In the Mid-Atlantic region we have two major types of voles--pine (Microtus pinetorum) and meadow (Mictrotus pennsylvanicsus). The pine vole is smaller, usually 4 to 6 inches long, while the meadow vole is 5 ½ to 7 ½ inches in length. Their habitat is slightly different: meadow voles tend to spend their time above ground building surface runways in long grass, while the pine voles tend to burrow down in subterranean runs. Voles are active both day and night and do not have a hibernation period.
Damage by the two species is slightly different. Pine voles feed on roots below the surface while meadow voles tend to feed around the base of the tree above the surface.
Do not confuse meadow vole damage with that caused by rabbits. Rabbit damage will extend up the trunk and typically shows more gnawing injury. An interesting note for orchards located in more northern areas where snow may persist for several weeks: you may see damage up into the tree when meadow voles can run across the snow surface.
Monitoring Vole Activity
The first step in any vole control program is to monitor the orchard to determine the extent of the population present. Monitoring consists of providing some "sheltered" locations in the orchard such as arched roofing shingles, tires cut in half, "PVC T-tubes", used aluminum soda cans or anything that can provide temporary shelter for the voles. Monitoring stations are best concentrated close to where orchard blocks adjoin woods or open fields but should also be scattered throughout large blocks. The apple index method is the most common method of monitoring. First place the "shelters" in the orchard, preferably where you may see or suspect vole runs. Make a grid map of the locations of the stations. Leave them in place for 3 to 5 days before baiting them. To bait them cut 0.5 inch square chunks of apples and place them under the shelter. Be sure to map the orchard as to the locations of the bait stations. Wait 24 hours and return to the bait stations and examine the apples for evidence of chewing on the apple or its absence. Marking the grid map you created with a + or - will give you a visual representation of the spread of the voles. Wherever there is a concentration of the vole population will be the area that you need to concentrate control measures.
Another method of determining the population is to set traps and monitor them. (Note: trapping is not an efficient control method in large orchards). For meadow voles, place the traps in runways, flush with the ground and perpendicular to the runway. Place the trigger end in the runway. For pine voles, locate a tunnel and place the trap within the tunnel and perpendicular to it. Put a cover such as a bent roofing shingle or box over the traps. This helps protect most non-target animals and makes the voles more likely to enter the site.
Cultural controls can be utilized to reduce populations and potential damage. The first line of defense is to mow the orchard row middles closely to reduce potential cover for the voles. A closely mown sod will expose the voles to attacks by predatory birds such as hawks and owls. Providing good nesting places for predatory birds can also help control the population. However, if you go this route, you probably should not be using poison baiting techniques.
Tree guards are another effective means to prevent damage to the trees. Wire mesh, perforated wire guards, and plastic wraps placed around the base of the tree can be effective deterrents to meadow vole damage. However, for the tree guards to be effective for pine voles they need to be buried several inches below the surface.
Habitat modification should also be a primary mechanism to control potential damage. Voles can live in dense populations in ditch banks, rights-of-way and water ways. Closely mowing adjacent fields and burning down weeds will help prevent voles from commuting between those areas and the orchard.
Repellents on a small scale may serve to reduce damage. Materials that contain thiram or capsaicin are available that can be applied directly to the trunks of trees. Protection is relatively short term. Due to changing chemical regulations, before applying a material containing thiram make sure it is labeled for use in fruit trees.
Indirect Chemical Management
As mention above in the cultural management section, voles dislike being exposed to predators and will tend to stay in tall vegetation. A vegetation free herbicide strip underneath the trees can help to reduce potential damage of meadow voles. This species will tend to shy away from feeding on trees where they are exposed to view and attack by predators.
Fall application of herbicides will not only help in your weed management program for next spring but also help reduce vole damage.
The application of toxic baits is probably the quickest and most effective method of reducing troublesome populations. These baits are applied in the fall after harvest preferably before the ground freezes and may need a re-application in the spring if monitoring shows a resurgence of the populations.
Used in conjunction with habitat modification and cultural controls, rodenticides are an important part of vole control. Two types of rodenticides are often used--one to provide a quick reduction in numbers (high toxicity and fast acting, a single-dose toxicant) and the other to provide protection throughout the winter (one of the anticoagulant baits). ZP Rodent Bait AG is a registered single dose toxicant and Ramik Brown is an anticoagulant. Both are currently registered for use in Pennsylvania orchards. To determine if a specific rodenticide can still be used, read the label very carefully. The label will provide information on rates and applications, and list legal uses for the product. Note any restrictions placed on the product. Most rodenticides may be used only during the dormant season when trees are not bearing fruit, and most are labeled as a restricted-use pesticides and can only be purchased and used by a certified pesticide applicator. If the label does not specifically state that it is legal for use in orchards, you can contact the Department of Agriculture, Division of Agronomic Services to find out if the product is registered for use in Pennsylvania orchards.
Acute rodenticides, zinc phosphide materials, are fast acting poisons usually only requiring a single feeding. In contrast, the anti-coagulant materials require multiple feedings over several days.
One strategy is to apply an acute rodenticide to knock down the population followed by an anticoagulant for protection through the winter. The anticoagulants are more toxic to voles than to birds and other mammals and thus pose less of a risk to non-target wildlife. Even with this fact every effort should be made to protect non-target wildlife. Zinc phosphide materials are equally toxic to all vertebrates and should be used responsibly to minimize non-target wildlife. The best use of these materials is through the use of bait stations. To minimize feeding by non-target bird species use only the pelletized formulations and not the grain based formulations.
Acute rodenticides can be applied at a rate of 2 lb/A when hand placing in runways or bait stations. The broadcast rate for these materials is 10 lb/A. Never make applications to bare ground. Anti-coagulant rodenticides should be applied at a rate of 10 lb/A when hand placing and 15 to 20 lb/A for broadcast applications.
Pine voles are not as active above ground, so bait should be placed directly in runways and burrow openings at two to four locations under infested trees. If runways and burrows cannot be found, roofing shingles, boards, or other objects placed on the ground at each placement site provide voles with shelters where they may build tunnels or nests. Place bait under these shelters after they have been in place for several weeks. For pine voles, baits must be placed in the underground runways.
Timing influences the success of control programs. Wet weather reduces the effectiveness of rodenticides, so apply baits when weather is likely to be fair and dry for at least 3 days. Baits are most effective when naturally occurring foods, such as green vegetation and fruit drops, are limited. Late fall is an important time to bait voles because it serves to reduce populations before the onset of winter, when vole damage is most severe and snow cover precludes rodenticide use.
When winter survival is high, baits should be applied in the spring before the breeding season and before renewed growth of ground cover diminishes bait acceptance.