Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org - vole Microtus spp. flowering dogwood, Damage: dieback and off color foliage
After a few weeks you walk through and notice some of the pots have half the media volume they had when you moved them! You lift the pots and notice large holes chewed into the plastic. Tunnels were made through the media, roots were eaten and the media that was removed is piled between the pots. Most likely, voles are the culprits.
Voles are small (3-7" long), chunky rodents with short tails, blunt noses and bead-like eyes. Adults are chestnut brown with some black or gray hairs, the young are gray. Several species of voles live in Pennsylvania, most notably the Meadow Vole (which nests and "runs" near or at the surface) and the Pine Vole (which lives in tunnels). Voles do not hibernate and are active night and day.
They aren't just trouble in the overwintering house: they also cause destruction in the greenhouse, nursery, and landscape bed throughout the year. They feed voraciously on plant vegetation in the growing season, and seeds and root systems (bulbs, rhizomes, tubers and woody plant roots and bark) in the dormant season.
In the greenhouse the runways may not be underground, but rather between pots, under the shuttle trays or flats. You may find clippings of stems, stripped of foliage, in tidy piles along their runways. In the nursery or landscape you may find girdling of the trunk under snow cover. Vole damage is distinguishable from rabbit damage in that the vole's teeth marks are tiny and irregular, occurring at various angles and in random patches, whereas the rabbit's teeth marks are larger and more uniform.
Voles are often mistaken for moles by growers and homeowners. Moles often get the blame for bulb and rhizome destruction, but moles rarely feed on plants, they prefer insects, grubs and earthworms. A major difference is the paws: moles have large spade-like front paws that they use for digging; voles have small paws. Moles also have pointy, naked snouts and appear not to have eyes or ears. To add to the confusion, voles will occasionally move into abandoned mole tunnels.
Expose voles to predators by maintaining a lawn or brush height of less than three inches around growing houses or fields. Exclude voles in the nursery by using plastic trunk guards or by placing hardware cloth (1/4" mesh or finer) around the trunks of young trees and shrubs. The mesh cylinder should be dug into the ground at least six inches and should extend well above the anticipated snow level.
Green-houses and overwintering houses should be surrounded by fine mesh screening, buried at least 6" underground, angled outward at 90°. Repellents such as hot pepper sprays and castor oil pellets or drenches are available. Often they need to be reapplied with time, plant growth or after rainfall events. While some growers experience good results with repellents, research concerning the efficacy of repellents is inconclusive. Read the labels of repellents for use restrictions.
The apple sign test was developed for Virginia orchards, but can be applied in the greenhouse or nursery as well. Use shingles or wood pieces that will blend into the surrounding soil. Slightly arch the shingle so that an animal can fit under it. Leave the shingles throughout the scouting area for five days, to help the wildlife adjust to them being there. After 5 days, place small cubes of apple under the shingles. Begin monitoring whether the apple bits have been nibbled upon or remove. This will indicate the areas of the nursery where control efforts should be practiced.
The legal status of vole is a nongame mammal and they are protected. However, they can be controlled when causing damage. Here are several strategies for controlling voles:
Mousetraps, baited with a peanut butter and oatmeal mix or apple bits, can be effective on a small scale. Place traps near holes or adjacent and perpendicular to runways. Cover the traps with a large flower pot or box, making sure the traps can still close, to protect non-target animals and humans from being trapped. Voles can also be live-trapped using the same bait. Release them at least a half-mile away in an overgrown field, or other places where they won't cause problems for other people. Wear gloves when handling voles, they can carry diseases that are harmful to humans.
One very effective, single-dose toxicant is Zinc Phosphide pellets or treated grains. Zinc Phosphide is a restricted use pesticide and must be applied by a certified applicator or under direct supervision of a certified applicator. Anticoagulant rodenticides can also be used; they may take as many as five to fifteen days to take effect. It is best to place toxic baits directly into runways or burrow openings, preferably inside a bait container. Bait containers could include: water repellent paper tubes, PVC pipe, or empty and dry beverage bottles. The opening of the bait container should be 1.5 inches in diameter.
The bait container will protect the bait from moisture, and reduce the chance that it will be consumed by non-target wildlife, domestic pets and wildlife. Bait containers should be labeled with a warning and flagged to alert humans to their location and contents and placed where non-target animals will not be able to access it. Toxic baits are poisonous to all forms of animal and human life and must be used with extreme caution, according to the instructions on the container label.