Contact Information: 

Dr. Gary Witmer 

Supervisory Research Wildlife Biologist

NWRC Headquarters 

4101 LaPorte Avenue 

Fort Collins, CO 80521

Phone: (970) 266-6335 

FAX: (970) 266-6157

gary.w.witmer@aphis.usda.gov

www.aphis.usda.gov/wildlife_damage/

nwrc/ 

Major Cooperators

 California Department of Food and 

Agriculture

 Colorado State University

 Global Materials Technology

 Island Conservation, Inc.

 University of California-Davis

 U.S. Department of Defense

 U.S. Fish and Wildlife Service

 U.S. National Park Service

Wildlife Services

Protecting People rotecting People

Protecting Agriculture rotecting Agriculture

Protecting Wildlife rotecting Wildlife

FY 2012

National Wildlife Research Center

United States Department of Agriculture

Animal and Plant Health Inspection Service

National Wildlife Research Center Scientists Assess and Develop Methods 

to Manage Native Rodents and Eradicate Introduced, Invasive Rodents

Wildlife Services’ (WS) National Wildlife Research Center (NWRC) is the only Federal research organization devoted exclusively to resolving confl icts between people and wildlife 

through the development of effective, selective, and socially responsible methods, tools, 

and techniques.

Rodents occur worldwide and have adapted to most types of ecosystems. Rodents provide 

many important ecosystem functions and although most rodent species do not cause serious damage problems, a small number of species do. Rodents damage ripening crops, 

forestry and nursery trees, rangelands, ornamental plants, and property, including cables 

and irrigation pipes. They also consume and contaminate stored food, transmit diseases, 

and contribute to the decline of native fl ora and fauna on islands. Many tools are used to 

reduce rodent populations and mitigate damage. NWRC researchers develop and evaluate 

rodenticides, barriers, and other tools to eliminate or reduce the damage caused by native 

and invasive rodents.

Applying Science and Expertise to Wildlife Challenges

Ground Squirrel Barriers for Underground Missile Sites—Richardson’s ground squirrels pass through or under chain link security fences, triggering sensors and undermining 

facility infrastructure at Intercontinental Ballistic Missile (ICBM) sites in the western U.S. 

Among potential solutions to ground squirrel intrusions at these remote sites are permanent barriers, extending both above and below ground. NWRC biologists tested several 

barrier systems in the rodent buildings at NWRC’s outdoor animal research facility in 

Colorado, using wild-caught Richardson’s ground squirrels from deactivated ICBM sites at 

Malmstrom Air Force Base, Montana. Several barrier systems were identifi ed that have 

a high potential to prevent movement by Richardson’s ground squirrels through or under 

standard, chain link-fenced areas. Effective above-ground barriers included clear, polycarbonate plastic, and a 2x4-inch woven wire fence with 2 strands of electrifi ed tape near 

the soil surface. Effective below-ground barriers included a pea gravel-fi lled trench, and 

a small-mesh expanded metal sheets. The barriers prevented both above- and belowground intrusions by ground squirrels. These barrier systems will be fi eld tested at ICBM 

sites to verify their effectiveness. 

Assessing Seedling Damage by Mice—Research has shown that house mice and deer 

mice may cause substantial damage to tree seedlings. In studies at NWRC, deer mice and 

house mice were placed in metal stock tanks with planted ponderosa pine and narrow-leaf 

cottonwood seedlings. Both rodent species damaged leaves and stems of cottonwood 

seedlings, with house mice damage resulting in the mortality of more than half of the cottonwood seedlings. Only slight damage was done by either species to the pine seedlings, 

and neither species damaged the roots of seedlings, despite extensive burrowing by house 

mice. Researchers conclude that management actions to reduce mouse damage at regeneration sites or in plant nurseries may be warranted. 

Effi cacy of a Cholecalciferol Plus Diphacinone Bait for California Voles—NWRC researchers determined the effi cacy of a new cholecalciferol plus diphacinone bait for use 

with California voles in California agricultural fi elds where they have developed resistance 

to chlorophacinone bait. In no-choice tests with captive voles, results showed that the pelleted bait was highly effective (100-percent mortality). Subsequent two-choice tests also 

showed a high effi cacy (80-percent mortality). Field effi cacy trials are planned.

Controlling Rodent Populations and Damage ontrolling Rodent Populations and Damage 

with an Emphasis on Invasive House Mice and ith an Emphasis on Invasive House Mice and 

Native Voles ative Voles

Groups Affected By These Problems

 Conservationists

 Farmers 

 Livestock producers

 Military bases

 Natural resource managers 

 Urban citizens
Assessment of Anticoagulant Rodenticide Hazards to NonTarget Animals—Anticoagulant rodenticides are important and 

widely used tools for managing rats, ground squirrels, voles, 

and other rodents that damage agriculture, impact native fl ora 

and fauna, transmit diseases, or otherwise confl ict with human 

interests. However, concerns about nontarget hazards to wildlife 

and other adverse environmental effects could limit the use of 

these rodenticides in the United States. There has been a growing concern that the use of anticoagulant rodenticides to control 

prairie dog populations may pose a signifi cant hazard to animals 

that feed on dead or dying prairie dogs. NWRC biologists assessed chlorophacinone residues in captive prairie dogs under 

controlled conditions. Chlorophacinone levels quickly peaked in 

prairie dogs after being fed Rozol® prairie dog bait. The highest 

levels were recorded from animals euthanized on the third day 

after being offered the bait. Levels quickly declined thereafter 

and were signifi cantly lower by the seventh day. The results also 

demonstrated that prairie dogs allowed unlimited access to the 

bait did not consume more bait nor did they have higher residue 

levels than those offered only 53g of bait. Results suggest that 

the highest risk of secondary exposure to chlorophacinone residues by non-target animals consuming prairie dogs exposed to 

the bait would occur within a few days after bait application and 

would drop quickly thereafter.

Preliminary Evaluation of Sodium Nitrite as a Rodenticide—

NWRC researchers evaluated sodium nitrite (a compound commonly used as a color fi xative and preservative in meats and fi sh) 

as a potential rodenticide. The preliminary trials involved blacktailed prairie dogs and Norway rats and used food and liquid bait 

containing encapsulated sodium nitrite. The lethal dose (LD-50) 

for both species was less than 200 mg/kg, which indicates that 

sodium nitrite has the potential to be an effective rodenticide for 

these species.

Development of a Food Bait Block for Use With Black-Tailed 

Prairie Dogs—Fertility control may be a useful tool for managing prairie dog populations in urban and suburban environments 

where rodenticide use is limited or socially unacceptable. However, an effective, oral delivery system is needed. NWRC biologists tested a food bait block delivery system that allows prairie 

dogs’ access to bait over several days. Prairie dogs readily 

consumed the bait blocks which were stacked on vertical metal 

poles during the day. However, rabbits and mice also consumed 

the food bait blocks, mainly at night. Researchers are working to 

modify the delivery device to eliminate bait access at night.

Selected Publications:

SAVIDGE, J., G. WITMER, S. JOJOLA, J. PIERCE, and P. 

BURKE. 2012. Genetic evaluation of an attempted Rattus rattus 

eradication on Congo Cay, U.S. Virgin Islands, identifi es importance of eradication units. Biol. Invasions 14:2343-2354.

SNOW, N., and G. WITMER. 2011. A fi eld evaluation of a trap 

for invasive American Bullfrogs. Pacifi c Conservation Biology 

17:285-291.

WITMER, G. W. and P. HALL. 2011. Attempting to eradicate 

invasive Gambian giant pouched rats (Cricetomys gambianus) in 

the United States: lessons learned. Pp. 131-134 in: Proceedings 

of the Symposium on Island Invasives: Eradication and Management. New Zealand.

WITMER, G. W., J. PIERCE, and W. C. PITT. 2011. Eradication 

of invasive rodents on islands of the United States. Pp. 135-138 

in: Proceedings of the Symposium on Island Invasives: Eradication and Management. New Zealand.

WITMER, G., N. SNOW, R. MOULTON, and J. SWARTZ. 

2012. An assessment of seedling damage by wild house mice 

and wild deer mice. Can. J. Forest Research 42:1168-1172.

WITMER, G., T. SHEFFELS, and S. KENDROT. 2012. 

The introduction, impacts, and management of a large, invasive, aquatic rodent in the United States. Pp. 49-89 in: D. 

Abreau and S. deBorbon (eds.). Marshes: Ecology, Management, and Conservation. Nova Science Publishers, Inc., NY.

WITMER, G., and W. PITT. 2012. Invasive rodents in the 

United States: ecology, impacts, and management. Pp. 47-

75 in: J. Blanco and A. Fernandes (eds.). Invasive Species: 

Threats, Ecological Impact and Control Methods. Nova Science Publishers, Inc., NY.

WITMER, G. W., and P. FULLER. 2011. Vertebrate species 

introductions in the United States and its territories. Current 

Zoology 57:559-567.

The U.S. Department of Agriculture is an equal opportunity employment provider and employer

Major Research Accomplishments:

• WS identifi ed effective barrier systems to prevent 

ground squirrel access and subsequent damage to 

underground missile sites.

• WS determined house mouse damage to cottonwood 

seedlings can be severe and lead to seedling 

mortality.

• WS evaluated the effi cacy of a new cholecalciferol 

plus diphacinone bait for use with California voles 

in California agricultural fi elds where voles have 

developed rodenticide resistance.

• WS assessed non-target hazards associated with 

chlorophacinone residues in dead or dying prairie 

dogs.

• WS determined sodium nitrite has the potential to be 

an effective rodenticide.