How to Manage Pests
UC Pest Management Guidelines
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.
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.
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.
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.
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.
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.
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.
UC IPM Pest Management Guidelines: Citrus