1

OFFICE OF 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

PC Codes: 076901

DP Barcode: D428698 

Date: November 13, 2015

MEMORANDUM 

SUBJECT: Problem Formulation in Support of the Registration Review for Strychnine 

(PC Code 076901) 

FROM: Nicholas Mastrota, Ph.D., Wildlife Biologist 

 

James Wolf, Ph.D., Environmental Scientist 

Rosanna Louie-Juzwiak, Risk Assessment Process Leader 

Environmental Risk Branch 3 

Environmental Fate and Effects Division (7507P) 

REVIEWED James Hetrick, Ph.D., Senior Advisor 

BY: Environmental Risk Branch 3 

 Environmental Fate and Effects Division (7507P) 

 

THROUGH: Dana Spatz, Branch Chief 

Environmental Risk Branch 3 

 Environmental Fate and Effects Division (7507P) 

TO: Kevin Costello, Branch Chief 

 Cathryn Britton, Team Leader 

Sue Bartow, Chemical Review Manager 

Risk Management and Implementation Branch 2 

Pesticide Re-Evaluation Division (7508P) 

1. Executive summary 

The Environmental Fate and Effects Division (EFED) has completed the problem formulation in 

support of the registration review of rodenticide uses of strychnine (PC Code 076901). Use of 

strychnine has not changed, and no new environmental fate or ecotoxicity testing has been 

submitted, since this rodenticide was previously assessed for reregistration in 1996. However, 

significant new information on the ecological risk of strychnine have become available in papers 

that have been published since 1996. In addition, more recent incident data are available that 

provide information on exposure and potential risk to non-target wildlife from current uses of 

strychnine. Of note are recent incidents of mortality involving the endangered San Joaquin kit 
2

fox (Vulpes macrotis mutica). These incidents also raise concern about possible secondary 

poisoning of other mammalian predators and scavengers. 

2. Nature of Chemical Stressor 

Strychnine is an alkaloid compound that has been used for vertebrate pest control for centuries. 

Today the use of the poison in the United States is restricted to control of the pocket gopher 

(Order Rodentia, Family Geomyidae), except in the state of Nevada where a special local need 

registration also allows its use to control the yellow-bellied marmot (Marmota flaviventris) and 

three species of ground squirrel. 

Strychnine is a powerful convulsant. Strychnine acts as a selective competitive antagonist to 

block the inhibitory effects of glycine at all glycine receptors. The convulsant action of 

strychnine results from interference with postsynaptic inhibition normally mediated by glycine, 

causing excessive excitation of muscles that manifests itself as convulsions. Tetanic convulsions 

caused by a lethal dose of strychnine can lead to rapid asphyxiation and death. Death in 

mammals occurs within an hour after exposure (Nolte and Wagner, 2001; MRID 43330301). 

Chemical names, structures, and other identifiers of strychnine are given in Table 1. 

Table 1. Chemical Nomenclature and Structure of Strychnine. 

Common Name Strychnine 

Synonyms Strychnidin-10-one 

Strychnine alkaloid 

CAS Reg. No. 57-24-9 

Empirical Formula C21H22N2O2 

SMILE O=C1N2c3c(C45C6N(CC4)CC4=CCOC(C(C4C6)C25)C1)cccc3

Molecular Structure 

3. Use Characterization 

Strychnine is registered for use as an active ingredient in rodenticide baits. Currently, all but one 

of the registered products containing strychnine are solid baits for controlling pocket gophers 

(Order Rodentia, Family Geomyidae). Numerous brands of strychnine gopher bait products are 

available, but they are similar in that they are solid bait products containing 0.5% strychnine that 
3

are designed to be placed within pocket gopher burrows. Above ground application of these 

products is prohibited. The bait products must be placed either inside underground runways of 

existing gopher burrows, or into artificial burrows that the gophers are expected to enter. 

Applications to existing burrow are made by digging an access hole, which is then sealed with 

soil after the bait is placed, or by using a probe to deposit the bait into the burrow. When used in 

artificial burrows, a piece of tractor-drawn ground equipment called a “burrow builder” is used. 

The burrow builder creates underground tunnels that intersect the gopher’s existing burrows, 

while dropping measured amount of bait into the tunnels. The baits may be used to control 

pocket gophers in various settings, including residential lawns, agricultural cropland, pastures, 

forestry land, and rangeland. Many of the labels also include “non-agricultural area” as a use site, 

and therefore may be used anywhere. Bait products may be used to control any of the numerous 

species of pocket gophers that occur in the United States (Thomomys sp. and Geomys sp.). 

Species of pocket gopher are distributed throughout central and western states, excluding Alaska 

and Hawaii, and in three southeastern states, Florida, Georgia, and Alabama. 

Up to 3 pounds of strychnine bait product, or 0.015 pound of strychnine active ingredient, may 

be applied per acre. The maximum rate for products designed for residential use (EPA Reg No. 

36029-1 and 909-2) range from 0.005 to 0.006 lb a.i./A. The maximum rate for other strychnine 

bait products range from 0.005 to 0.015 lb a.i./A. 

In addition to pocket gophers, strychnine may also be used under a special local need (24C) 

registration for control of yellow-bellied marmots (Marmota flaviventris), Richardson ground 

squirrel (Spermophilus richardsonil), Beldin’s ground squirrel (S. beldingi), and Piute ground 

squirrel (S. mollis) in the state of Nevada. This is a restricted use product containing 3.2% 

strychnine alkaloid paste registered by the Nevada Department of Agriculture (EPA Reg. No. 

NV830009). This product may also be used to control four species of pocket gopher that live in 

Nevada (Thomomys sp.). The maximum use rate is 0.032 lb a.i./A. 

EFED’s 2009 risk assessment for certain threatened and endangered species in California 

(USEPA, 2009) included a summary of strychnine use in California reported by the California 

Department of Pesticide Registration (CDPR) in their Pesticide Use Reporting (PUR) data from 

1999 to 2006. These data indicate that, in California, approximately 60% of the total pounds of 

strychnine applied was applied in nonagricultural sites. The nonagricultural sites included areas 

around domestic dwellings and other structures, landscape and recreational areas, and other 

“non-agriculture uncultivated areas/soils” such as ditch banks and right-of-ways. Agricultural 

sites in California with the greatest use of strychnine were grape vineyards, fruit and nut 

orchards, and alfalfa. Published papers indicate that strychnine bait is also commonly used to 

control pocket gophers in forestry sites in the Pacific Northwest, in particular in areas being 

reforested with conifer tree seedlings (Nolte and Wagner, 2001, El Hani et al., 2002, Arjo and 

Bryson, 2007). 

4. Conclusions from Previous Risk Assessments 

EFED’s most recent ecological risk assessment for strychnine was completed in 2009 to assess 

risk to three species inhabiting California that are federally listed as threatened or endangered 
4

species: the California red-legged frog (Rana aurora draytonii), California tiger salamander 

(Ambystoma californiense), and the San Joaquin kit fox (Vulpes macrotis mutica). The 

assessment made a “Likely to Adversely Affect” conclusion for all three species because of 

potential direct and indirect adverse effects from strychnine exposure due to labeled uses to 

control pocket gophers. The assessment concluded that the San Joaquin kit fox could be directly 

affected by oral exposure from consuming poisoned pocket gophers that it dug out of its burrow 

or found above ground, or by consuming soil invertebrates that have been contaminated by the 

strychnine bait. It predicted that a gopher could carry enough strychnine bait in its cheek pouches 

to be lethal to a kit fox if it were consumed. The assessment concluded that the California redlegged frog and the California tiger salamander also could be directly adversely affected, 

primarily from dermal exposure to strychnine if they were to use a strychnine-treated gopher 

burrow for shelter. It also concluded possible risk of secondary poisoning from these species 

consuming soil invertebrates that had been contaminated by the strychnine bait. Finally, the 

assessment concluded that indirect adverse effects were possible to all three species because use 

of strychnine bait could cause reduce prey abundance, and could indirectly adversely affect the 

red-legged frog and tiger salamander because of removal of gopher burrows which these species 

may use for shelter. 

Prior to this endangered species assessment, EFED conducted an ecological risk assessment for 

the Reregistration Eligibility Decision (RED)1

 for strychnine in March, 1996 (D216629, USEPA, 

1996). This assessment concluded that risk to plants and aquatic animals would be negligible, but 

non-target terrestrial animals could be at risk if they were able to access the strychnine bait. In 

addition, the assessment found that residues in the gastrointestinal tract of poisoned pocket 

gophers would pose a risk to predator and scavenger wildlife if they were to eat the carcasses. 

They also noted that studies have found that application of strychnine bait using mechanical 

burrow-builder equipment may pose a hazard to non-target wildlife because of the spillage of 

bait that occurs when the burrow-builder is lifted from the soil at the end of the rows. However, 

prior to the RED, additional instructions were added for burrow-builders stating that the 

applicator must pick up spilled bait to mitigate the risk from the burrow builder application. 

The RED concluded that prohibition of above-ground application of bait effectively reduces 

exposure to non-target wildlife, and that the use of strychnine to control the pocket gophers, 

according to the label directions, does not pose an unreasonable risk to non-target species, 

although it may cause harm to certain threatened or endangered terrestrial wildlife species. To 

mitigate risk to endangered species, the following warning statement was recommended for all 

labels of strychnine bait products: 

“ENDANGERED SPECIES CONSIDERATIONS: 

 

1

 USEPA. 1996. Reregistration Eligibility Decision (RED) Strychnine. United States Environmental Protection 

Agency, Washington, DC. EPA 738-R96-33. Available at:

www.epa.gov/oppsrrd1/reregistration/REDs/3133.pdf 
5

The killing of a member of an endangered species during strychnine baiting operations may 

result in a fine under the Endangered Species Act. Before baiting, the user is advised to contact 

the Regional U.S. Fish and Wildlife Service (Endangered Species Specialist) of the local Fish 

and Game Office for specific information on endangered species.” 

5. Environmental Fate and Transport 

Hydrolysis and soil photolysis are not significant transformation pathways for strychnine. 

Aerobic soil metabolism studies demonstrate a half-life of approximately 1 month. Batch 

adsorption/desorption data demonstrate strychnine’s strong binding to various soils. With the 

current use pattern of applying strychnine into below-ground burrows, strychnine is not expected 

to reach ground or surface water. 

Strychnine does not hydrolyze at pH 5, 7, or 9 (MRID 41122301). Strychnine is stable to soil 

photolysis (MRID 42973401), but degrades in soil under aerobic conditions with a half-life of 

26.7 days (MRIDs 42234201 and 43407601). The data indicate that strychnine is immobile; the 

parent is adsorbed to organic matter and clay. In the mobility study, unaged strychnine had 

Freundlich Kads in loamy sand, sandy loam, loam, and sandy clay loam soils of 39.79, 94.65, 

118.87, and 168.97 mL/g, respectively; adsorption increased with increasing cation exchange 

capacity (CEC). Desorption (Kdes) values were 55.0, 89.4, 114.6, and 146.1 mL/g for the loamy 

sand, the sandy loam, the loam, and the sandy clay loam soils, respectively (MRID 42366501).

Table 2 lists the environmental fate properties of strychnine. 

Table 2. Summary of Strychnine Environmental Fate Properties 

Study Value MRID # Study Status 

Solubility (20°C) 11.5 mg/L 

Dissociation Constant (pKa) 8.26 

Vapor Pressure (20°C) 2.9 x 10-9 mm Hg 

Henry’s Law Constant 6.0 x 10-14 atm 

m3

/mol 

Octanol/Water Partitioning 

Constant (pKow) 

4.0 at pH 7 

114.0 at pH 9 

Hydrolysis Stable 41122301 Acceptable 

Soil Photolysis Stable 42973401 Acceptable 

Aerobic Soil Metabolism 26.7 days 

42234201 

and 

43407601 

Supplemental 

Kd-ads / Kd-des 39-168 mL/g 42366501 Acceptable 
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6. Ecotoxicity 

a. Toxicity to Terrestrial Animals 

Available data on the toxicity of strychnine to birds and mammals are presented in Table 3. Data 

from studies that were submitted to the EPA by pesticide registrants are supplemented by acute 

wildlife toxicity data from testing conducted by the Environmental Contaminant Evaluation 

Program of the U.S. Fish and Wildlife (summarized in Hudson et al., 1984) and by the Denver 

Wildlife Research Center of the U.S. Department of Agriculture. Acute oral toxicity studies 

conducted by the U.S. Fish and Wildlife were considered acceptable for use in risk assessment 

when they were conducted using at least 20 test birds. To be consistent with the 850.2100 test 

guideline, only acute oral studies conducted with adult birds are classified as acceptable. For 

each study type, the lowest toxicity value derived from an acceptable study was selected to be 

used in the quantitative risk assessment. These values are shown in bold font in Table 3. 

Toxicity data classifies strychnine as very highly toxic to birds and mammals with acute oral 

exposure, and highly toxic to birds with subacute dietary exposure. These data indicate that a 

broad range of birds and mammals are highly sensitive to strychnine, including passerines, 

waterfowl, corvids, raptors, rodents, canids, and mustelids, whereas quail appear to be less 

sensitive. Acute exposure of rats to lethal doses caused rapid toxicity, with convulsions and death 

occurring within one hour after treatment (MRID 43330301). With chronic exposure, the mallard 

was the most sensitive test species, with reduced growth observed at dietary concentrations as 

low as 68.9 mg a.i./kg-diet (NOAEC = 33.2 mg a.i./kg-diet). Reduced egg production was 

observed in mallards at 141 mg a.i./kg-diet. 

Table 3. Summary of Available Data on the Toxicity of Strychnine to Terrestrial Animals 

Study Type Surrogate Species Toxicity Value1 

(95% C.I) Effects 

Author, 

Date 

MRID 

Acceptability

Birds 

Acute Avian 

House sparrow 

(Passer domesticus) 

LD50 = 4.18 mg/kg-bw 

(3.18-5.50) Mortality 

00160000 

(Hudson et 

al., 1984) 

Acceptable 

Japanese quail 

(Coturnix japonica) 

LD50 = 22.6 mg/kg-bw 

(11.9-42.9) Mortality 

00160000 

(Hudson et 

al., 1984) 

Acceptable 

Black-billed magpie 

(Pica hudsonia) 

LD50 = 2.84 mg/kg-bw 

(1.0-12.1) Mortality 40815201 Supplemental

Mallard, 36 hr 

(Anus platyrhynchos) 

LD50 = 2.62 mg/kg-bw 

(1.94-3.55) Mortality 

00160000 

(Hudson et 

al., 1984) 

Supplemental

Mallard, 1 week LD50 = 2.00 mg/kg-bw 

(1.51-2.65) Mortality 

00160000 

(Hudson et 

al., 1984) 

Supplemental

Mallard, 6 months LD50 = 2.27 mg/kg-bw 

(1.26-4.11) Mortality 00160000 Acceptable 
7

(Hudson et 

al., 1984) 

Subacute dietary 

Avian 

American kestrel 

(Falco sparverius) 

LC50 = 237 mg/kg-diet 

(188-363) Mortality 40815201 Supplemental

Black-billed magpie LC50 = 98.7 mg/kg-diet 

(65-129) Mortality 40815201 Supplemental

Bobwhite 

LC50 = 3536 mg/kg-diet 

(N.R.) 

NOAEC: 1250 ppm 

Internal bleeding, 

pale liver, gaseous 

intestines, loss of 

reflexes, 

immobility, death 

41322602 Acceptable 

Mallard 

LC50 = 212 mg/kg-diet 

(N.R.) 

NOAEC: 78 ppm 

Lethargy, loss of 

reflexes, muscle 

spasms, death 

41322601 Acceptable 

Chronic Avian 

Bobwhite 

NOAEC: 1113 mg/kgdiet 

LOAEC: > 1113 mg/kgdiet 

None 42716801 Acceptable 

Mallard 

NOAEC: 33.2 mg/kgdiet 

LOAEC: 68.9 mg/kgdiet 

Chick body weight 42716802 Acceptable 

Mammals 

Acute oral 

mammalian, 

TGAI 

Laboratory rat 

(Rattus norvegicus)

Females: 

LD50 = 2.2 mg/kg-bw 

Males: 

LD50 = 6.4 mg/kg-bw 

Mortality 40908901, 

41210701 Supplemental

Desert kit fox Vulpes 

macrotis arsipus LD50 = 0.75 mg/kg-bw Mortality 

Schitoskey, 

1975 

E35428 

Supplemental

Acute oral 

mammalian, 

0.5% bait 

(Wilco Gopher 

Getter Type 1 

Bait and Wilco 

Gopher Getter 

AG Bait) 

Laboratory rat 

(Rattus norvegicus)

Females: 

LD50: 2.9-8.7 mg/kg-bw 

Males: 

LD50 = 15.2 mg/kg-bw 

Combined: 

LD50 = 7.84 mg/kg-bw 

(4.36-13.6)

Mortality 43330301 Supplemental

Subacute dietary 

mammalian 

Red fox 

(Vulpes fulva) 

LC50 = 64 mg/kg-diet 

(37-113) Mortality 40296503 Supplemental

European ferret 

(Mustela putorius) 

LC50 = 198 mg/kg-diet 

(148-262) Mortality 40296502 Supplemental

1

 All toxicity values are expressed in terms of active ingredient. Values shown in bold were selected to be 

used in the ecological risk assessment for strychnine. 
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Terrestrial Field Studies 

Numerous field studies have been conducted to investigate the primary and secondary poisoning 

hazards to non-target wildlife associated with rodenticide use of strychnine. Table 4 summarizes 

papers available in the open literature that report on these field studies. The majority of studies 

evaluate risk to non-target small mammals, and secondary risk to scavengers and predators, 

associated with below-ground baiting to control pocket gophers in reforestation areas in the 

northwestern United States (Fagerstone et al., 1980; Anthony et al., 1984; Nolte and Wagner, 

2001; El Hani et al., 2002). All of these studies have shown that below-ground placement of 

strychnine bait in pocket gopher burrows result in some mortality to non-target small mammals, 

although the losses usually had little or no long-term effects on small mammal populations. 

These studies also indicate that some risk of secondary exposure exists because some of the 

poisoned small mammals, in particular non-target species, will die above ground, and because 

some predators, such as weasels, may enter into the burrows to scavenge pocket gopher 

carcasses. 

Arjo et al., 2006 evaluated residues in strychnine of insects feeding on carcasses. A risk 

assessment they conducted on insectivorous wildlife found little tertiary risk from consumption 

of strychnine-contaminated insects. Finally, Proulx and MacKenzie (2010) and Proulx (2011) 

studied the hazard to non-target wildlife associated with use of strychnine to control the 

Richardson ground squirrel in pastures and alfalfa fields in Saskatchewan. These studies showed 

similar conclusions as discussed in Arjo at al., 2006, with potential secondary exposures to 

poisoned mammals that died above ground, as well as to burrowing animals such as burrowing 

owls. 

Table 4. Field studies published in the open literature on the hazard of rodenticide use of 

strychnine to non-target wildlife 

Title	 Use	Studied Highlights	of	Paper Reference

Hazards	to	small	mammals	

associated	with	underground	

strychnine	baiting	for	pocket	

gophers	

Pocket	gopher	

control	in	pine	

reforestation	

areas	

● 		Field	study	on	mortality	and	

population	impacts	to	small	

mammals.	

●			Discussion	of	primary	and	

secondary	hazards	

Fagerstone	et	

al.,	1980	

Hazards	to	golden‐mantled	

ground	squirrels	and	associated	

secondary	hazard	potential	

from	strychnine	for	forest	

secondary	pocket	gophers	

Pocket	gopher	

control	in	pine	

plantation	

● 		Laboratory	toxicity	studies	with	

the	golden‐mantled	ground	

squirrel.	

●			Field	study	on	mortality	and	

population	impacts	to	the	golden‐

mantled	ground	squirrels.	

●			Secondary	poisoning	feeding	

study	with	the	mink,	hawks	and	

owls.	

Anthony	et	al.,	

1984	

Non‐target	impacts	of	

strychnine	baiting	to	reduce	

Pocket	gopher	

control	in	

● 		Field	study	on	mortality	and	

population	impacts	to	golden	

Nolte and 

Wagner, 2001 
9

pocket	gopher	populations	of	

forest	lands	in	the	United	States	

and

Response	of	non‐target	species	

to	underground	strychnine	

baiting	for	pocket	gopher	in	

southwest	Oregon	

reforestation	

areas

mantled	ground	squirrels	and	

yellow	pine	chipmunks.	

●			Observations	that	poisoned	

pocket	gophers	die	mostly	

underground.	

●			Discussion	of	secondary	

poisoning	hazard	to	weasels.

and

El Hani et al. 

2002 

Potential non-target risks from 

strychnine-containing rodent 

carcasses 

Pocket	gopher	

control	in	

reforestation	

areas

● 		Evaluated	removal	of	small	

mammal	carcasses	by	scavengers	

and	insects.	

●			Measurement	of	strychnine	

concentrations	in	insects	that	feed	

on	carcasses	

●			Assessment	of	risk	from	tertiary	

exposure	to	insectivores	feeding	

on	insects	that	feed	on	carcasses.	

Arjo et al. 2006 

Strychnine for the control of 

Richardson’s ground squirrels: 

efficiency and selectivity issues 

and

Field evidence of non-target and 

secondary poisoning by 

strychnine and chlorophacinone 

used to control Richardson’s 

ground squirrels in southwest 

Saskatchewan 

Richardson’s 

ground squirrel 

control in 

grasslands and 

alfalfa fields 

● 		Counted	Richardson’s ground 

squirrels found dead on surface 

following strychnine baiting in 

Canada. 

●			Monitoring	of	mortality	of	non‐

target	wildlife.

●			Discussion	of	secondary	hazard	

to	burrowing	owls.

Proulx and 

MacKenzie, 

2010 

and

Proulx, 2011 

In addition to these field studies results reported in the open literature, six field studies have been 

submitted to the EPA by registrants in support of strychnine registration (Appendix A). These 

studies have been reviewed and discussed in previous risk assessments. 

A field efficacy study conducted with strychnine baits (0.5, 1.15, and 1.8 % w:w) to control the 

Valley pocket gopher (Thomomys bottae) was assessed (MRID# 42488601; USEPA, 1994). 

Strychnine residues were found in the muscle tissue of the gophers; the mean was approximately 

0.5 ppm and ranged as high as 5.4 ppm. It was also found in the gastrointestinal tract with a 

mean of approximately 5 ppm and as high as 35.8 ppm. Three dead vertebrates were found: a 

horned lark (Eremophila alpestris), a brewer’s blackbird (Euphagus cyanocephalus) and a 

striped skunk (Mephitis mephitis). Strychnine residues in the horned lark were 0.3491 and 1.6133 

ppm for the muscle tissue and gastrointestinal tract, respectively and, for the brewer's blackbird 

they were 0.5606 and 23.3283 ppm for the muscle tissue and gastrointestinal tract, respectively. 

The skunk was not analyzed for strychnine residues. This study confirmed the possibility of 

secondary exposure (i.e., dead gophers were preyed upon by red-tailed hawks) and that direct 

exposure occurs to birds. 
10

b. Toxicity to Aquatic Animals 

Aquatic toxicity data on strychnine are limited to acute toxicity to freshwater fish and 

crustaceans. These tests classify strychnine as highly toxic to fish, and slightly to moderately 

toxic to aquatic invertebrates. 

Table 5. Summary of Available Data on the Toxicity of Strychnine to Aquatic Animals 

Study Type Surrogate Species Toxicity Value1

LC50/EC50 (95% C.I.) MRID Acceptability 

Freshwater fish and aquatic-phase amphibians 

Acute Rainbow· trout 

(Oncorhynchus mykiss)

2.3 mg/L 

(1.7-3.2) 41126501 Acceptable 

Bluegill sunfish 

(Lepomis macrochirus)

0.76 mg/L 

(0.61-0.96) 41126502 Acceptable 

Chronic Waived -- -- -- 

Freshwater invertebrate 

Acute Waterflea 

(Daphnia magna)

10.6 mg/L 

(9.0-12.4) 41126503 Acceptable 

1 All toxicity values are expressed in terms of active ingredient.

c. Toxicity to Plants 

No data are currently available on the toxicity of strychnine to terrestrial or aquatic plants. 

d. Ecological Incidents 

A query of the Ecological Incident Information System (EIIS, version 2.1) was completed in 

August of 2015 to identify ecological incident of adverse field effects associated with the 

rodenticide use of strychnine. There were 85 incidents in this database that were attributed to 

exposure to strychnine. The only incident involving aquatic organisms was an incident of fish 

dying in a reservoir from strychnine exposure (B0000-227-002). This incident was caused by 

intentional fouling of a reservoir with strychnine, and therefore does not reflect exposure from 

legal rodenticide use. 

The remaining 84 reported incidents involved mortality of non-target wildlife. The wildlife 

incidents that occurred prior to April 1988 are less relevant for assessing current use of 

strychnine because, prior to that date, above-ground uses of strychnine bait were permitted, and 

these uses were likely to have caused much more exposure to non-target species than the current 

below-ground uses. In addition, uses of strychnine to control other vertebrate pests besides 

pocket gophers were permitted during these early years but have since been cancelled. Of the 38 

reported incidents occurring since April 1988, 31 are either associated with a registered 

rodenticide use of strychnine, or the legality of the use could not be determined. These incidents, 

which are summarized in Table 6, will be the primary set of incidents used to evaluate non-target 

wildlife risks associated with current uses of strychnine. Reported incidents occurring since April 

1988 that have been identified as being the result of illegal use of strychnine are listed in Table 7. 
11

Further evaluation of these incidents listed in Table 6 may identify additional incidents that are 

associated with misuse of strychnine. 

Table 6. Wildlife Mortality Incidents Attributed to Strychnine Poisoning, Excluding 

Known Misuses, Occurring since 1988 

Incident ID Location Date Legality Certainty Species No. 

Dead 

I013810-022 San Diego, 

CA 3/15/1989 Unknown Probable Blackbird 1

I007651-002 

I013810-019 Yolo, CA 12/11/1990 Unknown Highly 

Probable 

Dunlin (36), 

killdeer (2) 38

I005525-002 Siskiyou, CA 1/29/1991 Unknown Probable Goose 6

I005525-003 Fresno, CA 2/5/1991 Unknown Highly 

Probable 

Sparrow (2), 

dove(1) 3

I003351-022 El Dorado, CA 6/23/1994 Unknown Highly 

Probable 

Canada Goose 

(32), California 

gull (1), mallard 

(1) 

34

I003351-023 Fresno, CA 7/7/1994 Unknown Highly 

Probable Goose (17) 17

I004993-007 

San 

Bernardino, 

CA 

5/30/1996 Unknown Probable Mallard (30) 30

I010002-003 Marin, CA 2/3/1999 Unknown Highly 

Probable Rock dove 17

I010002-016 Marin, CA 4/30/1999 Unknown Highly 

Probable Rock dove 4

I010002-023 

I010002-024 Marin, CA 9/23/1999-9/30/1999 Unknown Highly Probable Rock dove 11

I018980-021 Washburn, WI 4/1/2000 Unknown Highly 

Probable 

Brewer's 

blackbird (10), 

brown-headed 

cowbird (10), 

red-winged 

blackbird (10) 

30

I020627-027 Franklin, WA 5/19/2001 Unknown Highly 

Probable Dog 

I018980-016 Hall, HA 6/15/2002 Unknown Highly 

Probable Mallard 22

I018980-019 Sandoval, NM 9/26/2005 Unknown Highly 

Probable 

American coot, 

mallard 30

I019703-002 Maricopa, AZ 10/12/2007 Unknown Possible 

Dove (1), 

mourning dove 

(2), house 

sparrow (3), 

unidentified 

species (52) 

58

I019703-009 Maricopa, AZ 1/30/2008 Unknown Highly 

Probable 

Inca dove, 

mourning dove, 

pigeon 

20

I020995-002 Rice, CA 12/16/2008 Unknown Possible 

Canada goose 

(55), goose 

(47), mallard 

190
12

(1), whitefronted goose 

(87) 

I023104-001 Kern, CA 11/1/2010 Unknown Highly 

Probable 

San Joaquin Kit 

Fox 4

I023104-002 Kern, CA 1/10/2011 Registered 

Use 

Highly 

Probable 

San Joaquin Kit 

Fox 1

I023104-003 Kern, CA 1/27/2011 Registered 

Use 

Highly 

Probable 

San Joaquin Kit 

Fox 1

I023104-004 Kern, CA 1/31/2011 Registered 

Use 

Highly 

Probable 

San Joaquin Kit 

Fox 1

I024443-014 San Diego, 

CA 6/15/2011 Unknown Highly Probable Duck approx. 30

I024821-002 Kern, CA 6/27/2011 Unknown Highly 

Probable 

San Joaquin Kit 

Fox 1

I025049-001 

I026953-008 McClain, OK 3/3/2013 Unknown Highly Probable 

Meadowlark, 

mourning dove, 

red-winged 

blackbird 

80-100

I025151-001 Riverside, CA 3/3/2013 Unknown Highly 

Probable Goose Several

I026953-011 Midland, TX 7/12/2013 Unknown Highly 

Probable Dove 

I026953-001 Maricopa, AZ 1/15/2014 Unknown Highly 

Probable 

Dove, greattailed grackle, 

Inca dove, 

mourning dove, 

pigeon 

18

I026174-001 

I026953-002 El Dorado, CA 1/28/2014 Unknown Highly Probable 

Brewer's 

blackbird over 30

I026261-001 Trinity, CA 2/10/2014 Unknown Highly 

Probable Dog 1

I026953-005 Ector, TX 10/22/2014 Unknown Highly 

Probable 

White-winged 

dove 6

I024528-029 

I025150-007 Kern, CA 5/30/2012 Unknown Highly 

Probable 

Mourning dove 

(many), 

sparrow (few), 

dog (1), raptor 

(1) 

30-50

Table 7. Wildlife Mortality Incidents Attributed to Misuse of Strychnine Occurring since 

1988 

Incident ID Location Start Date Certainty Species No. 

Dead 

I001566-002 NE 10/1/1994 Probable Sparrow (285), owl (2), 

blue jay (1) 288

I006925-001 Wise, VA 3/11/1998 Highly 

Probable Crow 4

I011233-001 Henderson, 

TX 6/1/2001 Probable Birds (several), dog (1) Several

I017034-001 Tuolumne, CA 1/11/2006 Probable Red-winged blackbird 

(6), birds (>100) >100
13

I021792-003 Wabasha, MN 12/18/2009 Highly 

Probable Dog 2

I024152-010 San Luis 

Obispo, CA 4/9/2011 Highly 

Probable Red fox 1

The 47 reported incidents that occurred before April 1988 are shown in Table 8. While they may 

not represent non-target wildlife exposure from current below-ground uses of strychnine, they do 

provide information on the non-target species that might be impacted if they were to gain access 

to the strychnine bait, or ingest other animals that had access to the bait. 

Table 8. Wildlife Mortality Incidents Attributed to Strychnine Poisoning Occurring before 

April 1988 

Incident ID Location Date Legality Certainty Species No. 

Dead 

B0000-503-18 Yolo, CA 1/1/1966 Misuse 

(accidental) 

Highly 

Probable Deer 2

B0000-503-19 San Benito, 

CA 1/1/1968 Registered Use Probable Dove 50

B0000-503-10 Torrance, 

NM 2/7/1972 Misuse 

(intentional) 

Highly 

Probable Bald eagle 1

B0000-300-83 MI 5/4/1973 Misuse 

(intentional) Probable Birds 15

B0000-503-24 

San 

Bernardino, 

CA 

12/1/1973 Registered 

Use Probable Canada 

goose 

14

B0000-503-45 Siskiyou, 

CA 5/1/1974 Unknown Highly Probable 

Ring-billed 

gull 11

B0000-503-25 Canyon, ID 6/3/1976 Suspected 

misuse 

Highly 

Probable 

Starlings, 

robins, greathorned owl, 

short-eared 

owl, goshawk 

Thousands 

B0000-503-28 Whitman, 

WA 6/4/1976 Unknown Highly Probable 

Mourning 

dove 5

B0000-503-33 Douglas, 

WI 4/30/1976 Unknown Highly Probable Blackbird 6

B0000-503-09 Carbon, UT 2/22/1977 Unknown Probable Bald eagle 1

B0000-503-12 

B0000-503-13 

Conejos, 

CO 3/6/1977 Unknown Highly 

Probable Bald eagle 2

B0000-503-06 Kossuth, IA 4/9/1978 Unknown Highly 

Probable Bald eagle 1

B0000-503-36 St. Louis, 

MN 2/16/1978 Unknown Highly 

Probable 

Rough 

legged hawk, 

owl, pigeon 

3

B0000-503-16 Ketchikan, 

AK 6/21/1979 Unknown Probable Bald eagle 1

B0000-503-17 Ketchikan, 

AK 8/1/1979 Unknown Highly 

Probable Bald eagle 1

B0000-503-29 

B0000-503-42 

Franklin, 

OH 6/11/1979 Misuse (intentional) 

Highly 

Probable 

Mourning 

dove, 

Killdeer, 

other 

8
14

migratory 

birds 

B0000-503-50 Kiowa, CO 1/12/1979 Unknown Highly 

Probable Golden eagle 1

B0000-503-01 

Newport 

News City, 

VA 

12/29/1980 Unknown Highly 

Probable 

Peregrine 

falcon 1

B0000-503-04 Denver, 

CO 1/5/1980 Unknown Highly Probable Prairie falcon 3

B0000-503-08 Baltimore 

City, MD 11/1/1980 Unknown Probable Peregrine 

falcon 1

B0000-503-31 Butler, OH 12/12/1980 Registered 

Use 

Highly 

Probable N/R 17

B0000-503-47 Rawlins, 

KS 12/23/1980 Unknown Highly 

Probable Golden eagle 1

B0000-503-07 Baltimore 

City, MD 8/7/1981 Unknown Highly 

Probable 

Peregrine 

falcon 1

B0000-503-14 Modoc, CA 3/16/1981 Unknown Highly 

Probable Bald eagle 1

B0000-503-32 York, PA 8/4/1981 Misuse 

(accidental) 

Highly 

Probable 

American 

kestrel 3

B0000-503-35 Tom 

Green, TX 1/28/1981 Unknown Highly Probable 

Golden 

eagle, 

jackrabbit (2) 

3

B0000-503-39 Modoc, CA 3/13/1981 Unknown Highly 

Probable Golden eagle 1

B0000-503-41 Ramsey, 

MN 2/7/1981 Unknown Highly Probable 

Great-horned 

owl 4

B0000-503-48 Grant, KS 3/13/1981 Unknown Probable Golden eagle 1

B0000-300-88 

B0000-503-03 

Ferguson, 

MT 4/21/1982 Registered Use 

Highly 

Probable Bald eagle 1

B0000-503-26 Alameda, 

CA 10/20/1982 Unknown Highly Probable 

Canada 

goose 

8

B0000-503-27 King, WA 6/2/1982 Misuse 

(intentional) 

Highly 

Probable Mallard 1

B0000-503-34 Jefferson, 

NY 5/19/1982 Misuse 

(intentional) 

Highly 

Probable Bear 1

B0000-503-40 Ramsey, 

MN 3/26/1982 Unknown Highly 

Probable 

Great-horned 

owl 1

B0000-503-46 Pierce, WA 1/27/1982 Unknown Highly 

Probable 

Glaucous 

gull, mallard, 

rock dove 

9

I013810-017 King, WA 1/27/1982 Unknown Probable Peregrine 

falcon 1

I013810-018 Pierce, WA 5/1/1982 Unknown Probable Duck (10) 10

B0000-503-05 Talbot, MD 1/21/1983 Unknown Probable Bald eagle 1

B0000-503-15 Navajo, AZ 1/22/1983 Unknown Highly 

Probable Bald eagle 1

B0000-503-38 Baltimore 

City, MD 4/28/1983 Unknown Highly 

Probable 

Peregrine 

falcon 1

B0000-503-43 Hubbard, 

MN 5/4/1983 Unknown Highly Probable 

Red-tailed 

hawk 1

B0000-503-02 Judith 

Basin, MT 4/6/1984 Unknown Highly 

Probable Bald eagle 1
15

B0000-503-11 Judith 

Basin, MT 4/18/1984 Registered 

Use 

Highly 

Probable Bald eagle 1

B0000-503-30 Deschutes, 

OR 8/30/1985 Misuse 

(intentional) 

Highly 

Probable 

Blue jay, 

Brewer's 

blackbird, 

cowbird, 

chipmunk, 

green-tailed 

towhee, 

house finch, 

mourning 

dove 

15

B0000-503-44 Modoc, CA 3/21/1985 Misuse 

(intentional) 

Highly 

Probable 

Red-tailed 

hawk 3

B0000-503-49 Thomas, 

KS 12/4/1986 Unknown Highly 

Probable Golden eagle 1

A review of the Avian Monitoring Information System (AIMS), which is maintained by the 

American Bird Conservancy, identified 55 avian incidents associated with exposure to 

strychnine. All but four of these incidents were ones that are also recorded in the EIIS database. 

The four incidents not included in EIIS are described in Table 9. The incident occurring in 

Washburn, Washington in 2000 that resulted in the deaths of approximately 30 blackbirds and 

cowbirds (AIMS ID # 1741) was attributed to use of strychnine for pocket gopher control, 

although it is not known if the application was done legally. The strychnine uses associated with 

the other three reported incidents are unknown. 

Table 9. Bird Mortality Incidents Attributed to Strychnine Poisoning that Are Reported in 

the AIMS Database but not Reported in EIIS 

AIMS 

Incident ID Location Date Legality Certainty Species No. Dead 

2191 Washington, 

NY 4/23/1973 Unknown Possible 

Blue jay(2), 

common 

grackle (6) 

8

2251 Modoc, CA 4/1/1987 Unknown Probable Bald eagle 1

2252 Modoc, CA 3/19/1995 Unknown Probable Bald eagle 1

1741 Washburn, 

WA 4/1/2000 Unknown Highly 

Probable 

Red-winged 

blackbird, 

Brewer’s 

blackbird, 

cowbird 

30

In addition to the incidents recorded in EIIS and AIMS, the Agency maintains a database that 

stores counts of minor incidents that pesticide registrants record in quarterly aggregated incident 

reports. Ecological incidents recorded in these aggregated reports include those categorized as 

‘minor fish or wildlife’ (W-B), ‘minor plant’ (P-B), and ‘other non-target’ (ONT) incidents. A 

review of this database in August 2015 identify no reported fish or wildlife, plant, or other nontarget incident attributed to a product containing strychnine as the active ingredient. 
16

7. Exposure Pathways of Concern 

The primary route of exposure to strychnine bait would be from terrestrial wildlife directly 

consuming the bait (primary exposure), or by consuming animals that have fed recently on the 

bait and contain significant residues of strychnine within their bodies (secondary poisoning). 

Numerous field studies have documented primary poisoning of non-target animals from belowground application of gopher bait containing strychnine (Fagerstone et al., 1980; Anthony et al., 

1984; Nolte and Wagner, 2001; El Hani et al., 2002). Small fossorial rodents like ground 

squirrels and chipmunks, which may enter into pocket gopher burrows, are particularly at risk of 

primary exposure (Anthony et al. 1984, El Hani et al., 2002). Also, for the special local need use 

of strychnine in the state of Nevada (EPA Reg. No. NV830009), the label requires strychnine 

bait to be placed 8 inches down the opening of the burrows of the yellow-bellied marmot and 

ground squirrel; however, considering the large opening of the burrows of these species, this 

depth may not be enough to prevent some animals from accessing the bait. Primary poisoning of 

non-target species has been observed with use of strychnine for ground squirrel control in 

Canada (Proulx and MacKenzie, 2010; Proulx, 2011). Secondary exposure may occur when 

predators or scavengers consume a pocket gopher or non-target animal that has consumed the 

strychnine bait. Because strychnine is a very fast-acting poison, prey animals that consume a 

lethal dose of strychnine bait are expected to die before they have absorbed and assimilated a 

significant amount of strychnine residues into their tissue. However, those animals may die with 

a significant amount of strychnine remaining within their gastrointestinal tract, with a high 

enough dose that could be lethal to the predator/scavenger. Furthermore, pocket gophers and 

other seed-eating rodents with cheek pouches, such as ground squirrels and chipmunks, may 

have a significant amount of strychnine bait remaining stored within their cheek pouch when 

they die (Hegdal and Gatz, 1976). 

Pocket gophers poisoned by strychnine mostly die underground (Nolte and Wagner, 2001; El 

Hani et al., 2002), which would likely limit exposure to predators and scavengers. Secondary 

poisoning may occur to some predators that may access the dead and dying pocket gophers by 

entering into their underground burrows (e.g., weasels and snakes). Other predators and 

scavengers may dig to access the gophers underground, or may be exposed to the occasional 

gopher or other non-target animals that dies above ground. Ground squirrels poisoned by 

strychnine mostly die above ground (Anthony et al., 1984), and therefore, there is a higher 

potential for secondary poisoning from the use of strychnine in Nevada for ground squirrel 

control. 

Conceivably, tertiary exposure could occur if insects feed on the carcasses of poisoned rodents, 

and then insectivores feed on the contaminated insects. Arjo et al. (2006) measured strychnine 

residues in insects collected from carcasses of strychnine poisoned pocket gophers, and 

conducted a risk assessment to evaluate the potential for risk from this route of exposure to three 

species of insectivorous animals, the house sparrow, the house mouse, and the bullfrog. Risk 

quotients were calculated based on predicted dietary exposures from this route and estimated 

LD50 values for these species. Risk quotients ranged from 0.009 to 0.064, indicating negligible 

risk from this route. Therefore, tertiary exposure to insectivores will not be further considered as 

a significant route of exposure in the risk assessment. 
17

Strychnine is not expected to volatilize; therefore, very little inhalation exposure to strychnine is 

expected, even to animals that enter treated burrows. Some dermal exposure would be possible, 

but only to small mammals, amphibians, and reptiles which might enter into treated pocket 

gopher burrows. Dermal exposure may be particularly significant to amphibians because they are 

susceptible to absorbing contaminants through the seat patch, an area of absorbant skin on their 

lower abdomen used to absorb water. Dermal exposure would likely be less than exposure from 

direct consumption of bait, but may be important to amphibians which may enter into the treated 

burrows but would not consume the gopher bait. 

Use of strychnine is limited to bait placed below the ground within burrow systems, which is 

expected to greatly limit potential surface water contamination. Additionally, adsorption/ 

desorption data show that strychnine binds strongly to soil, limiting movement through the soil 

profile. Therefore, strychnine exposure to fish, aquatic invertebrates, and aquatic plants is not a 

significant exposure route of concern. 

Placement of strychnine bait in underground burrows of rodents is not expected to result in 

significant exposure to plants. Any absorption of strychnine residues from the soil would be 

minimal and extremely localized. As very little strychnine is expected to be absorbed and 

translocated into the flowers of plants, exposure to pollinators is assumed to be negligible. 

8. Analysis Plan 

The only stressor of concern that will be considered in the risk assessment is the parent 

compound, strychnine. The risk assessment will be focused on risk from primary exposure to 

non-target wildlife that consume strychnine bait, and secondary exposure to predators and 

scavengers that feed on small mammals (target and non-target) which feed on strychnine bait. 

Risk quotients for primary exposure will be calculated by dividing of the strychnine 

concentration in the bait (5000 mg a.i./kg) to dietary LC50 of birds and mammals measured in 

subacute dietary toxicity studies. In addition, dose-based terrestrial risk quotients will be 

calculated by predicting the doses (mg a.i./kg-bodyweight) that birds and mammals would ingest 

if they were to feed on strychnine bait, and then dividing those doses by the acute oral LD50

values obtained in acute toxicity tests with the mallard and the laboratory rat. The number of 

grains of strychnine bait that birds and mammals would need to consume to reach the LD50 will 

also be calculated as another index for assessing risk from primary exposure. 

Risk to terrestrial wildlife from secondary exposure will also be assessed with dietary- and dosebased risk assessments. First, the amount of strychnine that is potentially contained within the 

body of a small mammals will be estimated. The northern pocket gopher (Thomomys talpoides) 

and the golden mantled-ground squirrel (Spermophilus lateralis) will be used as model prey 

species. Strychnine residues in the prey will be based on the estimated amount of strychnine bait 

grains that could be present within the GI tract and cheek pouches of an animal that has been 

feeding on the bait, and the strychnine concentration in the bait (0.5%). A dietary-based risk 

assessment will be conducted by comparing the predicted whole-body concentration of 

strychnine in the modeled prey to the dietary LC50 of the red fox and the American kestrel. Dosebased risk assessments will be conducted for predatory mammals using the red fox and the red-
18

tailed hawk (Buteo jamaicensis) as model species. A daily dose (mg a.i./kg-bodyweight) will be 

predicted for these species based on the assumption that they consume the strychnine-killed prey 

at their average daily food intake rate. Acute risk quotients will be calculated by dividing the 

daily dose by the acute LD50 for birds and mammals. The acute oral LD50 measured in the desert 

kit fox (Vulpes macrotis arsipus), 0.075 mg a.i./kg-bodyweight (Schitoskey, 1975) will be used 

for the red fox RQ. Because no acute LD50 is available for a predatory bird species, the acute oral 

LD50 of the mallard will be used. In addition to these assessments, the secondary-poisoning risk 

assessments for owls, hawks and minks that were reported in Anthony et al. (1984) will be 

evaluated. 

Chronic risk quotients will be calculated for birds by dividing the chronic mallard NOAEC by 

the predicted dietary concentration of strychnine in strychnine bait (for primary exposure) and in 

strychnine-killed pocket gophers and ground squirrels (for secondary exposure). 

As in previous risk assessments, exposure to terrestrial plants, bees, and aquatic organisms is 

assumed to be negligible from underground strychnine bait uses, and therefore no risk 

assessment will be conducted to these groups of organisms. 

9. Status of Fate and Ecological Effects Data 

a. Environmental Fate

Table 10 identifies the environmental fate studies submitted for strychnine for each guideline 

requirement, as well as study classifications. With the limited dissipation or movement of 

strychnine expected to occur in the environment, no further fate data are needed in order to 

support the pending risk assessment. 

Table 10. Submitted Environmental Fate Data for Strychnine. 

OCSPP 

Guideline Data Requirement Submitted Studies (MRID)

Study 

Classifications 

Are data needed to 

conduct risk 

assessment? 

835.2120 Hydrolysis 41122301 Acceptable No 

835.2240 Aqueous photolysis -- -- No 

835.2410 Soil photolysis 42973401 Acceptable No 

835.4100 Aerobic soil metabolism 42234201 and 

43407601 Supplemental1

 No

835.4200 Anaerobic soil metabolism -- -- No 

835.4300 Aerobic aquatic metabolism -- -- No 

835.4400 Anaerobic aquatic metabolism -- -- No 

835.1230 

835.1240 Adsorption/ desorption and leaching 42366501 Acceptable No 

835.1410 Volatility – laboratory -- -- No 

835.6100 Terrestrial field dissipation -- -- No 

850.6100 Environmental Chemistry Method and 

Independent Laboratory Validation 

-- -- No 

850.1730 Fish bioconcentration -- -- No 
19

1

 The aerobic soil metabolism studies did not use radiolabeled material and thus material balances and degradate 

identification were not completed. However, sufficient information was provided to allow determination of the halflife of parent strychnine. 

b. Ecological Effects 

Available ecotoxicity data for terrestrial organisms are shown in Table 11. No additional 

terrestrial data are needed for completing the ecological risk assessment. Toxicity data on honey 

bees and terrestrial plants are not needed because the use of strychnine is limited to solid bait that 

is placed below ground for rodent control, and this use is not expected to result in significant 

exposure to insect pollinators or terrestrial plants. 

Table 11. Available Terrestrial Ecological Effects Data for Strychnine

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Test 

Species 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Current 

Additional Data 

Need 

 850.2100 Avian oral 

toxicity 

00160000 Mallard Acceptable 

No -- 

00160000 Japanese 

quail Acceptable 

00160000 House 

sparrow Acceptable 

40815201 

Blackbilled 

magpie 

Supplemental 

 850.2200 Avian dietary 

toxicity 

41322602 Bobwhite Acceptable 

No -- 

41322601 Mallard Acceptable 

40815201 American 

kestrel Supplemental 

40815201 

Blackbilled 

magpie 

Supplemental 

850.2300 Avian 

reproduction 

42716801 Bobwhite Acceptable 

No -- 

42716802 Mallard Acceptable 

Nonguideline Avian inhalation -- -- (N/A) No 

Because inhalation 

is not expected to 

be a significant 

route of exposure 

for below-ground 

bait use, an avian 

inhalation study is 

not requested. 

870.1100 

Acute oral 

mammalian, 

TGAI 

40908901, 

41210701 

Laboratory 

rat Supplemental No -- 
20

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

Test 

Species 

Study 

Classifications 

Are data 

needed for 

risk 

assessment? 

Current 

Additional Data 

Need 

870.1100 Acute oral 

mammalian, TEP 43330301 Laboratory 

rat Supplemental No -- 

870.3150 Subacute dietary 

mammalian 

40296503 Red fox Supplemental 

No 

40296502 European 

ferret Supplemental -- 

870.4100 Chronic 

Mammalian -- -- -- No -- 

850.3020 Honey bee acute 

contact toxicity -- -- -- No 

Because 

underground bait 

use is expected to 

result in negligible 

exposure to insect 

pollinators, no bee 

studies are being 

requested. 

Nonguideline Honey bee acute 

oral toxicity -- -- -- No 

Nonguideline 

Honey bee 

chronic larvae 

toxicity 

-- -- -- No 

Nonguideline 

Honey bee 

chronic adult 

toxicity 

-- -- -- No 

850.3030 

Honey bee 

residue on 

foliage 

-- -- -- No 

 850.3040 Field testing for 

pollinators -- -- -- No 

850.4100 

Seedling 

Emergence and 

Seedling Growth 

-- -- -- No 

Because 

underground bait 

use is expected to 

result in negligible 

exposure to 

terrestrial plants, no 

terrestrial plant 

studies are 

requested. 

850.4150 Vegetative Vigor -- -- -- No 

Available ecotoxicity data for aquatic organisms are shown in Table 12. Acceptable acute 

toxicity data are available for freshwater fish and freshwater invertebrates. Because the belowground rodenticide use of strychnine bait applied is not expected to result in significant exposure 

to aquatic ecosystems, all other aquatic toxicity data requirements have been waived, including 

studies with saltwater species and all chronic toxicity studies. 
21

Table 12 Submitted Aquatic Ecological Effects Data for Strychnine

OCSPP 

Guideline Data Requirement 

Submitted 

Studies 

(MRID) 

Test Species Study 

Classifications 

Are data needed 

to conduct risk 

assessment? 

850.1010 Freshwater invertebrate 

acute toxicity 41126503 Water flea Acceptable No 

850.1035 

850.1045 

Saltwater crustacean 

acute toxicity (Waived) -- -- No 

850.1025 

850.1055 

Saltwater bivalve 

acute toxicity (Waived) -- -- No 

 850.1075 Freshwater fish acute 

toxicity 

41126502 Bluegill sunfish Supplemental 

No 

41126501 Rainbow trout Supplemental 

 850.1075 Saltwater fish acute 

toxicity (Waived) -- -- No 

 850.1300 Freshwater invertebrate 

life cycle (Waived) -- -- No 

 850.1350 Saltwater invertebrates 

life cycle (Waived) -- -- No 

 850.1400 Freshwater fish 

early-life stage (Waived) -- -- No 

 850.1400 Saltwater fish 

early-life stage (Waived) -- -- No 

 850.1500 Fish life cycle (Waived) -- -- No 

850.4400 Aquatic plant Toxicity 

Test using Lemna spp. (Waived) -- -- No 

850.4500 Algal toxicity (Waived) -- -- No 

850.4550 Cyanobacteria (Waived) -- -- No 

10. Endangered Species Assessments 

Consistent with EPA’s responsibility under the Endangered Species Act (ESA), the Agency will 

evaluate risks to federally listed threatened and endangered (listed) species from registered uses 

of pesticides in accordance with the Joint Interim Approaches developed to implement the 

recommendations of the April 2013 National Academy of Sciences (NAS) report, Assessing 

Risks to Endangered and Threatened Species from Pesticides. The NAS report outlines 

recommendations on specific scientific and technical issues related to the development of 

pesticide risk assessments that EPA and the Services must conduct in connection with their 

obligations under the ESA and FIFRA. EPA will address concerns specific to strychnine in 

connection with the development of its final registration review decision for strychnine.

In November 2013, EPA, the U.S. Fish and Wildlife Service, National Marine Fisheries (the 

Services), and USDA released a white paper containing a summary of their joint Interim 

Approaches for assessing risks to listed species from pesticides. These Interim Approaches were 

developed jointly by the agencies in response to the NAS recommendations, and reflect a 
22

common approach to risk assessment shared by the agencies as a way of addressing scientific 

differences between the EPA and the Services. Details of the joint Interim Approaches are 

contained in the November 1, 2013 white paper, Interim Approaches for National-Level 

Pesticide Endangered Species Act Assessments Based on the Recommendations of the National 

Academy of Sciences April 2013 Report. 

 

Given that the agencies are continuing to develop and work toward implementation of the 

Interim Approaches to assess the potential risks of pesticides to listed species and their 

designated critical habitat, this ecological problem formulation supporting the Preliminary Work 

Plan for strychnine does not describe the specific ESA analysis, including effects determinations 

for specific listed species or designated critical habitat, to be conducted during registration 

review. While the agencies continue to develop a common method for ESA analysis, the planned 

risk assessment for the registration review of strychnine will describe the level of ESA analysis 

completed for this particular registration review case. This assessment will allow EPA to focus 

its future evaluations on the types of species where the potential for effects exists, once the 

scientific methods being developed by the agencies have been fully vetted. Once the agencies 

have fully developed and implemented the scientific methods necessary to complete risk 

assessments for listed species and their designated critical habitats, these methods will be applied 

to subsequent analyses of strychnine as part of completing this registration review. 

11. Endocrine Disruptor Screening Program 

Strychnine was not included in the first group of 67 chemicals issued an order to conduct Tier 1 

EDSP testing. For additional information the EDSP program, visit http://www.epa.gov/endo/. 

12. Clean Water Act Programs 

Strychnine has not been identified as a cause of impairment for any water bodies listed as 

impaired under section 303(d) of the Clean Water Act2

. In addition, no Total Maximum Daily 

Loads (TMDL) have been developed for strychnine3

. More information on impaired water 

bodies and TMDLs can be found at the Agency’s website4

. The Agency invites submission of 

water quality data for this pesticide. To the extent possible, data should conform to the quality 

standards in Appendix A of the OPP Standard Operating Procedure: Inclusion of Impaired 

Water Body and Other Water Quality Data in OPP’s Registration Review Risk Assessment and 

Management Process5 in order to ensure they can be used quantitatively or qualitatively in 

pesticide risk assessments. Aquatic benchmarks6 have not been established for these chemicals. 

Any data submitted or otherwise located as part of the registration review process may be used to 

update aquatic life benchmarks, if applicable. 

 

2 http://iaspub.epa.gov/tmdl_waters10/attains_nation_cy.cause_detail_303d?p_cause_group_id=885 3

http://iaspub.epa.gov/tmdl_waters10/attains_nation.tmdl_pollutant_detail?p_pollutant_group_id=885&p_pollutant_group_name=

PESTICIDES 4 http://www.epa.gov/owow/tmdl/ 5 http://www.epa.gov/oppsrrd1/registration_review/water_quality_sop.htm 6 http://www.epa.gov/oppefed1/ecorisk_ders/aquatic_life_benchmark.htm
23

13. References 

Anthony, R.M., G.D. Lindsey, and J. Evans. 1984. Hazards to golden-mantled ground squirrels 

and associated secondary hazard potential from strychnine for forest secondary pocket 

gophers. Proceedings of the Eleventh Vertebrate Pest Conference (1984). Paper 2. 

Available online: http://digitalcommons.unl.edu/vpc11/2. 

Arjo, W.M., K.K. Wagner, D.L. Nolte, S.S. Stahl, and J. J. Johnston. 2006. Potential non-target 

risks from strychnine-containing rodent carcasses. Crop Protection 25:182-187. 

Arjo, W.M. and D.T. Bryson. 2007. Reducing non-target hazards of rodenticides in forest settings. 

In Proceedings of the 12th Wildlife Damage Conference (D.L. Nolte, M. Argo, and D H. 

Stalman, Eds.) 

El Hani, A., D.L. Nolte, J.R. Mason, and S. Bulkin. 2002. Response of non-target species to 

underground strychnine baiting for pocket gopher in southwest Oregon. Western Journal 

of Applied Forestry. 17(1): 9-13. 

Fagerstone, K., V.G. Barnes, Jr., M. Anthony, and J. Evans. 1980. Hazards to small mammals 

associated with underground strychnine baiting for pocket gophers. Proceedings of the 9th

Vertebrate Pest Conference (1980). Paper 10. 

Hegdal and Gatz, 1976. Hazards to wildlife associated with underground strychnine baiting for 

pocket gophers. Proc. 6th Vert. Pest. Conf., Monterey, CA, p. 258-266. 

Hudson, R.H., R.K. Tucker and M.A. Haegele. 1984. Handbook of toxicity of pesticides to 

wildlife. USDI Resource Publication No. 153. Washington, DC. 92 pp. MRID 00160000. 

Nolte, D. and K. Wagner. 2001. Non-target impacts of strychnine baiting to reduce pocket 

gopher populations of forest lands in the United States. USDA National Wildlife 

Research Center – Staff Publications. Paper 607. 

Proulx, G., N. Mackenzie, K. Mackenzie, K. Walsh, B. Proulx, and K. Stang. 2010. Strychnine 

for the control of Richardson’s ground squirrels: efficiency and selectivity issues. Proc. 

24th Vertebr. Pest. Conf. (R.M. Timm and K.A. Fagerstone, Eds.) Univ. of Calif., Davis. 

2010 Pp. 125-128. 

Proulx, G. 2015. Field evidence of non-target and secondary poisoning by strychnine and 

chlorophacinone used to control Richardson’s Ground Squirrels in Southwest 

Saskatchewan. In Proc. 9th Prairie Conserv. and Endang. Species Conf. Winnipeg, 

Manitoba. 2010. Pp. 128-134. 

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U.S. Environmental Protection Agency. Washington, DC. EPA/600/R-93/187a
24

Appendix 

OPPIN Bibliography 

Ecotoxicity 

71-1 Avian Single-Dose Oral Toxicity 

MRID Citation Reference 

40815201 Garrison, M.; Schafer, E.; Harris, P.; et al. (1988) Dietary LC50 of Strychnine Alkaloid 

to the Black-billed Magpie and American Kestrel: BDC 896.39. Prepared by USDA, 

Denver Wildlife Research Center. 29 p. 

 

71-2 Avian Dietary Toxicity 

MRID Citation Reference 

41322601 Pedersen, C. (1989) Strychnine: Avian Dietary LC50 (8 Day) in Mallard Ducklings: Lab 

Project Number: 89/DC/128. Unpublished study prepared by Bio-Life Associates, Ltd. 

76 p. 

41322602 Pedersen, C. (1989) Strychnine: Avian Dietary LC50 (8 Day) in Bobwhite Quail: Lab 

Project Number: 89/QC/131. Unpublished study prepared by Bio-Life Associates, Ltd. 

78 p. 

71-4 Avian Reproduction 

MRID Citation Reference 

42716801 Pedersen, C.; DuCharme, D.; Lesar, C. (1993) Strychnine Alkaloid: Toxicity and 

Reproduction Study in Bobwhite Quail: Lab Project Number: 89 QR 37: QA-207. 

Unpublished study prepared by Bio-Life Associates, Ltd. 1053 p. 

42716802 Pedersen, C.; Lesar, C. (1993) Strychnine Alkaloid: Toxicity and Reproduction Study in 

Mallard Ducks: Lab Project Number: 89 DR 33: QA-206. Unpublished study prepared 

by Bio-Life Associates, Ltd. 1342 p. 

71-5 Simulated or Actual Field Testing 

MRID Citation Reference 
25

40022016 Marsh, R.; Schmidt, R.; Howard, W. (1986) Secondary Hazards to Coyotes of Ground 

Squirrels Poisoned with 1080 or Strychnine. Unpublished study prepared by Univ. of 

Calif., Davis. Wildlife and Fisheries Biology. 17 p. 

40936908 Hegdal, P.; Gatz, T. (1977) Hazards to Wildlife Associated with Underground 

Strychnine Baiting for Pocket Gophers. Final Report. Denver Wildlife Research 

Center. EPA-IAG-D4-0449, 53 pp. 

40936909 Hegdal, P.; Gatz, T. (1976) Hazards to Wildlife Associated with Underground 

Strychnine Baiting for Pocket Gophers. Proc Vertebrate Pest Control Conference 

7:258-266, 12 pp. 

41147801 Savarie, P.; Matschke, G. (1989) Secondary Feeding Hazard of Strychnine to the 

Domestic Ferret: Final Report: Project ID 612.31. Unpublished study prepared by 

Denver Wildlife Research Center. 87 p. 

42027201 Hegdal, P.; Gatz, T. (1977) Hazards to Wildlife Associated with Underground 

Strychnine Baiting for Pocket Gophers. Unpublished study prepared by Denver 

Wildlife Research Center. 53 p. 

42488601 Poche, R. (1992) Field Evaluation of Three Concentrations of Strychnine Baits (0.5, 

1.15, and 1.8% Strychnine) to Control the Valley Pocket Gopher (Thomomys bottae) 

in California Using an Artificial Burrow Builder: Lab Project Number: 91001. 

Unpublished study prepared by Genesis Labs. 108 p. 

72-1 Acute Toxicity to Freshwater Fish 

MRID Citation Reference 

41126501 Bowman, J. (1989) Acute Toxicity of Strychnine to Bluegill Sunfish (Lepomis 

macrochirus): Project ID: 37645. Unpublished study prepared by Analytical Biochemistry Laboratories, Inc. 79 p. 

41126502 Bowman, J. (1989) Acute Toxicity of Strychnine to Rainbow Trout (Salmo 

gairdneri): Project ID: 37646. Unpublished study pre- pared by Analytical Biochemistry Laboratories, Inc. 89 p. 

72-2 Acute Toxicity to Freshwater Invertebrates 

MRID Citation Reference 

41126503 Forbis, A. (1989) Acute Toxicity of Strychnine to Daphnia magna: Project ID: 

37647. Unpublished study prepared by Analytical Bio-chemistry Laboratories, Inc. 

80 p. 

93203006 Elias, D. (1990) H Interdonati Inc. Phase 3 Summary of MRID 41126503. 

Strychnine Alkaloid- Hazard Evaluation- Wildlife and Aquatic Organisms. 

Prepared by Denver Wildlife Research Center. 11 p. 

81-1 Acute Oral Toxicity in Rats 
26

MRID Citation Reference 

40908901 

Cerven, D. (1988) Strychnine Alkaloid Technical: Oral Toxicity in Albino Rats: 

Project ID MB 88-9165 A. Unpublished study prepared by MB Research 

Laboratories, Inc. 22 p. 

41210701 

Cerven, D. (1988) Strychnine Alkaloid Technical: Single Dose Oral Toxicity in 

Rats/LD 50 in Rats: Revised Final Report: Laboratory Project ID 88-9165 A. 

Unpublished study prepared by MB Research Laboratories, Inc. 24 p. 

43330301 

Baroch, J. (1994) Acute Oral Toxicity study with Rats (Rattus norvegicus) Using: 

Wilco Gopher Getter Type 1 Bait and Wilco Gopher Getter AG Bait: Lab Project 

Number: 94002. Unpublished study prepared by Genesis Labs, Inc. 35 p. 

82-1 Subchronic Oral Toxicity: 90-day Study 

MRID Citation Reference 

40296502 

Record, C. (1987) Tests To Determine the Dietary LC-50 of Strychnine Alkaloid to 

European Ferrets (Mustela putorius furo): Lab- oratory I.D. No. 112. Unpublished 

study prepared by Summit Lab- oratories. 19 p. 

40296503 

Record, C. (1987) Tests To Determine the Dietary LC-50 of Strychnine Alkaloid to 

Red Foxes (Vulpes fulva): Laboratory I.D. No. 113. Unpublished study prepared by 

Summit Laboratories. 19 p. 

Environmental Fate 

161-1 Hydrolysis 

MRID Citation Reference 

41122301 Mishalanie, E.; Griffin, D.; Bullard, R. (1989) Environmental Fate Study: Strychnine 

Alkaloid Hydrolysis: Laboratory Project ID: QA-57. Unpublished study prepared by 

Denver Wildlife Research Center. 42 p. 

162-1 Aerobic soil metabolism 

MRID Citation Reference 

42234201 Starr, R.; Hurlbut, D.; Timm, R. (1992) The Aerobic Degradation of Strychnine in Soil 

Under an Optimum Incubation Temperature and Moisture Regimen: Lab Project 

Number: QA-128. Unpublished study prepared by Denver Wildlife Research Center. 

167 p. 
27

43407601 Starr, R.; Timm, R.; Hurlbut, D.; et al. (1994) The Aerobic Biodegradation of 

Strychnine in Soil Under an Optimum Incubation Temperature and Moisture Regimen: 

Supplement: Lab Project Number: QA/346. Unpublished study prepared by Denver 

Wildlife Research Center. 192 p. 

163-1 Leaching/adsorption/desorption 

MRID Citation Reference 

42366501 Timm, R.; Starr, R.; Hurlbut, D.; et al. (1992) Mobility of Strychnine in Soil: The 

Sorption/Desorption of the Parent Compound in Four Soil Systems: Lab Project 

Number: QA-190. Unpublished study prepared by Denver Wildlife Research Center. 

353 p.