Voles may be small in size, but they can create big damage. Also known as meadow mice, voles are one of the most destructive pests to alfalfa and irrigation systems, especially in the West.

At the California Alfalfa and Forage Symposium held last year in Sparks, Nev., Roger Baldwin explained different vole control tactics and the variable results of each one. Overall, the professor of cooperative extension with the University of California, Davis, suggested voles will never be eliminated, but the harm these pests cause alfalfa can be mitigated.

Voles fall between house mice and pocket gophers on the spectrum of rodents by size, measuring 4 to 6 inches in length. The pests are typically dark gray or brown in color and are characterized by a blunt nose, small eyes, and short ears and legs. Whatever voles lack in stature, they make up for in number, producing five to 10 litters of three to five young per year. However, vole populations tend to exhibit irruptive growth patterns that peak every six to eight years.

“In some cases, maybe you don’t realize you have voles, and then conditions get just right and populations can explode. When that happens, it can feel like you are being overrun by voles,” Baldwin said. “It’s certainly very challenging to deal with in that situation.”

Voles feed on alfalfa taproots and aboveground biomass, and they also cause damage to stands with their burrows, which are about an inch and a half in diameter. One way to differentiate vole burrows from other rodent burrows are the well-worn paths they create by running between burrow openings on the soil surface — other rodent species don’t make the same trails. This can be an important identifier when pursuing control methods specific to voles.

Stand damage varies with vole density, but the pests cause an estimated 11% average revenue loss in California alfalfa when they are found in a field. Even though a complete eradication may not be possible, pest management should be taken seriously to protect stands to an extent.

Flooding and baiting

Current vole control recommendations are centered on an integrated approach using a combination of strategies; however, there is a limited arsenal of options. These include cultural practices, toxic baiting, and exclusion fencing. Fumigation, trapping, and repellents, on the other hand, are not very effective forms of vole control.

Where available, Baldwin suggested flood irrigation can be the initial tool to knock vole populations down a peg. Flooding either drowns voles out or forces them to the surface where they are subject to predators.

“If you’re an individual that still has the capability of using flood irrigation, this can be a good tool to use periodically to keep some of these burrowing species in check,” Baldwin said. “I think the loss of flood irrigation is one of the reasons we’ve seen the proliferation of rodents in these fields over the last few years.”

Pairing flood irrigation with chemical control options like zinc phosphide can enhance the efficacy of the latter practice. Baldwin explained that zinc phosphide is the only rodenticide that is registered for use directly in alfalfa. It’s an acute toxicant, so it can kill voles after a single feeding; however, it might not meet its consumers’ standards.

“It has a distinctive garlicky odor — and supposedly taste — that is associated with it that the rodents don’t necessarily like,” Baldwin said about zinc phosphide. Therefore, picky eaters may consume a few pieces of grain or pellets, become sick, but then fail to die. Then, voles associate sickness with the bait and avoid eating it again — also known as bait shyness. Because of this, farmers are only allowed two applications of zinc phosphide in alfalfa per year.

“Zinc phosphide is one of the primary tools we have, but it is a little hit or miss, and I’m sure many farmers have experience with it where sometimes it worked well and other times it didn’t work very well at all,” Baldwin said.

Tillage and fences

Habitat modification alters a habitat area to make it less desirable for pests. While this can be a viable control tactic in a variety of different cropping systems, Baldwin stated it is not ideal for alfalfa since the plants themselves are the preferred vole habitat, providing them food and cover. Therefore, destroying burrows with deep tillage after stands are terminated is a more effective way to eliminate burrow systems.

Exclusion fencing may not be the most practical solution, either, but it can be implemented along field borders to discourage vole movement into alfalfa. In his own exclusion fencing research in artichoke fields, Baldwin buried plastic mesh fence 6 inches belowground for optimal vole control. He suggested driving rebar or wooden stakes into the ground every 15 feet and extending the fence at least 10 inches aboveground.

“One of the nice things about voles is that they are not very good climbers,” Baldwin said. Maintaining a 1-foot vegetation-free perimeter outside the fence might further discourage voles. With that said, he acknowledged farmers can’t realistically fence entire fields this way. But exclusion fences can limit infestations to smaller areas and offer a vantage point to monitor surging populations. Fences might be good sites for rodenticide bait stations, too.

“You’re placing bait stations on a barrier that is funneling voles up and down that particular structure, so you won’t need near as many as if you were using bait stations throughout an entire field,” Baldwin said.

Birds of prey

There may be another option to eradicate rodents — barn owls. Biocontrol, otherwise known as the use of natural predators to control pest populations, shows some promise for alfalfa production, and barn owls may be the most viable raptor species to recruit to infested stands for two major reasons.

For one, barn owls are not territorial, which means farmers can artificially inflate their population densities in a given area by providing them with nesting structures — usually owl boxes. With those established, barn owls will stick around to hunt, which leads to the second reason they can be useful predators: Individual owls can consume up to 3,000 small rodents per year.

“The question, though, is if the removal rate that they engage in is able to overcome the reproductive output that a lot of these rodents engage in,” Baldwin said. “Voles in particular reproduce at a higher rate than pretty much all of the other rodent species.” Overall, there is not enough conclusive evidence to prove the effectiveness of barn owls for vole control in alfalfa.

This article appeared in the July 2025 issue of Hay & Forage Grower on page 30.

Not a subscriber? Click to get the print magazine.