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2 - Second Eastern Wildlife Damage Control Conference (1985) Eastern Wildlife Damage Control Conferences

September 1985

COMPARISONS OF STRYCHNINE AND ZINC PHOSPHIDE IN PRAIRIE DOG CONTROL

H. Todd Holbrook University of Nebraska Panhandle Research and Extension Center, Scottsbluff, Nebraska

Robert M. Timm University of Nebraska - Lincoln

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Holbrook, H. Todd and Timm, Robert M., "COMPARISONS OF STRYCHNINE AND ZINC PHOSPHIDE IN PRAIRIE DOG CONTROL" (1985). 2 - Second Eastern Wildlife Damage Control Conference (1985). 24. https://digitalcommons.unl.edu/ewdcc2/24

This Article is brought to you for free and open access by the Eastern Wildlife Damage Control Conferences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in 2 - Second Eastern Wildlife Damage Control Conference (1985) by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. COMPARISONS OF STRYCHNINE AND ZINC PHOSPHIDE IK PRAIRIE DOG CONTROL

by H, Todd Holbrooke and Robert M. Timm°o

ABSTRACT INTRODUCTION Efficacy and safety are primary The blackballed prairie dog considerations in registration and use (£3momy_3_ ludovicianua) is an abundant of toxicants for vertebrate pest rangeland pest of increasing concern to controlo Strychnine (0,5$) and zinc farmers and ranchers in the Great phosphide (2$) are currently registered Plains,, In western Nebraska, prairie by EPA for prairie dog control, but dog populations increased approximately continued registration is uncertain. 60? between 1970 and 1980 (Nebraska Two percent zinc phosphide bait has Game & Parks, unpubl, data). Although been suspected of producing lower and little data are available since 1980, more erratic results than strychnine landowner complaints suggest that an baito In our study in western Nebraska upward trend has continued to the in fall 198MD indices based on changes present. Notable increases also have in burrow activity showed no difference been observed in other areas of the in efficacy (£=0,66) or variability Great Plains (Fagerstone 1981), (£=0<,7) of control for strychnine and Prairie dogs and associated grazing zinc phosphide, however neither wildlife have been reported to reduce toxicant consistently gave effective total forage availability to livestock control of blackballed prairie dogso by 2k to 37? on established prairie dog Costs for proper control (prebait and towns (Hansen and Gold 19770 O'Meilia poison) were similar for strychnine and 1980, Hyde 1981), However, prairie zinc phosphide,, For clean=up of dogs increase the perennial grasses surviving prairie dogs, fumigation with that are heavily utilized by livestock aluminum phosphide was more effective (Bonham and Lerwick 1976, Gold 1976, than shooting and more cost effective Coppock 1980), Since the effects vary than shooting combined with fumigation,, among geographical areas and plant Observed nontarget wildlife losses (M communities (Fagerstone 1981), there is horned larks and 2 lagomorphs) were no consensus on the amount of rangeland small and of doubtful biological damage that is caused. In most significance. Availability of both situations economic loss is likely and strychnine and zinc phosphide is part long term rangeland damage is possible of the flexibility needed in an when substantial livestock grazing integrated approach to prairie dog occurs on active prairie dog towns. controlo Also, there is need for Poison grain bait is the most additional toxicants or methods practical and cost effective technique that will give more consistently for controlling prairie dogs (Boddicker efficacious control. 1983)o Strychnine and zinc phosphide are the only toxicants currently federally registered for this purpose, °University of Nebraska Panhandle although Compound 1080 was widely used Research and Extension Center, in the past and is still used in Scottsbluff, Nebraska 693615 current Colorado for prairie dog control. In address; Georgia Dept, of Natural 1980 the U0So Environaental Protection Resources, 2150 Dawsonville Hwy,0 Agency (EPA) proposed cancellation of Gainesville, GA 30501, strychnine for prairie dog control (EPA 1980a, 1980b)o The Scientific Advisory Q0 5 Department of Forestry0 Fisheries and Panel reviewing EPA s recommendations Wildlife, University of Webraska0 on strychnine withheld support for Lincoln, Nebraska 68583=0819,, cancellation in part because of

73 uncertainty concerning zinc phosphide's and containing a yellow dye (South efficacy. More recently EPA indicated Dakota Dept, of Agriculture, Pierre, intentions of suspending manufacture South Dakota)„ All baits were fresh and sale of zinc phosphide baits and in good condition. Baits were manufactured under U,S, Fish and applied in the manner and at rates Wildlife Service labels because of specified on the product labels. failure to provide EPA with specific Clean-up of surviving prairie dogs was supportive data (Henderson 1984s, Ro attempted with fumigation (aluminum Kelly USFWS perso commo)o phosphide, Phostoxin®, using 3 tablets Efficacy and safety are important per burrow entrance), shooting, or a considerations in future registration combination of the two. Only burrows and use of toxicants for prairie dog that apparently were active after toxic controlo An objective of this study grain treatments were fumigated. was to test the efficacy of 0»5? Prior to control, prairie dog towns strychnine and 2% zinc phosphide baits were surveyed for nontarget wildlife for prairie dog control. We also and assessed for potential problems. report on the value of various Landowners were questioned about techniques for clean-up following certain at=risk wildlife species poison grain treatments;, costs occuring the vicinity of the study associated with control operations,, and sites. No evidence of black-footed potential hazards to nontarget ferrets (Mustela jxtRrlp_e.s) was found wildlife. at any site. During the prebaiting The authors thank ROE. Marsh for his activities, landowners were trained helpful comments on an earlier draft of individually in the proper application this paper» We thank the Upper of toxic bait. Within 10 days of Niobrara-White and North Platte Natural prebaiting, the landowners applied the Resources Districts for providing toxicant. During the 1 to 4~day period materials used in the project, and following bait application, towns were Extension agents Mo Hendricks, D, Hulsp searched twice for dead nontarget Do O'Dea, D. Robinson, R, Roeber, and wildlife. Subjective evidence was used Fo Whetzal for aid in locating the in determining cause of death for any cooperating landownerso nontarget animals found. Necropsies were not conducted. METHODS Percent burrow activity on each town In fall 1984, 18 prairie dog towns was calculated by marking and plugging from 1 ha to 8 ha in size were selected a sample of burrows and counting the for study in the mixed-grass prairie of number reopened. Efficacy was western Nebraska, Prairie dog determined by the change in burrow populations were not determined, but activity before and after baiting towns contained 100 to 125 burrows/ha. (Boddicker 1983)« Change in burrow Approximately one-third of all burrows activity was also recorded on 9 were active. Prairie dog towns were untreated sites, located in Box Buttep Cheyenne, Morrillj, Scotts Bluff, and Sioux STATISTICAL ANALYSIS countieso Prairie dog towns for both the The towns were randomly divided into strychnine and zinc phosphide-treated 2 groups. Half were prebaited with groups were ranked by the percent untreated steam-rolled oats prior to reduction in burrow activity, A application of steaxa°rolled oats Wilcoxon Rank Sum test was used to treated with 2$ zinc phosphide (U,S, compare efficacy of strychnine and zinc Fish and Wildlife Service, Pocatello,, phosphide, Moses' test was used to Idaho), The remaining towns were determine if the variability in prebaited with untreated whole oats reduction of prairie dog activity was prior to the application of whole oats different for the two toxicants treated with 0,52 strychnine alkaloid (Hollander and Wolfe 1973)»

74 RESULTS STRYCHNINE There was no difference (£.=0o66) in percent reduction of burrow activity between strychnine (median = 69, X=69t 100 17o5 M=9) and zinc phosphide (median = 66, 87 16 1=66, 1=9) baits (Table 1, Figure 1). 85 15 A small reduction in burrow activity 84 14 was observed on the untreated towns 69 12 (median = 2, 1=12, 1=9). Standard 63 6 deviations in the reduction of burrow 50 5 activity were similar for strychnine 47 4 (21.ft?) and zinc phosphide (18.7?) 37 1.5 treatments (£=0,7)„ Shooting during fall and winter was mean (X) = 69 an ineffective clean-up technique;, median =69 removing less than 1 prairie dog per rank sum =91 town. Shooting in combination with fumigation was less cost effective than fumigation alone because it added labor ZINC PHOSPHIDE while removing few additional prairie dogSo Of the burrows that were Reduction 1tenk. fumigated (1=84), 12? were reopened by 100 17»5 prairie dogso 83 13 Cost of materials including prebait 68 10,5 was $3<,21/ha for strychnine and 68 10,5 $2«15/ha for zinc phosphide treatments,, 66 9 The prebaiting and toxic bait 65 8 application required 2O4 hrs/ha of 64 7 labor,, Clean~up fumigation with 44 3 aluminum phosphide cost $0»60/burrow in 37 1.5 materials and required 0o09 hrs/burrow of labor to find and fumigate the mean (X) =66 surviving prairie dogs. When labor median = 66 costs were considered to be $5<,00/hr, rank sum = 80 the total cost for control, including both labor and materials (prebaiting, toxic bait application, and fumigation) Table 1„ Percent reductions in burrow was $20o09/ha for strychnine treated activity and resulting nonparametric towns and $19.03/ha for zinc phosphide ranks for 0,5? strychnine and 2% zinc treated towns„ phosphide treated prairie dog towns in The total area treated was 1402 ha western Nebraska, fall with strychnine and 20„2 ha with zinc phosphide. Three horned larks (JSremophila aloestris),, a cottontail none were found on zinc phosphide rabbit (Svlvilagus fJLojp.idanus), and a treated towns. Although nontreated blacktail jackrabbit (Leous towns were not systematically searched, californicus) were found dead on no dead nontarget wildlife or prairie strychnine treated towns„ and 1 horned dogs were observed on nontreated towns lark was found dead on a zinc phosphide or on treated towns prior to treated town0 Timing of mortalities application of the toxic baito and proximity of carcasses to toxic bait suggest that these animals died from consumption of toxic bait,, Seven prairie dogs were found dead above ground on strychnine treated towns, and

75 1978), Subsequently;, 2 sites were controlled similarly resulting in 95,7? 2 Ira (3 and 95 <.6% reduction in activity for S ft ir 1?

76 (Boddicker 1983),, and that zinc This may have been a result of phosphide bait may produce more varied strychnine's faster action, causing results than strychnine (Sullins 1980)o poisoned animals to die on the study Although both toxicants were of sites while the slower action of zinc variable effectiveness, there was no phosphide permitted poisoned animals to difference (£=0o66) in efficacy between wander off the sites before dying. It the 0,555 strychnine and 2% zinc is also possible that the color, taste,, phosphide treatments,, Standard or odor of zinc phosphide bait made it deviations of reduction in burrow less attractive to certain species than activity for strychnine (21,451) and the yellow=dyed strychnine bait. zinc phosphide (18,7?)„ Moses' test for Strychnine baits have been effectively dispersion differences between these used in control of certain birds and treatments, and the box plots (Figure lagomorphs8 while zinc phosphide baits 1) indicated similar variances for have not. Recovering more dead prairie these toxicants. The high p=values dogs above ground on the strychnine^ imply that even with large sample treated towns was not consistent with sizes, no differences in efficacy or Sullins' (1980) work, but it again may variability of control would be found. be due to the slower action of zinc For most situations present in phosphide^ allowing poisoned animals to western Nebraska, we regard a level of return to their burrows before dying. control greater than 8055 to be We consider the nontarget losses adequate; in our study, this level of that we observed to be biologically efficacy was obtained on only k of 9 insignificant. The chance of negative strychnine treated sites and 2 of 9 impact on unendangered wildlife zinc phosphide treated sites0 populations from direct or secondary Reduction in prairie dog activity was poisoning while correctly using either less than 5051 on k of 18 sites (2 of 9 of these toxicants is remote. It is sites in each treatment group). The possible that prebaiting increased cause for these poor levels of control hazards to nontargets by attracting was unknown, but at three of the four them to the site and conditioning them sites where less than 50$ reduction was to consume the grain bait. Further, observed, grass appeared to be taller prebaiting approximately doubles the and more dense than on other sites. labor cost involved in control. Although vegetation measurements were Therefore, all other factors being not taken, we suspect these existing equal, a toxicant which is efficacious food supplies may have competed with without prebaiting may be less grain baits and thereby lowered bait hazardous to nontarget animals as well acceptance. as more cost-effective. In our study, control with Results of this study suggest that strychnine bait cost $1,06/ha more than both strychnine and zinc phosphide control with zinc phosphide bait, the baits give similar but variable results difference being due to bait costs. when used for control of black-tailed Labor costs were high because all prairie dogs. Fumigation with aluminum baiting was done on foot. Compared to phosphide can be used effectively as a the total cost of control of $20,09/ha clean~up method following toxic grain (including labor and materials for bait treatment. Cost differences for prebaitingj, toxic bait application,, and the two grain baits tested were small fumigation), this difference in bait in comparison to the total cost of the cost is relatively minor <, control operation. Hazards to The observed nontarget mortalities unendangered nontarget wildlife may apparently resulted from direct exist, but they are not likely to be of consumption of toxic bait. Finding biological significance. There is also more dead nontarget animals on need for additional toxicants or other strychnine treated towns was consistent cost effective control methods that can with results in Montana (Sullins 1980), b© used safely to provide consistent,

77 high levels of prairie dog control„ Gold, I.K, 1976, Effects of blacktail The best toxicant for integrated prairie dog mounds on shortgrass control of prairie dogs will depend on vegetation, M,S, Thesis, Colorado a variety of local considerations. At State University, Ft, Collins, CO, the current time, we believe both tO pp, strychnine and zinc phosophide need to be available,, Hansen, R.M, and I»K0 Gold, 1977, Blacktail prairie dogs, desert LITERATURE CITED cottontails,, and cattle trophic BoddickerD M0L. 1983= Prairie dogs, relations on shortgrass range, J, Pp, B75=B8J* ins Prevention and Range Manage, 30s210=214o Control of Wildlife Damage, Great Plains Agricultural Council and Henderson, Ro 1984, Zinc phosphide Nebraska Cooperative Extension manufacture may be suspended, Service, 632 pp, Kansas Pesticide Newsletter 7;70,

Bonham, CD, and A, Lerwick, 1976, Hollander, M, and D.A. Wolfe, 1973, Vegetation changes induced by Nonparametric Statistical Methods, prairie dogs on shortgrass range, John Wiley and Sons, New York, 503 J, Range Manage, 29:221=225,

Brakke, V, 1982, South Dakota rodent Hyde, R,M, 1981o Prairie dogs and control update. Proceedings of their influence on rangeland and WRCC=42 Meeting, Sacramento, CA, livestocko Proc, Gt, Plains Wildl, South Dakota Dept, of Agricultures, Damage Cont, Wkshp, 5s202=206, Pierre, SDO 20 pp, O'Meilia, M.E, I98O0 Competition Coppock, D,L, 1980, Bison-prairie between prairie dogs and beef cattle dog=plant interactions in a northern for range forage, M,S, Thesis, mixed~grass prairie. Bull, Ecol, Oklahoma State University, Soc. Am, 61;113» Stillwater, OK, 33 PP»

Environmental Protection Agency, Record,, C,R, 1978, Ground squirrel 1980a, Strychnine; preliminary and prairie dog control in Montana, notice of determination concluding Proc Vert, Pest Conf, 8;93~97, the rebuttable presumption against registration of pesticide products. Sullins, Mo 1977, Evaluation of Federal Register 48(216):73602~ prebait for improving bait 73608, acceptance by black=tailed prairie dogs. Report to Montana Dept, of Environmental Protection Agency, Livestock^ Vertebrate Pest Control 1980b, Strychnine; position Bureau, 6 ppo document 2/3, Office of Pesticide Programs, Office of Pesticides and Sullins, Mo 1980, Efficacy of Toxic Substances, EPA, Washington, strychnine and zinc phosphide baits D.Co 109 pp. for controlling blackballed prairie dogs. Report to Montana Dept, of Fagerstone, K.A, 1981, A review of Livestock,, Vertebrate Pest Control prairie dog diet and its variability Bureau, 3 pp, among animals and colonies, Proc, Gt, Plains Wildl, Damage Cont, Swick, Co Do 1976, A field evaluation Wkshp, 5 of strychnine, zinc phosphide and 1080 grain baits for prairie dog control, Montana Dept, of Livestock Report, 9 ppo

78 Tietjen, HOPO 1976, Zinc phosphide; its development as a control agent for black-tailed prairie dogso Fish and Wildlife Service, USDI, Washington,, DOC.P Special Scientific Report-Hildlife Noo 195o 14 pp0

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