When a young tree in an orchard suddenly collapses and dies from being girdled at the base of the trunk, gophers are often quickly blamed for the damage. While the damage may appear very similar, girdling by voles (meadow mice) can sometimes be more serious since quick population explosions can result in extensive damage over a wide area in a very short time.
Voles, not to be confused with moles that eat insects and grubs, normally live in areas with dense vegetation in shallow burrow systems. They are active both day and night and do not hibernate, thus can be seen year-round. Their burrow systems can be differentiated from gophers’ in that they do not have the large mounds of soil covering the burrow entrance. This is why their damage in young orchards often goes unnoticed until trees start to wither or collapse suddenly with no obvious above-ground evidence. This is especially true in organic orchards or in fields where dense vegetation is allowed to grow around the trunk area. I have even seen trunk damage occur quickly in young orchards when middle vegetation is suddenly destroyed by cultivating or spraying with herbicides. With their food source suddenly gone, the voles resort to feeding on the tender bark of the trees.
I can attest to the voles’ year-round activity, since during November or December when I am backhoeing out stumps for replacement trees, it is not uncommon to see several voles scurrying out from the disturbed trunk area of each removed tree.
Damage To Drip Lines
A recent problem with voles in orchards has been their damage to drip or microsprinkler lines. Most of the time they do not sever or chew entirely through plastic lines, but rather cause thin razor-like scratch marks with their teeth. When the lines are pressurized, thin streams of water can be seen spraying up from the hoses. In my own orchards, this has been a bigger problem than coyotes or squirrels chewing the hoses, even though I keep the tree rows weed free around the trunks. The majority of the damage is always right up next to the trunks, and every time we go to irrigate some repairs are necessary.
Control of voles is much more difficult than gophers, since vole populations can be quite large and not always easy to notice. Exclusion by barriers or trapping is logistically impractical in most commercial orchards. As with gophers, owl houses can help, but are generally insufficient on their own to control damaging populations. Fumigating burrows with materials such as aluminum phosphide is marginally effective due to the extensive amount of shallow burrowing and the many open entrances.
Baiting is usually the most practical method of dealing with large numbers of voles. Anticoagulant baits can be effective if used properly. Sufficient bait must be made available over a number of days (usually four to five days) to be effective. Since voles do not travel far, bait only needs to be broadcast in the immediate area of their tunnels and entrances. Make sure the bait used is specific for voles, since many of the mouse and rat baits available at the local home improvement stores are not designed for voles and can present secondary toxic hazards to pets and non-target predators. The acute toxicant baits such as zinc phosphide can also be highly effective if used properly. As with the anticoagulant baits, the label should be carefully followed to insure that use and application methods do not present a hazard to non-target animals.