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, and 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 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. Extensive damage may occur in orchards, particularly during peak population years.

Damage

Voles may cause extensive damage to fruit trees and orchards as a result of 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  may damage tree trunks as high as snow accumulates. 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 orchardists note weak, unhealthy trees, the damage is already extensive.

Monitoring

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 1/2 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.

The apple indexing method is a way to determine the distribution of voles in an orchard and their relative abundance. Place an apple with a slice removed into a meadow vole runway or in a pine vole tunnel. Check the apple after 24 hours for vole tooth marks. The presence of tooth marks will indicate where vole activity is highest and which trees are at risk. To obtain an estimate of the abundance of voles, weigh the apple before putting it out and after 24 hours. One pine vole consumes approximately 13 grams of apple in a 24-hour period and one meadow vole consumes about 20 grams.

Most orchardists do not need to know the exact number of voles present, but they may want to know whether the population is increasing or decreasing, or whether a particular treatment had an impact on population size. Monitoring vole numbers with the apple indexing method is a means of achieving these goals.

Trapping can also be used to assess the effectiveness of a vole-control program. Before initiating the control program, select approximately 10 trees and place four wooden-base (mouse-size) snap traps in runways near these trees (for trap placement see section on trapping below). Record the number of voles trapped in a 3- to 5-day period. After the control program is finished, set the traps in the same place and, for the same length of time, compare the number of voles caught   after treatment with the number caught before treatment. If the program has been successful, you should trap no more than two or three voles.

The number of voles that can be tolerated is a trade-off between cost of control and cost of damage, and it depends on the orchardist. A single vole may cause damage, but most damage occurs at high population levels. Monitoring vole populations enables growers to assess when populations are starting to increase and to begin control programs at that time.

Management

Biological control

Hawks, owls, shrikes, 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, orchardists 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.

Habitat modification

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, that is, reducing or eliminating grasses and cover, is one of the best long-term methods for 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. Too much delay between mowings results 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 for reducing thatch.

Establishing vegetation-free zones under tree canopies 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 may be established by mowing, applying herbicides, cultivating, or placing a layer of crushed stone or gravel 3 to 4 inches deep that extends 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.

Exclusion

Hardware cloth barriers can be used to keep voles from girdling small trees. Wrap a strip of 1/2-inch or less mesh hardware cloth around the base of small trees. 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 may 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 larger number of trees need protection.

Trapping

Trapping is not an efficient way of controlling voles in large orchards, but it is an effective and safe control method for 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 in the runway. For pine voles, locate a tunnel and place the trap within the tunnel and perpendicular to it.

Rodenticide Techniques

Please refer to pages 44-45 and page 72 in the 1998-1999 New England Apple Pest Management Guide for an overview of rodenticides and techniques for their use.  Note:  rodenticide baits may be attractive to domestic pets, birds, and other non-target wildlife.  Precautions should be taken to avoid problems.