Some Idaho alfalfa fields were so covered with voles last summer it appeared as though the fields were moving. Multitudes of the small rodents, trying to cross roads, nearly caused traffic hazards.

Those may sound like wild exaggerations, but they aren’t, say Idaho alfalfa growers who lived through the experience.

“It was unreal,” says Chris Taber, who manages the crop operations for Donley Farms, a family partnership near Shoshone. “The voles were in our small grains, hay, irrigated pastures and sugarbeets. They root-pruned the alfalfa. They cut through corn like a chainsaw cuts through timber. Small grains took a terrible hit.”

Taber jumped on the vole problem early. He treated every acre of alfalfa with a zinc phosphide rodenticide. For two treatment cycles, the product helped. Then the voles seemed to learn to avoid it, and in a growing season that wasn’t very good, they probably cost him between ½ and 1 ton/acre.

Even now, with snow on the ground, the voles are eating, worries Taber. “What I need is a winter hard enough to kill voles, one where moisture gets down to where the voles burrow and is then followed by unbearably cold weather that turns the moisture to ice.”

The area where Taber and his neighbors farm pretty much was ground zero for the vole explosion, confirms Christi Falen, a University of Idaho Extension educator for Lincoln County. “Alfalfa was hit really hard,” she says. “Multiple fields were taken out by voles burrowing and eating roots. New alfalfa seedings were hit super hard. The voles would eat the new growth like candy.”

Some growers lost 30-40% of their stands, she reports. Others replanted alfalfa or seeded another crop. Some even took complete losses.

“Our farmers are a die-hard, optimistic group. But it was a hard time.”
A mild winter with good snow cover sparked the spike in vole numbers, Falen believes.

Glenn Shewmaker, University of Idaho Extension forage specialist, agrees. While his area south of the Snake River had fewer vole problems, he fielded a lot of calls from producers wanting to know what to do about them.

“For three years, we’ve seen increasing numbers of voles, mostly due to favorable environmental conditions,” he observes. “Mild winters provide a snow cover that protects voles from predators like hawks and coyotes. The snow also protects forage residue – the voles’ food source – from mold much like a refrigerator protects food. To get rid of voles, we need a December snow followed by rain to turn the snow to ice. Ground frozen with a hard seal of ice will decimate the vole population.”

Voles come and voles go, says Bill Simon, an alfalfa grower near Fairfield, ID. His fields escaped the heaviest vole infestations, but he knows that someone else’s problem one year may be his the next.

“I’m counting on nature to step in,” says Simon. “When rodent populations get too thick, it seems that they develop disease problems, and most of them die off.”

That’s true, concurs Shewmaker, who notes that vole populations are very cyclic. But in serious vole situations, growers may not want to wait for nature to step in with ice or disease. In Idaho, they can bait fields with rodenticides such as zinc phosphide. However, this option is not labeled in many states.

He advises Idaho growers to scout their fields now and again in spring.
Look for tunneling, burrowing and runs. Put out bait stations where voles are most active. Bait stations housed in PVC pipes can be moved from field to field.

Start by prebaiting. Get voles accustomed to eating the bait as a food source, then add the rodenticide according to label. Baiting can be very successful when you adhere to the manufacturer’s instructions, Shewmaker emphasizes. But voles won’t take the bait if they figure out it’s poison.

Managing Voles And Other Rodents
Check these Web sites for more on voles, pocket gophers and other pests:
Managing Voles in Colorado;
Organic Alfalfa Management Guid;
Wildlife Damage Management Series;
Controlling Vole Damage;
Wildlife Damage Control Publications;
Garden Friends & Foes;
University of California IPM Online;
Controlling Voles;
Pocket Gopher Control and Management Informatio.