Description and Life History: Please bear with me as I go outside of my typical realm of the creepy-crawlies and give vertebrate pest management a shot. Thanks to our own Dave Pehling, WSU Snohomish Co., I think I can work through some IPM for pests with spines. With our mild winter last year, high vole populations could pose a problem for your woody landscape plants and bulb gardens this winter. The following will give you insight into vole biology and management the IPM way.
In the state of Washington, we have seven species of Microtus. However, in Whatcom County gardens, we are probably only dealing with two species: M. townsendii and oregoni. Voles are pear-shaped, portly, short-eared and short tailed critters. Townsends vole (M. townsendii) is about 6-10 inches in total length; tail included, and has a dark brown back with a gray underbelly. The creeping vole (M. oregoni) is smaller and about 4 ½-6 inches with a dark brown hide and a silver underbelly. Vole tails are scantily covered with hair and usually only about 2 inches long, with the exception of the long-tailed vole.
Voles can breed throughout the year, however most reproduction happens during favorable weather from March to September. Creeping voles peak reproduction occurs in spring, usually during April. Creeping voles have 3-4 babies per litter and an average of 4-8 litters per year. Townsends vole has a similar reproductive potential; litter sizes average 4-7 young with at least 2 litters per year. Gestation periods are short (less than a month) and females are ready to breed by 24 days old. As with insect or weed pests, you can see how these vertebrate pest populations can build rapidly under prime conditions.
Damage: Lets first clear the air from some misconceptions. Moles do not eat plant material. Voles do. Moles do burrow and make unsightly dirt mounds in turf, but they will not damage plants by feeding. Moles feed on insects, earthworms and other soil invertebrates. Pesky voles are likely responsible for most vertebrate feeding damage in your garden and landscape plants, not moles.
Voles feed on a variety of herbaceous plants and grasses. Voles feed on above and below ground plant parts such as foliage, seeds, stems, roots and bulbs. Voles are pretty benign and may go unnoticed during the spring and summer because of the availability of food during these times. For bulbs and plants with succulent roots, damage by voles can be immediate with obvious losses of plants and tubers. Small trees can also be very vulnerable, however symptoms may not be noticed until spring. As food becomes scarce and vole populations are high in fall and early winter, voles may seek the tasty cambium of small tree roots, crowns and trunks. Root-chewed trees are stunted, spindly and have very little foliage. Leaves can even show signs of reddening and other water-stress symptoms.
Damaged trees can look like theyve been whittled near the trunk. The chew marks made by a vole can be recognized by: the pattern, location, and the size of the bite marks. Voles feed close to the ground, if not below ground. Gnaw marks left by feeding voles are non-uniform, irregular and are at various angles. Other vertebrate pests like bunnies, feed like typewriters: uniform, regular and at consistent angles. Gnaw marks are anywhere from 1/16th - 1/8th of an inch while rabbit marks are much wider. Voles can also damage trees by tunneling extensively around the root system, causing air pockets.
Monitoring: Voles prefer meadows and dense vegetation to nest, feed and forage in. Voles burrow and build intricate runways through grasses and meadows. Sometimes, voles will use the burrows of moles to get around. Grass nests are constructed in burrows. Many vole families will share the runway systems. Generally, our voles foraging ranges cover about .1-.2 acres (not a very large area). Scout the area around your landscape for these vole highways. When populations are high, these runways are pretty obvious (see diagram). Also look for small burrows and tunnels. Some rebar works well for probing the soil for tunnels.
Baits work very well for determining vole activity and distribution. Ive used a small 10 section of PVC pipe (about 2-4 diameter) and placed an apple wedge inside. This pipe was set into a runway at ground level. You might need to make the opening a little smaller because rats or other varmints may eat the apple. If you dont want to use PVC, you can place the apple wedge under a roofing shingle or a square foot of cardboard. Check the wedge daily for feeding (be sure to wear gloves!). Place enough of these bait stations in your garden or yard to give you an idea of how many voles you have active and where they are traveling to and fro. If you have a sizable place to monitor in, draw a map of your bait stations and identify the areas of heaviest activity. These areas are where you need to concentrate your management practices. At the office, we have a chart showing the way to calculate vole population sizes based on a feeding index.
Management: Now that you know where your voles are coming from and how many of them you have, you can take steps to reduce their numbers. The key for vole management is habitat management. Control dense vegetation and weedy areas. Most voles will nest in these areas. In areas where you have vulnerable plants and trees, make a weed-free buffer of at least 3 feet. For trees and garden beds, make sure that mulch and loose soil is reduced directly around the bases of small trees or vulnerable plants. Deep mulch and loose soil make great building materials for vole tunnels.
For trees, there are protective barriers available. These tree guards can be purchased or homemade. Guards are made out of plastic, fabrics or screen. If you use a screen, use a mesh size of a quarter inch or less. Make sure that the height of the guard is at least 12 inches and also plant the base of the guard deep enough that voles cannot burrow beneath them, 6-10 inches is enough. Check these guards regularly! You may have just made a cozy vole home!
Where practical, you can also trap for voles. Be sure to check recent regulations about gripping type traps. With the passage of I-713 in December 2000, It is unlawful to use or authorize the use of any body-gripping traps to capture any animal. Most traps that snare vertebrate animal bodies are now illegal without a special permit, with the exception of common rat and mousetraps. Place traps at burrow entrances and along runways. Placement of these traps should probably be made around areas near vulnerable trees and plants. Having a mine field of mouse traps in your entire yard might make playing ball with the dog or kid a little more exciting than it needs to be.
Poison baits, fumigants and rodenticides are not registered for use on voles by homeowners, however there are some that are available for use by professional pesticide applicators. Make sure you read labels and/or check with the Cooperative Extension office before you attempt any chemical control method. Repellants may be useful for deterring vertebrate critters from chewing apart your landscape. We are currently testing the efficacy of caster oil and putrefied egg solids in potted containers for repelling voles. After diving into the weird realm of animal repellants, like dissolved animal parts or drunken hillbilly urine, we thought that it would be worthwhile to test some of these repellants and separate fact from fiction. Soon we can share the results with you. Regardless of these practices, vole management will not be as successful if you do not manage vole habitats and weedy areas.
Finally, there are many other animals that love to eat and terrify voles. Predators like house cats, dogs, hawks, owls, coyotes, snakes and shrews will love to take all the voles off your hands. If you have some land, encourage those predators that can do the work for you.
Another great site for vertebrate pest management is: Internet Center for Wildlife Damage Management
To reach Todd Murray please call (360) 676-6736 or e-mail him at: firstname.lastname@example.org