Report 16 Control of Kangaroo Rats With Poison Baits Range Improvement Task Force Agricultural Experiment Station Cooperative Extension Service
ABSTRACT Contents Description of study area... 1 Methods... 2 Results and discussion... 2 Kangaroo rat control... 2 Non-target animals killed... 2 Other observations... 2 Summary... 4 Literature cited... 5 Kangaroo rats (Dipodomys spp.) were treated with three poison baits in tobosa (Hilaria sp.) - grama (Bouteloua spp.) grasslands in southern Hidalgo County during 1981 and 1982. Two strychnine-oat baits (0.50% A1 and 0.16% AI) and zinc-phosphide (2% AI) pelleted bait were used in each of four treatment periods. Treatments were hand applied to 100 active kangaroo rat mounds for each bait in September 1981, and January, April and June, 1982, to determine seasonal control levels and differences between baits during each treatment period. The best overall period of control occurred in June when considering all three baits (74.9%). The highest rate of control (88.3%) was with 0.50% strychnine oats in June. When considering all baits, January produced the second highest control percentage of (72.8%). April produced the lowest levels of control for all baits. Overall, 0.50% strychnine oats gave the best control (avg. 80.4%) followed by zinc-phosphide pelleted bait (55.5%) and 0.16% strychnine oats (53.5%). New Mexico State University is an equal opportunity employer. All programs are available to everyone regardless of race, color, religion, sex, age, handicap, or national origin. January 1984 Las Cruces, New Mexico
Control of Kangaroo Rats With Poison Baits V. W. Howard, Jr. and Michael Bodenchuk* Kangaroo rats are, in many areas of the Southwest, serious range pests. Wood (1969) found that Ord's kangaroo rats (Dipodomys o r dii) ate about 1300 lbs air-dry of plant materiallsectionlyear based on average population densities on the Jornada del Muerto. He also found that banner-tailed kangaroo rats (D. spectablis), at average densities in the same area, ate an additional 336 lbs air-dry of plant materiallsectionlyear. Additional to these amounts are plant materials stored underground (2.9 tonslsectionlyear for D. spectablis alone) and forage production lost by rodent mounds (1 0.6% of the excellent condition rangeland). Poison baits are an efficient mechanism to control rodents on rangelands. In 1964 the Leopold Committee recommended compound 1080 be banned as a rodenticide on rangelands because of secondary hazards. This left strychnine as the only rodenticide labelled by the Environmental Protection Agency (EPA) for range rodents control. Personnel at the Denver Wildlife Research Center (DWRC) subsequently found that strychnine poses hazards to humans and nontarget wildlife. The EPA is now reviewing all strychnine labels under the Rebuttable Presumption Against Registration (RPAR) process. Suggested modifications would cancel the current strychnine (0.50%) label and require a new (0.16%) label registration. This concentration is untested for rangeland rodent control. Furthermore, chemical data indicate better toxicants (zinc phosphide) may be available. Wood (1965) investigated control of desert rangeland rodents by various techniques using *Professor, Department of Fishery and Wildlife Sciences and former Range Specialist, N.M. Department of Agriculture. respectively.. ' compound 1080 and 0.50% strychnine treated oats. He determined that den baiting was the most effective technique for kangaroo rats with both toxicants. The efficacy of zinc phosphide and 0.16% strychnine oats in field applications on kangaroo rats is unknown. The objective of this study was to determine the efficacy of zinc phosphide rodent bait and two concentrations of strychnine treated oats on kangaroo rats during different seasons of treatment. DESCRIPTION OF STUDY AREA Study plots were located on the Gray Ranch approximately 20 miles south of Animas, Hidalgo County in southwestern New Mexico. The area is a broad, gently sloping valley with mountain ranges to the east and west. Maker et al. (1970) describes the climate for the area as continental, arid to semi-arid, characterized by light (10 to 20 in) precipitation and large dirunal and annual temperature changes, distinct seasons, plentiful sunshine and low humidities. Half the annual rainfall occurs from July to September as brief, but often heavy, showers and thunderstorms. Average annual snowfall is less than 10 in., but more than 16 in. may fall in the surrounding mountains. Average annual temperatures are between 58 and 62 degrees F with daily ranges in excess of 30 degrees. Mean maximum and minimum temperatures are 73 and 42 degrees F, respectively. Soils in the area are the Eba-Cloverdale-Eicks association which support a good' cover of vegetation used by livestock and wildlife. Principal grasses include tobosa (Hilaria mutica),
fluff-grass (Erioneuron pulchellum), black grarna (Bouteloua eriopoda), blue grama (B. gracilis), sideoats grarna (B, curtipendula), three awns (An'stida spp.), vine mesquite (Panicurn obtusum) and ring muhly (Muhlenbergia torreyziz. Common shrubs are mesquite (Prosopsis juliflora), fourwing saltbush (A triplex canescens), white thorn (Accacia sp.) and mormon tea (Ephedra sp.). Thin, scattered stands of cholla cactus (Opuntia sp.) and sachuista (Nolina sp.) occur on the upper slopes near the outer fringes of this association. METHODS The field research was conducted during September 1981, and January, April and June, 1982. Plots for each of the three treatments were randomly assigned during each treatment period. Each plot was large enough to include 100 active rat mounds. The plots were adjacent to each other to include similar terrain and vegetation, but were separated by a 200-ft buffer strip to reduce possible crossing-over by kangaroo rats while gathering food. Plot corners were marked with a 3-ft wooden stake painted blaze orange for ease of location. Each mound within the plots received approximately 2 teaspoons of bait. The three baits included 0.50% active ingredients (AI) and 0.16% A1 strychnine treated oats, and 2% A1 zinc phosphide rat bait. Each bait was hand distributed in two areas on opposite sides of 100 mounds per treatment. Each treated mound was marked with surveyor's wire and flagging. A different color was used for each treatment to facilitate relocation of mounds and to separate data. All treatments remained in place for a minimum of 7 days before the mounds were checked for signs of kangaroo rat activity. These signs included fresh tracks, dropping and mounding activity. Presence of spider webs in burrow entrances also indicated no recent activity by kangaroo rats. Whenever no recent signs of activity were noted and bait was missing, the rat(s) occupying the mound were classified as dead. It was assumed the difference in death rates among mounds treated with the three baits was the result of difference in baits. Death losses from other causes were assumed to be similar among the three treatments. Additional data were recorded for each treatment. These included incidence of precipitation between treatment and checking for rates of control, presence of dead rats above ground, presence of dead nontarget species, and digging in mounds by scavenging carnivores. RESULTS AND DISCUSSION Kangaroo Rat Control The results of the four seasonal treatments are shown in table 1. Data in this table indicate 0.50% strychnine treated oats had the highest average control rate and the highest control rate during each of the four treatment periods. Zinc phosphide rodent bait gave slightly better results than did 0.16% strychnine oats, but results were variable with season. Neither of these treatments was as effective as the 0.50% (AI) strychnine oats. Overall, June was, the best period to treat kangaroo rats. Both 0.50% strychnine oats and zinc phosphide rodent bait had their highest control percentages during this period (88 and 70% control, respectively). The 0.16% strychnine oats were most effective in January, the second most effective treatment period of this study. April was the least effective treatment period as all three baits were least effective during this treatment period. Non-target Animals Killed Relatively few non-target species were observed dead on the study plots while checking mounds to determine the efficacy of each treatment. During the September treatment, one grasshopper mouse (Onychomys sp.) was found on the plot treated with 0.16% strychnine oats. During the January treatment, one male horned lark (Eremophila alpestris) was found on the plot treated with 0.16% strychnine oats and one silky pocketmouse (Perognathus flavus) was found on the plot treated with 0.50% strychnine oats. No dead non-target animals were found on any of the treated plots in April or June. The oats were apparently more attractive to some nontarget rodents and seed-eating birds than was the zinc phosphide treated rat pellets. Other Observations In September and January, light to moderate rain fell on the plots between application of the baits and subsequent readings for efficacy. No rain was recorded during either April or June sampling periods. Other observations such as numbers of mounds dug into by scavenging mammals, numbers of mounds with'bait remaining and numbers of dead kangaroo rats are shown in table 2. Most scavenging occurred dur-
Table 1. Results of experiments to control kangaroo rats in southern Hidalgo County, 1981-82 September 1981 January 1982 April 1982 June 1982 Mean Treatment Treat. Read Actv. Contr. Treat. Read Adv. Contr. Treat. Read Actv. Contr. Treat. Read Actv. Contr. Contr. no. no. m. % no. no. no. % no. m. no. % no. no. no. % % Zinc-Phoshide Rodent Bait 100 96 51 46.9 100 99 42 57.6 100 87 47 46.0 100 98 29 70.4 55.2 0.16% Strychnine Oats 100 95 45 52.6 100 92 23 75.0 100 90 74 17.8 100 99 33 66.7 53.0 0.50% Strychnine Oats 100 94 17 81.9 100 99 14 8'5.9 100 90 32 64.4 100 94 11 88.3 80.1 Average 60.5 72.8 42.7 75.1 Table 2. Incidental observations during experiments to control kangaroo rats in southwestern Hidalgo County, 1981-82 September 1981 January 1982 April 1982 June 1982 Totals Dug Rat With Dug Rat With Dug Rat With Dug Rat With Dug Rat With ' Treatment Into Carcasses Bait Into Carcasses Bait Into Carcasses Bait Into Carcasses Bait Into Carcasses Bait... number---------------------------------------------------------------------- Zinc Phosphide Rodent Bait 0 0 0 0 1 7 0 0 20 22 12* 0 22 13 27 0.16% Strychnine Oats 0 2 0 2 0 18 1 0 1 18 1 0 21 3 19 0.50% Strychnine Oats 0 2 0 2 2 4 2 0 2 8 0 9 12 4 15 Total 0 4 0 4 3 29 3 0 23 48 13 9 *Partial carcasses only.
ing the June treatment and was done by coyotes (Canis latrans). This activity coincides with the time adult coyotes would require large amounts of prey to feed rapidly growing pups. No dead coyotes were found on or around the study plots. A high incidence of zinc phosphide bait being left outside the burrows coincided with the low control percentage in April. However, this was not true for 0.16% strychnine oats in January when 18 mounds had bait remaining (table 2), but there was a 75% control rate recorded for the study plot (table 1). This control rate was the highest recorded for any season for this bait. It appears that some mounds believed to be active when baits were applied were either inactive or were receiving some use by kangaroo rats from neighboring mounds on that particular study plot. Similarly, there were nine mounds with 0.50% strychnine oats remaining in June (table 2) and simultaneously the highest percentage (88.390) of control (table 1) was recorded for any season with that bait. SUMMARY AND CONCLUSIONS Wood (1965) investigated control of desert rangeland rodents in southern New Mexico by various techniques using compound 1080 and strychnine treated oats. He determined that den baiting was the most effective technique for control of kangaroo rats with both toxicants. Wood (1969) also determined that 12 species of range rodents in southern New Mexico were removing almost 4 tons of vegetation per section per year. The banner-tailed kangaroo rats alone, in the climax grassland areas, kept 10% of the area out of vegetative production by denuding the areas of their mounds. Two kangaroo rat; species and two woodrat species were found to have similar ranges but different food and shelter requirements and, as a result, were able to more completely utilize the food resource with little competition for any specific food item. Food items stored in banner-tailed kangaroo rat dens included perennial grasses (18%), annual grasses (23%), snakeweed (Gutierrezia sp.) (43%) and pepper grass (Lepidium sp.) (7%). Athough rodents and livestock are not in direct competition for all their food items, any competition for perennial grasses or seasonal forbs would be expected to decrease stocking rates or prevent recovery of areas where previous overgrazing had caused a decline in successional stages. This would be particularly critical where the carrying capacity for livestock is already low, or- in areas where rest can enhance pasture recovery. Recent chemical data indicate zinc phosphide treated grain or pelleted bait could prove to be more effective and environmentally safe. However, zinc phosphide was not registered for control of kangaroo rats. It was registered for orchard mice (primarily Microtus sp.), prairie dogs (Cynomys spp.), nutria (Myocastor coypus), pocket gophers (Geomyidae) and rats (Sigmodon spp. and Rattus spp.) with some in-crop use permitted. Registration for range application required tolerances be set and LD50's be investigated. Personnel at the DWRC reported the approximate lethal dose (ALD) for banner-tailed kangaroo rats as 8.0 mglkg body weight. The currently marketed 2% A1 formulation was lethal to prairie dogs whose LD50 is 18.0 mglkg. Therefore, we could reasonably assume the 2% A1 formulation would be lethal to large portions of the baited populations of all Dipodomys spp. if applied at the proper season. Furthermore, the economy of the control with this product should depend largely upon the economy of the method of treatment. Results of this study indicate 0.50% strychnine oats is the best of the three baits tested. Neither the zinc phosphide bait nor the 0.16 % strychnine oats performed well during all seasons. Average percentages of control were 69% for zinc phosphide rodent bait and 66% for 0.16% strychnine oats, when compared to 0.50% strychnine oats. Mortality from these baits was 31 to 34% less than 0.5090 strychnine oats, making these alternative baits less desirable for controlling kangaroo rats on western rangelands. The added cost of a second (follow-up) treatment would probably add considerably to the cost of control which could be achieved with a single application of 0.50% strychnine oats. Because no measurable difference in hazard to non-target species was observed between the three baits tested, it would appear registration of 0.50% strychnine oats can be maintained and not replaced with one for 0.16% strychnine oats. In addition, we recommend the label for 2% zinc phosphide rodent bait be expanded to include Dipodomys sp. on rangelands. The EPA strychnine RPAR team requested our results from the first three seasonal treatments on 24 May 1982. The Draft Decision Document from EPA states the data from New Mexico indicated a less-than-desirable efficacy with the recommended 0.16% A1 concentrations and, because the agency had no data indicating adverse environmental impacts, the present label (0.50% AI) would be allowed.
/- LITERATURE CITED Maker, H. J., D. N. Cox, and J. U. Anderson. 1970. Soil associations and land classification for irrigation, Hidalgo County. N. Mex. Agri. Exp. Sta. Res. Rep. 177. 29 pp. Wood, J. E. 1965. Response of rodent populations to controls. J. Wildl. Manage. 29(3):425-438. Wood, J. E. 1969. Rodent populations and their impact on desert rangelands. N. Mex. Agri. Exp. Sta. Bul. 555. 17 pp.