REQUEST FOR DEROGATION TO USE STRYCHNINE FOR THE CONTROL OF POCKET GOPHERS

Transcription

1 REQUEST FOR DEROGATION TO USE STRYCHNINE FOR THE CONTROL OF POCKET GOPHERS The northern pocket gopher (Thomomys talpoides) is a rodent native to the northwestern United States. Pocket gophers are fossorial animals, spending most of their time solitarily in underground burrow systems (Engeman and Witmer 2000). Gophers feed on a variety of vegetation, and can be considered keystone species in some systems due to their ability to dramatically alter both soil and vegetation structure (Cantor and Whitham 1989, Reichman and Seabloom 2002). Pocket gophers, like many rodents, have high population growth rates and are able to quickly colonize and re-colonize suitable habitat even when their populations have been reduced (Engeman and Campbell 1999, Sullivan et al. 2001). Pocket gophers account for more damage to regenerating forests in the western U.S. than all other animals combined (Crouch 1986, Borrecco and Black 1990). Repeated complete failures at reforestation are not uncommon, sometimes resulting in non-forested conditions lasting for decades (Barnes 1978). Pocket gophers damage and kill large percentages of conifer seedlings in Idaho by feeding on their roots and stems (Graham and Kingery 1990, Ferguson and Adams 1994, Ferguson 1999). Regeneration failure is common in areas with high gopher densities. The Forest Stewardship Council (FSC) mission is...to promote environmentally appropriate, socially beneficial, and economically viable management of the world s forests. Good forest management should balance the three legs of FSC s mission. This mission is further articulated in FSC s 10 Principles and Criteria, of which Criteria 6.6 defines chemicals that are banned by FSC. Banned chemicals are those that are either: 1) WHO Type 1A or 1B (toxic to humans), 2) chlorinated hydrocarbons, 3) have high persistence (half life >100 days), 4) high toxicity (LD50 up to 200mg/kg), 5) carcinogens, mutagens, or endocrine disruptors, or 6) banned by international agreement. Although strychnine is banned by FSC due to its potential high toxicity to humans and other vertebrates, this application for derogation shows that it is the only feasible method for the control of pocket gophers in regenerating forests. Furthermore, we provide strong evidence that the application method used for pocket gophers virtually eliminates the risk of consumption by other vertebrates, and poses no adverse environmental impacts, and therefore its use in controlling pocket gophers is in the best interest of promoting FSC s goals. A variety of methods are available to control pocket gopher damage in regenerating forests, although all are not equally effective (Case and Jasch 1994, Engeman and Witmer 2000; see Table 1). Cultural methods include modifying habitat through harvest prescription or site preparation before regeneration, as well as planting unpalatable or genetically-resistant stock (Engeman and Witmer 2000). Harvest prescriptions that result in early-seral vegetative conditions also favor pocket gophers (Marsh and Steele 1992) by creating an abundance of preferred food composed of grasses, forbs, and conifer seedlings. These prescriptions are often necessary in order to restore the stand to the proper tree species composition. Harvest prescriptions that result in natural regeneration often perform better than planted stands (Marsh 1

2 and Steele 1992). However, successful natural regeneration is unpredictable, especially of ecologically-appropriate tree species. In our area, pocket gopher sites will have natural Table 1. Summary of relative cost-effectiveness of gopher damage management techniques. Method Cost Effectiveness Population Reduction Strychnine Baiting Low High (70-80% pop. reduction) Zinc Phosphide Baiting Low Mod Fumigants Low Low (20% pop. reduction) Trapping High (>6x baiting) Mod (45-60% pop. reduction) Enhance Predation Mod Low Habitat Manipulation Herbicides High Mod Burning High Mod Sheep/cattle grazing Low Mod Barriers Plastic tubing High High (some root deformities) Repellents High Low Silvicultural Natural Regeneration Mod-High Mod Planting Resistant Stock Low Low Larger Seedling Size High Low Increase Stocking Rate High Mod (Engeman and Witmer 2000, Brian Moser unpublished data) regeneration overwhelmingly dominated by grand fir (Abies grandis), while our ecologicallyappropriate target species mix includes western white pine (Pinus monticola), western larch (Larix occidentalis), ponderosa pine (Pinus ponderosa), and Douglas-fir (Pseudotsuga menziesii). Site preparation such as burning and herbicides can be used to reduce suitable gopher habitat (Engeman and Witmer 2000) by reducing their food supply. We prefer to use herbicides sparingly as they can be costly and also have the potential to reduce habitat for other wildlife. In addition, site preparation methods only suppress vegetation for 2-5 years, after which gopher habitat becomes suitable again. Currently, no gopher resistant tree varieties have been bred, although some species appear to be relatively more resistant (Ferguson 1999). Physical controls include both exclusionary devices and traps (Engeman and Witmer 2000). Vexar tubing has been used to effectively protect seedlings and their roots from gopher damage (Anthony et al. 1978, Engeman et al. 1997, Engeman et al. 1999, Pipas and Witmer 1999). However, the extremely high cost of this method precludes its use in most situations. In addition, the possibility of root deformation and constriction makes it unpractical (Ellis 1972). Trapping has long been used to control pocket gophers with mixed success (Marsh 1998a, Witmer et al. 1999). Potlatch research has shown that trapping is only 45-62% effective in reducing gopher populations, and costs over 6 times that of baiting (Morrow 2004, Potlatch Corp. unpublished data). 2

3 Biological control through predation has often been hailed as a natural approach to pest management (Moore et al. 1998). Although predator habitat can be enhanced (Moore et al. 1998), no studies have demonstrated effective control of gopher populations through predator enhancement (Marsh 1998b). Induced infertility is another method of biological control that is still in its infancy (Miller and Fagerstone 2000). Although this approach shows promise, there are many biological and regulatory hurdles before this becomes a cost-effective method to control gopher reproductive rates (Miller and Fagerstone 2000). Chemical controls include toxicants and repellents (Engeman and Witmer 2000). Repellents would appear to be a humane way to reduce gopher damage, although no studies have found a cost-effective method to do so (Witmer et al. 1998, Sterner et al. 1999), especially since gophers are fossorial and for a repellent to protect tree roots it must be applied underground and persist for many months including the spring snowmelt season when a significant flux of water moves through the soil. Toxicants are the most cost-effective chemical control, and strychnine alkaloid is the most costeffective gopher control method used in the western U.S. (Engeman and Witmer 2000). Strychnine is an alkaloid derived from the seeds of Strychnos nux vomica, a native tree of India. It is an acute toxicant that results in quick death when toxic amounts are consumed. Strychnine is banned by FSC only due to its potential toxicity to vertebrates. It is not a chlorinated hydrocarbon, has low persistence, is not a carcinogen, mutagen, or endocrine disruptor, and is not banned by international agreement. Although strychnine is highly toxic, its applicator exposure risk is low and it has a low hazard to non-target animals and for secondary poisoning. Strychnine is applied to grain bait entirely below ground. Application rates range from 0.25 to 1.0 lbs of 0.5% strychnine bait per acre. Once placed below ground the hole is covered with soil. Because gophers vigorously defend their burrows, intrusion of the burrow and consumption of bait by other species is unlikely. Furthermore, caches of bait degrade quickly. Strychnine alkaloid has a half-life of <30 days, and 90% degradation after 40 days (Starr 1993). Studies have shown that limited numbers of ground squirrels and other burrowing rodents may accidentally be poisoned, but that the impact on those populations is minimal (Fagerstone et al. 1980, Anthony et al. 1984, El Hani et al. 2002). Secondary poisoning can only occur if an animal consumes the digestive tract of a dead gopher, because strychnine does not accumulate in tissues. In fact, a scavenger would have to consume a lethal dose at one time, since strychnine does not accumulate over time. Scavenging of dead gophers is highly unlikely as most gophers die underground (Barnes et al. 1985). The only animals that might consume them are small mustelids, which are not usually scavengers. Birds of prey and threatened species such as grizzly bears are unlikely to be affected by strychnine baiting (Anthony et al. 1984, Barnes et al. 1985). Strychnine does not persist long in the environment. Furthermore, strychnine alkaloid binds strongly to soil particles and is not water soluble, thus there is limited movement of strychnine in the environment (Smith 1982). Vegetation does not take up strychnine in detectable quantities, 3

4 therefore the risk of an herbivore consuming strychnine is small (Smith 1982). Because strychnine is applied underground and away from water sources, binds to soil particles, and is not water soluble, the probability of water contamination is nearly zero. If small amounts did reach water, they would be so diluted as to not be toxic. Since strychnine does not accumulate in tissues, multiple sublethal doses would have no adverse effects on aquatic flora and fauna. Both the Clearwater and Nez Perce National Forests have conducted thorough Environmental Assessments for the use of strychnine baiting on National Forests, as required by the U.S. National Environmental Protection Act (U.S. Forest Service 1993a,b). Both have concluded that strychnine is not only the most cost-effective method of gopher abatement, but that it is also an environmentally-safe alternative. Without the use of strychnine, we have two alternatives. Either forgo the harvest on nearly 2000 acres each year, or accept failed regeneration on those acres. The latter is simply not possible in Idaho where forest lands must be regenerated by state law. The former represents a substantial amount of foregone revenue and over many years puts tens of thousands of acres on a trajectory for significant forest health issues that may ultimately be manifested in catastrophic wildfire. Because our acres subject to gopher damage tend to be geographically concentrated, the wildfire concern is even further heightened. We believe that neither of these alternatives are in the best interest of achieving FSC s mission. Literature Cited Anthony, R.M., and V.G. Barnes, Jr Vexar plastic netting to reduce pocket gopher depredation of conifer seedlings. Vert. Pest Conf. 8: Anthony, R.M., G.D. Lindsey, and J. Evans Hazards to golden-mantled ground squirrels and associated secondary hazard potential from strychnine for forest pocket gophers. Vert. Pest Conf. 11: Barnes, V.G., R.M. Anthony, K.A. Fagerstone, and J. Evans Hazards to grizzly bears of strychnine baiting for pocket gopher control. Wildlife Society Bulletin 13: Borrecco, J.E., and H.C. Black Animal damage problems and control activities on National Forest system lands. Vert. Pest Conf. 14: Case, R.M., and B.A. Jasch Pocket Gophers. Pages B17-B29 in S.E. Hygnstrom, R.M. Timm, and G.E. Larson, eds. Prevention and Control of Wildlife Damage. University of Nebraska Cooperative Extension, Lincoln, Nebraska. Cantor, L.F., and T.G. Whitham Importance of belowground herbivory: pocket gophers may limit aspen to rock outcrop refugia. Ecology 70: Crouch, G.L Pocket gopher damage to conifers in western forests: a historical and current perspective on teh problem and its control. Vert. Pest Conf. 12: El Hani, A., D.L. Nolte, J.R. Mason, and S. Bulkin Response of nontarget species to underground strychnine baiting for pocket gohper in southwest Oregon. Western Journal of Applied Forestry 17:9-13. Ellis, G.R Plastic mesh tubes constrict root development after two years. Tree Planter s Notes 23: Engeman, R.M., R.M. Anthony, H.W. Krupa,and J. Evans The effects of Vexar seeling protectors on the growth and developement of lodgepole pine roots. Crop Protection 16: Engeman, R.M., and D.L. Campbell Pocket gopher reoccupation of burrow systems following population reduction. Crop Protection 18: Engeman.R.M., R.M. Anthony, V.G. Barnes, Jr., H.W. Krupa, and J. Evans Evaluations of plastic mesh tubes for protecting conifer seedlings from pocket gophers in three western states. Western Journal of Applied Forestry 14: Engeman, R.M., and G.W. Witmer Integrated management tactics for predicting and alleviating pocket gopher (Thomomys spp.) damage to conifer reforestation plantings. Integrated Pest Management Reviews 5: Fagerstone, K.A., V.G. Barnes, Jr., R.M. Anthony, and J. Evans Hazards to small mammals associated with underground strychnine baiting for pocket gophers. Vert. Pest Conf. 9: Ferguson, D.E Effects of pocket gophers, bracken fern, and western coneflower on planted conifers in northern Idaho an update and two more species. New Forests 18: Graham, R.T., and J.L. Kingery Seedling damage and mortality of conifer plantations on transitory ranges in northern and central Idaho. Vert. Pest Conf. 14: Marsh, R.E. 1998a. One hundred years of pocket gopher traps and trapping. Vert. Pest Conf. 18: Marsh, R.E. 1998b. Barn owl boxes offer no solution to pocket gopher damage. Vert. Pest Conf. 18: Miller, L.A., and K.A. Fagerstone Induced fertility as a wildlife damage management tool. Vert. Pest Conf. 19: Moore, T., D. Van Vuren, and C. Ingels Are barn owls a biological control for gophers? Evaluating effectiveness in vineyards and orchards. Vert. Pest Conf. 18:

5 Pipas, M.J., and G.W. Witmer Evaluation of physical barriers to protect ponderosa pine seedlings from pocket gophers. Western Journal of Applied Forestry 14: Reichman, O.J., and E.W. Seabloom The role of pocket gophers as subterranean ecosystem engineers. Trends in Ecology and Evolution 17: Smith, H.G Strychnine residue studies and their implications in rodent control. Vert. Pest Conf. 10: Starr, R.I Strychnine aerobic soil metabolism: status of completed studies with recommendations for future aerobic soil research. USDA APHIS Denver Wildlife Research Center. Unpublished Report. Sterner, R.T., K.A. Hollenbeck, and S.A. Shumake Capsicum-laden soils decrease contact time by northern pocket gophers. Physiology and Behavior 67: Sullivan, T.P., D.S. Sullivan, and E.J. Hogue. Reinvasion dynamics of northern pocket gopher (Thomomys talpoides) populations in removal areas. Crop Protection 20: U.S. Forest Service. 1993a. Management for pocket gopher damage: environmental assessment. Clearwater National Forest. U.S. Forest Service. 1993b. Management for pocket gopher damage: environmental assessment. Nez Perce National Forest. Witmer, G.W., M.J. Pipas, and J.C. Bucher Field tests of denatonium benzoate to reduce seedling damage by pocket gophers (Thomomys talpoides Rich.). Crop Protection 17: Witmer, G.W., R.E. Marsh, and G.H. Matschke Trapping considerations for the fossorial pocket gopher. Pages in G. Proulx, ed. Mammal Trapping. Alpha Wildlife Research and Management Ltd., Sherwood Park, Alberta, Canada. 5