ORCHARD VOLES
Two species of voles cause serious orchard damage in New England. The meadow vole (Microtus pennsylvanicus) ranges throughout New England. The pine vole (M. pinetorum) is known to be present in orchards in the southern New England states, and in the southern parts of Vermont, New Hampshire, and Maine.
Meadow voles inhabit the orchard floor, developing a network of surface trails through the groundcover, and they feed primarily on grasses and fleshy herbs. Most of an individual meadow vole's activity is within an area of about 1/20th of an acre. In the winter when other food sources become scarce, meadow voles can invade new areas and damage apple trees where you were not previously aware of a problem. But damage is possible any time of the year. They chew away areas of bark and cambium that can be reached from the ground or from higher positions in or on snow cover. In some soils they will burrow, and sometimes are responsible for trunk girdling several inches below the ground surface.
Pine voles travel either in surface trails or in burrows at depths up to 3 feet or more, depending somewhat on soil conditions. In solid grass sods they may be almost totally subterranean, but where the groundcover contains a high percentage of broadleaf herbs, surface pine vole trails may be numerous. Most of an individual pine vole's activity is within a small area of about 400 square feet. During the cold months, their activity is largely limited to the underground burrows. On apple trees, pine voles feed upon bark and cambium primarily below the soil line, and chew off small roots up to about pencil diameter.
Commercial apple cultivars and rootstocks are very susceptible to vole feeding. Young trees (1-15 years old), and dwarfing rootstock are the most likely to be damaged. A selection of Malus sublobata has been found to be quite resistant, however. The selection was named "Novole" and has been introduced as an apple root and trunk stock for areas with severe vole problems. As a rootstock, "Novole" produces a large, vigorous tree. Use of a dwarfing interstem is suggested for tree size control.
IDENTIFICATION OF VOLE PESTS
It is important to determine if pine voles are present, because some of the management practices used for meadow vole are not effective against pine vole. Trapping is time consuming, but it is the best way to identify which species are present. Traps placed on the surface most likely will not capture pine voles. Find subsurface burrows by probing with your fingers 2 to 3 inches deep at the dripline, deeper near the trunk. Then, carefully excavate an area of sod or soil just large enough to allow a standard wood mouse trap to sit flush with the bottom of the underground runway. Place the trap lengthwise across the trail with the trip pan in the center. Use small chunks of apple, rolled oats, peanut butter, or a combination as bait. Cover each trap site with a shingle to prevent other animals from tripping it and to help relocate it 24 hours later. Trap for 3 or 4 nights, then repeat a week or two later. Record the number of each species caught at each trap.
Tail length is useful for identification. The pine vole tail is very short; about the same length as the hind foot (not leg!), measuring 3/4-inch or less. The meadow vole tail is about twice the length of its hind foot, reaching 1.5 to 1.75 inches on adults. Both species have chunky bodies, small beady eyes and ears are small and almost concealed in fur. Fur color is dark or gray-brown. If you catch a long-tailed specimen, it is likely to be a white-footed mouse (Peromyscus). This mouse's tail is well over 2 inches long, and all of its underparts are covered with white fur. It is reported to eat bark of young trees occasionally, but is generally considered a non-pest species in orchards. Your traps may also catch a shrew, which is a beneficial small mammal; or a mole. Shrews can be identified by their long pointed snout and needle-sharp front teeth, which are white at the base and dark brown at the tips. (Voles have chisel-shaped front teeth). Moles differ from the others in their large front feet with outward facing palms and prominent digging claws.
ESTIMATING VOLE ACTIVITY
Vole monitoring stations require less time than trapping. They can be used to identify areas with vole activity, to assess the vole threat in the orchard overall, or to check 2 to 3 weeks after a treatment to see if a follow-up treatment is needed.
A bait station consists of a shingle, split tire, or board, etc. in the grass at the edge of the herbicide strip or dripline. Establish the stations in the spring or summer to give the rodents time to find and tunnel under the board. Tree flagging helps to find the stations later.
After harvest, check under each station for tunneling. In each station with a run or tunnel, place a 1-inch thick apple slice in the runway or next to the hole, then recover it. Check the stations after 24 hours. If the percentage of apple slices that have tooth marks (or are entirely missing) exceeds 20-25%, this indicates potential for serious vole damage. A record of repeated assessments over a period of months or years gives a more accurate indication of vole activity. These vole monitoring stations can be used later as bait stations.
You can get a general estimate of vole activity by looking for vole runways on or just below the ground surface. Voles usually nest near the base of a tree trunk.
PREVENTION AND CONTROL METHODS
Hardware cloth guards, properly installed, are very effective in preventing most meadow vole damage unless snow depth exceeds guard height. Voles tunnel through snow to any depth. Embed guards at least 2 inches below the surface. Trunk guards do not prevent underground damage by pine voles. Keeping orchard floor vegetation below 10 inches (below 4 inches is best) discourages vole activity aboveground, because it leaves the voles visible to predators. Flail and rotary mowers do a much better job than sickle bar mowers at making orchard ground cover less suitable for voles. An herbicide strip to kill vegetation in the tree row, removing brush and weedy areas around the orchard, and removing or mowing dropped apples, are effective ways to reduce or even eliminate meadow voles from the orchard. Removing sucker growth, which attracts voles, will also help. To a lesser degree these practices can reduce pine voles which live primarily underground.
Poison baits are of two types: zinc phosphide and anticoagulant (see Notes on Rodenticides). Just 1 or 2 fresh grains or pellets of zinc phosphide baits can quickly kill a vole that eats them, but it may take several days of feeding on anticoagulant baits to kill a vole. Zinc phosphide baits have been more effective than anticoagulant baits against meadow voles. Recent research has found that zinc phosphide is most effective when delivered as treated steam-rolled oats or as pellets. Zinc phosphide-coated cracked corn was found to be less effective. Unwaxed bait is preferred to reduce hazard to wildlife. Zinc phosphide loses its toxicity rather quickly if the bait becomes wet. To preserve toxicity of unused zinc phosphide baits, place the opened package within a plastic bag and seal tightly.
CAUTION: Rodenticide baits may be attractive to domestic pets, birds and other non-target wildlife. Exposed bait, particularly waxed corn or grain-based pelletized bait on bare ground, increases the chances of non-target injury. As with all pesticides, use good judgement and take reasonable precautions to avoid problems.
RODENTICIDE TECHNIQUES
Broadcast
applications of baits are fast, particularly if applied with a fertilizer spreader, and can be effective against meadow voles. They are usually not effective against pine vole. Placing the baits within a weed-free herbicide strip will probably not be effective because voles avoid open ground. Most product labels limit treatments to the post-harvest dormant period. The presence of dropped apples can make baiting ineffective, however, as firm apples are a preferred food for voles. All sound drops should be removed before bait is broadcast. If the weather is wet and dark during the first few days after bait is broadcast, the baiting effort will have been wasted. Wet weather and dark days discourage vole activity, and wet bait loses potency and palatability. The best timing for bait application is soon after a postharvest mowing, and before a 3 day period of sunny, dry weather. The goal is to reduce the vole population just before winter.Brushy overgrown areas adjacent to a vole-infested orchard are likely to have a population of the same species present in the orchard. If these border areas are not baited, they will be a source of reinfestation to the treated orchard.
Baiting in Artificial Trails: Used properly, tractor-drawn mechanical trail builders allow for more efficient bait application that is effective against pine voles as well as meadow voles. A trail is made along each side of the tree rows, beyond the wheel tracks, beneath the drip-line in sod. Proper depth setting of 2 to 4 inches, timing and suitable soil conditions are critical for success. If there is vehicle and foot traffic in the orchard after the trails are built, or if the soil is too dry, the tunnels will collapse and bury the poison. If the soil is too wet the poison degrades quickly. This method is more expensive than the alternatives.
Hand-baiting: Hand-baiting means selective placement of baits under established bait stations, or where active trails or burrows are located. This method makes the most efficient use of baits, but requires the greatest time for distribution. When done properly it is likely to be the most effective method, particularly for pine voles. Teaspoon size, or larger, quantities of bait are placed at each spot, at the rate of 2 to 3 lbs. per acre. Some anticoagulant bait labels specify certain minimum amounts for each placement. To greatly speed bait placement, bait stations, such as asphalt roofing shingles or split tires should be distributed beneath the trees in sodded areas well in advance of baiting time. Over a period of weeks or months, voles develop trails under these bait stations; trails that can be quickly baited after harvest. Split tires are available in some areas from landfill operators. Split tires have the advantage of permitting bait to be placed in a shallow cup, under the tire so that bait does not readily deteriorate. T-tube bait stations can be made from 1.5 inch diameter PVC pipe.
Retreatment with baits: Where some voles have survived a rodenticide treatment after being sickened, acceptance of the same bait a second time within a few weeks will be poor. This seems to be a problem more with zinc phosphide baits than with anticoagulants. There are two ways of minimizing this problem: 1) do everything possible to favor nearly complete control with the first treatment; and 2) if a second treatment is needed, and another type of rodenticide is legal in your state, then use the different type for the second application.
For more specific information on voles, see Vole Management in Vermont Apple Orchards, available from the University of Vermont Extension.