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Wildlife Damage Management Conferences -- Wildlife Damage Management, Internet Center Proceedings for

2009

Vole Problems, Management Options, and Research Needs in the United States

Gary W. Witmer USDA-APHIS-Wildlife Services, [email protected]

Nathan P. Snow USDA-APHIS-Wildlife Services, [email protected]

L. Humberg USDA-APHIS-Wildlife Services

T. Salmon University of California Cooperative Extension-San Diego

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Witmer, Gary W.; Snow, Nathan P.; Humberg, L.; and Salmon, T., "Vole Problems, Management Options, and Research Needs in the United States" (2009). Wildlife Damage Management Conferences -- Proceedings. 140. https://digitalcommons.unl.edu/icwdm_wdmconfproc/140

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Vole Problems, Management Options, and Research Needs in the United States

GARY W. WITMER, USDA, APHIS, Wildlife Services, National Wildlife Research Center, Fort Collins, CO, USA NATHAN P. SNOW, USDA, APHIS, Wildlife Services, National Wildlife Research Center, Fort Collins, CO, USA L. HUMBERG, USDA, APHIS, Wildlife Services, Brooklyn, NY, USA T. SALMON, University of California Cooperative Extension-San Diego, San Diego, CA, USA

ABSTRACT Voles (Microtus spp.) are ubiquitous to the northern hemisphere. Numerous species occur in North America and several species cause significant damage of various types: food crops, livestock forage production (e.g., alfalfa), nursery trees, reforestation, orchards, rangeland forage, and damage to lawns, golf courses and ground cover. Much research has been conducted with voles and a number of management options have been developed, including habitat manipulation, rodenticides, traps, repellents, barriers, supplemental feeding, and increased natural predation. However, significant damage still occurs because voles are not easily managed. Voles are small and secretive, prolific, active year-round, able to exploit refugia, and cyclic with periodic irruptions. Currently there are no permanent solutions to managing voles, so long-term monitoring and management of populations is required. We review what is known about voles, the types and extent of damage they cause, advantages and disadvantages of management methods, and some research needs. Research needs include the development of effective repellents, effective rodenticide baiting strategies that minimize nontarget hazards, and cost-effective methods to protect the root systems of woody plants.

KEY WORDS Microtus, meadow mice, rodent management, vole, wildlife damage

There has long been controversy regarding information on voles and the damage they the members and taxonomy of the microtine cause. We also discuss management rodents (Carleton and Musser 1984). The strategies that can help reduce agricultural group has been classified as being within the damage by voles and new directions for family Cricetidae (Wilson and Reeder 2005) future research. while others place it within the family Muridae (Nowak 1991). When classified Ecology of Voles within Cricetidae, the group has been placed The biology, ecology, characteristics, and within the subfamily Arvicolinae which distribution of voles have been summarized contains approximately 27 genera, including by Pugh et al. (2003), O’Brien (1994), and voles, lemmings, and muskrats (Wilson and Tamarin (1985). Voles are small, secretive Reeder 2005). Species of this group are rodents. They have small eyes and ears, circumboreal around the northern short tails, and dark fur. Most species of hemisphere and many are serious pests Microtus are <20 cm in total length and (Prakash 1988, Witmer et al. 1995). In North weigh <75 g. Most species of voles live in America, many of the pest species belong to colonies. They occupy a variety of habitats, the genus Microtus, commonly called voles but are mostly associated with grasslands. or meadow mice. About 7 of the 23 species Voles are semi-fossorial with elaborate of Microtus cause damage in various parts burrow systems. Pugh et al. (2003) noted of North America. In this paper, we review that vole nests are usually only about 12 cm the literature and provide background below the surface. The burrows provide

Proceedings of the 13th WDM Conference (2009) 235 J. R. Boulanger, editor shelter from inclement weather and cover (Birney et al. 1976), density- predators, a place to raise young, and a place dependent season length (Smith at al. 2006), to store food stuffs. The openings to the breeding performance (Mihok and Boonstra burrows are about 4 cm in diameter and are 1992), defense mechanisms from food plants connected by a series of surface runways (Massey et al. 2008), disease outbreaks that are about 2.5–5 cm wide. Careful (Wolff and Edge 2003), and body condition examination of the runways will often reveal of individuals in a population (Agrell et al. clipped plants and fecal droppings. 1992); but not related to stress hormone Voles are active year-round and have levels (Boonstra and Boag 1992). Lambin et many foraging bouts throughout the 24 hour al. (2006) explained that the reasons for day. They feed on a variety of plant cycles likely differ by geographic region, materials and their feeding preferences shift and multiple reasons should be considered. through the seasons. Succulent grasses and Johnson and Johnson (1982) described forbs are used when first available in the how the irruptions of vole populations in spring and throughout the summer. From western USA resulted in damage of late summer through the fall, seeds are agricultural crops, orchards, and rangeland heavily used. During the winter, voles and forest resources. Normally, across a primarily feed on woody species as variety of habitats, densities of voles at herbaceous foods are not readily available. about 10–100 per ha are common (O’Brien Roots and tuberous materials are fed on 1994). Densities during these irruptions throughout the year. often reach several thousand animals per ha Voles have a high reproductive potential. (Johnson and Johnson 1982, O’Brien 1994). They reach sexual maturity in a few months Low densities are common in the winter and and females can have 5 or more litters per spring and then increase substantially year with 3–6 (maximum 11) young per through summer and fall due to annual litter. Voles are known to reproduce during reproduction and recruitment. The annual the winter under snow cover, especially if mortality rates of voles are quite high with green foods are available (Negus et al. 1977, 70+% dying within a year of birth (O’Brien Jannett 1984, Johnson 1987). Dispersal by 1994). A large variety of mammalian and young animals into surrounding areas, avian predators prey upon voles and vole including crop fields, often occurs at this survival rates are lowest where abundant, time. Overwinter survival depends greatly dense cover was not available (Pugh et al. upon weather severity and food availability. 2003). Vole densities can dramatically vary throughout the year. Also, voles are known Studying Voles to undergo multi-year cycles throughout the Numerous methodologies have been northern hemisphere (Stenseth 1999). implemented for studying voles. Radio- Cycles of peak densities occur every 3–5 transmitters have been widely used to years, but despite intensive research efforts, estimate movement patterns and home ecologists are conflicted about what causes ranges of voles (Herman 1977, Webster and them (Krebs 1996, Ylonen et al. 2003). Brooks 1981, Hansteen et al. 1997, Russell Researchers have suggested the cycles are et al. 2007). However, transmitter-carrying related to: resource limitation (Ford and voles have shown reduced activity and lower Pitelka 1984, Hornfeldt et al. 1986), survival from predation (Hamley and Falls predation pressures (Korpimaki et al. 1991, 1975, Webster and Brooks 1980), therefore, Korpimaki and Norrdahl 1998), vegetation this may not be the most effective study

Proceedings of the 13th WDM Conference (2009) 236 J. R. Boulanger, editor technique. Thus, researchers have employed driven by the practices of agricultural other techniques, like tracking voles marked production in the Czech Republic. with radioactive material, with success (Godfrey 1954, Miller 1957, Ambrose Types of Damage 1973). Live-trapping with mark-recapture In the course of their winter foraging techniques (Getz et al. 2005, Wiewel et al. activity, voles can cause substantial damage 2007), live-trapping with timer mechanisms to berry bushes, orchards, woody (Drabek 1994), and pitfall-trapping ornamentals, Christmas tree plantations, and (Boonstra and Krebs 1978) have also been reforestation efforts (Askham 1992). The used to access various biological aspects of bark and vascular tissues of trees in fruit voles. orchards across North America sustain significant damage from voles (Askham Benefits from Voles 1990, Sullivan et al. 2000). Damage to We found very few reports of benefits to woody species may not be readily noticed humans from voles. However, Frischknecht because the roots are gnawed over time and and Baker (1972) reported that voles can be stem girdling often occurs under snow used as a biological control to reduce cover. sagebrush in order to increase grazing Voles are often implicated in damage to productivity in rangelands. Also, voles are certain field crops such as alfalfa, grains, an important prey species to numerous soybeans, and sprouting corn. Several species of wildlife, especially raptors (Baker researchers have described the substantial and Brooks 1981). Similarly, the importance loss in corn yield and other crops that can of cyclic rodent species, such as voles, have occur when vole and other rodent been described by Goszczynski (1977) and populations are large (Clark 1984; Hines Andersson and Erlinge (1977). 1993, 1997; Hygnstrom et al. 1996, 2000; Hines and Hygnstrom 2000). Clark (1984), DAMAGE Johnson and Johnson (1982), and O’Brien Most reported vole damage centers on (1994) describe the nature of vole damage agriculture and forestry (O’Brien 1994, and give examples of substantial economic Pugh et al. 2003). Voles also cause losses to apple and alfalfa production. Voles structural damage (i.e., such as undermining and other rodents can dig up seeds, although of dikes, levees, and irrigation ditches by damage often involves foraging on the burrowing; or gnawing on cables and plastic newly-emergent seedlings several weeks tubing) and aesthetic damage (i.e., such as after planting (Hines and Hygnstrom 2000). destroying lawns, golf courses, and In some cases, these rodents cause vegetative ground covers). Although voles significant damage to root vegetables (e.g., are susceptible to a number of diseases, they carrots, sugar beets, and potatoes) especially rarely pose a health threat to people, pets, or in small gardens that border good vole livestock (Pugh et al. 2003). The cyclic habitat. During peak density years, voles nature of vole populations can impact may deplete forage intended for livestock on agricultural fields drastically and quickly. pastures and rangeland. However, reducing the damage sustained by Much of the damage in no-till crops in agriculture could be a circular predicament, the Midwestern and Pacific Northwestern because Janova et al. (2008) found evidence portions of the United States occurs to that the cycles of voles could, in part, be sprouting plants in the late winter or spring (Clark and Young 1986, Johnson 1986,

Proceedings of the 13th WDM Conference (2009) 237 J. R. Boulanger, editor Witmer et al. 2007). Based on estimates of conducted by Byers (1985) for orchards, vole food requirements and densities, Askham (1992) for silviculture, and Pelz Grodzinski et al. (1977) surmised that voles (2003) and Witmer and VerCauteren (2001) had little impact on winter wheat production for agriculture fields. The traditional during low population years and that only 2– approaches to vole population and damage 3% of the crop was destroyed in periods of management have relied on direct reduction high density. Conversely, Witmer et al. of the vole population using rodenticide (2007) found damage levels of up to 9% in baits or rodent traps, and the reduction of winter pea fields during a low vole density habitat carrying capacity for voles by habitat period with damage appearing to occur manipulation (Johnson and Johnson 1982, mostly during winter under snow cover. Clark 1984, O’Brien 1994). Today, many Johnson (1987) suggested that high levels of approaches focus on management efforts damage can occur by voles in no-till fields that are environmentally benign (Singleton during high population densities. et al. 1999, Pelz 2003). These techniques High vole densities can attract raptors to have had varying degrees of success. airports resulting in an increase in bird- Importantly, managers must consider the aircraft strike hazards (Witmer and location, species, and type of damage before Fantinato 2003). When vole populations are choosing an effective management strategy. controlled at airports, raptor use of the area Each method has advantages and and the number of bird strikes both decline disadvantages, and generally using an (Robert Johnson, Kansas City International integrated pest management (IPM) approach Airport, unpublished data). will involve several methods woven into an effective damage reduction strategy (Table Economics of Damage 1; Witmer 2007). Most cooperative The physical impacts from voles in extension county offices or state agricultural agriculture and forestry industries can have universities have booklets available on high economic costs (Askham 1992, rodent control specific to particular species O’Brien 1994, Pugh et al. 2003). For and areas. example, the cyclic fluctuations of the levant vole (M. guentheri) in middle-eastern Monitoring countries resulted in more that 50% decline The importance of pest population in yield of cereal and fodder crops, and up to monitoring or “scouting” as a component of a 16–25% lose of alfalfa yield (Wolf 1977). IPM has received considerable attention in Also, in Washington State (USA), Askham recent years (Matthews 1996). This certainly (1988) reported a production decrease in a applies to vole populations because of their large apple orchard of 36% during a 2-year high reproductive potential and because period of an extremely high population of once high densities ( ≥200/ha) are achieved, montane voles (M. montanus). Therefore, substantial damage is generally inevitable. A controlling for voles is often implemented, variety of methods have been developed for but can have high costs to reach desired monitoring vole populations: use of live- or levels of damage reduction (Byers 1984). snap-traps along grids, use of apple slices or other food removal methods, and the MANAGEMENT counting of active colonies per acre (Tobin Voles and their damage pose many et al. 1992; Tobin and Richmond 1993; management challenges. Reviews of Hines 1993, 1997; Clark 1994; Hines and management and control have been Hygnstrom 2000; Witmer and VerCauteren

Proceedings of the 13th WDM Conference (2009) 238 J. R. Boulanger, editor Table 1. Methods to reduce damage by voles.

Population Management Habitat Management Other Approaches

Rodenticide baits Eliminate vegetative cover Physical barriers Fumigants Manage or remove refugia Repellents Traps Disrupt burrows Frightening devices Encourage predation Plant unpalatable vegetation Supplemental feeding Endophytic grasses Fertility control Crop/tree species/variety selection

2001) . Vole populations should be crop fields are inadequate to support many monitored in late winter or early spring, rodents and also sustain voles during lows in after snow-melt, and again just prior to their population cycles. Management actions planting. Managers and landowners should can include mowing, burning, grazing, look for fresh trails in the grass, burrow plowing, herbicide application, and the use openings, droppings, and evidence of of rodenticides (Witmer and VerCauteren feeding. They should pay particular attention 2001, Brown et al. 2004). Also, normal to adjacent fallow areas that have heavy farming practices can reduce vole vegetation because voles can build up in populations in fields, such as mowing, these areas and quickly invade agricultural mulching, harvesting and plowing. fields. Some general guidelines indicate that However, plowing was the only effective vole population or damage management farming strategy found for reducing activities may be required if trap success is common voles in Germany (Jacob 2003). >10% or if >12 active colonies per ha are But, also in Germany, Jacob and Hempel observed (Witmer and VerCauteren 2001). (2003) found that reducing vegetation height quickly reduced the home range size of Habitat Management voles by 42%, and likely increased the Habitat manipulation has long been used as amount of predation risk for voles. Mowing a way to lower the carrying capacity for should be combined with removal of plant voles. Voles need tall, protective vegetative residues and those residues can provide cover and researchers have noted the good cover and travel corridors for voles importance of grassy areas to voles and (Witmer and Fantinato 2003, Witmer et al. other rodents (Randall and Johnson 1979, 2007). Combining habitat management with Witmer et al. 2007). Several researchers another form of management (e.g., see have also noted the importance of grassy section on Biological Control) may also be borders as refugia for rodents and the need an effective strategy for maintaining low to manage the refugia to help reduce the numbers of voles. Various methods of influx of rodents into crop fields during habitat management are especially useful in growing seasons (Clark 1984, Edge et al. no-till agriculture fields, because without 1995, Martinelli and Neal 1995, Chambers tilling, intact burrow systems and crop et al. 1996, Witmer et al. 2007). Perhaps the residues are maintained, and surrounding most important approach for preventing areas provide suitable habitat for rodents rodent damage to crop fields is lowering the (Witmer et al. 2007). Therefore, the carrying capacity for rodents in agricultural potential exists for substantial increases in fields and refugia surrounding fields. The rodent populations with subsequent crop refugia provide harborage for rodents when damage (Johnson 1987, Bourne 1999).

Proceedings of the 13th WDM Conference (2009) 239 J. R. Boulanger, editor Crop selection is another consideration where significant damage is occurring or for agricultural producers that may be expected to occur. Some areas should be left experiencing vole damage, because some unmanaged to help support biodiversity. crop species or varieties may be less Also, reducing vole populations and damage susceptible to damage by voles and other can be problematic on some conservation rodents (Witmer et al. 1995, Witmer and lands where severe habitat management is Fantinato 2003). This is also true for not an option (Lee Humberg, personal reforestation situations where types of tree observation). In high public visibility areas, species are important. Additionally, planting there may be socio-political pressures to not large seedlings can help assure the trees use certain methods such as rodenticides, achieve large size quickly, becoming less snap-traps, or management methods that prone to vole and other rodent damage lessen the aesthetics of the landscape. (Askham 1992). Endophytic grasses offer a potential Supplemental Feeding method to reduce vole populations. The Voles have been found to switch their fungi in endophytic grasses produce feeding from agricultural crops and orchards alkaloids that are known to reduce to suitable alternative foods; given the herbivory. Endophytic fescue and perennial alternative foods are more palatable rye grasses may reduce rodent carrying (Sullivan and Sullivan 1988). Broadcast capacity, but further investigation is needed whole or cracked corn, or soybeans have (Fortier et al. 2000). Witmer (2004) found a been used as a supplemental food source to lower abundance of small mammals on reduce damage by voles, especially in no-till endophytic grass fields than non-endophytic. corn and soybean fields, (Hines and Endophytic grass fields could be maintained Hygnstrom 2000, Hygnstrom et al. 2000). around agricultural fields or at sites such as These can be applied at a rate of about 125. airports. One of the difficulties with this 5 kg per ha at the time of planting or several approach, however, is that a near weeks post-planting, depending on when monoculture of the endophytic grass species serious damage is anticipated. It also must be maintained at the site. Other species appeared that rodent damage to seedlings in of unpalatable plants may offer a similar reforestation efforts can be reduced by using approach to lowering the rodent carrying sunflower seeds (Sullivan and Sullivan capacity of a site. For example, 1982) or alfalfa pellets (Sullivan et al. 2001) meadowfoam (Limnanthes macounii) is a during the winter when most woody plant native herbaceous plant of the Pacific damage occurs. However, these efforts have Northwest that seems to be unpalatable to not always been particularly effective on many rodents (Gary Witmer, USDA, some sites (Sullivan and Sullivan 1988, APHIS, Wildlife Services, unpublished Sullivan et al. 2001). observation). An issue of using supplemental feeding However, managers should consider that to reduce damage by rodents is that the unmanaged habitats tend to support the addition of nutritious food may increase the highest densities of small mammals which, survival and reproduction rate of the in turn, support various predator species that rodents. Desy and Thompson (1983) depend on small mammals (Aschwanden et demonstrated this effect in a field study in al. 2007). Therefore, any manipulation of Illinois. However, the addition of refugia habitats to reduce vole populations supplemental food did not prevent the should be restricted to the most crucial areas

Proceedings of the 13th WDM Conference (2009) 240 J. R. Boulanger, editor population crash of this cyclic vole around a tree reduced the incidence and population a year later. severity of damage. Other types of barriers such as a circle of pea gravel around Exclusion and Barriers seedlings to discourage burrowing to the Excluding voles from large areas is often root system are being investigated (Gary difficult and rarely practical. However, wire- Witmer, USDA, APHIS, Wildlife Services, mesh barriers placed both above- and below- personal communication). This is ground for gardens and around individual particularly important because much damage trees have been useful in certain situations to seedlings and orchard trees occurs from (O’Brien 1994). It is important that voles burrowing to the root system and individual barriers around trees do not damaging it. Normally, once signs of the damage the tree bark or its root system or damage are noticed by growers, the trees cause any growth problems or deformities. have already received substantial damage. This approach to protecting individual plants or clumps of plants is more effective and Biological Control economical than fencing entire gardens or Voles have many predators, many of which fields (Marsh et al. 1990), and is especially are birds of prey (Pugh et al. 2003). Also, effective for reducing damage to seedlings weasels (Mustela spp.) and other (Zimmerling and Zimmerling 1998). mammalian predators are known to prey Barriers made of metal, plastic, and wire heavily on voles. Modifying sensitive areas mesh that were approximately 25 cm high (e.g., agricultural fields, pastures, and were effective in reducing damage in a pen orchards) to support higher numbers of trial, and even more effective in predators has had varying success for combination with a repellent (Witmer et al. reducing vole populations (Sullivan and 2001). Pelz (2003) suggested that some Druscilla 1980, Askham 1990, Pelz 2003). barriers stop movements of voles, especially To increase raptor use of the areas, nest when placed in combination with poison bait boxes and artificial perches can be added; stations. However in a field trial, Witmer et whereas, to increase use of the area by al. (2007) found that 25-cm-tall metal mammalian predators, hay bails or other barriers extending above and below ground types of protective cover can be added were not effective at excluding voles from (Witmer et al. 2008). areas of no-till agriculture areas. Rodents Research is underway in the area of can burrow under barriers or get over them wildlife fertility control (e.g., Miller et al. by climbing the tall crop plants as they grow 1998), but it will probably be years before a higher than the barriers. It is also difficult registered commercial product is available and costly to maintain barriers in for any species of rodent. Early studies with agricultural fields because of all the activity voles looked at the compounds with large farming equipment and vehicles. diethylstilbestrol (German 1985) and Broadleaved trees guarded with rigid indomethacin (Seeley and Reynolds 1989) plastic tubes that were at least 25 cm tall and with promising results. Fertility control in pushed into the ground did not sustain rodent species that are continuous breeders damage from voles, whereas shorter guards poses technical difficulties, cost- contained some damaged trees (Davies and effectiveness issues, and nontarget issues Pepper 1989). Davies and Pepper (1989) (Tyndale-Biscoe and Hinds 2007, McLeod also found that using chemical weed control et al. 2007). There has been some in a 1 meter diameter swath as a barrier preliminary investigation of the ability of

Proceedings of the 13th WDM Conference (2009) 241 J. R. Boulanger, editor altered light cycles to influence vole odorous repellent could significantly reduce reproduction (Haim et al. 2004). This study feeding. in Israel found that flashes of light throughout the night reduced reproduction in Trapping voles and affected their thermoregulatory Snap-traps can be used to reduce vole system. They suggested that this method populations, but are labor-intensive and also may help regulate population size and help not very practical over large acreages. They avoid vole population irruptions. are used mostly for population monitoring Managers and crop growers often raise and for research purposes. However, they the question of using a species-specific can be used where the use of rodenticides disease to control rodent populations in their are not desirable or allowed, such as in fields. Technical difficulties, public backyards or garden areas. Traps should be concerns, and legal-regulatory issues placed throughout the area of active vole suggest that this will not be an option in the colonies with a trap spacing of about 3–10 m near future in the USA (Witmer 2007). between traps. Peanut butter, oatmeal, or apple slices make excellent baits for many Repellents species of voles. Often, no bait is needed Some chemical repellents are registered for because voles will trigger the trap as they vole damage control, but these are only pass over it while running along their partially effective and not practical over runway. Snap-traps should be placed at right large areas. EPA-registered vole repellents angles to in these runways and flush with the contain the active ingredient capsaicin or ground. thiram (O’Brien 1994). Additionally, the use of repellents on food crops is usually Rodenticides restricted or not allowed. Some researchers When voles are numerous or when damage have suggested, however, that predator occurs over large areas, it may be necessary odors (e.g., from urines, feces, or anal to use a rodent toxicant to reduce the glands) may help exclude rodents from population and therefore, the damage levels. areas, although success rates are dependent When using toxic baits, it is essential to upon cover availability and other factors assure the safety of children, pets, and non- (Sullivan et al. 1988, Merkens et al. 1991). target animals. This is mainly done by Some electronic and magnetic devices have carefully following the EPA label appeared on the commercial market, but instructions. The options for rodenticide use these have not been found effective in on agricultural lands are somewhat limited, eliminating rodents from fields or buildings especially during crop production cycles. (Timm 2003). Witmer et al. (2001) showed This is to reduce the likelihood of pesticide that various repellents did reduce vole residues in foods. Rodenticide use is less breaches of barriers and food consumption, restricted for rangelands, orchards, along including blood meal, capsaicin, castor oil, fencerows, on right-of-ways, and in and coyote urine, quebracho, and thiram. around buildings. A variety of rodenticides However, high concentrations of the are available (EPA-registered for use in the repellents were needed. Salatti et al. (1995) U.S.) for vole population control: showed that the herbicide Casoron also was a potential repellent. Voles in Central Asia • Acute rodenticides were mainly attracted to food via the odor of o Zinc Phosphide (2% a.i. ) the plants (Fan et al. 1992), thus an effective

Proceedings of the 13th WDM Conference (2009) 242 J. R. Boulanger, editor • Anticoagulant rodenticides (1st brodifacoum (Kaukeinen 1984) could be Generation only) investigated for registration for vole control. o Diphacinone (0. 005% a.i. ) Zinc phosphide can be applied in a o Chlorophacinone (0. 005% variety of formulations: coated grain and a.i. ) pelleted products are available. In recent o Warfarin (0. 025% a.i. ) years, zinc phosphide pellets, when applied • Fumigants at 4.5–6.75 kg per ha, have proven effective in reducing rodent populations in no-till corn o Aluminum Phosphide (56% a.i. ) when applied in-furrow before planting or at planting time. Some EPA registrations for Detailed information on rodenticides and zinc phosphide use in corn, milo, and their use in the USA was provided by Jacobs soybeans have been obtained (Hygnstrom et (1994), Timm (1994), and Witmer and al. 2000). It should be noted, however, that Eisemann (2007). Zinc phosphide is an zinc phosphide is known to sometimes cause acute toxicant, meaning that the animal “bait-shyness” in rodents. Consequently, generally consumes a lethal dose in one bait efficacy can be improved by pre-baiting feeding and dies relatively soon thereafter. with a formulation that is very similar to the On the other hand, the anticoagulants are toxic bait, but does not contain the toxicant often called multiple-feeding or slow-acting (Sterner 1999). Alternatively, one can toxicants because the animal feeds on the switch to a different toxicant if efficacy is bait for several days, eventually too low or decreases over time. accumulating a lethal dose and dying from The anticoagulants occur as pellets or internal hemorrhaging as its blood clotting blocks. The blocks are generally intended to ability declines. The anticoagulants are be used in bait stations. Rodenticides subdivided into two groups: the first- intended for voles can be applied in a variety generation compounds (e.g., warfarin, of ways: broadcast by hand or seed spreader, chlorophacinone, diphacinone) are less toxic placed by hand in runways and burrow and have less persistent residues in tissues openings, or placed in bait stations. As a than the more recently developed second- side note, some researchers have shown generation compounds (e.g., brodifacoum, ground sprays of anticoagulant solutions to bromadiolone, difethialone). The second- be effective in controlling vole populations generation anticoagulants were developed (Byers 1975). Generally, baits can be because of the increased incidence of applied at various times of the year, but it is genetic resistance to the first-generation good to monitor populations and apply the compounds (Buckle et al. 1994). Of the baits before the population becomes sizable anticoagulants, only first-generation and significant damage begins to occur. compounds are EPA-registered for use to Often, baits are applied in the late winter or control vole populations in the U.S. Salmon early spring when the vole population is at and Lawrence (2006a) recently reported its lowest level, natural foods are scarce, and resistance to first-generation anticoagulants high levels of reproduction have not yet in voles in California artichoke fields. begun. Ecologically-based baiting strategies Hence, it may be important to investigate have been developed and are thoroughly alternative rodenticides for vole control in discussed by Ramsey and Wilson (2000), this crop type (Salmon and Lawrence who have studied Australian rodent 2006b). Other rodenticides such as irruptions which have become a serious cholecalciferol (Moran 2003) and problem in grain and other crops.

Proceedings of the 13th WDM Conference (2009) 243 J. R. Boulanger, editor While broadcast-baiting can effectively diffuses into the air, hence, there are few and quickly reduce rodent numbers (Witmer secondary hazards from these formulations and Fantinato 2003), the effect does not last (Johnson and Fagerstone 1994). However, very long and rodent populations can zinc phosphide is very toxic to most birds recover within a year or two. Johnson (1987) and mammals, so it can present a primary noted that zinc phosphide treatment resulted hazard to some nontarget animals. This is in only a brief population decline in voles in why it must be used very carefully and why the Pacific Northwest. In studies in the accidental spills of bait must be cleaned up midwestern U.S., Hygnstrom et al. (2000) quickly. There have been recent die-offs of noted that in-furrow drilling of zinc geese in the Pacific Northwest that have phosphide pellets reduced vole damage by been attributed to the use of zinc phosphide- 7–34%. treated grain for vole control on hay and It is also important to treat the fallow grass seed production fields (Rose lands (i.e., refugia) around croplands Kachadoorian, Oregon Department of because many of these areas have dense Agriculture, personal communication). A vegetation and support vole and other rodent researcher at the National Wildlife Research populations. Croplands do not support Center is investigating the potential addition abundant rodent populations during portions of a bird repellent (anthraquinone) to zinc of the year (after harvest or during winter), phosphide-treated grain to reduce the hazard but the rodent populations subsist in the to birds (Scott Werner, USDA, APHIS, bordering habitats and “invade” the cropland Wildlife Services, personal communication). each year when the crops begin to grow, With the first-generation anticoagulants providing food and protective cover. registered for vole control, there has been Dispersing young animals are especially concern raised recently about possible losses likely to invade, hence, strategies to keep of predatory animals (both avian and rodent densities low in refugia can help mammalian) to the secondary hazard of reduce crop damage. consumption of rodents that have consumed Fumigants can be used for control of rodent baits (Peter Gober, U.S. Fish and rodent populations in situations where well- Wildlife Service, personal communication). developed burrow systems occur. With While the toxicity levels and persistency many vole species, unfortunately, the durations of first-generation anticoagulants burrows are complex, shallow, and often make them less of a hazard than the second- have numerous openings. This situation generation compounds, nonetheless, they allows the fumigant gas to escape and can pose a secondary hazard (Mendenhall results in poor effectiveness; therefore, and Pank 1980, Brakes and Smith 2005). As fumigants are generally not recommended a result, the residue levels and persistency for vole control. durations in the tissues of rodents are being There are two types of hazards to non- investigated. target animals from the use of rodenticides: primary hazards result from the direct RESEARCH NEEDS consumption of the rodenticide bait, while Despite many decades of research on voles secondary hazards result from consuming and thousands of publications, Pugh et al. rodents that have previously consumed the (2003) noted that there is still much to learn rodenticide bait (Witmer and Eisemann about voles. Researchers should continue to 2007). Zinc phosphide baits convert to seek ways to reduce rodent populations and phosphine gas inside the rodent and the gas damage to agriculture (Witmer et al. 1995).

Proceedings of the 13th WDM Conference (2009) 244 J. R. Boulanger, editor Some promising areas of research include Askham, L. 1990. Effects of artificial perches and the use of endophytic (alkaloid-producing) nests in attracting raptors to orchards. Proceedings of the Vertebrate Pest Conference grasses in non-production areas (Fortier et 14:144–148. al. 2000) and fertility control (Miller et al. Askham, L. R. 1992. Silvicultural methods in relation 1998). Other areas of research could to selected wildlife species. Pages 187–205 in include: Black, H. C., editor. Silvicultural Approaches to Animal Damage Management in Pacific Northwest Forests. Portland, Oregon, USA. • Predicting vole outbreaks/irruptions Baker, J., and R. Brooks. 1981. Distribution patterns (as per house mice in Australia; of raptors in relation to density of meadow voles. Krebs et al. 2004). Condor 83:42–47. • Protecting root systems from damage Bergeron, J., and L. Jodoin. 1994. Comparison of by tunneling voles. food habits and of nutrients in the stomach contents of summer- and winter-trapped voles. • Effective rodenticides and methods Canadian Journal of Zoology 72:183–87. to further reduce nontarget animal Birney, E., W. Grant, and D. Baird. 1976. Importance hazards. of vegetative cover to cycles of Microtus • Effective and durable repellents. populations. Ecology 57:1043–1051. Boonstra, R., and C. J. Krebs. 1978. Pitfall trapping • Species interactions with other native of Microtus Townsendii. Journal of Mammalogy and non-native rodent species. 59:136–148. • Food safety issues in agricultural Boonstra, R., and P. T. Boag. 1992. Spring declines areas. in Microtus pennsylvanicus and the role of steroid hormones. Journal of Animal Ecology 61:339–352. Clearly, managing vole populations and the Bourne, J. 1999. Controlling wildlife damage in damage they cause will continue to direct seeding systems. Agdex 519-16. Alberta challenge researchers and land managers for Agriculture, Food and Rural Development, a long time to come. Edmonton, Alberta, Canada. Brakes, C. R., and R. H. Smith. 2005. Exposure of non-target small mammals to rodenticides: short- LITERATURE CITED term effects, recovery and implications for Agrell, J., S. Erlinge, J. Nelson, and M. Sandell. secondary poisioning. Journal of Applied 1992. Body weight and population dynamics: Ecology 42:118–128. cyclic demography in a noncyclic population of Brown, P., M. Davies, G. Singleton, and J. Croft. the field vole (Microtus agrestis). Canadian 2004. Can farm management practices reduce the Journal of Zoology 70:494–501. impact of house mouse populations on crops in Ambrose, H. W. 1973. An experimental study of an irrigated farming system? Wildlife Research some factors affecting the spatial and temporal 31:597–604. activity of Microtus pennsylanicus. Journal of Buckle, A., C. Prescott, and K. Ward. 1994. Mammalogy 54:79–110. Resistance to the first and second generation Andersson, M., and S. Erlinge. 1977. Influence of anticoagulant rodenticides. Proceedings of the predation on rodent populations. Oikos 29:591– Vertebrate Pest Conference 16:138–144. 597. Byers, R. E. 1975. Effect of hand baits and ground Aschwanden, J., O. Holzgang, and L. Jenni. 2007. sprays on pine vole activity. HortScience Importance of ecological compensation areas for 10:122–123. small mammals in intensively farmed areas. Byers, R. E. 1984. Economics of Microtus control in Wildlife Biology 13:150–158. the eastern U.S. orchards. Pages 297–302 in Askham, L. R. 1988. A two year study of the Proceedings of the Conference on the physical and economic impact to voles (Microtus Organization and Practice of Vertebrate Pest montanus) on mixed maturity apple (Malus spp.) Control. A. Dubock, editor. ICI Plant Protection orchards in the pacific northwestern United Division, New Hampshire, England. States. Proceedings of the Vertebrate Pest Byers, R. E. 1985. Management and control. Pages Conference 13:151–155. 621–646 in Tamarin, R. H. editor. Biololgy of New World Microtus. American Society of

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