There has been growing concerns about vole damage in apple and peach orchards throughout northern and southern New Jersey counties. Vole populations are often cyclic, and can reach high populations every 2-4 years. This, combined with poor weed control in orchards, can lead to disastrous effects on trees.
Identification and Habitat
Meadow voles and pine voles are two common types of voles. Pine voles have small eyes and small, almost inconspicuous ears, while meadow voles have larger eyes and ears. Meadow voles primarily feed in open vegetation, while pine voles prefer underground burrow systems just below the ground surface. Vole diets include many primary food sources in addition to tree bark. However, they can attack fruit trees during fall and winter months when other food sources are scarce.
Meadow vole damage is often clearly visible at the ground surface in the form of girdling (Figure 1) or gnawing (Figure 2) at the base of the trunk. Pine vole damage is difficult to notice until the tree growth begins to decline by which time it is difficult to save the tree. Most pine vole damage will be seen as root damage and crown girdling just under the ground surface. However, when feeding has occured under a deep snow cover, damage can appear at the ground surface.
Meadow voles create an extensive network of surface runways in the grass which are about 1.5 inch wide, and are often visible after close mowing. Bits of leaves and vole droppings in pathways are the surest signs of meadow vole presence. Runways are often spread out through the cover crop in orchard aisles, and under poor weed control, often cross back and forth to the tree row. Since pine vole burrows and runways are just underground, there is a “soft” feel of the soil under foot, which is one indication of pine vole presence. A simple monitoring method is a two-step process. First place a grid of ‘shelters’ in the orchard. These can be pieces of roofing shingles, heavy tar paper or roof matting, or scraps of wood. After leaving them for 3-5 days so the voles can find them, place apple chunks or slices beneath each shelter. Return in 24 hours and assess which pieces have been chewed. This will give you a rough idea where controls need to be focused.
Non Toxicant: Regularly mowing the ground cover down to 3 to 5 inches is a recommended practice to include as part of an integrated vole damage management program. It not only limits the availability of food for voles’ survival, but also make them exposed to predators. Maintaining a vegetation-free zone below the trees discourages voles from living close to the trunk where they can cause great damage. This is the MOST IMPORTANT aspect of routine vole prevention. Sanitizing the orchard floor by removing leaves and pruned twigs which attract the voles is also helpful.
The addition of a 12 inch diameter, 3-4 inch deep gravel layer around the trunk will also help. Gravel collapses in on a vole runway, and helps prevent vole access to the trunk and crown area of the tree.
Cylinder-shaped wire guards made from hardware cloth mesh of 1/4 inch placed around the tree trunks can protect young trees. However, make sure to bury the hardware cloth at least 6 inches below the surface to control both meadow and pine voles. This exclusion method may not be very practical or cost effective in large orchards.
Toxicants: Sometimes to control large scale vole populations, chemical methods are the only practical option available. When vole populations get too high, toxicants within and near the herbicide strip in the tree rows is the most effective method to control vole populations. There are two types of toxicants available; 1.) zinc phosphide, which is a fast acting stomach poison, and 2.) chlorophacinone and diphacinone, which are slow acting anticoagulants. Zinc phosphide is extremely toxic. These materials are sold under a number of brand names including for Zinc Phosphide: Prozap Zinc Phosphide Pellets, Prozap Zinc Phosphide Oat Bait, Bonide Orchard Mouse Bait, and Zinc Phosphide Rodent Bait AG. Anticoagulants include Ramik Oats, Ramik Brown, and Rozol Vole Bait. Make sure you have the necessary pesticide applicator license to handle these chemicals.
The reliance of one product over another is discouraged, since voles will get accustomed to grain baits with anticoagulants, or become bait shy of anticoagulants in general. Bait use is most effective when used in simple bait stations such as under roofing shingles, boards, and pieces of scrap plate steel or in PVC pipe bait stations. Hand placement in areas of high vole activity, or depositing bait with a trail maker are also effective. Broadcast placement of toxicants is discouraged, since it is inefficient and can put wildlife at risk. Bait stations can reduce the chances of bait getting into contact with non-intended animals or humans. Bait stations are easy to make using 2 to 3 inch PVC pipes in an ‘L’ or ‘T’ shape in which the horizontal pipe provides an opening for voles to enter, and the vertically connected pipe contains the toxicant.