Wildlife Damage Management
Voles in Horticultural PlantingsVoles are small mammals, commonly called mice, that live in field and shrub habitats. In the wild, voles forage on native vegetation and provide a valuable food source for predators such as weasels, hawks, and snakes. In horticultural plantings, including flower and shrub plantings and home orchards, however, voles can cause damage by eating flower bulbs, girdling the stems of woody plants, and gnawing roots. Plants not killed outright may be invaded by diseases or die from water stress during periods of drought.
Surveys of wild habitats across North Carolina and reports by North Carolina Cooperative Extension Service agents indicate that voles are likely to occur in just about any upland habitat in the state. Furthermore, Extension agents report that voles have caused moderate to severe damage in home landscapes and gardens in about 90 percent of the state.
Although voles have value in the natural world, homeowners and managers of horticultural plantings may need to use measures to control damage from voles. This publication will help you identify voles and the damage they cause. Besides providing information on controlling vole populations to reduce the damage, this publication also outlines an early warning system that you can use to prevent problems from becoming severe.
Characteristics of Voles
There are two kinds of voles in North Carolina, the pine vole (Microtus pinetorum) and the meadow vole (Microtus pennsylvanicus). Both of these can cause damage in orchards, ornamental nurseries, and other horticultural plantings.
Physical characteristics. Pine voles have small eyes and ears that are hidden by their fur. The tail is shorter than the hind legs. The fur is reddish brown. The adult pine vole is about 3 inches long and weighs 1 ounce or less.
Reproduction. Female pine voles have a gestation period of 24 days, have an average litter size of 2.8, and produce four to six litters per year. They reach sexual maturity at 37 to 38 days and have a reproductive life span of 15 to 18 months.
Ecology. Pine voles spend most of their lives under the ground in burrow systems. They feed on plant roots, flower bulbs, and the growing tissue (cambium) of tree roots. Pine voles tend to stay in an area as small as 1,000 square feet for their entire lives. At night, they come above the ground and feed on fruit and tender green vegetation. Soils with substantial clay content are more likely than sandy soils to support pine vole populations because the clay in soils permits relatively permanent tunnel systems and nest chambers. The typical natural habitat for voles is the shrubby edge between the woods and meadow openings.
Physical characteristics. Meadow voles' eyes are not covered by fur, and their ears (partly covered by hair) are also visible. The tail is longer than the hind legs. Their fur is dark brown, often silvery on the underside. The adult meadow vole ranges from 3.5 to 5 inches in length and weighs 1 to 2.5 ounces.
Reproduction. Female meadow voles have a gestation period of three weeks, have an average litter size of five, and produce four to five litters per year. They reach sexual maturity at 40 days and have a reproductive life span of 1 to 2 years.
Ecology. Meadow voles spend most of their lives above the ground, living in and feeding on grasses. They have larger home ranges than pine voles and may travel as far as 1/4 mile in a week. The typical habitat for meadow voles is a grassy meadow, particularly in places where the grasses grow in clumps. Tall fescue in orchards, lightly grazed pastures, and old fields are typical habitats.
Signs of Vole Activity
It is important to be alert for signs of vole damage. If vole activity is detected, the nature of the damage will reveal the type of vole present. As discussed later, that information is essential in selecting a control strategy.
Pine voles damage trees and plantings below the ground (Figure 1A). When the damage to a particular tree, shrub, or broad-leaved plant is extensive, the plant will be severely weakened and may die. The trunks of small trees or shrubs may be severed from the roots, making it possible to pull the top of the plant out of the soil. Upon close inspection of the plant, gnawing marks can be seen just under the soil line. In apple orchards, the damage to the tree may not be sufficient to kill the tree, but damaged trees produce less fruit. Careful observation beneath the tree may reveal piles of earth (3 to 4 inches wide) and tunnels that are about 1 1/2 inches in diameter. Look under the tree for hollow shells of apples, eaten from the underside. If pine voles are living under the tree, a network of tunnels approximately 3 inches under the soil can be located by probing with a 1/2- to 3/4- inch- diameter stick or rod.
Signs of meadow voles are found mostly above the ground (Figure 1B) in taller grasses and cover. Look for trails in the grass and grass clippings, and check for feces at the base of large clumps of grass. The feces may be brown or green in color, are shaped like wheat grains, and are frequently left in small piles.
Typically, meadow voles girdle trees and saplings at the ground line. Close inspection of the damage will reveal paired grooves left by their chisel- like teeth. The grooves will be about 1/16 inch wide. Girdling completely around the tree trunk will kill the tree, so any indication of above- ground damage is cause for instituting a control program.
Rabbits also chew on young trees, but the girdling begins several inches above the soil line. Rabbits have much larger incisor teeth than voles, which will be reflected in the size of grooves on the girdled tree. Rabbit damage can be controlled with a plastic tree guard, but these devices will not prevent meadow vole damage.
Integrated Pest Management (IPM) for Voles
Sound principles of integrated pest management (IPM) require that pest populations be monitored before any control measures are taken. Homeowners and managers of grounds with shrub and flower plantings can use the apple sign test to determine if voles are present. Control measures can then be restricted to those locations.
Developed in Virginia, the apple sign test has been verified at the Mountain Horticulture Crops Research and Extension Center at Fletcher, North Carolina. This test for voles is inexpensive and does not require much time once the monitoring stations are established.
The Apple Sign Test
The apple sign test was developed to monitor vole populations in commercial orchards. The test permits the grower to detect vole populations before damage becomes severe. It also encourages economy and reduces exposing nontarget animals to control activities. Because the test shows where control is needed, areas without voles are not treated, saving time, money, and environmental risk. For these reasons, anyone who has invested in ornamental landscaping or a home orchard should establish and maintain an apple sign test.
The apple sign test is easy to do. The original method used 1-foot-square pieces of asphalt shingles, placed throughout the orchard and particularly where old fields, woods, and shrubs joined the orchard boundary. The gardener can use brown shingles that will blend in with the mulch or leaves or sections of 1- to 2-inch-thick pieces of board painted to match the background color of their flower garden or plantings.
Step 1. Prepare enough of these shingles or wooden pieces to scatter them strategically along the edges and throughout plantings at 15-foot intervals. Sketch a map of the grounds, especially if you have extensive plantings.
Step 2. To establish a test site, place a shingle on the ground, if possible over a hole caused by a vole. If you are monitoring for meadow voles, the shingle must be rounded in a tent-like fashion or propped up 3 to 4 inches off the ground so that the animal can go under it.
Step 3. After 5 days, place a 1/2- inch cube of apple under each shingle. After 24 hours, check whether or not the apple has been removed or eaten. On the map prepared in step 1, mark a simple + (to indicate that voles are present) or - (to indicate that voles are not present). Leave the shingles in place for future monitoring (Figure 2).
Step 4. When monitoring has been completed, you can determine the locations where vole damage may occur and can direct control activities to those areas rather than treating the entire planting.
Step 5. File the recording sheets with the dates of monitoring and the locations at which control measures have been used.
Step 6. Conduct the apple sign test in the fall and spring each year and 21 to 30 days after each rodenticide application.
Trapping for Positive Identification
Trapping is an effective way to determine if one or both kinds of voles are present. A snap- type mouse trap used with a small piece of apple for bait works well. The trap should be placed under a shingle. To trap pine voles, some excavation will be needed to place the trap down in the run. Place the trap at a right angle to the run. Bend the shingle to form an arched roof over the trap so that the spring will clear the shingle (Figure 3). Meadow voles can be caught by setting traps at right angles to their runways in the grass. No excavation is necessary because meadow voles live above ground. Under North Carolina law, a depredation permit must be obtained from an agent of the North Carolina Wildlife Resources Commission before trapping voles.
Controlling Vole Damage
Currently, trapping or rodenticides are the only ways to control pine vole populations in home or institutional plantings and orchards. Careful and routine application of the apple sign test will reveal the locations of pine vole activity. Trapping or rodenticides should be applied only in those areas. After control measures have been taken, the apple sign test should be repeated. Make the test at least twice each year, once in the fall and once in the early spring.
Two alternative strategies can be used to control meadow voles. First, if the damage is extensive, you may need to treat the planting immediately with a rodenticide. Second, following successful rodenticide treatment, you can reduce or remove grass thatch to deter meadow vole populations. In mature orchards, a 4- foot grass- free strip on each side of the tree row is recommended, but narrower bands may be used on steep slopes to reduce soil loss. Close mowing will also prevent meadow vole populations from becoming established.
Using a Rodenticide to Control Voles
Currently, chlorophacinone formulated as paraffinized pellets (sold as Rozol Rat and Mouse Killer Pellets) is recommended for use by homeowners and managers of horticultural landscapes for controlling voles. It is effective and safe if used according to the following directions. However, household pets should be prevented from coming into contact with this or any other pesticide.
Place 2 tablespoons of pellets under a covered runway that is actively used by voles. Establish these covered bait stations at 10-foot intervals throughout the infested area. After approximately 21 days, repeat the baiting. After another 21 days, conduct the apple sign test to confirm control. Repeat the baiting process only in those areas still showing vole activity.
After control has been achieved, repeat the apple sign test twice annually, in the spring and fall.
Using Traps to Control VolesTrapping has been used to eliminate pine vole infestations. Meadow voles have much larger home ranges than pine voles, making it impractical for homeowners to control meadow voles by trapping. As noted above, damage from meadow voles can be all but eliminated in most cases by close mowing or removal of grass cover.
It takes persistence as well as skill to be a successful trapper. Individual traps should be set as outlined previously under the identification section. Remember, when trapping pine voles it is essential that no light from the sky reach the trap site. Another tip is that bait is not necessary if the trap is set across the runway and the trap trigger is expanded. To do this, fix a piece of cardboard (such as is found on the back of a writing tablet) to the trigger. The new trigger should be just slightly smaller than the wooden base of the trap. Traps should be set at 10-foot intervals throughout the damaged planting. They should be checked daily and reset until no voles are caught over a one-week period. In large landscaped areas, you can concentrate trapping in a particular plant bed, achieve control there, and then move the trapping effort to another area.
All rodenticides are designed to kill mammals. Take all reasonable precautions to prevent exposure to humans, pets, and nontarget mammals, birds, and fish.
Peter T. Bromley, Specialist-in-Charge, Wildlife Extension
Graphic Artist: Sandy Shultz
Helpful reviews of the manuscript were provided by George Barthalmus and J. G. Vandenbergh. The photos of the voles and Figure 2 were reprinted with permission of the Virginia Cooperative Extension Service.
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension Service agent.
Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
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