Vole explosion

Farmers in Oregon’s Willamette Valley hope that populations of voles recede after reaching high levels in recent years.

With the population of voles apparently peaking in Oregon’s Willamette Valley, farmers hope the destructive rodent’s numbers will soon take a cyclical nosedive.

Though estimating the pest’s specific population isn’t feasible, damage to crops indicates their regional numbers are “astronomical” — likely in the millions, said Nicole Anderson, field crops Extension agent with Oregon State University.

“We’ve definitely been on that high level for the past several years,” Anderson said.

Vole populations naturally fluctuate, with populations spiking for two to three years at intervals of about five to eight years apart, she said.

By that pattern, the rodent should be headed for a cyclical decline soon, she said. “It should be imminent. I hope so.”

Typically, the vole population will “explode” but then encounter an outbreak of disease that rapidly brings down the rodent’s numbers, said Denny Thorud, an agronomist with Valley Agronomics.

More recently, though, the pest’s populations have appeared to resist steep plunges, he said. “We haven’t had a real die-off of voles in probably the last 10 years.”

Voles live underground and form groupings of connected tunnels from which they can consume plant roots, depriving crops of water nutrients until they eventually die, said Anderson of OSU.

The rodents can reduce yields by up to 30 to 40 percent in field crops, she said.

A vole becomes ready to breed roughly three weeks after birth and can then produce a litter of four to seven offspring after another three weeks, she said.

“They have an incredible reproductive potential,” Anderson said.

While voles are often associated with crops, such as grass seed and clover seed, they’re not averse to attacking newly planted hazelnut trees or blueberry bushes, said Thorud of Valley Agronomics.

When farmers plant hazelnut trees in fields of grass, voles begin feeding on the green young bark when surrounding grass strips go dormant, he said.

“We saw a lot of orchards get girdled,” Thorud said. “There were a lot of trees that had to be replanted.”

Plowing a field is a direct way for farmers to kill voles, but that’s not a realistic solution for perennial crops, including grasses and clovers that produce crops for several years, Anderson said.

Mechanical cultivation also isn’t a permanent solution, as the rodents will eventually recolonized fields, she said. “They will get more populated the further from tillage you get.”

Crop rotation is one option to suppress vole numbers, as they don’t seem to thrive in crops such as meadow foam, wheat and oats, Anderson said.

Installing “raptor poles” on which predatory birds can perch can reduce vole populations, as can “owl boxes” for nesting, though they won’t completely solve the problem, she said.

Zinc phosphide, a rodenticide, will also reduce vole populations, though it’s not without complications, she said.

“We’ve noticed its effectiveness has declined over time,” Anderson said.

The substance is applied in the form of bait that’s dropped into the holes created by voles to keep their numbers in check, said Mike Vandehey, a grass and clover farmer in Washington County.

“Even then, they can overwhelm you,” Vandehey said. “It’s been a challenge, I would say.”

The chemical causes a vole’s internal gases to build up until the rodent perishes, which is a safer method of killing them because it won’t affect birds or mammals that may eat the carcasses, he said.

As for the reduced efficacy of the rodenticide, Vandehey blames the aversion that voles develop for its taste after surviving non-lethal doses.

It’s also possible some have developed a tolerance to the chemical, he said.

“When you use the same bait for 30 years, it wouldn’t surprise me,” he said.

I've been working at Capital Press since 2006 and I primarily cover legislative, regulatory and legal issues.

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