Two species, the meadow vole (Microtus pennsylvanicus) and the pine vole (Microtus pinetorum), might damage fruit trees and become serious pests in orchards. The meadow vole is approximately 5.5 to 7.5 inches long. It has brown fur mixed with black, and its tail is approximately twice the length of its hind foot. The meadow vole is also called the meadow mouse. The pine vole is Pennsylvania's smallest vole. It is 4 to 5 inches long and has chestnut or auburn fur and a short tail approximately as long as the hind foot.
Voles are vegetarians, feeding on grasses, tubers, and seeds. They also consume the bark of young trees. Unlike many other small mammals, voles do not hibernate. Instead, they are active throughout the year, both day and night, with peak activity at dawn and dusk.
Meadow voles create surface runways in the grass; in winter, they are active in these runways beneath the snow. Pine voles build underground tunnels in loose, crumbly soil. As they build the tunnels, they push out dirt, producing small conical piles of soil on the ground surface. Both types of voles build large globular nests of dry grasses and leaves. The nests are located close to tree trunks, in tussocks of grass, and at the end of burrows.
Voles are extremely prolific. Their peak breeding activity occurs between March and October, but when winters are mild, voles may breed all year long. A female meadow vole could potentially produce over 70 young in a year, and the young voles become sexually mature at the age of 1 month. As a result, under ideal conditions vole populations can reach densities as high as 270 voles per acre. Scientists have found that voles exhibit regular population fluctuations at approximately 4-year intervals. Populations apparently crash to levels as low as 10 voles per acre after peak years and then begin to build up again. Voles can cause extensive damage to orchards, particularly during peak population years.
Voles can cause extensive damage to fruit trees and orchards by girdling seedlings and trees and damaging roots. Damage occurs primarily during winter when other types of food are scarce. The most common form of tree injury caused by meadow voles is trunk girdling at or near the ground surface. Since voles burrow in the snow, they might damage tree trunks as high as the accumulated snow. Young trees are especially susceptible to attack. Occasionally, meadow voles will burrow in the soil and damage roots, resulting in weak, unhealthy trees.
Damage from pine voles is harder to detect because it occurs underground as voles consume small roots, girdle large roots, and eat bark from the base of trees. By the time growers note weak, unhealthy trees, the damage already is extensive.
Signs of Voles
The most easily identified sign of meadow vole presence is a system of surface runways in the grass. Meadow voles create these runways by their feeding activities and keep them free of vegetation. The runways are generally about 1.5 inches wide. Bits of freshly cut vegetation and accumulations of vole droppings (brown or green in color and shaped like rice grains) in the runway are positive evidence they are being used. Vegetation, small roots, or mold in the runways indicate that the voles are no longer using them. Pine voles do not use surface runways, so their presence is much harder to detect. In apple orchards, tiny, elongated tooth marks on apples on the ground are signs of both meadow voles and pine voles.
Voles are classified as nongame mammals and can be controlled when causing damage.
Natural controls such as hawks, owls, snakes, weasels, raccoons, foxes, opossums, and house cats all feed on voles. These predators are beneficial in orchards because they help keep vole populations under control.
Whenever possible, growers should encourage these predators, or at least not harass or kill them. When natural controls are inadequate, artificial methods must be used to control vole populations. The fall is the best time for initiating control programs. A number of different control methods are listed below. The greatest success is usually achieved by using a variety of techniques at once.
In orchards, the major food sources for voles are normally not the fruit trees, but roots and stems of grasses and other groundcover. As a result, habitat modification (e.g., reducing or eliminating grasses and cover) is one of the best long-term methods of controlling voles. Repeated mowings that maintain groundcover at a height of 3 to 6 inches serve to limit both food and cover and expose voles to predators. Where possible, mow both between trees in a row as well as along tree rows. Delays between mowings result in excessive vegetation, which, when cut (especially with a sickle-bar mower) forms a thatch layer that protects voles. A flail or rotary mower is preferred to reduce thatch.
Establishing vegetation-free zones that extend at least 2 feet from tree trunks will discourage voles from living near the bases of trees, where they cause the most damage. Vegetation-free zones can be established by mowing, applying herbicides, cultivating, or placing a layer of crushed stone or gravel 1 to 2 inches deep extending 15 to 18 inches from the trunk. Do not allow mulch, prunings, or decaying vegetation to accumulate around the bases of trees or in tree rows.
Hardware cloth barriers can be used to keep voles from girdling small trees. Wrap a strip of 0.25- inch or smaller mesh hardware cloth around the base of each small tree. The hardware cloth should be set 4 to 6 inches into the ground and be approximately 18 to 24 inches high. Use higher guards where snow might be deep. Tree guards should be large enough to allow for 5 years of growth. This method is very effective but extremely labor intensive and expensive when a large number of trees need protection.
Several repellents are registered for voles. The primary active ingredient in these products is capsaicin (the "hot" in hot peppers). It is not clear how effective these products are at deterring voles. If you need to protect a large number of plants, repellents could become too expensive. As with most repellants, they should be applied before damage is significant, and always follow label directions.
Trapping is not an efficient way of controlling voles in large orchards, but it is an effective and safe control method in small orchards or around selected trees. Use standard wooden-base snap traps (mouse size) and bait them with peanut butter, oatmeal, or apple slices. For meadow voles, place the traps in runways, flush with the ground and perpendicular to the runway. Place the trigger end directly in the runway. For pine voles, locate a tunnel and place the trap within the tunnel and perpendicular to it.
Keep the grass mowed as if it were your front lawn. This is an excellent way to control rodents. Rodents require lots of cover to keep their natural predators--hawks and cats--from finding them. If the cover is removed, voles won't stay in the area. Use snap traps to help maintain populations at low levels.