Vexed by Voles

When Idaho’s vole populations spike, environmental factors and a corresponding surge in predators usually bring their numbers back into equilibrium by the following year.

In the Mud Lake area of Jefferson County, however, the mouse-like, burrowing rodents have been plaguing farmers for three consecutive seasons, with no relief in sight.

Last fall, the problem grew so acute in the rural eastern Idaho community that area farmers called an emergency meeting attended by congressional leaders, state lawmakers, regulators and University of Idaho Extension staff. Shortly after the meeting, Idaho Governor Brad Little flew over the area in a helicopter to survey the damage.

UI Extension experts who have been contemplating how to address the problem in Mud Lake and other pockets of the state where voles are running rampant acknowledge there’s no “silver bullet” solution, but they’ve identified several simple practices to help growers better weather the storm.

UI Extension Educator Joseph Sagers, Jefferson County, believes it might be worthwhile for growers to consider pre-baiting. Zinc phosphide, the most used toxicant for vole control, is commonly applied to wheat, oat or millet seed and broadcast onto fields for voles to consume. Broadcasting untreated grain seed onto fields, known as pre-baiting, aims to increase voles’ appetite for the bait, thereby leading them to eat more of it when growers spread treated kernels.

Sagers also plans to host a demonstration project for area growers, setting up barn owl boxes to support predators. UI Extension Educator Jason Thomas, Minidoka County, has promoted the use of predators to control voles and has posted free plans online for farmers who would like to make their own boxes.

Sagers distributed a survey to farmers at last fall’s vole meeting to assess the scope of the problem. Yield losses in the hardest-hit crop, alfalfa, averaged 27% among the survey’s 30 respondents. Many farmers said they were forced to take fields out of alfalfa after only two years, rather than the five to eight years of alfalfa they expected to get.

“They had 27% yield losses. That’s really bad for operations working on single-digit profit margins,” Sagers said. “They’ve reached this critical mass in the alfalfa fields that voles are spreading out into other fields.”

In potatoes, even a small amount of vole damage may cause tubers to turn green and decrease crop value. Voles have also been decimating small grains, rangeland and sugar beets.

Sagers plans to conduct another survey of area farmers to gather additional information, and he’ll be working with UI Extension agricultural economist Patrick Hatzenbuehler to evaluate the economic impact of voles in the county. Sagers may also set up trail cameras in certain fields to help quantify vole numbers.

Terreton farmer Steve Shively advocates for a local disaster declaration to fund the widespread distribution of zinc phosphide bait.

“I think it’s going to take a disease to get them to thin out,” Shively acknowledged. “We’ve had to tear up some of our alfalfa just because it was chewed up too bad and too rough. We had some fields we were tilling two or three times trying to thin them out and planting winter wheat, and the voles were still in there.”

Some of the same shifts in practices that have made Idaho farmers better stewards of the environment have also given voles an advantage. Throughout the past few decades, vole outbreaks have gotten worse as growers have dramatically reduced tillage to improve soil health, while also installing highly efficient sprinkler systems in fields that were formerly flood irrigated. The combination of deep, moldboard plowing and flood irrigating formerly disturbed a lot more vole habitat and subterranean dens.

“We’re having alfalfa that survives longer and we’re doing better with no-till, which is amazing for soil health, but as we build these efficient crop systems it’s making things better for the voles,” Sagers said.

In southeast Idaho and the Magic Valley, the vole population explosion started last fall, thanks to perfect habitat conditions and an abundance of grass and forage. Danielle Gunn, a UI Extension educator based at the Fort Hall Reservation, believes producers should begin gathering hard data on vole losses in support of the establishment of a vertebrate task force to help tackle the problem.

“If we can begin working more cohesively with growers and vertebrate pest specialists, we can compile vole-loss statistics, track populations by year and develop additional solutions to this significant problem,” Gunn said.

Gunn’s primary advice to growers for the time being is to carefully follow product labels when deploying vole-control toxicants, making sure to use bait at the proper timing. For example, voles prefer to eat surrounding vegetation and ignore bait when stubble height in fields is greater than 2 inches.

In addition to modern tillage and irrigation practices, Gunn believes the proliferation of cover crops, development pressure, land conservation programs and the shift toward mild, open winters and wet springs and falls due to climate change have contributed to worse vole outbreaks. Typically, vole populations experience a minor peak every four to six years, with epidemic population increases occurring every 10 to 12 years.

Gunn advises farmers who have the flexibility to rotate out of alfalfa in favor of crops with fibrous root systems, such as wheat, when voles become a problem. Gunn recommends cultivating a 15- to 30-foot perimeter of bare ground surrounding fields, as voles are wary of crossing areas where there’s no cover due to predator concerns. Furthermore, growers may wish to use anticoagulant bait adjacent to those perimeters, taking care not to use it directly in crops.

Removing weeds surrounding fields and burning vegetation along ditch banks can also help farmers eliminate vole habitat and food sources. In some foreign countries where voles have caused major crop losses, producers have buried 5-gallon buckets in fields, each filled with a little water to trap the rodents that fall in them. Gunn believes the approach is worth a look in Idaho.

The bad news for farmers is that the U.S. Environmental Protection Agency has been reviewing some of the crucial toxicants used in vole control, and new restrictions may be forthcoming.

“It’s really important that we start being more proactive with the problem,” Gunn said.

Published in Catching Up with CALS