High Yield Grass Seed Production and Water Quality Protection Handbook

Vole Control

Gray-tailed vole

In some years, voles (meadow mice or field mice) can cause serious crop damage in grass seed fields. Their presence is evident from numerous small holes, about the size of a quarter, often in the center of a denuded area. Voles construct an extensive system of surface runways and burrows, frequently using the burrows of other rodents. The surface runways are usually hidden in tall vegetation.

There are several species of voles in Oregon that can cause damage in field crops, orchards and landscapes. The problem species in grass seed fields is usually the gray-tailed vole (Microtus canicaudus). This vole is 5 to 6 inches long, with a short tail 1 to 1.75 inches long. It eats mostly grass leaves and roots, but it also climbs the stalks and clips off seed heads when these are available. Clover is another favorite food. The gray-tailed vole is endemic to the Willamette Valley and Clark County, Washington. Formerly this species lived in the prairie grasslands that resulted from annual burning by native Americans. Now it lives primarily in agricultural lands - grass seed and grain fields, pastures and grassy waste places. Another species, the Townsend's vole (Microtus townsendii), which usually inhabits wet meadows and riparian thickets, can become a problem in the wetter grass seed fields.

Species biology

Voles are capable of reproducing at fantastic rates. Female gray-tailed voles go from birth to pregnancy in 28 days when food is abundant. Gestation is 21 days and litter size is typically four to six. Their life spans, however, are extremely short - not more than 12 months. Most voles live less than a month because they become food for owls, hawks, snakes, coyotes, foxes, skunks, house cats and great blue herons.

Although gray-tailed voles are active and capable of breeding all year round, they stop breeding when there is little food. When grass seed fields are harvested, their population crashes. They are also more vulnerable to predators at this time because of the lack of cover. With fall green-up, they start to reproduce and build their numbers. In most years, they die off again in the winter to only one to two animals per acre. A series of mild winters, however, can allow their population to build to pest proportions.

Cultural practices that make an area less hospitable to voles, such as eliminating vegetative cover and encouraging predator habitat, are helpful in some situations but may not be practical for grass seed fields. Some growers have tried installing perches for hawks near fields. There is no research as yet to verify how effective this might be.

Chemical control

Voles can be killed with poison bait, and zinc phosphide bait is the most effective. However, in wet conditions, zinc phosphide escapes into the air as phosphene gas. There are currently Oregon 24(c) Special Local Needs labels for broadcast application of zinc phosphide oat bait and pellets in grasses grown for seed. Because zinc phosphide is toxic to birds and other wildlife, there are use restrictions. The most significant restriction is that it can only be applied between April 30 and September 1. The labels limit the application to an area of not more than 20 acres as a solid block, with the total treated acreage not exceeding 30 percent of the field. The bait should be concentrated in areas with the heaviest vegetative cover. It should not be broadcast onto bare ground where wildlife are likely to be attracted to it.

A good thing about zinc phosphide is that there is little danger of poisoning to wildlife eating the dead voles, as there is from anti-coagulant or strychnine baits.

Susan Aldrich-Markham; Oregon State University Extension Service