Orchard Wildlife - Integrated Management of Voles in Orchards

Voles are primarily vegetarians, feeding on grasses, tubers, and seeds. They also consume the bark of young trees.
Orchard Wildlife - Integrated Management of Voles in Orchards - Articles
Orchard Wildlife - Integrated Management of Voles in Orchards

Voles may cause extensive damage to fruit trees and orchards as a result of girdling seedlings and trees and damaging roots.

Unlike many other small mammals, voles do not hibernate. Instead, they are active throughout the year, both day and night, with peak activity at dawn and dusk.

Identification

Voles are small rodents with short legs, stocky bodies, small eyes and ears, and short tails. Two species, the meadow vole (Microtus pennsylvanicus) and the woodland or pine vole (Microtus pinetorum), may damage fruit trees and become serious pests in orchards.

The meadow vole is approximately 5.5 to 7.5 inches long. It has brown fur mixed with black, and its tail is approximately twice the length of its hind foot. The pine vole is Pennsylvania's smallest vole. It is 4 to 5 inches long and has chestnut or auburn fur and a short tail approximately as long as or shorter than the hind foot.

Distribution

The meadow vole is one of the most widespread mammals in Pennsylvania. It abounds in grassy fields, moist meadows, orchards, or any area with a dense ground cover of grasses. Pine voles are most abundant in southeastern Pennsylvania, where they are common in old fields, thickets, gardens, orchards, and the edges of agricultural land, particularly where the soil is loose and sandy.

General biology and behavior

Meadow voles create surface runways in the grass, and in winter, are active in runways beneath the snow. Woodland voles build underground tunnels in loose, crumbly soil. As they build the tunnels they push out dirt, producing small conical piles of soil on the ground surface. Both voles build large globular nests of dry grasses and leaves. The nests are located close to tree trunks, in tussocks of grass, and at the end of burrows.

Voles are extremely prolific. Their peak breeding activity occurs between March and October, but when winters are mild, voles may breed all year long. A female meadow vole could potentially produce over 70 young in a year. Voles become sexually mature at between 4 and 6 weeks of age. As a result, under ideal conditions vole populations can reach densities as high as 270 voles per acre.

Scientists have found that voles exhibit regular population fluctuations at approximately 4-year intervals. Populations apparently crash to levels as low as 10 voles per acre after peak years and then begin to build up again.

Extensive damage may occur in orchards, particularly during peak population years.

Damage

Voles may cause extensive damage to fruit trees and orchards as a result of girdling seedlings and trees and damaging roots. 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.

Monitoring

The most easily identified sign of meadow vole presence is a system of surface runways in the grass. Meadow voles create these runways by their feeding activities and keep them free of vegetation. The runways are generally about 1.5 inches wide. After a close mowing, the pattern of runways is often visible. Bits of freshly cut vegetation and accumulations of vole droppings (brown or green in color and shaped like rice grains) in the runway are positive evidence they are being used. Vegetation, small roots, or mold in the runways indicate that the voles are no longer using them. 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 woodland vole runs close to the surface.

The apple indexing method is a way to determine the distribution of voles in an orchard and their relative abundance. Place a slice of apple into a meadow vole runway or in a pine vole tunnel. Check the apple after 24 hours for vole tooth marks. The presence of tooth marks will indicate where vole activity is highest and which trees are at risk. To obtain an estimate of the abundance of voles, weigh the apple before putting it out and after 24 hours. One pine vole consumes approximately 0.5 ounce of apple in a 24-hour period and one meadow vole consumes about 0.7 ounce.

Most orchardists do not need to know the exact number of voles present, but they may want to know whether the population is increasing or decreasing, or whether a particular treatment had an impact on population size. Monitoring vole numbers with the apple indexing method is a means of achieving these goals.

Trapping can also be used to assess the effectiveness of a vole-control program. Before initiating the control program, select approximately 10 trees and place four wooden-base (mouse-size) snap traps in runways near these trees (for trap placement see section on trapping below). Record the number of voles trapped in a 3- to 5-day period. After the control program is finished, set the traps in the same place and, for the same length of time, compare the number of voles caught after treatment with the number caught before treatment. If the program has been successful, you should trap no more than two or three voles.

The number of voles that can be tolerated is a trade-off between cost of control and cost of damage, and it depends on the orchardist. A single vole may cause damage, but most damage occurs at high population levels. Monitoring vole populations enables growers to assess when populations are starting to increase and to begin control programs at that time.

Management

Biological control

Hawks, owls, snakes, weasels, raccoons, foxes, and coyotes all feed on voles. These predators are beneficial in orchards because they help keep vole populations under control. Whenever possible, orchardists should encourage these predators, or at least not harass them.

When natural controls are inadequate, artificial methods must be used to control vole populations. The fall is the best time for initiating control programs. A number of different control methods are listed below. The greatest success is usually achieved by using a variety of techniques at once.

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. Repeated mowings that maintain ground cover at a low level serve to limit both food and cover and expose voles to predators. Too much delay between mowings results in excessive vegetation, which when cut forms a thatch layer that protects 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. Vegetation-free zones may be established by mowing, applying herbicides, cultivating, or placing a layer of crushed stone or gravel 3 to 4 inches deep that extends 15 to 18 inches from the trunk. Do not allow mulch, prunings, or decaying vegetation to accumulate around the bases of trees or in tree rows. Drops should be cleaned up quickly.

Exclusion

Hardware cloth barriers can be used to keep voles from girdling small trees. Wrap a strip of ¼ 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 5 years of growth. This method is very effective but extremely labor intensive and expensive when a larger number of trees need protection.

Trapping

Trapping is not an efficient way of controlling voles in large orchards, but it is an effective and safe control method for small orchards or around selected trees. Use standard wooden-base snap traps (mouse size) and bait them with a peanut butter and oatmeal mixture or apple slices. 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 nontarget animals and makes the voles more likely to take the bait. In some situations it may be necessary to drill a hole in the corner of the trap, attach a string or wire, and use a large nail to stake it in the ground. Occasionally, cats will remove the vole and the trap.

Repellents

The repellent Thiram 24/7 (a fungicide) or repellants containing capsaicin (the ingredient that makes chili peppers hot) are registered for vole control. 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.

Thiram 24/7 is labeled for use on tree seedlings, shrubs, ornamental plantings, nursery stock, and non-bearing fruit trees and vines. Most labels allow thiram to be used on fruit trees only during the dormant season. Thiram 24/7 is a liquid formulation with 4.0 lb. of thiram per gallon or 480 g/L. The product may be sprayed on or painted on the tree trunks. For spray applications mix 1 quart of product to 3 quarts of water with the addition of 1 pint of latex based sticker for greater retention and residual efficacy. The brush on application can be made with paintbrush undiluted with the addition of 4 ounces of latex based sticker per gallon for increased residual efficacy. Thanks to our next door neighbor Win Cowgill's sharp eyes.

Capsaicin-based products are labeled for use on ornamental trees, fruit and nut trees, fruit bushes and vines, nursery stock, shrubs, and lawns. Capsaicin should be applied only before the fruit sets or after the harvest. Capsaicin is registered for use on vegetable plants and agricultural crops only before edible portions and/or heads begin to form. The organic products used for voles are the same as mole repellent.

To prevent a feeding pattern from developing, apply repellents before damage becomes significant or, in the case of monitored populations, before damage occurs. They must be reapplied after a rain, heavy dew, or new plant growth. Always follow label directions for the repellent being used. Never apply repellents to any portion of a plant likely to be eaten by humans or livestock unless the label permits it.

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. 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 lists 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 pesticide 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, 717-772-5211, and ask them to check if the product is registered for use in Pennsylvania orchards.

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 feeding to achieve a lethal dose. In contrast, chronic rodenticides, which include anticoagulants such as those found in Ramik Brown, require multiple feedings over a period of several days before a lethal dose is achieved. Some growers use an acute rodenticide to knock the population down and an anticoagulant for protection through the winter.

Zinc phosphide is acutely toxic to all vertebrates and therefore presents risks to nontarget wildlife. The anticoagulant baits are more toxic to rodents than other birds and mammals and pose less of an overall risk to nontarget wildlife. But in both cases, efforts should be made to protect nontarget wildlife. 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.

Bait shyness occurs when animals consume sublethal doses of acute toxicants, then develop an aversion to the bait. Therefore, growers are advised not to apply zinc phosphide baits more often than once every 6 months. Ideally, growers can reduce the pest population with an initial application of a zinc phosphide bait and then after 2 days conduct an apple-slice index to assess the need for a follow-up application with an anticoagulant bait.

Recommended application rates for acute rodenticides are 2 pounds per acre when hand-placing zinc phosphide pellets in runways and 10 pounds per acre for broadcast application. Do not apply to bare ground. Recommended application rates for chronic rodenticides are 10 pounds per acre when hand-placing pellets or 15 to 20 pounds per acre for broadcast applications. Chronic rodenticides may be reapplied 30 to 60 days later if the vole problem persists.

Bait placement is critical to the success of a control program. Broadcast distribution of pellets and hand placing of pellets at recommended rates will work, but the best results are achieved by using bait stations. In addition, bait in stations is less available to nontarget wildlife.

Bait stations can be made from discarded beverage cans. Enlarge the opening in the end of the can so that it is about 1.5 inches in diameter. Dent the side of the can. Put bait in the can and place it dented side down in the area to be protected. Mark the bait containers with flags or stakes so they can be relocated.

Another type of bait station that has been successful is made from an automobile tire split longitudinally. Tires are placed with the hollow side down, and the bait is placed in a small cup under the tire. The tire halves are then distributed one per tree or one every 10 yards throughout the area. Discontinue use if nontarget animals are coming into contact with bait.

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.

Timing also 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.

Most rodenticide labels stipulate that bait can only be applied during the dormant season, after harvest, and before bud burst in the spring.

Source: Penn State Tree Fruit Production Guide.

For additional information on controlling voles, see the Wildlife Damage Control 9: Volesarticle.

Sources of supply

Bell Laboratories, Inc.
3699 Kinsman Blvd.
Madison, WI 53704
608-241-0202
(ZP Rodent Bait AG)

Hacco, Inc.
110 Hopkins Drive
Randolph, WI 53956
(Ramik Brown)

Miller Chemical and Fertilizer Corp.
Box 333, 120 Radio Road
Hanover, PA 17331
717-632-8921
(Hot Sauce Animal Repellent)