Kim Wagner

Voles (a.k.a. meadow mice) are small, stocky, short-tailed rodents that can cause extensive damage in timber plantings. A study in British Columbia demonstrated that vole damage was the primary reason why over 40 percent of the plantations examined had inadequate restocking rates. Voles eat a wide variety of vegetation, and are attracted to areas with good cover and a variety of plants. The plant communities that develop within a few years after fire or logging are often highly productive vole habitats. Vole populations follow a cyclic pattern with population levels reaching a peak every two to five years. In general, vole populations range from 20 to 35 animals per acre, but may increase to over 1,000 animals per acre in high quality habitats. Vole damage is most common in fall and winter when other foods are limited.

Recognizing Vole Damage

Vole activity can be detected by looking for a combination of tree damage and above-ground runways. Voles build extensive burrow systems and above-ground runways between openings to their burrows. The presence of vole runways is a cue to start looking for vole damage. Vole runways are one to two inches wide and may have droppings or plant clippings in the runway. In areas with heavy ground cover, the runways may look like above-ground tunnels. Plants may be clipped close to the ground in areas around well-traveled runways.

Voles may damage trees above and below the ground. The most obvious damage is to stems of trees where vole tooth marks will be at various angles and may have a whorled or circular appearance. Tooth marks will be about 1/8 inch wide, 3/8 inch long and 1/16 inch or more deep. Rabbits and porcupines leave larger tooth marks and may leave larger chips at the base of the plant. When clipping plants, voles leave a somewhat pointed tip at the end of the stem. Rabbits and porcupines will leave an angular cut. Vole damage to tree roots may not be detected until the needles start turning brown. In these instances, it may not be possible to determine the cause of damage without pulling up the tree and examining the roots. Voles damage roots by peeling the cambium off the root and then eating the root. As with clipped stems, voles tend to leave pointed root ends. In contrast, pocket gophers eat the entire root at once and leave angular root ends.

Managing Vole Damage

Habitat management is one of the most effective ways to handle vole problems. By limiting vegetation and plant litter, you reduce the amount of available food, increase the risk of predation, and minimize the attractiveness of the site for new immigrants. If possible, leave time for the population to decline in response to the reduction in vegetation before you plant. Reducing vegetation at the time of planting may leave the voles with little to eat other than your trees.

Plowing or disking is ideal because it destroys vole food, cover and burrows. Vegetation reduction with herbicides, fire or mowing is also effective options. If some vegetation is needed, spot treatments can be used. Spot treatments involve removing vegetation and plant litter from around the base of the tree. Spot treatments help keep burrowing activity away from your trees because voles are unlikely to dig in areas where they don't find food. Try to keep vegetation and debris at least three feet from the base of the tree. Cut the remaining vegetation short and remove plant litter to limit food and shelter. If the area between spot treatments contains a high number of voles, competition for food will eventually cause hungry voles to cross the spot treatments.

Toxicants are an alternative if vegetation management fails to reduce damage to tolerable levels. Toxicants may also help quickly reduce vole population on sites where you cannot leave time between vegetation reduction and planting, or where trees are already established. Remember that voles multiply rapidly in good habitat, so you will have the longest period of population reduction if you combine use of toxicants with vegetation management.

Zinc phosphide is the most common toxicant used to control voles in timber. However, it is not legal in all areas, so check with your local agriculture extension agent to determine product availability and licensing requirements. Using poison bait within two to three days of plant removal and conducting your baiting program during a season when voles are feeding on above-ground vegetation will improve your success. Keep bait dry and fresh or the voles will reject it. Placing pieces of shingle, tar paper or board over the section of runway with bait will help keep the bait dry and reduce the risk to other animals. Voles will be more likely to find your bait if you hand place bait in runways and burrow openings, or reduce plant cover so that broadcast bait can reach runways.

Planting larger seedlings will not reduce vole numbers or prevent them from eating trees, but larger plants are better able to withstand partial damage. Larger seedlings also spend less time in the size range that is most vulnerable to damage by small rodents. However, it should be noted that voles can completely girdle trees several inches in diameter.

Repellents and barriers usually will not stop vole damage. Voles can climb over and into most tree shelters and wraps. Barriers cannot prevent underground damage to roots. Most repellents only protect the plant parts they touch and cannot protect root systems. There are some systemic repellents that will get the repellent to all parts of the plant, but these products use a bittering agent that has not performed well in tests to reduce damage by herbivores.

Remember to keep monitoring your site. Plant communities and vole populations can change relatively quickly. An effective damage management program may require periodic repetition of your treatments.

Kim Wagner is a research wildlife biologist with the U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Wildlife Services, National Wildlife Research Center Field Station, Olympia, Wash. She can be reached at kimberly.k.wagner@usda.gov.

This article appeared in the Northwest Woodlands Magazine, Spring 2001- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.