|Pine voles (microtus pinetorum) attack |
the root systems of woody plants in the winter.
Photo from scienceblogs.com.
To learn more about the pine vole and how to control it, see the following excerpt from Wildlife Damage Notes from the North Carolina Cooperative Extension Service.
|Figure 1. A: Damage below the ground indicates pine vole activity;|
B: Damage above the ground indicates meadow vole activity.
Illustration by Sandy Shultz.
Voles in Horticultural Plantings
Published by North Carolina Cooperative Extension Service
Peter T. Bromley, Specialist-in-Charge, Wildlife Extension
William T. Sullivan, Jr., Research Assistant, Department of Zoology
Michael L. Parker, Extension Specialist, Horticulture
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).
Figure 2. Tunnels made by pine voles under a shingle at an IPM monitoring site.
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.
Figure 3. A trap properly set for pine 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 Voles
Trapping 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.
For the full notes on pine voles, see data sheet: 2/94-10M-DWD-240073