Voles killing hardwood saplings in tree tubes
We're converting an old horse pasture to mixed forest. Two years after planting I decided to use tree tubes and ground fabric to protect the hardwoods. They grew like crazy last summer and I was really happy, until this spring when I found 15-20% of the trees have been killed by voles. They seem to be nesting under the fabric, actually using the fabric for nesting material. Should I remove the fabric now, or wait until next fall? I plan to put aluminum screen around the base, inside the tubes, for each tree. In our heavy clay soil, I doubt I can dig the screen in very far. Should I also use repellent of some kind?
Midland County Michigan
You are not alone--voles are often prevalent where we use ground cover fabrics or mulch--or even have tall grass that provides cover and a pocket under the snow. As food becomes more available in Spring and summer, the voles likely won't bother the trees. As colder weather returns for fall and winter, they will again feed on bark. Voles really only eat bark when they are relegated to choose from limited options.
Voles are extremely prolific, but short-lived with high rates of mortality caused by predators including snakes, hawks, owls, fox and coyotes. Their peak breeding activity occurs between March and October, but when winters are mild, voles may breed all year long. A female vole could potentially produce over 70 young in a year. Scientists have found vole populations expand and contract on a four-year boom-bust cycle. This is why control is so difficult. Luckily, forest tree species will reach a large enough trunk diameter (3" diameter and larger) after which voles typically stop eating the bark or don't do substantial girdling damage.
Until your trees get big enough to naturally become unattractive to voles, there are some important points to consider in terms of management:
Legal status: In Michigan, voles are classified as nongame mammals and can be controlled without a permit when causing damage.
Strategies: No single management method to prevent vole damage works all the time or in all settings. Generally, multiple management methods should be combined so that one method enhances the effects of another.
Monitoring: Vole damage can be overlooked due to the inconspicuous nature of voles. Since vole populations can increase rapidly, damage can appear to come on suddenly. Damage from one species of vole—the pine vole—is harder to detect because it burrows just below the ground surface and girdling damage to trees can go unnoticed. By the time orchardists note unhealthy trees, the damage is often already extensive. Monitoring is critical to minimize vole damage.
Exclusion: Hardware cloth or plastic cylinders can effectively exclude voles from seedlings and young trees. The mesh should be 1/4 inch or less in size. Bury the wire 6 inches to keep voles from burrowing under the cylinder. It is probably not cost-effective to attempt fencing large areas to exclude voles.
Alternative methods: Modifying environmental factors to moderate vole populations can be useful. Burning, mowing, using herbicides, cultivation or selecting low growing plants to reduce vegetative cover can help make a site less attractive to voles. Controlling ground cover also exposes voles to raptors, coyotes and other predators. Removing plant cover in strips at least 15 feet wide surrounding an agricultural area may help slow new voles from moving into a site. Consider encouraging raptor predators through perches or nest boxes. Ideally, adjacent landowners can work together to manage large areas of land to prevent high vole populations from becoming established.
Repellents: Various repellents made with thiram, capsaicin, castor oil and other active ingredients are available for vole control in ornamental plants. However, these products are not labeled for use on food crops. Repellents are relatively expensive and provide only short-term protection. Wet weather can quickly degrade repellents, creating a need for frequent reapplication. When vole foods are in short supply, such as in winter, repellents’ effectiveness usually decreases.
Toxicants: Using rodenticides is an important component of an integrated vole control program, but should not be relied upon as a stand-alone control. Voles have a relatively short lifespan and reproduce quickly, making lethal control strategies effective for only a limited time and further enforcing the need for an integrated approach.There are a limited number of rodenticides labeled for use in cropping systems and availability varies by state. Carefully review labels to ensure the intended site is listed before application. Use all rodenticides according to the label. In general, rodenticide application to bearing crops is only permitted during the dormant season. Application to non-food crops is generally less restricted. Many products recommend using bait stations to help protect nontarget organisms. Read and follow all label directions to reduce risk of pesticide exposure to yourself, others, non-target organisms and the environment. You can find these products at many hardware, home improvement or garden centers. Because these are toxicants that affect mammals--you need to closely follow the instructions on the label to keep yourself and non target organisms (cats, dogs, other wildlife etc.) safe.
Trapping: Trapping is not effective in controlling large vole populations because labor costs become prohibitive. However, mouse snap traps can be used to control a small population. Place the trap perpendicular to the runway with the trigger end in the runway. A peanut butter-oatmeal mixture or apple slices make good baits. Covering traps and the nearby runway with a tunnel of opaque material may improve effectiveness. Many vole species are easiest to trap in fall and late winter.
As in most situations with vertebrate pests, a combination of methods may be more effective than relying on any one method for controlling vole damage. Most problems with voles in urban and backyard areas probably involve small populations that can be controlled with habitat modifications, exclusion or trapping. Non-urban damage situations may involve larger populations of voles spread over greater areas, and can be resolved using habitat modification and toxic baits. Voles don’t always cause significant damage to property. Populations of voles, however, can increase quickly and be cause for concern. Generally, a direct relationship exists between populations of voles and the expected overall level of damage.
Before undertaking control, consider the extent of the problem in relation to control cost. For example, a few voles could damage a highly valued tree or flower bed and warrant control. At other times, they may go virtually unnoticed, making control unnecessary. Given their incredible reproductive rate, new infestations can seem to appear overnight. Usually, it is more cost-effective to respond quickly to signs of damage than to wait until damage becomes severe. The complete elimination of rodents from agricultural systems is neither necessary nor feasible.
You can read more about voles here https://www.canr.msu.edu/outreach/wildlife-voles-web.pdf