VOLES
Voles often called "meadow mice," are small, mouse-like rodents that measure 4 to 8.5 inches long and weigh 0.8 to 3 ounces and vary in color from brown to gray. Their underfur is dense and covered with thicker, longer guard hairs. They are pudgy, with blunt faces and small eyes, and sometimes inconspicuous ears, short legs, and a short and scantily haired tail (the long-tailed vole is an exception).
GENERAL BIOLOGY AND BEHAVIOR
Voles occupy a wide variety of habitats, depending on the species. Generally, voles prefer areas with heavy ground cover of grasses, grass-like plants, or litter. They are active both day and night and throughout the year. Voles eat a variety of plants and animals. They frequently forage on grasses, forbs, roots, bulbs, tubers, bark, snails, and insects. To find food, voles construct tunnels and surface runways with many burrow openings. Several adults and young can live in these runway systems. This intricate network of tunnels and burrows provide voles with excellent shelter from the weather and protection from predators. Voles store seeds and other plant matter in underground chambers.
Surface runway system of the vole.
Although voles spend considerable time aboveground and may occasionally be seen scurrying about, most of their time is spent below ground in their burrow system. The clearest signs of their presence are the well-traveled, aboveground runways that connect burrow openings; the runways are usually hidden beneath a protective layer of grass or other ground cover. The maze of runways leads to multiple open burrows that are each about 11/2 to 2 inches in diameter. The runways are easily found by pulling back overhanging ground cover. Fresh clippings of green grass and greenish-color droppings about 3/16 inch long in the runways and near the burrows are further evidence of voles. With age, the droppings lose the green coloring and turn brown or gray.
Voles are poor climbers and do not usually enter homes or other buildings. Instead, they inhabit wild lands or croplands adjacent to buildings, or gardens and landscaped sites with protective ground cover. Most problems around homes and gardens occur during times of large meadow mouse populations.
Voles usually live between 2 and 16 months. Their home ranges usually are less than 1/4 acre and vary with season, food supply and population density. Population densities of voles vary from species to species. Large population fluctuations that range from 14 to 500 voles per acre are common. Their numbers generally peak every 3 to 5 years. Factors that influence population levels include dispersal rates, food quality, climate, predation, physiological stress, and genetics.
Voles breed throughout the year, with peaks occurring during spring and summer. Voles can produce 3 to 12 litters per year with 3 to 5 being average. Litters range in size from 1 to 11 young per litter, with 3 to 6 being the average. Although gestation periods vary slightly among species, 21 days is the average length of gestation for voles. Young are weaned by the time they are 21 days old, and females are sexually mature within 40 days.
VOLES IN COLORADO
Eight species of voles are distributed widely throughout various ecosystems of Colorado.
SOUTHERN RED-BACKED VOLE (Clethrionomys gapperi)
They are found in moist and well-developed coniferous forests. They are most abundant in lodgepole pine stands, usually between 8,000 and 11,000 feet.
MEADOW VOLES (Microtus pennsylvanicus)
These voles primarily occur along the Front Range and southcentral Colorado. They tend to live in or near damp marshy areas or wet meadows.
MONTANE VOLES (Microtus montanus)
The montane voles are primarily found in the western half of Colorado in moist meadows and valleys and in grassy areas from 6,000 feet to above timberline.
LONG-TAILED VOLES (Microtus longicaudus)
They occur just below 5,000 feet elevation to above timberline in the western half of Colorado. They are most abundant in streamside meadows.
MEXICAN VOLES (Microtus mexicanus)
These voles are associated with grassy areas of Ponderosa pine forests of southwest Colorado in Mesa Verde National Park.
PRAIRIE VOLES (Microtus ochrogaster)
They are found along streams and on irrigated lands in northeastern Colorado.
SAGEBRUSH VOLES (Lemmiscus curtatus)
They occupy the driest of all vole habitats in Colorado. Sagebrush voles occur between 5,000 and 9,000 feet in the northwest area of Colorado.
HEATHER VOLES (Phenacomys intermedius)
These voles are found from 7,000 to 12,000 feet in the forested mountains of central Colorado. They occupy a variety of habitats but are most abundant along streams.
DAMAGE
Voles can cause extensive damage to forest, orchards and ornamental plants by girdling trees and shrubs. They prefer the bark of young trees but will attack any tree, regardless of age, when food is scarce. Most damage occurs in the winter when voles move through their grass runways under the protection of snow. The greatest damage seems to coincide with years of heavy snowfall.
Vole damage to trees and shrubs is characterized by girdling and patches of irregular patterns of gnaw marks about 1/16 to 1/8-inch wide. Gnawed stems may have a pointed tip. Do not confuse vole damage with damage by rabbits, which includes stems clipped at a smooth 45-degree angle and wider gnaw marks. Stems browsed by deer usually have a rough jagged edge. Voles also girdle the roots of trees and shrubs.
Voles cause damage by feeding on a wide range of garden plants including carrots, beets, turnips, sweet potatoes, cabbages, lettuce, celery, spinach, tomatoes, brussel sprouts, and cauliflower. Landscape plantings such as lilies and dichondra may be eaten by voles. Voles damage lawns and golf courses by constructing runways and burrow systems.
Fortunately, voles pose no major public health problems because of infrequent contact with humans. However, they can harbor disease organisms, such as plague and tularemia. For this reason, voles should never be handled. If you have to handle a vole, or any other species of wildlife, you should wear the appropriate protective clothing (e.g., leather gloves).
CONTROL
Methods to prevent and control damage are habitat management, exclusion, repellents, trapping, and poison baits. Voles are classified as non-game wildlife in Colorado and may be captured or killed when they create a nuisance or cause property damage.
HABITAT MANAGEMENT
The elimination of weeds, ground cover, and litter around lawns and ornamental plantings can reduce habitat suitability for voles and lead to a decreased likelihood of vole damage. For example, lawns should be mowed regularly and mulch should be cleared 3 feet or more from the base of trees. Additionally, soil cultivation destroys vole runway-systems and may kill voles outright. For these reasons, plots of annual plants often are less susceptible to vole damage than perennial plants.
Damage to lawns can be reduced by close mowing in the fall before snow arrives and by mowing and removing tall grassy cover near lawns. To repair damage to lawns from runway construction, rake, fertilize and water the affected area. If suitable, plant crown vetch (a legume unpalatable to voles) around the boundaries to reduce vole populations.
EXCLUSION
Cylinders made of hardware cloth (available at most hardware stores) are often effective in excluding voles and protecting individual plants. The mesh size of the hardware cloth used to construct cylinders should be no larger than 1/4 inch in size. The cylinder should be buried at least 6 inches below the ground surface to ensure that voles will not burrow under the hardware cloth and gain access to the plant and should project 18 inches above the ground. Although this technique will protect individual plants, fencing typically is not effective in protecting large areas (e.g., lawns) and probably is cost-prohibitive. But a 1/4 inch hardware cloth fencing may work around a small garden area to help keep out the voles.
REPELLENTS
The agency responsible for regulating chemicals used on wildlife, the Environmental Protection Agency (EPA) currently approves two chemicals for use in repelling voles. These two repellents may contain thiram (a fungicide) or capsaicin (the chemical that makes peppers "hot") and act by altering the taste of plants and making them unpalatable to voles. Although these repellents may provide temporary protection for plants, their effectiveness usually is short-lived. The repellents may be washed off by rain, sprinklers, or even heavy dew, so you must keep reapplying them. Voles may become accustomed to such repellents and forage on plants even after treatment. For a more long-term prevention effort, other techniques should be considered.
TRAPPING
When voles are not numerous or when the population is concentrated in a small area, trapping may be effective. Use a sufficient number of traps to control the population: for a small garden a dozen traps is probably the minimum number required, and for larger areas at least 50 or more may be needed. A simple, wooden mousetrap baited with a peanut butter-oatmeal mixture or apple slice is commonly used. Often, no bait is needed because voles will trigger the trap as they pass over it.
Trap placement is crucial. Voles seldom stray from their runways so set traps along these routes. Look for burrows and runways in grass or mulch in or near the garden or flowerbeds. Place baited traps at right angles to the runways with trigger end in the runway. Examine traps daily and remove dead voles or reset sprung traps as needed. Continue to trap in one location until no further voles are caught, then move the trap to a new location 15 to 20 feet away. Destroy old runways or burrows to deter immigration of new voles to the site.
Bury dead voles or place them in plastic bags in the trash. Because voles may carry infectious pathogens or parasites, do not handle them without gloves; you can use a plastic bag slipped over your hand and arm as a glove. Once the vole is removed from the trap, hold it with your "bagged" hand and turn the bag inside out while slipping it off your arm and hand. Be sure to keep small children and pets out of the area where you have set traps.
POISON BAITS
Rodenticides usually are short-term solution to damage by voles. Habitat management usually is more long-term. Two- percent zinc phosphide baits are available in pellet form, on oats, and on corn. One study indicated that pelleted zinc phosphide baits provide greater control of voles than zinc phosphide placed on oats or corn. Be sure to follow label direction carefully when using any poison bait.
To minimize hazards to birds, do not apply zinc phosphide bait to bare ground, areas without vegetation, or in piles. Also, do not apply to crops destined for use as food or feed. Zinc phosphide can be applied to orchards and groves only during the dormant season after harvest.
The best time of year to use zinc phosphide baits on lawns is during fall before snow cover. Unpredictable rain and snowfall will severely limit the lifespan of baits exposed to the elements. Zinc phosphide can be absorbed in small amounts through human skin. Wear rubber gloves to avoid contact with the chemical and take extra care to avoid breathing zinc phosphide dust.
Anticoagulant baits are also an effective means of reducing vole populations. As with zinc phosphide baits, anticoagulants can be broadcast over an area or placed by hand in runways and burrows. Additionally, anticoagulant baits are often glued to the inside of a water-repellent paper tube to make an effective, disposable bait container. Anticoagulant baits work much slower than zinc phosphide and death is delayed for several days following the ingestion of a lethal dose. This slow action offers an important safety advantage where pets or livestock frequent because it provides time to administer the antidote (Vitamin K1) to an effected animal. Like zinc phosphide baits, anticoagulants can also be toxic to humans. Therefore, you should take precautions to prevent children from gaining access to anticoagulants baits.
BIOLOGICAL CONTROL
Many predators, including coyotes, foxes, badgers, weasels, cats, gulls, and especially hawks, and owls eat voles. However, in most cases predators cannot keep vole populations below damaging levels. Many predators have a broad-based diet and readily shift to alternative prey when the number of voles declines. Predators rarely, if ever, take every last vole; thus, a residual population remains. With their astonishing reproductive potential, any remaining voles could repopulate an area in a short period. With this potential, for severe damage, a homeowner or gardener cannot afford to wait for a predator to appear, but must take immediate action to prevent the loss of valuable plantings.
As with all animals, natural constraints limit vole numbers. Because populations will not increase indefinitely, one alternative is to do nothing and let nature limit the voles. Experience has shown, however, that around homes and gardens the natural population peak is too high and damage will be above tolerable limits.
OTHER METHODS
Frightening devices have been shown to be ineffective in reducing vole damage. Fumigants usually are not effective in controlling voles because the complexity and depth of vole runways and burrows allow the fumigant to escape before voles are exposed to it. Electromagnetic or ultrasonic devices and flooding are also not effective against voles. In the event that a vole invades your house (which is a rare event), individuals can be removed with snap traps or live traps as you would for house mice.