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CORVALLIS - A vole population explosion, aided by warm, wet spring weather and the vole's prodigious reproductive capacity, has hit grass seed fields throughout the Willamette Valley, resulting in crop losses that may reach severe levels in some areas, according to Oregon State University experts.

This year skyrocketing numbers of grey-tailed voles are inflicting varying degrees of crop damage on 40-50 percent of the grass seed acreage in the valley, causing some growers to describe the situation as the biggest vole outbreak in recent memory, said Mark Mellbye, an Extension field crops agent in the Linn County office of the OSU Extension Service.

Grass seed is one of the state's top agricultural crops, earning over $300 million in commodity sales last year. Oregon is a leading grass seed producer in the United States, and Linn County is one of the top grass seed producing counties in the Willamette Valley.

A task force is being formed to address the epidemic, officials say.

Voles are members of the rodent family. Many species live in Oregon, including grey-tailed voles that are commonly found in valley grass seed fields. They feed on grass seed and on leafy plant growth, which stunts the plants, preventing them from producing seed.

Often referred to as meadow mice or field mice, voles grow to no more than six inches in length and breed rapidly. Female voles can produce a new litter of 4-8 young every three weeks, and they breed again immediately after delivering their young.

"This is the worst vole problem in grass seed crops that I've ever seen in my 20 years of working in the central and southern valley area," said Mellbye. "This clearly appears to be the case in Linn County where some fields have suffered up to 60 percent seed yield loss."

Mellbye began to get a rough idea of the scope of the problem while examining grass seed experimental trial plots that he and other OSU Extension field crops faculty have established throughout the valley.

"We typically set out trial grass seed plots in growers' fields in several locations to evaluate new fertilizer and plant disease control approaches," Mellbye said. "This year the trial plots have become a great tool in helping us measure the impact of vole damage on crop yields."

The current flood of grey-tailed voles swarming through grass seed fields is due in part to the animal's eruptive population cycle.

"Voles will go from a population low to a population peak every 3 - 5 years," said Bruce Coblentz, professor in the OSU Department of Fisheries and Wildlife. "They're obviously in a peak year in the Willamette Valley now.

"In addition, the warm, wet weather we've had so far this year has produced an abundance of thick vegetation in crop fields," Coblentz said. "This provides a plentiful food source and thick habitat cover that helps voles hide from predators."

In most years, animals that prey on voles, including hawks, owls, coyotes, foxes, dogs and cats, would influence the rate of increase and the eventual peak density of vole populations, but in a year like this the number taken by predators is minuscule in comparison to the rate of reproduction of these voles, Coblentz explained.

Responding to increasing concerns about the vole epidemic, the OSU Extension Service is coordinating the formation of a Vole Task Force.

"This group will include OSU Extension faculty, grass seed industry representatives and Oregon Department of Agriculture officials," said Mellbye. "Our top priority will be to survey the extent of crop yield damage caused by voles throughout the valley. The task force will also examine alternative pest management methods and take steps to ensure that rodent baits are available and help growers to use them carefully according to application requirements."