Voles are small mouselike rodents. In Maryland, two species, the meadow vole (Microtus pennsylvanicus) and the pine vole (Microtus pinetorum), eat roots, bark, and bulbs; even at low population levels, they cause significant damage to forest plantations, orchards, nurseries, and landscapes.

This fact sheet explains how to identify these injurious rodents and provides information on vole biology and management for forest plantations. In recent years, many landowners have established forest plantations and experienced poor survival due to damage by voles. This damage has occurred in newly established plantations and those that are 4-5 years old. Much of the research on voles has taken place in apple orchards where the vegetation is mowed and herbicide strips maintained to reduce vole habitat. Forest plantations commonly are not well maintained after planting so many of the recommendations in this fact sheet are intended to be rigorous enough to provide adequate protection, but not so intensive that they would not be implemented.

The meadow vole, also called the meadow mouse or the field mouse, is a small, compact rodent approximately 4 1/2 to 7 inches in length, with small round ears and a tail about twice the length of the hind foot (Figure 1). The back of the mature meadow vole is chestnut brown mixed with black; the belly is dark gray. Young meadow voles are uniformly gray (as are the young of most other small rodents).

Meadow voles live in grassy habitats where they construct a complex network of surface runways. To identify active runways, look for clipped grass and small piles of droppings that resemble green or tan grains of rice. Nests are well-formed balls of interwoven dry grass, usually built at ground level. Where ground cover is sparse or on recently cultivated sites, meadow voles sometimes dig burrows and nest underground.

The pine vole, also called the pine mouse, spends nearly all of its life in an extensive system of trails and burrows located 1 inch to 2 feet below ground. The adult pine vole is smaller than the meadow vole, approximately 4 to 5 inches long, with a shorter tail, smaller eyes, and a more blunt nose (Figure 2). Pine voles have smooth chestnut brown fur on their backs with no black shading.

Moles and shrews may be confused with voles, and they are occasionally caught in traps set for voles. Unlike voles, moles and shrews primarily feed on soil insects and worms and do not damage plants. In fact, they can be beneficial predators because they feed on pest insects, such as Japanese beetle grubs. However, moles often are viewed as pests because their raised tunnels in lawns are considered unsightly. Moles can be identified by their large front feet and claws and by their outward facing palms. Shrews can be distinguished from voles by their pointed snouts and shorter tails.

Vole populations vary greatly from year to year, depending on climate, food, habitat availability, disease, rodenticide use, and the presence of predators. Like most rodents, voles breed at a very high rate. Meadow voles produce 5 to 10 litters per year, with an average of five young per litter. Gestation is approximately 23 days, and females can mate again the same day that their young are born. Pine voles produce fewer litters than meadow voles and average only three or four young per litter. However, pine voles probably have a higher survival rate because they live underground, which helps protect them from predators. Young voles grow rapidly, are weaned at only 2 to 3 weeks, and are sexually mature within a month or two. Thus, if the habitat is available, voles can increase to damaging levels within a single growing season. The economic threshold for damage by voles is at a very low population level. A single animal living near a tree or shrub may cause sufficient damage to kill the plant or reduce production or landscape values. Population levels are not uniform, even within small areas, making it difficult to predict economic thresholds for damage.

Voles are vegetarians that cause considerable plant damage. During the growing season they eat green, succulent vegetation and fruits. During the fall, winter, and early spring, they feed on roots, stems, and bulbs. Strawberry plants, trees and shrubs, apple trees, pine seedlings, and herbaceous ornamentals are subject to vole feeding. Ornamental bulbs, such as tulips, also are favorite foods. Voles feed on both roots and stems of herbaceous flowering plants. Voles kill trees by girdling them, eating the bark at ground level, as well as the roots. Pine seedlings in commercial forests and Christmas tree plantations sometimes are pulled down into vole tunnels and consumed entirely. Tree shelters provide good cover for voles.

Assessing the presence of voles should be done during the initial inspection of the planting site. By just walking through a potential or established forest plantation, you won't be able to see much evidence of vole activity unless you get down on your hands and knees and pull back all that grass or other ground cover.

In an established planting site, pull up the tree shelter and pull back the vegetation. First, check the seedling itself for girdling of the bark or root system. Pull up on the seedling to make sure it is firmly rooted. In many cases, the voles will eat the majority of the roots below ground although the upper stem looks okay and the leaves may still be green.

Start near the seedling and work your way out. If you find any well-worn trails about 3/4 to 1-1/2 inches in width meandering along the ground surface, you've found the characteristic sign of meadow voles. If you find small holes (about 1" diameter) somewhere near the seedling and a few feet out with small piles of soil near these holes (but no real evidence of trails), you've probably got pine voles. However, just because you've found this evidence provides no indication of whether the trail or burrow system is active or has been abandoned. If you notice small rootlets, mold, or fine grass shoots growing in the trail, chances are it has been abandoned. Vole droppings (feces), small fresh grass clippings, or cached apple material are good indicators of recent vole activity. Because the home range (area of daily activity) of a vole typically falls within the area covered by only a few trees, you cannot assume that voles are not present in other areas of the plantation if you didn't find any evidence by looking only at one or two trees.

Simply finding evidence of vole presence does not indicate whether that population is large enough to be considered a problem to your operation or just an annoyance. To make that determination, you must periodically monitor vole populations.

To affect proper management of vole populations and the damage caused by them, you must undertake a reasonable monitoring program. Most of the monitoring programs have been developed in apple orchards where voles are used to apples as a food source. Many forest plantations are in areas where there are no apples and it may take longer than the usual 24-hour monitoring period for the voles to find the bait and eat it. Rather than using apples, another bait that works well is peanut butter. The strong smell attracts rodents and its longevity can be enhanced by mixing in oatmeal to make a kind of paste, as well as a small amount of birdseed. The mixture can be spread on a 2-inch square piece of the shingle and evaluated like a piece of apple based on how much of the mixture is eaten off the shingle.

There are several ways to develop a monitoring program by using the: Apple Activity Index (AI), Feeding Activity Index (FI), or periodic trapping. The first two methods are quite similar, but offer slight variations in the usefulness of the end product. Trapping is more time consuming, labor intensive, and has a tendency to underestimate vole numbers and thus may not provide a reliable assessment of the vole population. However, it is the best technique to use to distinguish between the species (you will capture an individual that will allow in-hand identification).

The Apple Activity Index (AI) was first presented by Dr. R. E. Byers of the Virginia Polytechnic Institute in the early 1970's as a means to quickly assess the presence or absence of voles and also estimate the potential severity of their activity within orchards. The AI technique has been adapted in this fact sheet for use in forest plantations. It does necessitate spending some time and labor effort, but the amount usually is not significant and represents time well spent.

In this technique, monitoring stations are established throughout the orchard and are periodically checked to gauge the status of the vole population. For simplicity, it is best to establish permanent monitoring stations during the initial site assessment prior to planting that can be used for many years until crown closure occurs. To be effective, you should think about establishing 4-8 monitoring stations per acre in the plantation. Roofing shingles or pieces of wood or metal can be used for monitoring stations. Roofing shingles are composed of three sections that can be separated and used to establish monitoring stations. Tires that are cut in half longitudinally and placed open side down also work well to monitor voles and can be used as bait stations later if needed.

Find the most active runways or holes and place a 1"-thick slice of apple or a 2"-square piece of shingle treated with peanut butter ("baited shingle") in the runway or adjacent to a hole at each station. If using apples, you just carve one up as you walk from station to station. Many produce managers at grocery stores will sell outdated apples for a low cost. Cover the bait and runway or hole with a shingle and mark its location with flagging to facilitate finding them later. After a 24-hour period, check each apple or baited shingle at each site for signs of voles feeding (tooth marks). At some of the more active sites, an entire apple slice or baited shingle may be consumed or cached by voles. Therefore, you must record both the evidence of tooth marks and slices that have been removed. Because the "baits" are relatively small in size, it is essential to properly cover each site to prevent other animals from feeding on or removing apple slices.

In forest plantations, voles may not be accustomed to apples or other baits and it may take a number of days for them to find the bait and feed on it. If no activity is seen after 24-hours it is wise to revisit the site after one or two more days to recheck the baits.

At its most simple level, this technique tells you which areas of the plantation have active vole populations. To gauge the extent of population activity over the entire plantation, divide the total number of stations by the number of stations where apples or baited shingles had been fed on (i.e., calculate what percentage of the plantation supports active populations). Because of the explosive breeding potential of voles, an index greater than 20-25% usually indicates a potential for serious damage and a need for vole management.

To more accurately assess vole abundance, and thus the potential for damage, Dr. L. R. Askham at Washington State University refined the AI to include a more objective estimate of feeding activity and called the new method the Feeding Index (FI). The amount of feeding on each apple slice is broken into 1 of 5 categories and is used to predict population abundance and damage potential. The categories of feeding activity are defined, and an example is given below.

Once you have visited and inspected all stations, the total number of apple slices that fell in each category is multiplied by their category value (e.g., 5 slices in category 4 = 20). Add these values together, and then divide by the total number of stations to obtain a rating for the plantations as a whole. The following example shows what a typical assessment worksheet might look like.

When several assessments are made over a period of months (using the same monitoring stations during each trial), you have an index as to whether the population is increasing, decreasing, or staying about the same. The FI indicator can be used to determine when and what type of control to employ in the plantation.

It is important to make a positive identification of a pine or meadow vole through trapping. Relying only on the presence of surface tunnels or underground runways can be confusing; according to some research, meadow voles will tunnel and cause damage like pine voles. In other cases, pine and meadow voles have been found using the same tunnels. This setup of traps, as well as monitoring stations, can be done during the initial site inspection and visited the next day to determine the species and activity. Traps should not be set near monitoring stations. The use of conventional snap traps is recommended because they are inexpensive and easy to use.

Since vole populations may shift from one part of the plantation to another, you need to lay out a grid of traps that will cover the entire plantation. Intensive trapping is not likely to be implemented by foresters or landowners, but a minimum of 2-4 traps per acre is recommended.

A caution needs to be mentioned here. Because pine voles spend so little time at or near the surface, traps placed on the surface most likely will not capture them, even where they are present. First, you must find their subsurface burrows (by probing with your fingers about 2-3" deep at the dripline, deeper near the bole). Then excavate an area of sod or soil large enough to accommodate a standard wooden-based mousetrap, flush with the bottom of the underground runway. Try not to disturb the trail system too much during excavation. Place the trap perpendicular to the trail with the trip pan in the center of the trail. Traps should be baited with peanut butter. Cover each trap site with a shingle to prevent other animals from tripping the trap and to facilitate locating trap sites 24 hours later. Be sure to record your catch at each station accurately, including which species was captured and the number of individuals of each species caught. Results from trapping can often provide an indication of whether the composition of the vole population is changing (e.g., are the numbers of pine voles going up while meadow voles go down?). It can also be used to detect the spread of a vole infestation within the plantation area. Voles will also invade quickly from surrounding areas to repopulate an area where vole populations have been controlled.

Another type of trap is the Sherman trap, which is a collapsible metal trap that is baited with apple or peanut butter. The vole goes into the trap and is captured live. The Sherman trap can be purchased for about $15 each from forestry and/or wildlife supply companies. It will work quite well for meadow voles but may not be effective for the more secretive pine vole.

Dead rodents and traps must be handled carefully. Serious respiratory diseases in humans in the Southwest and Florida and other states have been attributed to microorganisms found in rodent feces and urine. The deer mouse, common in Maryland, is considered a possible host for these diseases, but voles have not been identified as hosts. When handling dead rodents and traps, it is very important to wear disposable gloves, and wash hands well afterward. Treat dead rodents and traps as possible carriers of disease. Bury the dead rodents immediately.

There is no "magic pill" for vole control; no single, simple remedy solves the problem. Following positive identification of the rodent, integrated pest management (IPM) - the appropriate combination of habitat reduction, trapping or poison baiting, and predators - is the best known approach to reducing vole populations. Monitor vole sites in early spring and again each fall to detect rising numbers before plants are damaged.

In many situations, natural resource managers carry out survival surveys of forest plantations in the fall. Many are surprised to find a high mortality rate compared to survival the previous fall. If a check of monitoring sites and visual assessments indicates high vole populations, then vole populations can be reduced that fall using a rodenticide to minimize winter losses. It is imperative that vole populations be monitored prior to any replanting of new trees. If voles are present at high populations, replanting without treatment is a waste of time and money. Once a high vole population is established, poison baits or rodenticide treatments are the quickest and most effective way to control them.

Trees planted in old fields, pasture, and other areas with permanent sod cover are more susceptible to vole damage, and management of ground cover is essential to manage vole populations. Any accumulation of vegetation about the base of the tree provides ideal habitat. The widespread use of tree shelters provides excellent cover for voles, but the addition of thick vegetation only increases the problem.

A general rule of thumb to follow is to remove all vegetation within 3 feet of the tree (either as a circle or strip. This can be accomplished by cultivation or careful use of an approved herbicide. Be sure to remove all remaining dead plant material: even though it's dead, it still provides standing cover. This is a problem in older plantings that have not been maintained and then are herbicided. Vegetated strips between rows should be mowed frequently and maintained. Because you are not likely to get rid of all the voles, you want to try to confine them to the strips - not by the trees. If adequate food exists within the strips to support that portion of the population that you can't eliminate, they normally will not venture out into the open. However, monitoring of the population is necessary.

Herbicides suitable for orchards, nurseries, or forest plantations can be used to clear vegetation from a 4-foot circle at the base of trees in the fall to help keep voles away from the tree trunks and tree shelters. Be careful not to leave a depression when removing weeds from under young trees. It may hold water that could freeze and suffocate roots. Where a heavy growth of vegetation is present, herbicide applications should be timed to prevent a thick accumulation of dead material, which could harbor mice. If possible, rake off leaves and grass from around trees.

If clearing around trees is not possible, frequently mowing grass will be helpful. Even if areas around trees are cleared, frequent mowing of areas between trees will help to further reduce protective cover for voles.

Many orchard growers protect young tree trunks from meadow voles by embedding cylindrical wire guards up to 3 inches into the soil at tree bases. This is not practical for most forest plantings, especially where trees shelters are used. However, wire guards may be used in smaller plantings so the details for their use are provided below.

Guards should be taller than average snow depth so that voles cannot crawl over the top and girdle the stems. Guards are less effective against pine voles because they can tunnel under the guard. These guards usually are made from 1/4-inch mesh galvanized hardware cloth. Bend the wire mesh into a cylinder large enough to allow for 5 years' growth. After the guard is in place, use three short pieces of wire to secure an overlap of approximately 1 inch. Installing guards when planting minimizes damage to tree roots. Check guards in the fall to be sure their bases are still buried properly. Other materials, such as rolled roofing, aluminum foil, sheet metal, and specially made plastic spiral wraps, are also used for tree guards. However, none of these are as satisfactory as wire mesh.

Encouraging vole predators is often underestimated as a control measure. It is true that many predators are not present until voles have already reached damaging levels; however, encouraging predators can form the basis for long-term control. House cats can eat voles all year long.

Hawks, owls, and crows also feed on voles. Owls hunt at night, whereas hawks and crows hunt in the daytime; all consume large numbers of small rodents. In many landscapes, a lack of proper perch sites overlooking vole-infested areas limits the effectiveness of hawks, owls, and crows because they hunt by scanning the ground from a perch, then swooping down to capture prey. Installing a perch can assist in long-term rodent control. A perch can be constructed using a wooden pole properly secured in the ground, with a 2- to 3-foot cross tee at 8 to 10 feet above the ground. One or two per acre spaced to provide maximum viewing of the ground area should suffice.

Black snakes and king snakes also are highly effective predators of small rodents, and they do not harm humans. They should not be killed or disturbed if seen.

Commercial rodenticides are used in acute situations when populations of voles must be reduced quickly to minimize loss of trees or prepare a site for planting. If an initial monitoring of the planting site indicates high vole populations, rodenticides can be used to reduce the population and then vegetation management (mowing, herbicide strips) can be employed along with monitoring to keep the vole population in check.

Rodenticides are available from agricultural supply stores. Over-the-counter formulations vary widely in their effectiveness. However, in Maryland you must have a certified commercial pesticide applicator's license in order to obtain and use the most effective rodenticides. When using poison baits, always carefully follow the instructions on the label to avoid injury to yourself, other people, or pets and wildlife.

Rodenticides are classified as either acute or chronic in their action. Acute rodenticides (e.g., zinc phosphide) are fast-acting poisons that usually will quickly kill voles after one feeding. Zinc phosphide works by forming a phosphide gas in the stomach of the rodent after it is consumed. The gas kills instantly and then dissipates so there is no incorporation into muscle tissue. This minimizes problems of secondary poisoning of predators that may feed on voles. The bait is sold mixed with

In contrast, chronic rodenticides, which include most anticoagulants, require multiple feedings over a period of about 5 days to kill voles. Unlike zinc phosphide, there is an effective antidote, Vitamin K1. These are available to certified pest control applicators through pesticide distributors. When you use poison baits on a regular basis, it is suggested that you rotate rodenticides so that the voles do not develop bait shyness, a common problem with zinc phosphide.

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.

Timing can affect the success of a poison-bait program. The optimum times to apply poison baits are in the fall and in the winter and early spring. Early spring applications can reduce populations before most females begin breeding for the year, but vole populations can quickly recover before the next winter. As the spring progresses and natural foods become more abundant, baiting becomes less effective. During the fall, bait acceptance increases again. Late fall applications reduce vole populations just before winter, when these pests do the most damage. Unfortunately, voles may quickly reinvade from surrounding areas and still cause significant winter damage, especially under the cover of snow. Winter is an ideal time to deliver bait, when most damage occurs and bait acceptance is greater as a result of natural food shortages.

Bait placement is critical to the success of a rodenticide program. The contrasting living habits of meadow and pine voles have important implications for their detection and control. Dispersing bait over an entire area, or broadcast baiting, is more effective against meadow voles because they live on the ground and forage more widely than pine voles. However, in most forestry applications the tall grass makes broadcast baiting ineffective. The bait will never get through the ground cover to a place where the voles will reach it. Instead, it will remain suspended in the brush, get wet quickly, and be ineffective. Also, broadcast baiting increases the opportunity for non-species to eat the bait. For these reasons, it is recommended that hand baiting or bait stations be used.

A major problem with all baits is they quickly absorb moisture, and many applications do not last for more than a few weeks. Placing bait for pine voles in tunnels or under roofing shingles, slabs of wood, split automobile tires, or other similar materials with help protect the bait from moisture and non-target species. Bait stations placed above active runways or tunnels give the best results for pine and meadow voles.

An effective bait station can be constructed from an automobile tire split in half longitudinally. Tire splitters are available commercially, or local tire companies can split tires at a nominal cost. Arrange tire halves hollow-side down, and distribute one per tree or one every 10 yards throughout the infested area. There is no need to provide entrances. Place bait in small cups under the tires. The rounded shape of the tire prevents moisture from dripping into the bait cup, and heat inside the tire can help to attract the voles.

Another bait station design consists of three pieces of 40-gauge, 1 1/2-inch-diameter PVC tubing joined in the shape of an inverted "T" by a PVC tee joint. The vertical tube is 12 inches long and covered at the top with a PVC cap or 12-ounce soft drink can opened at one end to keep out rain and snow. Each of the bottom horizontal pieces is 6 inches long with the outside end cut at a 45-degree angle. The pieces are cemented together with PVC cement. Bait is placed in the vertical tube, and voles enter through the side tubes and feed. Each station may be secured to a tree with rope and placed in natural or manufactured tunnels. This bait station has been effective in some Northeastern orchards during winter and early spring. About 5-7 bait stations per acre are recommended.

Many growers who have experienced a problem with voles and then applied some form of control, never conduct a follow-up to see if their treatment had any impact on vole numbers. One treatment may not be sufficient to manage a severe infestation of voles. Monitoring treatment effects is quite easy and simple. In fact, simplified versions of the original assessment techniques (AI, FI, Trapping) will fulfill this need.

One method involves placing apple slices at 10 more sampling stations in an area containing active runs before treatment begins. Count the number of slices that were chewed overnight and calculate a percentage using the total number of sampling sites (e.g., 5 of 10 slices chewed = 50%). Wait one week and repeat the process using the same sites, if possible. Check the slices the next morning and recalculate your percentage (e.g., 3 of 10 slices chewed = 30%). The reduction in the vole population is determined by calculating the change in percent (i.e., 50% to 30% = 20% reduction).

You may also use traps to monitor population reductions. Prepare and bait traps as outlined earlier. Set your traps just prior to and just after treatment. If voles are captured at more than 1 or 2 sites, reapplication may be warranted. Voles will continue to breed into late fall and early winter. Remember that voles remain especially when the weather remains mild. Also, these animals remain active throughout the winter beneath the snow. Therefore, don't feel comfortable that an early treatment will eliminate your problem for the entire winter. You may want to periodically look for evidence of vole the winter by watching for tunnels in the snow. You can hand-bait these trails throughout the winter to maintain some level of control.

Another factor to consider is reinvasion of voles from adjacent fields after a chemical treatment. Research in apple orchards indicates that less than a year after treatment, voles may reinvade a site in significant numbers.

• Figures 1 & 2 were provided by Ross E. Byers, Extension and research horticulturist, Department of Horticulture, Virginia Polytechnic Institute and State University, Blacksburg.

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