Six species of voles, genus Microtus, occur in California. The most widespread species in the state is the California vole (Microtus californicus), which occurs in the Central Valley and throughout the length of the coast range. Most damage to potatoes occurs in the Klamath Basin, where the montane vole (M. montanus) is found.
Voles are 4 to 6 inches long when full grown, with a blunt nose, small eyes, and short, furry ears. The tail is less than half as long as the body and is slightly hairy. Voles are active day and night, and may be seen scurrying around aboveground although usually they are hidden in vegetation. The clearest indication that voles are present is the network of aboveground runways that connect burrow openings that are about 1.5 to 2 inches in diameter. Droppings are about 0.18 inch (4.5 mm) long and greenish when fresh, turning brown or gray with exposure to the environment. Sometimes fresh leaves or other cuttings are found in these trails.
Biology and Behavior
Voles, also called meadow mice or field mice, live in colonies in areas such as irrigated pastures, fence-rows, or weedy ditchbanks, where the soil is suitable for burrowing and where vegetation provides cover. They usually avoid the sandy soils in which potatoes are commonly grown. The soil of the Tule Lake Basin of northern California is more favorable for voles, and there is also abundant natural habitat where vole populations can remain active year-round. In this area vole populations reach levels that require control every 7 to 10 years. Controls are required less frequently in other parts of the Klamath Basin.
Voles move into potato fields when infested grain or alfalfa fields are harvested, usually in August or September, burrowing into hills for shelter and to feed on tubers. Their feeding damages tubers directly and their burrows may expose tubers to sunlight or freezing temperatures, causing additional losses. Damaged tubers have gnaw marks about 0.12 inch (3mm) wide and 0.36 inch (9 mm) long at various angles. Damage may also be caused by predators that dig into potato hills in search of voles. If vole populations are high at harvest, they may be carried into storage, where they will continue to feed on tubers and also contaminate the crop.
In the Tule Lake Basin, ditchbanks are monitored with visual inspection and traps to determine the relative numbers and presence of voles each spring by the county agricultural commissioners' offices. Visual inspections take into consideration the density of the vole population, cluster size, and distribution over the assessed area. Inspections at 30-day intervals provide information on the movement and magnitude of change of the populations.
If monitoring programs in your area indicate that voles may become serious, begin checking your fields for vole activity when removing irrigation pipe at the end of the season or after nearby grain or alfalfa fields are harvested. Look for active burrows—ones with fresh soil around the entrance. Check closely around clumps of wild oats, where vole burrows are most likely to be found. A trap index of 6 to 10 or more mice per 100 trap nights is indication of a pending problem and is, therefore, often used as a threshold for initiating vole control.
When trap monitoring or visual estimates of vole activity indicate damage is likely, poison bait may be applied to ditchbanks in spring to slow or prevent the population from expanding into the crop. (Bait can legally only be applied directly to potato fields after vine death.)
In other Klamath Basin growing areas, there are no regular monitoring programs because voles rarely reach damaging levels. However, a routine check for vole activity near the end of each season is a good way to identify if voles may become a problem during the next growing season. If you find vole activity in your fields at the end of the season, check with your farm advisor or agricultural commissioner's office to see if bait application is recommended.