Voles
Introduction
A vole is a small rodent resembling a mouse but with a stouter body; a shorter, hairy tail; and smaller ears and eyes.
Identification
Voles, also called meadow mice or field mice, belong to the genus Microtus. Voles are compact rodents with stocky bodies, short legs, and short tails. Their eyes are small and their ears partially hidden. Their underfur is generally dense and covered with thicker, longer guard hairs. They usually are brown or gray, though many color variations exist.
There are 23 vole species in the United States. According to the Department of Environmental Protection, two varieties live in New Jersey: the meadow vole and the pine vole.
Meadow Vole (M. pennsylvanicus). The meadow vole is the most widely distributed Microtus species in the United States. Its total length is 5 1/2 to 7 1/2 inches and its fur is gray to yellow-brown, obscured by black-tipped hairs. Northern subspecies may also have some red in their fur. Its underparts are gray, at times washed with silver or buff. The tail is bicolored.
Pine or Woodland Vole (M. pinetorum). The pine vole is a small vole. Its total length is 4 to 6 inches. Its brown fur is soft and dense. The underparts are gray mixed with some yellow to cinnamon. The tail is barely bicolored or unicolored.
Habitat
Voles occupy a wide variety of habitats. They prefer areas with heavy ground cover of grasses, grasslike plants, or litter. When two species are found together in an area, they usually occupy different habitats. Though voles evolved in “natural” habitats, they also use habitats modified by humans, such as orchards, windbreaks, and cultivated fields, especially when vole populations are high. Characteristic habitat descriptions for the two New Jersey species follow:
Meadow Vole. The meadow vole is found in the northern United States and Canada. It prefers wet meadows and grassland habitats. When in association with the montane vole or prairie vole, it is generally in moister habitats.
Pine Vole. The pine vole is found in the eastern United States. It inhabits a variety of habitats such as deciduous and pine forests, abandoned fields, and orchards. Heavy ground cover is characteristic of these habitats.
Food Habits
Voles eat a wide variety of plants, most frequently grasses and forbs. In late summer and fall, they store seeds, tubers, bulbs, and rhizomes. They eat bark at times, primarily in fall and winter, and will eat crops, especially when their populations are high. Occasional food items include snails, insects, and animal remains.
General Biology, Reproduction, and Behavior
Voles are active day and night, year-round. They do not hibernate. Home range is usually 1/4 acre (0.1 ha) or less but varies with season, population density, habitat, food supply, and other factors. Voles are semifossorial and construct many tunnels and surface runways with numerous burrow entrances. A single burrow system may contain several adults and young.
Voles may breed throughout the year, but most commonly in spring and summer. In the field, they have 1 to 5 litters per year. They have produced up to 17 litters per year in a laboratory. Litter sizes range from 1 to 11, but usually average 3 to 6. The gestation period is about 21 days. Young are weaned by the time they are 21 days old, and females mature in 35 to 40 days. Lifespans are short, probably ranging from 2 to 16 months. In one population, there was 88% mortality during the first month of life.
Large population fluctuations are characteristic of voles. Population levels generally peak every 2 to 5 years; however, these cycles are not predictable. Occasionally during population irruptions, extremely high vole densities are reached. Dispersal, food quality, climate, predation, physiological stress, and genetics have been shown to influence population levels. Other factors probably also play a part.
Many voles are excellent swimmers. The water vole, in fact, escapes predators by swimming and diving. The climbing ability of voles varies. The long-tailed vole, for example, is a good climber while the pine vole is a bit clumsy in this regard.
Voles are prey for many predators (for example, coyotes, snakes, hawks, owls, and weasels); however, predators do not normally control vole populations.
Damage and Damage Identification
Voles may cause extensive damage to orchards, ornamentals, and tree plantings due to their girdling of seedlings and mature trees. Girdling damage usually occurs in fall and winter. Field crops (for example, alfalfa, clover, grain, potatoes, and sugar beets) may be damaged or completely destroyed by voles. Voles eat crops and also damage them when they build extensive runway and tunnel systems. These systems interfere with crop irrigation by displacing water and causing levees and checks to wash out. Voles also can ruin lawns, golf courses, and ground covers.
Girdling and gnaw marks alone are not necessarily indicative of the presence of voles, since other animals, such as rabbits, may cause similar damage. Vole girdling can be differentiated from girdling by other animals by the non-uniform gnaw marks. They occur at various angles and in irregular patches. Marks are about 1/8 inch wide, 3/8 inch long, and 1/16 inch or more deep. Rabbit gnaw marks are larger and not distinct. Rabbits neatly clip branches with oblique clean cuts. Examine girdling damage and accompanying signs (feces, tracks, and burrow systems) to identify the animal causing the damage.
The most easily identifiable sign of voles is an extensive surface runway system with numerous burrow opening. Runways are 1 to 2 inches in width. Vegetation near well-traveled runways may be clipped close to the ground. Feces and small pieces of vegetation are found in the runways.
The pine vole does not use surface runways. It builds an extensive system of underground tunnels. The surface runways of long-tailed voles are not as extensive as those of most other voles.
Voles pose no major public health hazard because of their infrequent contact with humans; however, they are capable of carrying disease organisms, such as plague (Yersinia pestis) and tularemia (Francisilla tularensis). Be careful and use protective clothing when handling voles.
Legal Status
Voles are classified as nongame mammals and can be controlled when causing damage. Contact your local state wildlife agency for details regarding applicable codes and regulations.
Damage Prevention and Control Methods
Exclusion
Recommended to protect trees, ornamental plants, and small areas.
Habitat Modification
Eliminating ground cover reduces populations. Soil cultivation destroys burrows and reduces cover.
Frightening
Not effective.
Repellents
Effectiveness uncertain.
Toxicants
Zinc phosphide. Anticoagulants (registered in most states).
Fumigants
Not usually effective.
Trapping
Snap traps. Live traps (Sherman or box-type traps).
Economics of Damage and Control
Jameson (1958) calculated that 100 meadow voles per acre destroyed about 4% of an alfalfa crop, which amounted to about 1,000 pounds per acre (1,136 kg/ha) over 7 months.
Populations of 1,700 voles per acre (4,250 voles/ha) in Washington State apple orchards decreased production by 35%. This amounted to a loss of $3,036 per acre ($7,590/ha) due to reduced fruit quality and quantity. One year after eliminating voles, the production in the orchard increased but was still below the production of orchards that had not incurred vole damage. Total losses for the 2-year period were estimated at $6,100 per acre ($15,250/ha) (Askham 1988). Similar apple orchard loss figures were calculated for pine voles in New York. Known densities of voles (0, 109, 218, and 436 per acre [0, 273, 545, and 1,090/ha]) were stocked in fenced blocks of McIntosh trees for 2 years. There was little impact the first year. The second year, the highest vole population reduced fruit yield 65.5% and increased undersized fruit from 3.1% to 57.5%. These factors caused a $2,745 per acre ($6,863/ha) reduction in income. In addition, survival of the trees through a third year was considered unlikely. The worst vole outbreak in the United States probably occurred in Nevada in 1908 and 1909. Ten thousand acres (400 ha) of alfalfa were completely destroyed. Vole populations were estimated at 25,000 per acre (62,500/ha).
Often a control program may not appear to be justified in comparison to the damage being incurred. It should be remembered, however, that the “ounce of prevention” rule frequently applies in vertebrate pest control. Preventive control measures that at first appear too costly may eventually prove to be a bargain.
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