 

2.4.1 Toxicokinetics

Strychnine is absorbed through the gastrointestinal tract and nasal passages (Borges et al. 1989). 

Studies on dermal absorption of strychnine are limited. One report summarized nonlethal 

exposure in a woman whose left hand contacted a solution of strychnine with an assumed 

concentration of 2% and did not wash her hand until 30 minutes after exposure (Greene and 

Meatherall 2001). She spilled strychnine and cleaned it with a cloth, using sodium hypochlorite 

(bleach). Bleach is strongly alkaline and converts the strychnine to a free base, more easily 

absorbed through the skin. About 22 hours after exposure, she experienced progressive muscle 

spasms, pain in her lower limbs, and jaw stiffness. After 24 hours, she experienced dermal 

hypersensitivity. Two days after medical treatment, the woman had some muscle discomfort but 

had no symptoms within a month. 

Strychnine does not accumulate in the body; instead, the liver metabolizes and detoxifies 

nonlethal doses of strychnine, and the body excretes it in urine (Borges et al. 1989). Survival from 

strychnine poisoning is unlikely to cause long-term health effects unless damage from poisoning 

occurs, such as kidney failure or brain damage from low oxygen levels (Centers for Disease 

Control 2018). 

2.4.2 Acute Toxicity 

The USEPA (2015a) Office of Pesticide Programs (OPP) classifies strychnine as a toxicity 

category I (indicating the greatest degree of acute toxicity) for oral and inhalation effects 

(presumed inhalation toxicity) (Table 5). The inhalation toxicity category I was assigned based on 

the acute oral toxicity. This protective assumption to human health is conservative because 

strychnine has a low vapor pressure, and only particulates pose potential hazards. Mammalian 

strychnine toxicity studies found that females appeared more sensitive than males, and young 

animals appeared more sensitive than mature animals. 

USEPA (1996b) classified strychnine as a toxicity category I eye irritant because of rabbit mortality 

in the eye irritation study, which indicated rapid absorption of strychnine through the eye. 

For dermal contact, USEPA (1996b) classified acute dermal toxicity as category III and dermal 

irritancy as category IV. These classifications are due to strychnine's poor absorption through the 

skin based on the dermal toxicity study in rabbits in which neither mortality nor signs of toxicity 

were noted at a limit dose of 2,000 mg/kg body weight (USEPA 1996b; Glaza 1997a). In addition, 

after direct dermal contact with a strychnine solution (estimated at 2% strychnine), one woman 
9

did not show evidence of dermal irritation (Greene and Meatherall 2001). The woman was 

asymptomatic for about 12 hours. After which, she experienced shaking and spasms of the arms 

and legs, muscular pain, and hypersensitivity to touch, the same strychnine symptoms of oral 

systemic toxicity. Strychnine is a fast-acting toxin through the oral route of exposure, and the 

delayed toxic effects from dermal exposure indicate a slower rate of absorption (USFS 2010). A 

comparison of acute mammalian oral toxicity data between the technical active ingredient and 

formulations shows that the formulations are less toxic than the technical active ingredient (Table 

5). Other available comparable acute endpoints show similar or less hazard between the technical 

a.i. and formulated product, with the exception of the primary dermal irritation study in rabbits. 

Table 5. Acute mammal toxicity data for strychnine alkaloid technical (purity = 99.42%) and grain oat baits 

containing 0.5% strychnine. 

Test Results (99.42%) Toxicity 

Category

Grain baits 

(0.5% strychnine)

Toxicity 

Category

Acute oral LD501, lab rat 6.4 mg/kg males 

2.2 mg/kg females 

Death within 1 hour 

I 440-541 mg/kg2 IV 2

Acute dermal LD50, lab 

rabbit 

2000 mg/kg, No signs of 

toxicity 

III >2,000 mg/kg 3 III 

Acute LD50 inhalation N/A I* >2.11 mg/L 4 I 2

Primary eye irritation, rabbit Irritation and mortality I Temporary 5 IIB 2

Primary dermal irritation, 

rabbit 

No irritation, mortality, or signs 

of toxicity 

IV Slight irritation to skin 6 II 2

Skin sensitization – Guinea 

pig 

Not a sensitizer7 N/A IB 2

* Based on high acute toxicity of oral exposure route N/A = Not Available 

1

 LD50 – Lethal dose 50% = amount of a chemical sufficient to kill 50% of a population; 2 USDA (2019); 3 (Glaza 

1997a); 4 MRID 44708001: Wnorowski (1998); 5 MRID 44708002: Moore (1998); 6 Glaza (1997b); 7 USEPA (2015a) 

2.4.3 Subchronic and Chronic Toxicities 

Chronic and subchronic strychnine toxicity studies are limited. In a subchronic study, rats were 

exposed to a 0.2% solution of strychnine hydrochloride intubated into the stomach at a daily dose 

of 5 mg/kg and 10 mg/kg in males and 2.5 mg/kg in females over 28 days (Seidl and Zbinden 

1982). After each treatment, animals exhibited slight tremors, which subsided within the first hour. 

Treated rats did not differ from control rats in weight gain, food or water consumption, 

hematological values, urinalysis, eye examination, electrocardiogram test, organ weight, 

behavior, or coordination. No other symptoms were observed. Based on this study, the lowestobserved-adverse-effect level (LOAEL) for rats is 2.5 mg/kg/day (USEPA 2002). 

2.4.4 Effects on Nervous, Immune, and Endocrine Systems and Reproduction and 

Development 

Strychnine is a neurotoxin. Acute toxic doses of strychnine cause muscular hyperactivity, 

convulsions, respiratory failure, and death. Studies on the immunological effects of strychnine 

exposure are lacking in the literature. 

Studies on the effects of strychnine on the endocrine system are limited, and multigenerational 

exposure studies are not available. In the 28-day oral toxicity rat study summarized above, no 

histological changes occurred in the adrenal gland, pancreas, pituitary gland, ovary, testicle, and 

thyroid gland, and no significant weight changes occurred in the adrenals or testes (Seidl and 

Zbinden 1982). 
10

Multigenerational studies and reproductive toxicity studies are mostly lacking in the literature. In 

a developmental study, García-Alcocer et al. (2005) found the embryos of pregnant rats given a 

single oral dose of 2.5, 5.0, or 8.0 mg/kg strychnine on day 8 developed neural tube defects. 

2.4.5. Human Incidents 

USEPA (2015b) evaluated human incidents and epidemiology studies related to strychnine using 

the USEPA/OPP Incident Data System (IDS), the Centers for Disease Control, National Institute 

for Occupational Safety and Health Sentinel Event Notification System for Occupational RiskPesticides (SENSOR), and the National Pesticide Information Center (NPIC) databases between 

1998 and 2011. The incidents reported in IDS and SENSOR during this period following the 

implementation of the mitigation measures recommended in the 1996 reregistration eligibility 

decision were of low frequency and severity. Mitigation measures included only allowing readyto-use grain baits at levels of no greater than 0.5% strychnine concentration for manual baiting 

by homeowners and child-resistant packaging, as well as label modifications on personal 

protective equipment (PPE). Strychnine is not included in the Agricultural Health Study, so no 

reporting data is available. 

3.0 DOSE-RESPONSE ASSESSMENT 

3.1 Human Health 

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

human health. The lethal dose in humans varies. Human poisoning incidents include both 

accidental and intentional exposures. Fatal doses (acute oral toxicity) measured from individuals 

under medical intervention ranged between 1.4 and 80 mg/kg bw, and non-fatal doses ranged 

between 1 to 25 mg/kg bw (U.S. Forest Service (USFS) 2010). The oral lethal dose for humans 

ranges from 30 to 120 mg (Borges et al. 1989). Most incidents involving young children are 

associated with accidental ingestion of strychnine medicine tablets. Children are more sensitive 

to strychnine with lethal doses possibly as low as 15 mg (Goodman and Gilman 1985). 

The USEPA (1996b) estimated a single strychnine bait (0.5% strychnine) would be lethal to a 10 

kg child based on a calculated dose of 1.1-1.8 mg/kg of strychnine. Because of strychnine’s high 

acute oral toxicity, the USEPA (1996b) did not calculate an acute oral reference dose (RfD). 

The USEPA Office of Research and Development (USEPA 1987) derived a chronic oral exposure 

RfD of 0.0003 mg/kg bw/day based on a LOAEL of 2.5 mg/kg/day from the 28-day gavage rat 

study (Seidl and Zbinden 1982). An uncertainty factor of 10,000 was applied to account for a) 

extrapolation from a less-than-chronic to a chronic exposure study, b) extrapolation from animals 

to humans, c) differences in sensitivity among the human population, and d) using a LOAEL 

instead of a no-observable-adverse-effect level (NOAEL) to derive the RfD. The RfD is not used 

in this human health risk assessment because very few animals were involved, no NOAEL was 

identified in the study, and no human toxicity data was considered. The USFS (U.S. Forest 

Service (USFS) 2010) used a surrogate RfD of 0.02 mg/kg bw for their risk assessment based on 

the threshold limit value recommended by the American Conference of Governmental Industrial 

Hygienists. This threshold limit value is intended to be protective in both acute and longer-term 

exposures based on human data. USDA-APHIS uses this surrogate RfD for this risk assessment. 
11

The National Institute for Occupational Safety and Health (NIOSH) set a recommended exposure 

limit (REL) time-weighted average (TWA) of 0.15 mg/m3 for occupational exposure (exposure 

routes of inhalation, ingestion, skin, or eye contact) for a 10-hour workday during a 40-hour 

workweek (Centers for Disease Control 2019). This value is used to minimize the potential for 

neurotoxic and central nervous system effects. 

3.2 Ecological 

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

effects, including acute and chronic toxicity. Many toxicity studies have been conducted on 

species to determine the relative toxicity of strychnine to them and their potential to others as 

surrogate species (Table 6). In general, strychnine is highly toxic to vertebrates and invertebrates.

3.2.1 Terrestrial Dose-Response Assessment 

Mammals 

Strychnine has high acute toxicity in mammals (Table 6). Strychnine sensitivity varied from mink 

with an LD50 of 0.6 mg/kg to the California ground squirrel with an LD50 of 38 mg/kg (Table 6). 

Strychnine (99.9% a.i.) has median lethal concentration (LC50) values of 70 ppm and 198 ppm 

for red fox and European ferret, respectively (USEPA 1996b) (Table 4). In acute oral studies using 

the domestic brown (Norway) rat, females (LD50 = 2.2 mg/kg) were more sensitive than males 

(LD50 = 6.4 mg/kg) (USEPA 1996b). In these studies, death occurred within one hour. 

In a field study, exposure of plains pocket gophers to strychnine bait at 0.35%, 0.75%, and 1.30% 

caused mortality in 66.7%, 96.3%, and 89.7% of the animals, respectively (Ramey et al. 2002). 

Pocket gophers can absorb strychnine through their cheek pouches, where they may store treated 

bait, and through their stomach and intestinal tract (Record and Marsh 1988). 

In a subchronic study, brown rats treated by gavage with 0 to 10 mg/kg strychnine over 28 days 

did not display differences from the control rats in weight gain, food or water consumption, 

hematological endpoints, urinalysis, eye examination, electrocardiogram test, organ weight, 

behavior, or coordination (Seidl and Zbinden 1982). After each treatment, animals exhibited a 

slight tremor, which subsided within the first hour. 

Birds 

As with other animal species, there is variability in strychnine sensitivity in bird species, but it 

shows its high toxicity. Strychnine is acutely toxic to avian species (USEPA 1996b) (Table 6) with 

mortality occurring between one and five hours after poisoning (Hudson et al. 1984). In acute 

toxicity studies, signs of poisoning appeared about 10 minutes after dosing and include ataxia, 

salivation, tremors, convulsions, and immobility (Tucker and Crabtree 1970, Hudson et al. 1984). 

In subacute dietary toxicity studies, strychnine (technical grade) is slightly toxic in northern 

bobwhite, a quail species, with an LC50 of 3,536 ppm and no-observable-effect-concentration 

(NOEC) of 1,250 ppm. Strychnine is highly toxic in mallard, black-billed magpie, and American 

kestrel, with LC50 values of 212 ppm, 99 ppm, and 234 ppm, respectively, and a NOEC of 78 

ppm in the mallard (USEPA 1996a). 

 
12

Table 6. Strychnine acute oral and dietary toxicity values for vertebrates and invertebrates. 

Strychnine Acute Oral and Dietary Toxicity Values for Mammals and Birds

Species (Age) Test Result (Age) Reference

Mammals

Golden-mantled Ground Squirrel LD50 3.6 mg/kg Anthony et al. 1984 

California Ground Squirrel LD50 38 mg/kg 

Schafer and Bowles 2004 

Hispid Cotton Rat (Sigmodon hispidus) LD50 32 mg/kg 

Meadow Vole LD50 22 mg/kg 

House Mouse ♀ LD50 10 mg/kg 

Ricefield Rat (Rattus argentiventer) LD50 5.9 mg/kg 

Brown (Norway) Rat LD50 14 mg/kg 

Brown (Norway) Rat LD50 5.8 mg/kg 

Marsh Rice Rat LD50 5.8 mg/kg 

Nutria LD50 27 mg/kg 

Black-tailed Jackrabbit LD50 4.4 mg/kg 

Mule deer ♀ (8-11 months old) LD50 17-24 mg/kg 

Mink ♀ LD50 0.6 mg/kg Anthony et al. 1984 

European Ferret LC50 198 ppm USEPA 1996b Red fox LC50 70 ppm 

Birds

Mallard LD50 2.9 (♂ & ♀, 6 mos.) Tucker and Crabtree 1970, Tucker 

and Haegele 1971 

2.62 (36 hrs.) 

2.0 (1 week) 

5.88 (1 mo.) 

2.27 9 (♂ & ♀, 6 mo.)

Hudson et al. 1984 

Japanese Quail (Coturnix japonica) LD50 12 Schafer and Bowles 2004 

22.6 (♀ 2 mos.) Tucker and Crabtree 1970, Tucker 

and Haegele 1971 

California Quail (5-6 mos.) LD50 112 Hudson et al. 1984 

Chukar Partridge ♂ & ♀ (5-7 mos.) LD50 16 Tucker and Crabtree 1970, Tucker 

Ring-necked Pheasant ♂ (10-23 mos.) LD50 24.7 and Haegele 1971 

Rock Pigeon ♂ & ♀ LD50 21.3 

Golden Eagle LD50 5.0 (approximate) Tucker and Crabtree 1970 

House Sparrow ♂ LD50 4.18 

European Starling LD50 <5.0 Schafer and Bowles 2004 

Red-winged Blackbird LD50 6.0 

Fish

Bluegill 96-hr LC50 0.76 mg/L Bowman 1989a 

Rainbow Trout 96-hr LC50 2.3 mg/L Bowman 1989b 

Japanese Rice Fish (Oryzias latipes) 48-hr LC50 5.7 mg/L Rice et al. 1997 

Invertebrates

Water Flea (Daphnia magna) 96-hr LC50 

48-hr EC50

10 ppm 

11 mg/L 

USEPA 1996b, Forbis 1989 

In 28-day subchronic dietary studies, northern bobwhite had a NOEC of 972 μg/g and mallards 

had a NOEC of 91 μg/g (Sterner et al. 1998). Bobwhites fed more than 1,870 μg/g strychnine 

displayed ataxia, wing-beat convulsions, tremors, muscle spasms, and sitting posture, but of 

bobwhites displaying signs of toxicosis only one died (Sterner et al. 1998). Intestinal hemorrhage 

was found in post-exposure necropsies of one exposed to 972.6 μg/g and one exposed to 6,083.3 

μg/g (Sterner et al. 1998). Ducks were more sensitive to strychnine than quail. In ducks, signs of 

toxicosis occurred at 235, 484, and 972 μg/g strychnine diets. In addition to the signs observed 

in quail, ducks exhibited unkemptness, unsteady gait, falling, and other mobility problems (Sterner 

et al. 1998). Post-exposure necropsies of three ducks fed diets with strychnine levels between 75 

μg/g and 250 μg/g had hemorrhagic testes and four ducks had gaseous intestines (Sterner et al. 

1998). 
13

In chronic studies, reproductive effects in mallards included smaller testes at the lowest 

observable effect level of 33 ppm (USEPA 1996b). A reduction in chick body weight occurred on 

day one in the 68.9ppm dose group. At the 140.9ppm dose group, chicks had reduced body 

weight, and adult females had reduced egg production and body weight (USEPA 1996b). 

In chronic dietary reproductive toxicity studies, 140-day exposure of mallard hens to 140.9 μg/g 

strychnine diets (highest dose) caused a statistically significant decrease in egg production, 

normal egg formation, and hatching success as well as observed mortality of some ducklings that 

was not statistically significant (Pedersen et al. 2000). Clinical signs of toxicosis without mortality 

included loss of righting reflex, tremors, spasms, and wing-beat convulsions occurred between 

weeks 6 and 16 for ducks exposed to 68.9 and 140.9 μg/g (Pedersen et al. 2000). Female ducks 

fed a 140.9 μg/g strychnine diet significantly gained less body weight (99 g less) compared to the 

control females at week 20 (Pedersen et al. 2000). Based on this study, a no observed adverse 

effect level (NOAEL) on reproduction for subchronic strychnine-dietary exposure in mallard duck 

was 33.2 μg/g. Northern bobwhite did not display any reproductive effects from diets containing 

between zero and 1,113.6 μg/g strychnine (Pedersen et al. 2000). A NOAEL on reproduction for 

subchronic strychnine-dietary exposure in bobwhites was 1,113.6 μg/g (Pedersen et al. 2000). 

Surface-applied strychnine bait to control mice in an agricultural field resulted in direct and 

secondary poisoning of birds (WS applications are belowground only) (Brown and Lundie-Jenkins 

1999). Carnivorous birds gathered during the study likely died from eating poisoned mice, as one 

or two mouse carcasses were in their crop or gizzard. The recovery of a low number of mice from 

the crop or gizzard indicates a potential low level of strychnine is enough to cause death or the 

birds regurgitated mice when they became ill (Brown and Lundie-Jenkins 1999). Estimated lethal 

doses in raptors range from 0.94 mg/kg bw for snowy owls (Redig et al. 1982) to 10.75 mg/kg bw 

for red-tailed hawks (Anthony et al. 1984). Exposure of great horned owls to 2.1 mg/kg bw caused 

convulsions (Anthony et al. 1984), and exposure of one red-tailed hawk to strychnine by feeding 

it a mouse dosed with 2.3 mg/kg strychnine caused incoordination and agitation (Cheney et al. 

1987). In a secondary exposure study, researchers fed mice dosed with strychnine to two great 

horned owls (Cheney et al. 1987). Both owls displayed loss of motor coordination, including 

difficulty balancing on a perch, falling to the ground, extended flapping of the wings, and body 

trembling after eating a mouse dosed with 1.0 mg/kg and 1.5 mg/kg strychnine (Cheney et al. 

1987). In the wild, birds falling from their perch could be fatal, and motor coordination problems 

could make birds vulnerable to predation or affect foraging behavior. 

Reptiles and Terrestrial Phase Amphibians 

Studies on the effects of strychnine on reptiles and amphibians are limited. Research was 

conducted that involved feeding a strychnine-poisoned rodent every day for 14 days to 12 gopher 

snakes (Brock 1965, Pauli et al. 2000). The amount of strychnine ingested by the snakes was 

unknown, but the estimated average amount of strychnine consumed by the rodents was 1.64 

mg (range 0.03 – 11.70 mg) (Brock 1965). The snakes showed signs of toxicity, including tremors 

and irritability. Five snakes died, likely from strychnine poisoning. The average dose of 3.6 mg/kg 

bw for gopher snakes used in the study was based on the average body weight of the gopher 

snakes and the average amount of strychnine fed to the mice (Brock 1965, Pauli et al. 2000, 

USFS 2010). USFS (2010) estimated a fatal dose of 7 to 18 mg/kg bw for a prairie rattlesnake 

based on calculations of strychnine residues in pocket gophers (Evans et al. 1990) and collection 

of a prairie rattlesnake with signs of toxicity (Campbell 1953). 
14

For amphibians, the acute oral LD50 for the terrestrial-phase bullfrog is 2.21 mg/kg bw (Tucker 

and Crabtree 1970). Data is limited on the effects of strychnine on aquatic phase amphibians. 

Common toads (Bufo vulgaris) were immobilized at strychnine concentrations of 50 mg/L, and at 

5 mg/L, toads developed abnormalities of the digestive tract and eyes (Cuomo et al. 1978). 

Common toads were considered possibly a strychnine-tolerant species. 

Terrestrial Invertebrates 

Toxicity values for terrestrial invertebrates are limited. Field observations indicated toxicity to 

insects would not be significant (Nolte and Wagner 2001). Ants given strychnine orally (0.1 to 0.5 

mg/kg) displayed motor coordination problems but no mortality (Kostowski et al., 1965). In one 

controlled field study, Hymenoptera (ants, wasps, and others), Coleoptera (beetles and weevils), 

and Diptera (true flies) insects, which fed on pocket gopher carcasses that had died from eating 

strychnine bait, had detectable levels of strychnine (Stahl et al. 2004). Based on calculated 

exposure scenarios for pigeons, mice, and bullfrogs, the detected levels were not expected to 

pose a risk to insectivores. The study did not report any insect mortalities. 

Terrestrial Plants and Terrestrial Microorganisms 

No dose-response assessment is estimated for plants and microorganisms, given the lack of a 

hazard as well as a lack of toxicity data. 

3.2.2 Aquatic Dose-Response Assessment 

Aquatic Vertebrates and Invertebrates 

In freshwater fish, strychnine (99.9% a.i.) is moderately toxic to rainbow trout with a 96-hour LC50 

of 2.3 mg/L (Bowman 1989b) and is highly toxic to bluegill (Lepomis macrochirus) with a 96-hour 

LC50 of 0.76 mg/L (Bowman 1989a) (Table 6). Strychnine is moderately toxic to freshwater 

invertebrates; the freshwater cladoceran, Daphnia magna, has a LC50 value of 10 ppm (USEPA 

1996b) and a 48-hour EC50 of 11 mg/L (Forbis 1989). 

Japanese rice fish (also known as medaka) (Oryzias latipes) had a 48-hour (static) LC50 of 5.7 

mg/L (Rice et al. 1997). Fish experienced a loss of equilibrium and convulsions. Fish with severe 

strychnine intoxication stopped swimming or responding to external stimuli, including the failure 

to display a startle response. Stress symptoms appeared 2 hours after receiving 10 mg/L or more 

strychnine and 24 hours after receiving 1 mg/L or more strychnine (Rice et al. 1997). In a 

laboratory study, 96-hour exposure of zebrafish (Dania rerio) embryos to strychnine caused a 

reduction in hatching in response to doses greater than 50 micromolar (μmol)/L (Yu et al. 2014). 

Embryo development abnormalities occurred at 200 μmol/L strychnine, including curved spines 

and pericardial edema. 

In laboratory experiments, short exposure (18 and 29-hours) of zebrafish embryos to a 

concentration of 1.5 millimolar (mM) strychnine caused changes later in adulthood in swimming 

speed and diving behavior (a predatory avoidance response) (Roy et al. 2012). These long-term 

changes in behavior would place fish at risk of predation. 

Aquatic Phase Amphibians 

Research on the effects of strychnine on the aquatic phase of amphibians and aquatic reptiles is 

limited or not available. Common toad embryos exposed to a strychnine concentration of 50 mg/L 
15

were immediately immobilized (Cuomo et al. 1978). At the 5 mg/L concentration, the embryos 

developed abnormal digestive tracts and eyes. 

Aquatic Plants 

Information is unavailable on the toxicity of strychnine to aquatic plants. Based on strychnine's 

mode of action, the toxicity to aquatic plants is expected to be low. 

4 EXPOSURE ASSESSMENT 

4.1 Human Health 

Exposure assessment estimates the potential exposure of humans to strychnine. The exposure 

assessment begins with the use and application methods of strychnine products. A complete 

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

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

dermal contact by which contact can occur. In this way, the potentially exposed human 

populations and complete exposure pathways are identified. Finally, exposures for the identified 

human populations are evaluated qualitatively or quantitatively for each identified exposure 

pathway. 

4.1.1 Potentially Exposed Human Populations and Complete Exposure Pathways 

All strychnine formulations are restricted to belowground uses in rangelands, pastures, croplands, 

forests, and non-agricultural areas. Baits made under the various formulations are for application 

by burrow builders or hand-baiting with a probe that has a bait dispenser or through the use of a 

long-handled spoon. Based on the registered uses and specific application methods of strychnine, 

workers (i.e., handlers including loaders and applicators) in the program are the most likely human 

population group to be exposed to strychnine; mixing is not required by applicators. Applicators 

handle pre-mixed bait in either manual or mechanical applications. The complete exposure 

pathways for these workers include direct contact (i.e., incidental ingestion, inhalation, and dermal 

contact) to strychnine baits during normal belowground applications of each bait formulation. 

The general public is not identified as a potentially exposed human population because their 

exposure to strychnine mixed in bait formulations is extremely unlikely since they would have to 

dig up the gopher burrows. Therefore, an exposure pathway for the general public is not 

evaluated. 

Similarly, a complete exposure pathway is not identified for dermal contact with contaminated 

vegetation because this scenario does not apply to strychnine subsurface applications. A 

complete exposure pathway is also not identified for strychnine residues on dietary items. This is 

because of its nonfood and belowground use pattern, where strychnine residues in soil are not 

likely to be taken up by plants and therefore consumed in vegetables or fruits (see Section 2.3). 

A complete exposure pathway is not identified for drinking water. Strychnine is immobile in the 

soil and has low water solubility resulting in a negligible leaching potential from subsurface soil to 

groundwater (see Section 2.3). Surface runoff is not expected to occur due to the belowground 

use pattern (see Section 2.3). The use pattern and properties of strychnine indicate contamination 

of water sources is unlikely. 
16

4.1.2 Exposure Evaluation 

This section qualitatively evaluates the worker exposures in direct contact pathways associated 

with handling and applying the strychnine belowground formulations. It also quantifies worker 

inhalation exposure for the burrow builder and hand baiting applications. 

As discussed in Sections 2.4 and 3.1, strychnine has high acute toxicity via oral and ocular routes 

and probably the inhalation route of exposure. The direct contact exposures via incidental 

ingestion, inhalation, and dermal contact to workers (i.e., handlers, particularly applicators) are 

minimized with proper worker hygiene and properly functioning personal protective equipment 

(PPE) as required by the label. 

Accidental exposures may occur should PPE not function properly or for those who do not follow 

label directions regarding the use of PPE or washing shortly after strychnine applications are 

completed. WS personnel are responsible for stringently following label directions to ensure their 

safety. 

Inhalation exposure doses to strychnine for workers making applications using hand baiting with 

a probe or the use of a burrow builder were estimated using a PF10 safety factor for a respirator 

(Table 7). These values were estimated to determine the potential for risk to workers, which is 

discussed further in the risk characterization, Section 5. 

An inhalation exposure dose was quantified for the following exposure scenarios: 

1) a burrow builder label application rate of 0.005 lb a.i./acre treating 200 acres, and 

2) a hand baiting label application rate of 0.005 lb a.i./acre treating 1 to 8 acres daily. 

An inhalation dose is calculated using the following equation: 

Inhalation Dose = (Application Rate x Area Treated Daily x Inhalation Unit 

Exposure x Conversion Factor)/Body Weight 

PPE levels with a respirator of PF10 are used for an inhalation unit exposure. A PF10 respirator 

is assumed to reduce inhalation exposure by 90% (USEPA 2020). The inhalation doses for these 

exposure scenarios are summarized in Table 7, and detailed calculations are given in Table 8. 

Table 7. Inhalation doses for workers using a burrow builder and hand baiting applications. 

Worker 

Activity

FormulationApplication 

Method

Application 

Rate 

(lb a.i./acre)

Area 

Treated 

Daily 

(acre)

Inhalation Unit 

Exposure 

(μg/lb) 

Inhalation 

Dose 

(mg/kg/day) 

Application Bait Hand Baiting 0.005 1/8 38 (PF10) 2.4E-06/ 

1.9E-05 

Loading Granule Burrow Builder 0.005 200 0.825 (PF10) 1.0E-05 

4.2 Exposure Assessment for Nontarget Species 

WS personnel did not report any nontarget impacts to species other than the target species 

between FY16-FY20. However, it is acknowledged that nontarget species take likely occurred, 

as discussed below. 
17

Table 8. Inhalation margin of exposures for hand baiting and burrow builder applications using the inhalation 

dose equation1. A PE10 is a PF10 respirator, which reduces inhalation exposure by an assumed 90%. 

Parameters Units Hand Baiting Burrow Builder

PF 10 PF 10 PF 10 

Application rate lb a.i./acre 0.005 0.005 0.005

Area treated daily acre 1 8 200 

Inhalation unit exposure2 ug/lb 38 38 0.825 

Conversion factor mg/ug 0.001 0.001 0.001 

Body weight kg 80 80 80 

Inhalation doses mg/kg/day 2.4E-06 1.9E-05 1.0E-05 

Surrogate RfD mg/kg/day 0.02 0.02 0.02 

Inhalation MOE mg/L 8421 1052 1939 

1

 Inhalation Dose = (application rate * area treated daily * inhalation unit exposure * conversion factor)/ body weight 

2

 Source for inhalation unit exposure is USEPA (2020) 

4.2.1 Direct Exposure 

Direct or primary exposure of nontarget species to strychnine could occur if a nontarget species 

ingests strychnine baits. Strychnine baits could be available from accidental spills, 

misapplications, or nontarget species accessing a treated burrow. The belowground application 

reduces direct exposure to mammals, birds, and other species living aboveground. 

Spillage on the ground surface could occur during application with the burrow builder. Spillage 

during hand baiting is less likely to occur. Bait could fall out of the burrow builder if the bait clogs 

the nozzle or the builder is removed from the burrow before all the bait is dispensed. The 

strychnine labels state that bait should not be left exposed on the soil surface; thus, applicators 

must pick up any spilled material to reduce exposure. Hegdal and Gatz (1977) found a 

mourning dove that apparently died from strychnine milo spillage during a burrow builder 

application, which shows that it is a potential hazard. However, this is a rare occurrence, 

especially if the application process is monitored closely. 

The application of strychnine bait to inactive pocket gopher burrows is generally unlikely because 

applicators look for evidence of activity, such as fresh gopher mounds prior to treatment. Pocket 

gophers are highly territorial and do not typically share their burrow systems with other species 

(Fagerstone et al. 1980). WS personnel do not treat in areas where threatened or endangered 

species could be affected; thus, their take should not be a big concern. More nontarget species 

are expected to be taken with burrow builder applications than hand baiting because the runways 

produced with burrow builders are more likely to have nontarget use than existing pocket gopher 

burrows (Hegdal and Gatz 1977, Fagerstone et al. 1980). 

Other animal species, including small mammals, lizards, snakes, and insects, may enter active 

gopher burrows, and exposure to strychnine could occur if they eat the bait (Howard and Childs, 

1959, Fagerstone et al. 1980, El Hani et al. 2002). Aboveground searches for nontarget carcasses 

found strychnine residues in meadow and montane voles, North American deer mice, goldenmantled ground squirrels, yellow pine chipmunks (Neotamias amoenus), and western harvest 

mice (Reithrodontomys megalotis) from the application of strychnine bait to pocket gopher 

burrows (Arjo et al. 2006 cites several studies; Fagerstone et al. 1980, El Hani et al. 2002). The 

presence and population size of nontarget species will change depending on the habitat. In one 

study, daily aboveground searches did not find nontarget animal carcasses after applying handapplied strychnine baits to pocket gopher burrows (Ramey et al. 2002). 
18

4.2.2 Secondary and Tertiary Exposure 

Secondary exposure to nontarget species occurs if predators or scavengers consume species 

that have consumed strychnine. Field studies on secondary exposure from belowground baiting 

of strychnine are limited, but it is possible that mammalian predators of pocket gophers could be 

at risk of exposure (Fagerstone et al. 1980, El Hani et al. 2002, Anthony et al. 1984, Nolte and 

Wagner 2001). Strychnine-poisoned pocket gophers are more likely to die underground (Nolte 

and Wagner 2001; Ramey et al. 2002), which reduces secondary exposure to predatory or 

scavenger species that do not or cannot access burrows, except for species that dig, such as 

badgers and bears. However, a few small animals that access the burrows and consume the bait 

may be more likely to die aboveground, where they are available to avian and mammalian 

predators, scavengers, and decomposers such as insects (Anthony et al. 1984, El Hani et al. 

2002, Arjo et al. 2006). In one study, aboveground carcasses of ground squirrels and chipmunks 

were mostly gone within 48 hours through decomposition and consumption by insects (El Hani et 

al. 2002). This reduces the availability of the carcasses to scavengers. 

Animals can rapidly excrete strychnine; however, death is rapid, indicating toxic levels of 

strychnine can remain in the body (Record and Marsh 1988). Strychnine is mostly concentrated 

in the gut, given the limited time for distribution in the body as a result of a rapid time to death 

(Anthony et al. 1984). Predators, including raptors and scavengers that do not consume the 

gastrointestinal tract when feeding, are unlikely to experience impacts from strychnine (Howard 

and Childs, 1959, Record and Marsh 1988, James et al. 1990). One laboratory study found toxicity 

in owls and red-tailed hawks from secondary exposure to be minimal due to avoidance of 

gastrointestinal portions and regurgitation (Anthony et al. 1984). Another study found that 

Swainson’s hawk nestlings and adults fed Richardson’s ground squirrels killed with strychnine did 

not impact their survival as a result of the evisceration of the ground squirrels before feeding them 

to the nestlings (Schmutz et al. 1989). In a field study, a burrowing owl fed on a strychninepoisoned ground squirrel avoided the gastrointestinal tract and did not show toxicity after feeding 

on them (James et al. 1990). In another field study, 662 hectares were treated with 0.5% 

strychnine-treated bait using a burrow builder. Populations of small rodents (other than pocket 

gophers) declined in the treated area but increased in the control area (Hegdal and Gatz 1977); 

the researchers using radio-telemetry monitored red-tailed hawks, American kestrels, great 

horned owls, badgers, striped skunks, red foxes, and coyotes on treated fields found that all 

survived. They also monitored red-winged blackbirds and did not detect any effects on this seedeating species. A mourning dove was found that apparently died from eating spillage from the 

burrow builder. Marsh et al. (1987) studied the effects of secondary exposure in coyotes fed 

poisoned ground squirrels in controlled experiments. The squirrels were fed lethal doses of 1.0–

2.7 g (low dose) or 5.0-7.5 g (high dose) of strychnine bait. The estimated strychnine amount in 

the dosed squirrels ranged from 0.275 to 1.059 mg/kg at the low dose and 1.321 to 2.860 mg/kg 

at the high dose. The amount of strychnine coyotes ingested was likely lower due to their 

metabolism of strychnine and the tendency of coyotes to reject portions of the gastrointestinal 

tract when they contained strychnine. The four coyotes fed the low dose survived, and three of 

the four coyotes fed the high dose survived. 

Pocket gophers, ground squirrels, and other animals may store bait in their cheek pouches, which 

may expose predators or scavengers to strychnine bait (Anthony et al. 1984, Arjo et al. 2006). 

Scavengers that store carcasses may increase their exposure should they store a large number 

of carcasses containing strychnine (Arjo et al. 2006). Many factors influence the availability of 

strychnine-killed animals aboveground, including the rate of decomposition and the ability of a 

scavenger to find the carcass, as smaller carcasses are more difficult to detect (Arjo et al. 2006, 

Sullivan 1988). 
19

Insectivore species (e.g., insect-eating birds, terrestrial amphibians, etc.) exposure could occur if 

insects scavenge on animal carcasses killed by strychnine (El Hani et al. 2002, Arjo et al. 2006). 

In field experiments, measured strychnine concentrations in insects that fed upon strychninepoisoned small mammal carcasses were below the threshold level considered harmful to animals, 

including birds and amphibians that may feed on the insects (Stahl et al. 2004, Arjo et al. 2006). 

Ants exposed to strychnine-baited pocket gophers had a mean and maximum strychnine 

concentration of 0.130 and 0.338 μg/g. However, as a source of tertiary exposure to insectivores, 

the level of strychnine does not appear to be a significant risk (Stahl et al. 2004). 

5 RISK CHARACTERIZATION 

This section characterizes risks associated with adverse human health and nontarget species 

qualitatively and quantitatively, where appropriate. Under the WS use patterns, strychnine in 

belowground applications for controlling pocket gophers in rangelands, pastures, croplands, 

forests, and non-agricultural areas poses minimal risks to human health. 

5.1 Human Health 

Exposure will be limited to WS applicators and handlers since strychnine baits are placed directly 

belowground into burrows that are not accessible to the public. Adherence to required PPE and 

other label directions will minimize exposure and risk to applicators. Inhalation and ocular 

exposure to dust when loading a bait-dispensing probe or a burrow builder hopper could 

potentially occur. However, using goggles or a face shield and a label-approved respirator will 

reduce this exposure pathway and minimize risk. 

Inhalation margin of exposures (MOEs) were calculated by comparing an estimated inhalation 

dose for each exposure scenario to a benchmark toxicity value or reference dose. USEPA (2016) 

uses a MOE of 1,000, below which there is a presumption of risk. Table 9 summarizes the 

inhalation MOEs for the hand baiting and burrow builder applications. As shown in Table 9, the 

inhalation MOEs for an applicator hand baiting a low range (1 acre) and a high range (8 acres) 

with a PF10 respirator, and for an applicator using a burrow builder for a loading activity with a 

PF10 respirator are higher than the MOE of 1,000 with the protection of label-required respirator. 

A surrogate RfD of 0.02 mg/kg/day based on the threshold limit value recommended by the 

American Conference of Governmental Industrial Hygienists is used for the MOE calculation. This 

is a conservative approach because the acute inhalation LC50 of >2.11 mg/L from an acute 

inhalation exposure limit study (MRID 44708001) is more than 100x higher than the surrogate 

RfD. 

Table 9. Inhalation MOEs for workers under the hand baiting and burrow builder applications. 

Although strychnine is highly toxic to humans due to its high acute toxicity via the oral, ocular, and 

inhalation routes, there is a low potential for exposure and risk to applicators due to the 

Worker 

Activity

FormulationApplication 

Equipment

Application 

Rate 

(lb a.i./acre)

Area 

Treated 

Daily 

(acre)

Inhalation 

Dose 

(mg/kg/day) 

Inhalation 

MOE 

Application Bait Hand Baiting 0.005 1/8 2.9E-06/ 

2.4E-05 (PF10) 6809/851 

Loading Granule Burrow Builder 0.005 200 1.0E-05 (PF10) 1939 
20

belowground use of strychnine baits and the use of appropriate PPE described on each strychnine 

label. WS maintains records on exposure incidents of WS personnel that conduct WDM activities 

and has no incident reports of workers exposed during strychnine applications. 

Accidental exposures may occur should PPE not function properly; the potential for accidental 

exposure during belowground hand baiting and burrow builder applications is limited and minimal. 

The baits are premixed, so no accidental exposure will occur during applications from mixing or 

preparing baits. Accidental inhalation and ocular exposure from dust for loaders are unlikely 

because each bait formulation will generate minimal dust. If accidental exposure occurs for 

applicators, the exposure route is most likely to be dermal contact. Acute dermal toxicity (Category 

III – 2,000 mg/kg through 5,000 mg/kg) is at least two orders of magnitude less toxic than acute 

oral toxicity (Category I – up to and including 50 mg/kg). The strychnine labels require applicators 

to wear gloves, long sleeves and pants, shoes plus socks, and goggles or a face shield to reduce 

dermal exposure. In addition, the labels require waterproof gloves when loading a bait-dispensing 

probe, disposing of non-rigid containers (e.g., bags), and loading more than 3 lbs. of bait into 

mechanical equipment. USEPA 6(a)(2) reports show two adverse effects incidents between 1994 

through 2013 related to strychnine use. Still, neither of these incidents was associated with 

occupational exposure to WS personnel or exposure during application from WS use. 

5.2 Ecological 

5.2.1 Terrestrial Risk Characterization 

Adverse effects are likely and documented in the scientific literature for rodents that share burrows 

with pocket gophers. One field study demonstrated a reduction in a golden-mantled ground 

squirrel population shortly after hand-baiting western pocket gopher burrows with 0.5% 

strychnine-treated oats. However, the population recovered within a month or two, indicating a 

lack of long-term effects (Anthony et al. 1984). Deisch et al. (1990) conducted an experimental 

use of strychnine baits for black-tailed prairie dogs. They reported no adverse impacts to North 

American deer mice after making field applications with 0.5% strychnine-treated oats. Deer mice 

populations were variable but increased after treatment, possibly due to the low efficacy of 

strychnine treatments in black-tailed prairie dogs. Prairie dogs provide habitat for deer mice, and 

the lack of impacts on prairie dogs allowed mouse populations to increase over time. Deisch et 

al. (1990) did not follow other small rodent populations, such as Ord’s kangaroo rats or pocket 

mice (Perognathus spp.), and suggested these species should be monitored. 

Other terrestrial vertebrates and invertebrates that use the burrows or feed upon treated pocket 

gophers or pocket gopher food caches are also at risk. For example, golden-mantled ground 

squirrels and yellow pine chipmunks, which can access burrows, would be at risk if they feed on 

strychnine bait (Anthony et al. 1984). Canid and raptor predators and scavengers could also be 

at risk from secondary exposures. However, the belowground applications would limit their 

exposure to poisoned pocket gophers making the risk of adverse effects low. In one study, pocket 

gophers that died after their burrows were hand-baited with strychnine bait died at least 15 cm 

underground (Ramey et al. 2002). Similarly, another field study found that all pocket gophers 

killed with strychnine died underground in their burrows (Nolte and Wagner 2001). In addition, the 

preference of not eating the gastrointestinal tract of prey, where strychnine and treated bait may 

be concentrated, reduces the risk of exposure to certain canid (Marsh et al. 1987) and raptor 

species (Anthony et al. 1984). Other nontarget animals directly poisoned by strychnine may 

commonly die aboveground, as was the case in one study looking at the effects on goldenmantled ground squirrels after treatment of gopher burrows (Anthony et al. 1984). Anthony et al. 

(1984) noted badgers killed two treated squirrels with no apparent secondary exposure effects. 
21

The treatment of ground squirrels with colocated burrowing owls did not cause death or 

reproductive effects to the owls, but adults did have reduced body weight, possibly from a 

reduction in available squirrels as a food source (James et al. 1990). 

Direct access to treated bait inside the burrow is negligible for most bird species. Insectivorous 

bird species and terrestrial amphibians are unlikely to receive significant exposure by eating 

insects exposed to strychnine (Stahl et al. 2004, Arjo et al. 2006). 

5.2.2 Aquatic Risk Characterization 

Aquatic risk to strychnine exposure is not expected based on the application method for 

strychnine and its environmental fate profile. Strychnine applications are made belowground 

eliminating drift and reducing the probability of significant runoff into water bodies containing 

aquatic vertebrates, invertebrates, and plants. The label states that the product should not be 

used in areas where surface water is present and applicators should also use caution if rain events 

around the time of baiting could cause runoff. In addition, strychnine solubility in water is below 

the range of effect concentrations for aquatic vertebrates and invertebrates. Strychnine also 

degrades and binds tightly to soil and sediment, reducing the potential for significant residues that 

would be available to most aquatic organisms. 

6 UNCERTAINTIES AND CUMULATIVE EFFECTS 

Uncertainties associated with the risk evaluation are discussed qualitatively in this section. 

Toxicity data is unavailable for all possible species that could be exposed to strychnine bait 

treatments. The use of a few species to represent the toxicity for an entire group of species has 

inherent uncertainties. Strychnine is highly toxic to numerous mammals, birds, and aquatic 

vertebrates, and our risk evaluation assumes high toxicity to all species within these animal 

groups. This section qualitatively evaluates the potential cumulative effects associated with 1) 

repeated worker exposures to strychnine, 2) co-exposures to other pesticides within the program 

with respect to their toxicity, and 3) exposures to other chemicals impacting the toxicity of 

strychnine. 

Repeated exposure to strychnine is not expected in the WS program because, with PPE, worker 

exposure to strychnine is not expected. The improper use of PPE could lead to accidental 

exposure, as described earlier. Exposure to a non-lethal dose of strychnine will not lead to a 

substantial accumulation in the body since strychnine is rapidly excreted (See Section 2.4.1 and 

3.1 for toxicokinetics and dose-response information). WS has not reported any accidental 

exposures for more than 20 years, suggesting a low probability of risk to workers. 

Strychnine inhibits glycine and causes convulsions. The WS Program does not use chemicals 

with the same mode of action, so the potential for co-exposure is negligible. The other rodenticides 

used in the WS Program, not necessarily for control of pocket gophers, are mostly zinc phosphide, 

anticoagulants, and aluminum phosphide fumigants. 

Cumulative effects involving other chemicals that impact strychnine toxicity are unlikely. WS does 

not mix strychnine with other chemicals or apply other chemicals during the same treatment 

period. Strychnine degrades in the environment and is not expected to persist. Zinc phosphide is 

also a labeled toxicant available to WS for the control of pocket gophers, but WS does not use 

both at the same location. The application of strychnine baits belowground also reduces its 
22

potential interaction with chemicals that may be applied above ground by other land managers 

for other purposes. Other land management agencies, such as USFS, may conduct pocket 

gopher control; however, this would not occur on the same lands treated by WS during the same 

treatment period. WS does not knowingly conduct treatments of species where others are 

conducting WDM using toxicants. 

In California, an average of 506 lbs. a.i. of strychnine was used for calendar years 2016-2018 by 

the public and agencies including WS (California Department of Pesticide Regulation 2021). Most 

of the applications made in California involved crops, landscaping, and turf. Applications to 

timberland and rangelands were minimal in comparison. WS also assists a lot with crops and 

landscaping, but additionally, WS works on lands surrounding runways at airports to reduce the 

gopher population that attracts predators that could become wildlife strike hazards for aircraft and 

damage underground electrical cables for runway and taxiway lighting. However, from FY16-

FY20 nationally, WS annually averaged the use of 0.5 pounds a.i. (0.1% of California use), which 

suggests that WS would not add significantly to the cumulative take of pocket gophers in California 

or nationally. 

7 SUMMARY 

Strychnine baits are used by WS to control pocket gophers. WS personnel either hand-bait pocket 

gopher burrows or use burrow builders to treat areas. Strychnine is a highly lethal acute toxicant 

for pocket gophers. Formulations contain 0.5% of strychnine a.i. WS personnel have not had any 

known exposures or exposed the public or pets to strychnine. Thus, it is assumed that the risk to 

human health from strychnine baits used by WS in WDM has been at the most minimal. WS 

applicators used an annual average of 98.9 pounds of baits (0.5 pounds a.i.) of five formulations 

of strychnine in 10 states to take 3,535 pocket gophers of four species from FY16 to FY20. Plains 

and northern pocket gophers were the two species (68%) taken most. WS applicators attempt to 

minimize nontarget take by treating burrows with recent mounds. WS did not record any nontarget 

species taken but believes that it has occurred, especially for other small rodents that use pocket 

gopher burrows or tunnels. 

This risk assessment evaluated the human health and ecological risk of strychnine under the WS 

use pattern for pocket gopher control. Although strychnine is highly toxic to humans, the risk to 

human health is low because ingestion, inhalation, and dermal exposure risks are slight for 

underground applications. WS applicators wear proper PPE according to the pesticide label 

requirements, reducing exposure to strychnine poisoning and lowering the risk of adverse effects. 

Label restrictions protect the public from exposure to the toxicant when used accordingly. 

Label restrictions, use patterns, environmental fate, and aquatic toxicity data for strychnine 

suggest low risk to aquatic species and their habitats. Strychnine uptake risks to terrestrial plants 

are also low. The highest risk is to nontarget small mammals that will consume the baits. 

Nontarget terrestrial vertebrates and invertebrates that consume treated pocket gophers or 

nontarget species taken could be killed from secondary exposure but are considered low risk. 

Target and nontarget species killed by strychnine typically die within the treated burrows reducing 

the chance for secondary risk. Tertiary risks, primarily from species that consume insects that 

scavenge animals that have succumbed to strychnine poisoning, appear to be minimal. 

Restrictions on all labels lower the chance of exposure and risks to humans and nontarget 

terrestrial species. 
23

It should be noted that WS has been using strychnine since all uses went belowground and have 

not had any known impacts on people or pets. WS acknowledges that nontargets have been 

taken, but this would be relatively few compared to targets. WS also does not use strychnine 

where a sensitive species may be taken and, therefore, believes that WS personnel have not 

impacted these species. 

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gopher populations on forest lands in the United States. Pages 59-70 In Pelz, H.J., D.P. 

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Okuno, I. 1994. Product chemistry - octanol/water partition coefficient of strychnine alkaloid. 

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Report. 56 pp. 

Oliver, D.P., R.S. Kookana, S.L. Rogers, R.G. Sinclair, G. Roberts, and C. Cook. 2000. Strychnine 

uptake by cereals and legumes grown in the glasshouse and field. Journal of Environmental 

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efficacy of three strychnine alkaloid bait concentrations for hand-baiting control of plains 

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squirrels. Journal of Raptor Research 23:147-151. 

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strychnine N-oxide on zebrafish embryos. Chinese Journal of Natural Medicines 12:760-767. 

9 PREPARERS 

9.1 APHIS-WS Methods Risk Assessment Committee 

Writers for “Use of Strychnine 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. Four years of 

experience in preparing environmental analyses in compliance with the National 

Environmental Policy Act. 

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-two years of service in APHIS Wildlife Services, including wildlife 

damage management operations and research. Expert in preparing environmental documents 

for WS programs to comply with the National Environmental Policy Act and the Endangered 

Species Act. For strychnine specifically, I have used strychnine in WDM and supervised 

employees that used it in their duties. 

Writer: Fan Wang-Cahill 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Environmental Health Specialist, Riverdale, MD 

Education: B.S. Biology and M.S. Hydrobiology - Jinan University, Guangzhou, China; Ph.D. 

Botany (Ultrastructure/Cell Biology) – Miami University 

Experience: Joined APHIS in 2012, preparing human health risk assessments and providing 

assistance on environmental compliance. Prior experience before joining APHIS includes 18 

years environmental consulting experience specializing in human health risk assessments for 

environmental contaminants at Superfund, Resource Conservation and Recovery Act 

(RCRA), and state-regulated contaminated facilities. 

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 
28

Experience: Eight 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 Strychnine in Wildlife Damage Management Risk 

Assessment”: 

Editor/Contributor: Ryan Wimberly 

Position: USDA-APHIS-WS, Operational Support Staff, Staff Wildlife Biologist, Madison, TN 

Education: BS Wildlife Management and Ecology – Northwest Missouri State University 

Experience: Special expertise in wildlife biology, ecology, and damage management. Seventeen 

years of service with APHIS Wildlife Services, including operations and research, 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. Expert in preparing environmental documents for WS 

programs to comply with the National Environmental Policy Act and the Endangered Species 

Act. 

Data Contributor: Joey Millison 

Position: USDA-APHIS-WS Information and Technology (IT), Junior Applications Developer 

Education: Information and Technology coursework from various sources 

Experience: Eleven years of experience in APHIS, WS Management Information System (MIS) 

Group. Retrieves WS field data from the MIS for writers, reviewers, and editors. 

9.2 Internal Reviewers 

USDA APHIS Wildlife Services 

Reviewer: Emily Ruell 

Position: USDA-APHIS-WS, NWRC: Registration Specialist, 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 MS 

Biology, University of Nevada, PhD Botany/Ecology University of Hawaii 

Experience: Five years of experience with WS NWRC preparing and reviewing vertebrate 

pesticide registration data submissions and other registration materials, and 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. 

Reviewer: Shelagh DeLiberto 

Position: USDA-APHIS-WS Operational Support Staff, Environmental Management Coordinator, 

Fort Collins, CO 

Education: BA Biology and Environmental Science – Ithaca College; MS Wildlife Biology – 

Colorado State University 

Experience: Nineteen years of service in APHIS conducting wildlife research. Six years of 

experience in preparing categorical exclusions and environmental analyses in compliance 

with the National Environmental Policy Act. 
29

9.3 Peer Review 

The Office of Management and Budget requires agencies to have peer review guidelines for 

scientific documents. The APHIS guidelines were followed to have "Use of Strychnine in Wildlife 

Damage Management" peer reviewed. WS worked with the Association of Fish and Wildlife 

Agencies to have experts review the documents. 

9.3.1 Peer Reviewers Selected by the Association of Fish and Wildlife Agencies 

University of Tennessee/Tennessee Wildlife Resources 

Louisiana Department of Wildlife and Fisheries 

Wildlife Veterinarian, Arizona Game and Fish Department 

9.3.2 Comments 

1. Comment: Louisiana black bears are no longer federally protected as a T&E species but 

are protected by the state.

Response: Louisiana black bears are a protected species in southeastern states and 

within the range of the plains pocket gopher in eastern Texas, western Louisiana, and 

southern Arkansas, recently delisted as a federal T&E species. Protected species are 

considered when putting out bait. The maximum rate, 2.5 pounds of bait per acre (5,670 

mg active ingredient) underground, minimizes the opportunity for nontarget animals such 

as bears to get a lethal dose. Bears are omnivores and would eat bait or dead pocket 

gophers and can dig easily to get to either. However, it would take more strychnine than 

is somewhat available. To extrapolate, a yearling cub weighing 50 pounds (23 kg) with an 

LD50, possibly similar to a red fox (70 mg/kg), would require 1,610 mg or eating 0.7 pounds 

of bait to reach the point where it is lethal for 50%. The method of broadcast (probes or 

burrow builders) would make that very difficult to achieve. These are always a concern for 

WS personnel, and they consider the potential for taking nontarget species. With that said, 

WS did not apply strychnine baits within the range of the Louisiana black bear from FY11-

FY20. 

2. Comment: I found, at least to my understanding, a notable discrepancy in the amount of 

bait applied by WS. In the executive summary it stated that WS applied 99 lbs. of bait 

between FY 16 and FY 20 but page 4 states that WS applied 266 lbs. annually. Also, on 

page 4 it states 1,41 oz., an obvious typographical error.

Response: While updating data from FY11-FY15 to FY16-FY20, some data was 

overlooked. All data in the text and tables has been verified to be accurate for the updated 

FY16-20 time period. 

3. Comment: Although mentioned, one concern would be the incidental take of predatory 

snakes. In areas where there are endangered snake species that would prey upon the 

target species, I would avoid use since it is difficult to quantify the incidental take. 

Response: We have included language in Section 2.1 regarding the potential hazard of 

strychnine to all T&E species (including snakes). Label restrictions require applicators to 
30

contact the USFWS to determine if strychnine can be used in a given area where T&E 

species are present. WS personnel contact the USFWS to identify sensitive species within 

any potential application area. We believe these requirements will prevent exposure to 

sensitive species, including snakes.

4. Comment: Given that there is some evidence of stream contamination during rains, I 

would recommend avoiding application during known “wet” seasons to avoid immediate 

flooding of tunnels and possible movement of baits out of the tunnels prior to consumption. 

Along these lines avoid applying before rainstorms etc. that may lead to flash flooding 

before baits are fully consumed.

Response: We have included language in Section 5.2.2, “Aquatic risk characterization,” 

referencing the label restrictions about using where surface water is present and limiting 

application when rain events could cause runoff at the treatment site. However, rain makes 

treatments less effective; therefore, WS personnel will wait for the appropriate days to 

treat a site. 

5. Comment: The individual components of the MRA appear complete with the exception of 

the lack of consideration of the risks to non-targets species and human health should 

product sold by USDA-WS be applied or used incorrectly. While the product is reported to 

have been sold to certified pesticide applicators, no mention is made of any monitoring or 

accountability for those applicators. Since the most effective method for reducing risk is 

application underground, and applicators distribute more than seven times the amount of 

strychnine that WS did, this could be a significant error in the risk calculation for public 

and non-target species exposure. In the past 2 years, I have identified strychnine as a 

cause of avian mortality in 5 events with more than 75 total individuals affected. Most were 

doves however red-winged blackbirds were affected in one event. An additional 5 events 

were identified between 2014 and 2020 (a cottontail and ground squirrel, a javelina, a 

killdeer, multiple bird species, and doves). The event with multiple bird species included a 

great-horned owl and a kestrel and it occurred a city park. The killdeer was found dead in 

association with a spill event. These event suggest that some licensed pesticide 

applicators do not follow label directions.

Response: When WS sells products to the public and certified pesticide applicators, it is 

considered technical assistance and outside the scope of this Risk Assessment. Certified 

Pesticide Applicators and the public are required to abide by the product labels without 

monitoring or supervision by WS. They can face consequences if they use the products 

illegally. Since strychnine baits are required by the label to be applied underground, the 

bait is likely illegally applied if birds are found dead due to strychnine. WS is not a 

regulatory agency. Any suspected violations of the Federal Insecticide, Fungicide, and 

Rodenticide Act by private applicators should be reported to the State Department of 

Agriculture. New labels for strychnine will likely make all strychnine baits Restricted-Use, 

including hand-baiting labels, which are not currently restricted use.

6. Comment: While I agree the impact to non-target species is likely minimal, it is not 

appropriate to assume that avoiding consumption of the gastrointestinal tract occurs in all 

predation and scavenging events.

Response: We reference several studies in Section 5.2.1 that indicate species avoid 

gastrointestinal tracts when consuming prey. We identify that these preferences reduce 

the risk of exposure but do not assume it does not occur. It should be noted that pocket 
31

gophers most always die belowground, minimizing risks to terrestrial species. Fossorial 

(underground dwelling) or semi-fossorial scavengers, such as weasels that use gopher 

tunnels for preying on gophers, have the highest possibility of being taken as nontarget 

species. These scavengers would have direct access to pocket gophers that died from 

strychnine baiting. Weasels would have to consume the gut contents to get a high dose 

of strychnine toxicant, and the gopher would have had to consume enough bait to have 

enough remaining. A northern pocket gopher (average weight 110 g) that consumed 8% 

of its body weight (9 g) would have 45 mg a.i. This likely would be enough to kill a longtailed weasel (average weight 7.5 oz.). It should be noted that pocket gophers are highly 

territorial and often do not tolerate other species in their tunnels except young. Therefore, 

it Is unlikely for several gophers to be found in a single burrow. Most scavengers would 

likely have to dig up many pocket gophers to scavenge a lethal dose of strychnine.

7. Comment: The methodology of estimating the take of pocket gophers based on estimated 

consumption seems as though it has the potential to be grossly inaccurate although I do 

not have a better method. Ultimately, as long as the estimate corresponds with an 

appropriate decline in sightings/new burrows/damage I suppose it is accurate enough. 

Other methodologies in the document seem appropriate to arrive at the desired 

conclusions.

Response: This method of estimating take is standard practice. USEPA lays out a 

framework for this type of analysis in their Wildlife Exposure Factors Handbook (USEPA 

1993; EPA/600/R-93/187). Although gophers are not a species discussed in this 

handbook, a goal of the handbook is to “foster a consistent approach to wildlife exposure 

and risk assessments.” This handbook identifies body weight, food ingestion rate, and 

home range size, among other factors for each species, similar to what has been done in 

this risk assessment.

8. Comment: See Warnock and Schwarzbach 1995 Incidental kill of Dunlin and Killdeer by 

Strychnine, J Wildlife Dis 31:566-569. 

Response: This reference indicates that shorebirds were exposed to strychnine baits 

due to “strychnine-laced wheat seeds broadcast on a field for rodent control” prior to 1995. 

Broadcast and aboveground application of strychnine baits are no longer label-approved 

application methods. This reference is not within the scope of our current use patterns. 

Comments received not requiring a response. 

1. Comment: The document was thorough and incorporated all of the pertinent information 

to assess delivery methods, efficacy estimates, by-kill data, and human and animal safety. 

2. Comment: All of the components/ subjects of the document are complete to my 

satisfaction. 

3. Comment: Common sense methodologies are used to reduce non-target impacts and I 

am satisfied with the extent of the methodologies used to protect humans and other 

animals for unintended strychnine exposure. 

4. Comment: The references look appropriate to support the statements in the document. 
32

5. Comment: Overall a good document with noted exceptions. I support this risk 

assessment. 

Peer reviewers provided editorial comments on the manuscript. These were appreciated and 

incorporated into the final document. 

 
33

APPENDIX 1. WS Strychnine Use for Pocket Gophers for FY11-FY15. 

For comparison, take tables and strychnine used for FY11-FY15 are given for comparison. These 

compare with Tables 2 and 4 in Section 1. 

Table 1a. The annual average number of target pocket gophers killed with strychnine by APHIS-WS in 

WDM activities for FY11 to FY15 throughout the United States and the pounds of strychnine used. No 

known nontarget take occurred in this time. 

ANNUAL AVERAGE SPECIES KILLED WITH STRYCHNINE AND FORMULATIONS USED 

Species Target Lbs. Used % of Spp. Take States Used 

Yellow-faced Pocket Gopher 349 17 4% TX 

Plains Pocket Gopher 447 12 5% CO KS MN NE TX 

Botta’s Pocket Gopher 4,887 185 56% AZ UT 

Camas Pocket Gopher 172 6 2% OR 

Northern Pocket Gopher 2,828 47 33% CO ND OR UT WA 

Avg. Ann. Take (5T – 5 spp.) 8,683* 267* 

Formulation Target Lbs. Used % of Pounds 

Used

States Used 

Omega Gopher Grain Bait 44 1 0.4% OR

Strychnine Milo – Burrow Builder 3,033 98 42% AZ UT WA 

Strychnine Milo – Hand Bait 1,658 33 14% CO KS MN NE ND OR TX UT 

Strychnine Oats – Burrow Builder 2,036 91 26% TX UT WA

Strychnine Oats – Hand Bait 1,909 43 18% MN OR TX WA 

Avg. Annual Take and Use 8,680* 266* 

If take was not estimated, numbers of targets taken was estimated at about 8 per pound of bait for burrow builders or 

1 per ounce for hand baiting. 

* Rounding errors gave different numbers for summing data. 

Table 1b. The annual average number of target pocket gophers killed with strychnine and ounces used in 

each State by APHIS-WS in WDM activities for FY11 to FY15. No known nontarget take occurred in this 

time. 

ANNUAL AVERAGE STRYCHNINE USED WITH TAKE BY STATE FOR FY11-FY15 

State 

Strychnine Burrow Builder Baits Strychnine Hand Baits Omega Baits 

Milo Oats Milo Oats Grain 

Target Oz. Target Oz. Target Oz. Target Oz. Target Oz. 

Arizona 130 48 0 0 0 0 0 0 0 0

Colorado 0 0 0 0 342 100 0 0 0 0 

Kansas 0 0 0 0 19 8 0 0 0 0 

Minnesota 0 0 0 0 15 6 7 3 0 0 

Nebraska 0 0 0 0 329 143 0 0 0 0 

North Dakota 0 0 0 0 2 0.4 0 0 0 0 

Oregon 0 0 0 0 347 91 175 102 44 12 

Texas 0 0 105 80 18 8 244 188 0 0 

Utah 2,595 1,440 1,931 1,383 213 79 236 64 0 0 

Washington 309 81 0 0 374 98 1,247 328 0 0 

Total 3,034 1,569 2,036 1,463 1,659 533 1,909 685 44 12 

States 3 2 9 5 1 

If take was not estimated, numbers of targets taken was estimated based on parameters from Table 1a.