University service in Cooperative Extension ACTION no. 6.507 Managing in Colorado

1 William F. Andelt and Sayed Ahmed

Quick Facts Eight of voles are found in Colo- rado. They often are called meadow, field, or pine mice. Voles are small that cause damage by girdling seedling and mature trees in orchards, shelterbelts and forests. They also damage field crops and frequently construct runways in lawns. Damage by voles can be reduced by habitat modification, exclusion, repellents, above standing water from 8,000 feet elevation to trapping, and poison grain baits. above timberline. Long-tailed voles ( longicaudus) occur below 5,000 feet elevation to above timberline in the western half of Colorado. They are most abundant in streamside meadows. Voles are small that measure 4 to 8.5 Mexican voles (Microtus mexicanus) are associ- inches long and weigh 0.8 to 3 ounces. They are ated with grassy areas of Ponderosa pine (Pinus pudgy, with blunt faces and small eyes, small and ponderosa) forests of southwest Colorado in Mesa sometimes inconspicuous ears, short legs, and a Verde National Park. Prairie voles (Microtus short (the long-tailed is an exception) and ochrogaster) are found along streams and irrigated scantily haired tail. They vary in color from brown lands in northeastern Colorado. Sagebrush voles to gray. (Lagurus curtatus) occupy the driest of all vole Eight species of voles are distributed widely habitats in Colorado. They occur between 5,000 throughout various ecosystems of Colorado. They and 9,000 feet elevation in the northwest. Heather often are found in heavy ground cover of grasses, voles (Phenacomys intermedius) range from 7,000 grass-like plants, and litter. Southern red-backed to 12,000 feet elevations in the forested mountains voles (Clethrionomys gapperi) are found in moist of central Colorado. They occupy a variety of and well-developed coniferous forests. They are habitats, but are most abundant along streams. most abundant in lodgepole pine (Pinus contorta) stands, usually between 8,000 and 11,000 feet elevation. Meadow voles (Microtus 2 pennsylvanicus) primarily occur along the Front William F. Andelt, Colorado State University Range and southcentral Colorado. They tend to Cooperative Extension wildlife specialist and live in or near damp marshy areas or wet mead- associate professor; and Sayed Ahmed, former ows. Montane voles (Microtus montanus) primarily graduate student, fishery and wildlife biology (12/91) are found in the western half of Colorado in moist © Colorado State University Cooperative Extension. meadows and valleys and in grassy areas well 1991.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Kenneth R. Bolen, director of Cooperative Extension, Colorado State University, Fort Collins, Colorado. Cooperative Extension programs are available to all without discrimination. To simplify technical terminology, trade names of products and equipment occasionally will be used. No endorsement of products named is intended nor is criti- cism implied of products not mentioned. Voles eat a variety :of grasses, forbs and agri- diminish the amount of available habitat and cultural crops. They also eat bark on trees and reduce their numbers. shrubs, especially during fall and winter. Meadow voles are active during the day within their runways under thick grass and vegetation. Biology, Reproduction and Behavior Removal of all vegetation in summer, within about 2 feet of the trunks of fruit trees, will provide some Voles are active day and night throughout the protection because voles avoid exposed areas. year and do not hibernate. They usually live Damage to lawns can be reduced by close between 2 and 16 months. Their home ranges mowing in the fall before snow arrives and by usually are less than 1/4 acre and vary with mowing and removing tall grassy cover near lawns. season, food supply and population density. Voles To repair damage to lawns from runway construc- construct many surface runways and under- tion rake, fertilize, and water the effected area. ground tunnels with numerous burrow entrances. Close mowing and weed control in grassy borders A single burrow may contain several adults and adjacent to agricultural crops will reduce the young. habitat for voles and should reduce damage. If Population densities of voles vary from species suitable, plant crown vetch (a legume unpalatable to species. Large population fluctuations that to voles) in orchard and field boundaries to reduce range from to 500 voles per acre are common. vole populations. Their numbers generally peak every three to five Important predators of voles are short-tailed years. Population size is influenced by dispersal, shrews, badgers, coyotes, foxes, bobcats, barn food quality, climate, predation, physiological owls, great homed owls, long-eared owls, short- stress, and genetics. eared owls, barred owls, screech owls, and some Voles have three to six young per litter and snakes. Predators will not likely keep an orchard three to 12 litters per year. Their gestation period vole-free, but they can help to reduce the vole ranges from 20 to 23 days. They breed almost year population. Orchardists should be tolerant of around, although most reproduction occurs in predators and protect them if they do not consti- spring and fall. Females may become pregnant at tute a3 pest problem. weeks of age. To protect against vole damage, encircle young trees and shrubs with 1/4-inch mesh hardware Damage and Control cloth or 3-inch diameter Vexar plastic-mess cylin- ders. This barrier should project 18 inches above Voles can cause extensive damage to forests, the ground and 3 to 6 inches below the surface orchards and ornamental plants by girdling fees and shrubs. They prefer the bark of young trees above others, but when food is scarce, any tree Repellents regardless of age, may be attacked. Most damage Only a few repellents (including thiram and occurs in the winter when voles move through Hot Sauce (capsaicin}) are manufactured to protect their grass runways under the protection of snow. trees, shrubs and vegetable crops from voles. Little The greatest damage seems tocoincide withyears data is available on the effectiveness of repellents f o heavy snowfall. to deter vole damage. However, in one study, Damage to crops, such alfalfa, clover, thiram was reported to reduce damage to apple potatoes, carrots, beets and turnips is common stems by 80 percent. and moot evident when voles are at high popula- Thiram is manufactured by various companies tion levels. Runways and tunnel systems con- and sold under various trade names. Products structed in agricultural fields can divert irrigation such as Nott Chew-Not Repellent (Nott water. Voles often damage lawns and golf courses Manufacturing Co.), Bonide Rabbit-Deer Repellent by constructing runways and burrow systems. (Bonide Chemical Co.), and Science Deer and Vole damage to trees and shrubs is character- Rabbit Repellent (Miller Chemical and Fertilizer ized by girdling and patches of irregular patterns Corp.) are labeled for protecting most of the follow- of gnaw marks about 1/16 to 1/8-inch wide. This ing from voles; tree seedlings, shrubs, ornamental damage can occur on stems and roots. Other signs plantings, nursery stock, and fruit trees. Most of damage by voles includes: 1) 1- to 2-inch wide labels limit the use of thiram on fruit trees to the runways through matted grass and burrows; 2) dormant season. Scram 42-S (Wilbur-Ellis Co.) visual sightings; 3) hawks circling overhead and and Gustafson 42-S (Gustafson, Inc.) are labeled diving into fields; and 4) spongy soil from burrow- for protecting pine seeds from voles. ing activity. Trees that appear to be suffering from Capsaicin (Hot Sauce Animal Repellent, Miller disease or insect infestation may be suffering from Chemical and Fertilizer Corp.) is labeled for use on unseen vole damage. ornamental trees and shrubs, fruit and nut trees, Voles are classified as non-game wildlife in fruit bushes and vines, and nursery stock to Colorado. Voles may be captured or killed when protect them from vole damage. Limit application they create a nuisance or cause property damage. to fruit bearing plants to the period before fruit Methods to prevent and control damage are sets or after the fruit has been harvested. Hot habitat management, exclusion, repellents, Sauce also is registered for use on beans, com, trapping and poison grain baits. cucumbers, melons, peas, brussels sprouts, To reduce vole damage to orchards, eliminate squash, peppers, tomatoes, cabbage, and cauli- ground cover of weeds and tall grasses by repeated flower before edible portions and/or heads begin mowing, tilling and herbicide application. This will to form. Trapping When hand baiting around trees, place 1 teaspoon (4 grams) of bait at two to four locations Use mouse snap traps to remove small popula- around each tree in surface trails or at the mouth tions of voles from backyard lawns. Place traps of holes leading to underground burrow systems. perpendicular to runways with the trigger end in In non-crop areas, hand baiting generally consists the runway and bait with small amounts of rolled of placing 1 teaspoon of bait around each active oats or peanut butter. Set traps in the fall before burrow or runway. To broadcast bait, spread 4 to most damage occurs. 10 pounds of bait per acre with a cyclone seeder or by hand. Bait also can be spread with a Poison Grain Baits trailbuilder. A trailbuilder is usually pulled by a tractor, creates a burrow in the ground, and Two percent zinc phosphide is the only legal deposits 1 teaspoon of bait at 4 to 5 feet intervals. grain bait for controlling voles in Colorado. Zinc Two to 3 pounds of bait per acre usually is recom- phosphide baits are available in pellet form (Bell mended. Laboratories' ZP Bait AG, Chempar's Ridall Zinc phosphide baits are limited for use only Zinc) on oats (Bell Laboratories' ZP Rodent Bait on voles of the Microtus. Some of the zinc AG, USDA/APHIS/ADC Zinc Phosphide on steam- phosphide products and/or usages are limited to rolled oats) and on corn (Hopkin's Zinc Phosphide meadow and pine voles. Bait). One study indicated that pelleted zinc phosphide baits provide greater control of voles To minimize hazards to birds, do not apply zinc than zinc phosphide placed on oats or corn. Most phosphide bait to bare ground, areas without of these baits are labeled for use in orchards and vegetation, or in piles. Also do not apply to crops groves, nurseries, ornamental and non-bearing destined for use as food or feed. Zinc phosphide fruit trees, grapevine yards, and non-crop areas can be applied to orchards and groves only during such as lawns, ornamentals, golf courses, parks the dormant season after harvest. and nurseries. The labeled method of application The best time of year to use zinc phosphide varies somewhat among manufacturers. However, baits on lawns is during fall before snow cover. most of these products are labeled for hand bait- Applying bait during spring, after snow melt, ing, broadcast baiting, and/or trailbuilder baiting usually is ineffective. in orchards and groves, nurseries, and ornamental Because zinc phosphide is toxic to , and non-bearing fruit trees. In grapevine yards, store it away from humans and pets. Zinc phos- these products are labeled for broadcast baiting. phide can be absorbed in small amounts through The Chempar product is also labeled for hand human skin. Wear rubber gloves to avoid contact baiting. In non-crop areas, these products are with the chemical and take extra care to avoid labeled for hand baiting in conjunction with a breathing zinc phosphide dust. prebait. Zinc phosphide baits are classified as re- stricted use pesticides, which means landowners To prebait, scatter 4 or 6 pounds (see label must obtain private certification from the Environ- instructions) of untreated oat groats, rolled oats or mental Protection Agency before they can pur- barley (see label instructions) per acre two to four chase or use these products. Certification materi- days before placing a toxic bait. Prebaiting encour- als as well as sources of supplies are available ages consistent acceptance of bait. from county Cooperative Extension offices. Fumigants usually do not work for control of voles because their burrows are too shallow and complex.