How to Manage Pests

UC Pest Management Guidelines

Citrus

Voles (Meadow Mice)

Scientific Name: Microtus spp.

(Reviewed 9/08, updated 9/08)

In this Guideline:


Description of the Pest

Voles, also called meadow mice, are larger than house mice but smaller than rats. Compared to deer mice, however, voles have a more robust body, less obvious ears, and a relatively shorter tail. Voles' ears are at least partly obscured by the hair in front of them and voles' tails are about one-half to one-quarter the length of their head and body combined. Deer mice have relatively large and prominent, fleshy ears and their tail is more than 70% the length of their head and body.

Voles are active both day and night and all year round. Females bear several litters each year, with peaks of reproduction in spring and fall. Populations go through cycles, climaxing every 4 to 7 years and then declining fairly rapidly. Grasses and other dense ground covers provide food and cover that favor the buildup of vole populations. You can recognize vole activity by the presence of narrow runways in grass or other ground cover, connecting numerous shallow burrows with openings about 1.5 inches in diameter. Voles seldom travel far from their burrows and runways.

Damage

Voles can be a particular problem where dense vegetation or cover crops grow around the base of trees. Voles damage trees by feeding on bark around the root crown, and sometimes they chew holes in irrigation lines. Small trees are most susceptible to being completely girdled and killed by voles. Large trees are susceptible to damage, for instance after severe pruning, when sufficient light penetrates the tree canopy for vegetation to grow near trunks, but vole damage to large citrus trees is uncommon and rarely kills the tree.

Management

Vegetation management and the proper use of trunk guards on young trees usually keep damage to a minimum. Bait (either multiple anticoagulants or zinc phosphide) can control populations that reach harmful levels. Zinc phosphide is a restricted use material that requires a permit from the County Agricultural Commissioner for purchase or use.

Monitoring

Starting in midwinter, monitor monthly for active runways in cover crops or weedy areas. Look for fresh vole droppings and short pieces of clipped vegetation, especially grass stems, in runways. Look for burrow openings around the bases of orchard trees. If you find burrows, remove the soil from around the base of the tree and look for bark damage. Voles usually start chewing on bark about 2 inches below the soil line and then move upward to about 2 to 4 inches aboveground.

If you do not check carefully, you may not notice damage until late spring or summer, when it may be too late to prevent significant injury to the trees. Make sure to monitor ditch banks, fence rows, roadsides, and other areas around the orchard where permanent vegetation favors the buildup of vole populations.

Habitat Management

Voles travel only a few feet from their burrows, so any destruction of vegetation will make the area less favorable to them. Use a hoe, herbicides, or other methods to keep an area reaching about 3 feet out from trunks free of vegetation. If you maintain ground cover or resident weeds in the row middles, keep it mowed fairly short to be less attractive to voles.

A vegetation-free zone 30 to 40 feet wide between the orchard and adjacent areas helps reduce the potential for invasion by voles, but such a wide area is rarely practical. Bare soil borders may be undesirable where off-site movement of contaminated soil and water must be prevented with a vegetative border to filter runoff.

Tree Guards

Use wire or plastic trunk guards to protect young trees from voles and rabbits. An effective guard can be a 24-inch-tall cylinder made of 0.25- or 0.5-inch mesh hardware cloth that is of sufficient diameter to allow several years' growth without crowding the tree. Bury the guards' bottom edge several inches below the soil surface if possible to discourage voles from burrowing beneath them. Plastic, heavy cardboard, or other fiber materials can also be used to make trunk guards. These materials are less expensive, also provide sunburn protection, and are more convenient to use; however, they provide less protection against vole damage since the voles can chew through them.

Regularly check underneath any tree guards for evidence that voles are burrowing underneath them to gnaw on the tree trunk, looking also for the presence of other pests such as snails. Good weed control around trunks improves the effectiveness of trunk guards.

Baiting

If you find damaging infestations or population increases within the orchard, poison baits can greatly reduce the vole population. Baiting can also reduce populations in adjacent areas before they have a chance to invade the orchard. Single- and multiple-dose baits are available, but there may be baiting restrictions in some areas to protect endangered species. It is extremely important to understand and follow the label directions for use.

For small infestations, scatter the bait in or near active vole runways and burrows according to the bait's label directions. For larger areas and where the bait label permits it, you can make broadcast applications. In non-cropland, apply bait in fall or spring before the voles' reproduction peak. Bait acceptance will depend on the amount and kind of other food available.

Other Controls

Trapping is not practical for voles because so many individuals have to be controlled when they are causing problems in commercial orchards. Fumigation is not effective because of the shallow, open nature of vole burrow systems and the large number of voles. Repellents are not considered effective in preventing damage.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441

Vertebrates

  • R. E. Marsh (emeritus), Wildlife, Fish and Conservation Biology, UC Davis
  • T. P. Salmon (emeritus), UC Cooperative Extension, San Diego County
Acknowledgment for contributions to Vertebrates:
  • M. W. Freeman, UC Cooperative Extension, Fresno County

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