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Great Basin Naturalist 50(4), 1990, pp. 347-353

EFFECTS OF DOG RODENTICIDES ON DEER MICE IN WESTERN

Michele S. Deisch1, Daniel W. Uresk2, and Raymond L. Linder3

ABSTRACT.-Mortality of nontarget small was determined after application of three black-tailed prairie dog (Cynomys Zudovicianus) rodenticide treatments (prebaited zinc phosphide, prebaited strychnine, and strychnine alone) in western South Dakota. Immediate (September 1983) and long-term (September 1983 through August 1984) impacts on deer mouse (Peromyscus maniculatus) relative densities were evaluated, and the three rodenticide treatments were compared for efficacy. The three treatments had no significant ((x < .lO) immediate impacts on deer mouse relative densities, although zinc phosphide did lower them; that impact was not, however, long term. Long-term impacts of the two strychnine treatments were variable, with an increase in deer mouse densities with the strychnine only treatment. Overall, comparisons among the three treatments indicatedthat zincphosphidewasmore effective than either strychnine treatment in reducing deer mouse densities.

Considerable time and money have been suitable for some small mammals, such as spent on control of prairie dogs to reduce the deer mice, that are associated with sparse, agricultural damage they cause (Collins et al. heterogeneous vegetative cover. Prairie dog 1984). However, efforts to evaluate the im- provide security cover and nesting pact of prairie dog control methods on the for small mammals. When prairie total biotic communities of prairie dog towns dog activity ceases,burrows are no longer have been limited. For example, immediate maintained, soil erodes into the holes, and and long-term rodenticidal effects on non- vegetation recaptures the mounds (Klatt target wildlife such as deer mice (Peromyscus 1971, Potter 1980). maniculatus ) have not been fully evaluated. Rodenticides used for prairie dog control Appliers, when selecting toxic baits, often include zinc phosphide and strychnine. Zinc overlook information on the margin of safety phosphide is an acute rodenticide that ap- to nontarget wildlife. pears to have limited environmental impact Small mammals are important components (Hilton et al. 1972). Its increased use in recent of prairie dog towns. Their fossorial activities years (Schenbeck 1982) has resulted in im- mix and enrich soils; their food habits may proved formulations and application rates affect vegetation, seed, and invertebrate dis- (Tietjen 1976). Secondary poisoning from zinc tribution and abundance; and they provide a phosphide poses minimal threat to predators food base for predators. When small mammals and scavengers that feed on poisoned ingest rodenticides used to control prairie carcasses (Bell and Dimmick 1975, Schitoskey dogs, incidental loss may change the ecologi- 1975, Hegdal et al. 1981). cal balance on prairie dog towns. Nontarget wildlife that consume strych- Rodenticides, in addition to causing direct nine bait or strychnine-poisoned carcasses are mortality to nontarget wildlife, may impact at risk (Rudd and Genelly 1956, Schitoskey them indirectly by removing or reducing 1975, Hegdal and Gatz 1977, Deisch et al. prairie dog populations. Prairie dogs create 1989). Apa (1985),, in a companion study, niches for small mammals in rangeland eco- found that strychnine used for prairie dog systems (Koford 1958, Allen 1967, O'Meilie control reduced Horned Lark (Eromophila et al. 1982, MacCracken et al. 1985, Agnew alpestrus) densities. et al. 1986). For example, prairie dogs act as Little information is available on repopula- regulators by maintaining habitat tion of small mammals following rodenticide

1Institute for Wildlife Studies,Box 2500, California 2USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Rapid City, South Dakota 57701. 3Retired, Cooperative Fish and Wildlife Research Unit, South Dakota State University, Brookings, South Dakota57006.

347 348 M. S. DEISCH et al. [Volume 50 treatment (Wood 1965). Such information is May through August 1984 (posttreatment). needed to formulateguidelines for federal, Eighteen permanent 100 X 100-m (1.0-ha) state, and private landowners for minimizing sampling sites were established on 15 prairie nontarget wildlife losses caused by prairie dog dog towns. Rodenticide treatments were clus- rodenticides. A program to control black- tered into three separate groups to prevent tailed prairie dogs (Cynomys ludovicianus) in cross-contamination with respect to wide- western South Dakota provided us the oppor- ranging nontarget species (6 sites per rodenti- tunity to assess and compare immediate cide treatment) 13 and 16 km apart. Each (direct) and long-term (indirect) impacts on rodenticide treatment had 3 control and deer mouse densities of three prairie dog con- 3 treated sites. Only zinc phosphide treat- trol treatments: prebaited zinc phosphide, ments were applied to the park sites because prebaited strychnine, and strychnine alone. strychnine use is forbidden. Prebaited strych- nine and strychnine alone were applied to the sites. Study Area Relative densities of small mammals This study was conducted on the Buffalo (unique mammals/trap session) were deter- Gap National Grasslands and in the Badlands mined for each of the 18 sites. A trapping grid National Park of western South Dakota at ele- included 64 Sherman live traps 10 m apart and vations of 820-900 m. Geological formations a 10-m buffer border. Trapping began in May consisted of sharp pinnacles, towers, steep of each year and continued at four-week inter- gorges, and faults. Vegetated tabletop buttes vals. Each trap session consisted of one night and gently rolling mixed grasslands scattered of prebaiting followed by four consecutive throughout the area supported prairie dog nights of trapping (256 trap nights/session). towns. Traps were baited with a peanut butter-rolled The National Grasslands, located in Conata oats mixture. Captured were identi- Basin, is grazed by cattle from mid-May to late fied to species, assigned a unique number by October each vear. Native include toe amputation (Taber and Cowan 1969), then black-tailed (Lepus californicus) and white- released. Density was measured as the num- tailed jackrabbit (L. townsendii), eastern cotton- ber of unique captures. tail (Sylvilagus floridanus) , (Antilo- Rodenticides and Bait Application capra americana ), (Odocoileus hemionus), and various small mammals. The Steam-rolled oats used for prebait and poi- Badlands National Park excludes cattle, but soned baits were formulated at the U. S. Fish American bison (Bison bison) are present. and Wildlife Service Pocatello Supply Depot. Dominant grasses are western wheatgrass Zinc phosphide was applied to steam-rolled (Agropyron smithii) , blue grama (Bouteloua oats at a concentration of 2.0% by weight gracilis), buffalograss (Buchloe dactyloides), active ingredients. (Alcolec S, used as an ad- and needleleaf sedge (Carex eleocharis) . Prai- hesive, was made by American Lecithin Co., rie dogweed (Dysodia papposa ), Patagonia Inc.) Strychnine alkaloid was applied to oats Indianwheat (Plantago patagonica), buck- at 0.5% by weight. Nontreated steam-rolled horn (Plantago spinulosa), scarlet globemal- oats (4 g) were applied as prebait for zinc low (Sphaeralcea coccinea), and prostrate phosphide and for one strychnine treatment bigbract verbena (Verbena bracteata) are during 20-21 September 1983. Prebaited dominant forbs. areas were visited prior to baiting to assure Climate is semiarid-continental with ex- that most of the prebait had been consumed. tremely cold winters and hot, fluctuating Active rodenticides on oats (4 g) were applied summer temperatures. Average annual pre- three days after prebaiting (22-24 September cipitation is 39.7 cm, most of which falls as 1983) in accordance with federal instructions. high-intensity thundershowers from April Both prebait and rodenticides were applied through September. from bait dispensers affixed to Honda 3-wheel ATV's (Schenbeck 1982). Methods and Materials Statistical Aspects Small mammals were sampled from May Small mammals, includingnontarget deer through October 1983 (pretreatment) and m ice, weresampled on eachof 18 sites one

352 M. S. DEISCH ET AL. [Volume 5050 response-stimulating odor that proved attrac- (Rodentia). The Americann Society of Mammalo- gists. Special Publication No. 2. tive to small mammals, but strychnine did not BELL, H. B., AND R. W. DIMMICK. 1975. Hazards to have an attractive effect on rodents. Based on predators feeding on prairie voles killed with zinc these findings, discontinuation of zinc phos- phosphide. Journal of Wildlife Management 9: phide for prairie dog control is not recom- 816-819. CARMER, S. G. 1976. Optimal significance levels for appli- mended or required, but land management cation of the least significant difference in crop plans should include considerations for possi- performance trials. Crop Science 16: 95-99. ble nontarget deer mouse losses. We found CARMER, S. G., AND M. R. SWANSON. 1973. An evaluation of that use of strychnine alone or prebaited ten pairwise multiple comparison procedures by Monte Carlo methods. Journal of American Statis- strychnine generally showed a long-term tical Association 68: 66-74. increase in deer mouse densities. Use of these COLLINS, A. R., J. P. WORKMAN, AND D. W. URESK. 1984. two strychnine treatments for prairie dog An economic analysis of black-tailed prairie dog control appears to impose the least threat (Cynomys ludovicianus) control. Journal of Range Management 37: 358-361. to nontarget deer mice. CRABTREE, D. G. 1962. Review of current vertebrate pes- While this study addressed direct effects ticides. Pages 327-362 in Proceedings, Vertebrate of rodenticides (zinc phosphide, prebaited control conference, California Vertebrate strychnine, and strychnine alone) on deer Pest Control Tech, Sacramento. DEISCH, M. S. 1986. The effects of three rodenticides on mouse densities, impacts on other nontarget nontarget small mammals and invertebrates. Un- small mammals could not be evaluated be- published thesis, South Dakota State University, cause of the small populations observed. We Brookings. 149 pp. suspect that granivores, such as Perognathus DEISCH, M. S., D. W. URESK, AND R. L. LINDER. 1989. Effects of two prairie dog rodenticides on ground- spp. and Dipodomys spp., found on prairie dwelling invertebrates in western South Dakota. dog towns in western South Dakota, may also Pages 166-170 in A. J. Bjugstad, D. W. Uresk, be affected by rodenticides. Further investi- and R. H. Hamre, eds., Ninth gations are needed to assess nontarget losses wildlife damage control workshop proceedings. USDA Forest Service General Technical Report of small mammals other than deer mice. RM-171. Fort Collins, Colorado. 181 pp. Edgington, E. S. 1980. Randomization tests. Marcel Dekker, Inc., New York. ACKNOWLEDGMENTS Falls, J. B. 1968. Activity. Pages 543-567 in J. A. King, ed., Biology of Peromyscus (Rodentia). The Amer- This study was funded under cooperative ican Society of Mammalogists. Special Publication agreement IAG-57 with the USDA Forest No. 2. Service, Rocky Mountain Forest and Range FLAKE, L. D. 1973. Food habits of four species of rodents Experiment Station, Nebraska National For- on a short-grass prairie in Colorado. Journal of Mammalogy 54: 636-647. est, USDI Fish and Wildlife Service, the GREEN, R. H. 1979. Sampling design and statistical meth- South Dakota Cooperative Fish and Wildlife ods for environmental biologists. John Wiley and Research Unit, and the National Pesticide Sons, New York. 257 pp. Impact Assessment Program (NAPIAP). HAYNE, D. W. 1976. Experimental designs and statistical analyses in small population studies. Thanks are extended to Nebraska National Pages 3-13 in Populations of small mammals un- Forest and Badlands National Park for provid- der natural conditions. Pymatuning Laboratory of ing study areas. . Special Publication Series Volume 5. University of Pittsburgh. HEGDAL, P. L., AND T. A. Gatz. 1977. Hazards to seed LITERATURE CITED eating birds and other wildlife associated with sur- face strychnine baiting for Richardson’s ground AGNEW, W., D. W. URESK, AND R. M. HANSEN. 1986. Flora . EPA report under Interagency Agree- and fauna associated with prairie dog colonies and ment EPA-IAG-D4-0449. adjacent ungrazed mixed-grass prairie in western HEGDAL, P. L., T. A. Gatz, AND E. C. FITE. 1981. Sec- South Dakota. Journal of Range Management 39: ondary effects of rodenticides on mammalian 135-139. predators. Pages 1781-1793 in J. A. Chapman and ALLEN, D. L. 1967. The life of and plains. D. Pursley, eds., The world furbearer conference McGraw-Hill, Inc., New York. 232 pp. proceedings. Apa, A. D. 1985. Efficiency of two black-tailed prairie dog HILTON, H. W., W. H. ROBISON, AND A. H. TESHIMA. rodenticides and their impacts on non-target bird 1972. Zinc phosphide as a rodenticide for rats in species.Unpublished thesis, South Dakota State Hawaiian sugarcane. Entomology 1972: 561-571. University, Brookings. 71 pp. JONES, J. K., JR., D. M. ARMSTRONG, R. S. HOFFMANN, AND BAKER, R. H. 1968. and distribution. Pages C. JONES. 1983. Mammals of the northern Great 98-122 in J. A. King, ed., Biology of Peromyscus Plains. University of Nebraska Press, Lincoln. 1990] EFFECTS OF RODENTICIDES ON MICE 353

Klatt, L. E. 1971. A comparison of the ecology of active Sieg, C. H., D. W. URESK, AND R. M. HANSEN. 1986. Sea- and abandoned black-tailed prairie dog (Cynomys sonal diets of deer mice on bentonite mine spoils ludovicianus) towns.Unpublished thesis, Colo- and sagebrush grasslands in southeastern Mon- rado State University, Fort Collins. tana. Northwest Science 60: 81-89. Koford, C. B. 1958. Prairie dogs, whitefaces and blue Taber, R. D., AND I. MCT. COWAN. 1969. Capturing grama. Wildlife Monographs 3. 78 pp. and marking wild . Pages 277-317 in R. H. MacCracken, J. G., D. W. Uresk, and R. A Hansen. Giles, ed., Wildlife management techniques. 3rd 1985. Rodent-vegetation relationships in south- ed. The Wildlife Society, Washington, D.C. eastern Montana. Northwest Science 4: 272-278. TACHA, T. C., W. D. WARDE, AND K. P. Burnham. 1982. Use and interpretation of statistics in wildlife jour- Marsh, R. E., W. E. Howard, AND S. D. PALMATEER. 1970. Effects of odors of rodenticides and adherents on nals. Wildlife Society Bulletin 10: 355-362. attractiveness of oats to ground squirrels. Journal Terman, C. R. 1968. Populations dynamics. Pages 412- of Wildlife Management 34: 821-825. 445 in J. A. King, ed., Biology of Peromyscus (Rodentia). The American Society of Mammalo- Metzgar, L. H. 1980. Dispersion and numbers in Per- gists. Special Publication No. 2. omyscus populations. American Midland Natural- Tietjen, H. P. 1976. Zinc phosphide: its development ist 103: 26-31. as a control agent for black-tailed prairie dogs. O'Meilia, M. E., F. L. KNOPF, AND J. C. LEWIS. 1982. U.S. Department of International Fish and Wild- Some consequencesof competition between life Service.Special Science Report Wildlife prairie dogs and beef cattle. Journal of Range No. 195. 14 pp. Management 35: 580-585. URESK, D. W. 1985. Effects of controlling black-tailed Potter, R. L. 1980. Secondary successional patterns fol- prairie dogs on plant production. Journal of Range lowing prairie dog removal on shortgrass range. Management 38: 466-468. Unpublished thesis, Colorado State University, URESK, D. W., R. M. KING, A. D. Apa, M. S. Deisch, Fort Collins. AND R. L. LINDER. 1988. Rodenticidal effects of ROMESBURG, C. 1981. Randomization tests. Resource zinc phosphide and strychnine on nontarget spe- Evaluation Newsletter. Pages l-3 in Technical cies. Pages 57-63 in D. W. Uresk, G. L. Schen- Article 1. USDI Bureau of Land Management, beck, and R. Cefkin, eds., Eighth Great Plains Denver Federal Center, Denver. wildlife damage control workshop proceedings. RUDD, R. L., AND R. E. GENELLY. 1956. Pesticides: their USDA Forest Service General Technical Report use and toxicity in relation to wildlife. California RM-154. Fish and Game Bulletin No. 7. 208 pp. URESK, D. W., R. M. KING, A. D. Apa, AND R. L. LINDER. SCHENBECK, G. L. 1982. Management of black-tailed 1986. Efficacy of zinc phosphide and strychnine prairie dogs on the National Grasslands. Pages for black-tailed prairie dog control. Journal of 207-217 in R. M. Timm and R. J. Johnson, eds., Range Management 39: 298-299. Fifth Great Plains wildlife damage control work- WOOD, J. E. 1965. Response of rodent populations to shop proceedings, University of Nebraska, Lin- controls. Journal of Wildlife Management 29: coln. 425-438. SCHITOSKEY, F., JR. 1975. Primary and secondary hazards of three rodenticides to kit fox. Journal of Wildlife Received 20 December 1990 Management 39: 416-418. Accepted 28 January 1991