Monitoring and Managing Vole Populations in Orchards
Posted: November 5, 2010
Damage occurs primarily during winter when other types of food are scarce. The most common form of tree injury caused by meadow voles is trunk girdling at or near the ground surface. Since voles burrow in the snow, they may damage tree trunks as high as snow accumulates. Young trees are especially susceptible to attack. Occasionally, meadow voles will burrow in the soil and damage roots, resulting in weak, unhealthy trees. Damage from pine voles is harder to detect because it occurs underground as they consume small roots, girdle large roots and eat bark from the base of trees. By the time orchardists note weak, unhealthy trees, the damage is already extensive.
The most easily identified sign of meadow vole presence is a system of surface runways in the grass. Pine voles do not use surface runways, so their presence is much harder to detect. In apple orchards, tiny, elongated tooth marks on apples on the ground are signs of both meadow voles and pine voles. Probing the area under the tree with your fingers may help determine if there are vole runs close to the sur- face. Monitoring vole populations enables growers to assess when populations are starting to increase and to begin control programs at that time.
Hawks, owls, snakes, weasels, raccoons, foxes and coyotes all feed on voles. When natural controls are inadequate, artificial methods must be used to control vole populations. The fall is the best time for initiating control programs. The greatest success is usually achieved by using a variety of strategies, including habitat modifi- cation, exclusion, repellents and/or toxicants.
(PA Tree Fruit Production Guide, G. San Julian; detailed recommendations at http://agsci.psu.edu/tfpg)
Vole Management Programs
Habitat modification. In orchards, the major food sources for voles are normally not the fruit trees, but roots and stems of grasses and other ground cover. As a result, habitat modification, that is, reducing or eliminating grasses and cover, is one of the best long-term methods for controlling voles. Establishing vegetation-free zones under tree canopies that extend at least 2 feet from tree trunks will discourage voles from living near the bases of trees, where they cause the most damage.
Exclusion. Hardware cloth barriers can be used to keep voles from girdling small trees. Wrap a strip of 1⁄4 inch mesh hardware cloth around the base of small trees. The hardware cloth should be set 4 to 6 inches into the ground and be approximately 18 to 24 inches high. The guards should be at least 4 inches higher than anticipated snow depth. Tree guards should be large enough to allow for five years of growth.
Repellents. Little data is available on the effectiveness of repellents to deter vole damage; therefore, repellents should not be used as the sole method of vole control. To prevent a feeding pattern from developing, apply repellents before damage becomes significant or, in the case of monitored populations, before damage occurs.
Toxicants. Used in conjunction with habitat modification, rodenticides are an important component of most control programs because they provide the quickest and most practical means of bringing large populations of voles under control. Several rodenticides (ZP Rodent Bait AG, Rozol Paraffinized Pellets, Ramik Brown, Hopkins Zinc Phosphide Mouse Bait, Vole Whacker, and Hopkins Zinc Phosphide Pellets) are currently registered for use in Pennsylvania orchards. Product labels will provide information on rates and applications, and list legal uses. Note any restrictions placed on the product. Bait type is an important consideration in vole control programs. Acute rodenticides, such as those containing zinc phosphide, are fast-acting poisons that usually require only a single feed- ing to achieve a lethal dose. In contrast, chronic rodenticides, which include anticoagulants such as those found in Rozol pellets, re- quire multiple feedings over a period of several days before a lethal dose is achieved. Both acute and chronic rodenticides are available in pelleted bait formulations, which are superior to grain baits because they are more effective against voles and are not as hazardous to ground-feeding birds and other nontarget wildlife.