Damage and Damage Identification

Pine vole, Microtus pinetorum (left), and prairie vole, M. ochrogaster left.

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 (0.3 cm) wide, 3/8 inch (1.0 cm) long, and 1/16 inch (0.2 cm) 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.

Figure 6. Surface runway system of the prairie vole.

The most easily identifiable sign of voles is an extensive surface runway system with numerous burrow openings (Fig. 6). Runways are 1 to 2 inches (2.5 to 5 cm) 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.

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 thou-sand 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.