Vole populations in Utah peaking

A small furry rodent species – the vole – is showing up throughout Utah this year. Orchards, residential lawns, pastures and golf course lawns are now feeling most of the brunt of this rodent’s activity.

According to Terry Messmer, USU Extension wildlife specialist, voles are hands down the world’s most prolific mammals. Females can breed when they are a month old and produce litters of three to10 pups every three weeks for the rest of their lives. They are also known for their boom-bust population cycles, with population levels generally peaking every two to five years, but these cycles are not predictable.

“Voles pose no major public health problems because of infrequent contact with humans,” he said. “However, they can harbor disease organisms, such as plague and tularemia. For this reason, wear protective leather gloves if handling them becomes necessary.”

Messmer said the gnaw marks and girdling damage voles cause are similar to many other species of wildlife, particularly rabbits. This, coupled with the vole’s small size and inconspicuous nature, often lead homeowners to believe vole damage is caused by another wildlife species. Vole girdling is characterized by non-uniform gnaw marks at various angles in irregular patches. In contrast, rabbits clip branches with neat, clean cuts. The gnaw marks left by voles are about 1/8 inch in width and 3/8 inch in length; gnaw marks caused by rabbits are usually larger. Careful examination of girdling damage can help identify the animal.

“The most prominent sign of vole damage in yards and fields are their extensive runway systems,” he said. “Runways are 1 to 2 inches in width and vegetation is often clipped close to the ground next to well-traveled routes.”

Messmer said removing weeds, ground cover and litter around lawns and plantings can reduce habitat suitability for voles and help decrease their damage. Lawns should be mowed regularly, and mulch should be cleared 3 feet or more from the base of trees. Soil cultivation destroys vole runway systems and may kill voles outright. Because of this, annual plant areas are often less susceptible to vole damage than perennial plant areas.

Cylinders made of hardware cloth (found at most hardware stores) can be effective in excluding voles and protecting individual plants. The mesh size of the cloth should be no larger than 1/4 inch. The cylinder should be buried at least 6 inches below the ground to ensure that voles will not burrow under the cloth and get to the plant. This will protect individual plants, but fencing is often not effective in protecting large areas such as lawns and is also cost prohibitive.

“There are two chemicals approved by the EPA for use in repelling voles,” he said. “These two repellents may contain thiram, a fungicide, or capsaicin, the chemical that makes peppers ‘hot’ and alters the taste of plants, making them unpalatable to voles. Although these repellents may provide temporary protection for plants, their effectiveness is usually short lived. For more long-term prevention, other techniques should be considered.”

Messmer said for fields and other large rural areas, the EPA currently approves two toxicants that may be used to lethally control vole populations — zinc phosphide and anticoagulants. Of these, zinc phosphide (2 percent) is more commonly used and is available in pelleted and grain bait formulas and typically is broadcast at 6 to 10 pounds per acre. Hand placing the bait in burrows and runways greatly reduces the risk to other species. Zinc phosphide is toxic to humans when ingested and may be absorbed through the skin, so always wear gloves when handling it and dispose of the gloves in a safe manner. Zinc phosphide is a restricted-use chemical, and those who apply it must be certified applicators. To learn more, contact a local USU Extension office.

“Anticoagulant baits are also an effective means of reducing voles,” Messmer said. “They are often used to reduce rodent populations in general. Approximately 95 percent of mouse and rat control is performed with anticoagulants. Anticoagulants can be broadcast over an area or placed by hand in runways and burrows. Baits are often glued to the inside of a water repellent paper tube. Anticoagulants work much slower than zinc phosphide, and death is delayed for several days following the ingestion of a lethal dose. These can also be toxic to humans.”

Trapping can be effective in controlling small vole populations, but because of their high reproductive rates, it is not cost-effective in larger areas, he said. In the event that voles invade a house (which is unusual), they can be removed the same as house mice with snap traps or live traps.

For more information about vole baits and control, contact your local USU Extension office or access the publication entitled “Voles” online at http://extension.usu.edu/files/publications/publication/NR_WD_009.pdf

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