Orchard Mouse Damage to Fruit Trees
Dr. Guy Witney, Former WSU Area Extension Faculty
High mouse populations in the fall, combined with early and heavy snowfall in winter can result in significant mouse damage. Many growers will note high orchard mouse populations in the fall and take steps to control them. However, early and heavy snow accumulation in winter can surprise other growers, and also provided cover for the mice from predators.
Orchard mice will often feed on tender prunings left on the orchard floor before feeding on relatively tough tree trunks. However, growers may do little pruning by the first snow and so this source of food may not be available, leaving trunks vulnerable. Prunings on top of the snow cover are not accessible by mice. As the snow melts in late winter, more and more growers may report tree damage.
More on orchard mice:
The short-tailed meadow mouse (Microtus spp.) is the name of the rodent most growers refer to as orchard mice or voles. The animal has a stout 1.5 – 2.0 oz body covered with loose, fairly long gray/black hair, small shiny black eyes, and small fur-covered ears. This is a short-tailed mouse with a hairy tail about 1/3 body length.
These mice are very prolific. Females are sexually mature at four weeks old, and produce 8 -10 litters each year with an average of six young per litter. One breeding pair can produce 3,000 offspring in eight months. Their populations are cyclic with peaks every four or five years. At peak populations numbers range from 1,000 to 3,000 per acre. They consume their body weight in food each day.
Orchard mice have a lot of natural enemies including hawks, owls, shrikes, snakes, badgers and skunks. Preserve these predators where ever possible. If other control methods are required, this is generally done by poison baiting with zinc phosphide by hand or with a mechanical applicator.
A well-mowed orchard with a clean herbicide strip will have lower mouse populations than a grassy or weedy orchard. Vegetation cover adjoining orchards can also be a source of mice. Try to eliminate as many areas of mouse refuge as possible including weeds around buildings, vacant areas, and drive rows. Coordinate this with your neighbors.
These mice damage trees by chewing and peeling bark from tree trunks and roots near the soil surface, often girdling trees. This can result in reduced tree health or death, either directly from the feeding or by later entry of diseases like Phytophthora crown rot.
Right: Orchard mouse damage on a mature apple trunk. The upper damage occurred under heavy snow cover, while the lower damage occurred later as the snow melted . The upper damage circles about 40% of the trunk and may not require bridge grafting. The lower damage girdles the entire trunk and will require 5 – 8 bridge grafts. If left unattended, the tree would gradually die as the root system starved.
Trees that have more than 50% of their trunk circumference girdled by orchard mice should be bridge grafted with one graft per four inches of trunk circumference damage. Experienced orchardists have shown that ‘Golden Delicious’ provides the best apple graftwood for this purpose, but most apple wood is suitable. Select graft wood in early spring that is one year old and about 1/2 inch in diameter with bark slipping (indicating spring activity). The trunks to be repaired should also have bark slipping at this time. Cut both ends of the graft wood stick (scion) with a long smooth, sloping cut about 1 1/2 inches long. Going completely through the scion at both ends creating a point at each end. Make a flap cut the width of the scion in the bark of the trunk just below the injury. Loosen the flap and insert the basal end of the scion under the flap until the cut surface on the scion is covered. Drive a nail through the flap and scion to secure the graft. Repeat the process, attaching the scion above the injury. Protect the new grafts with wax or similar protective material (ask your nurseryman for a high quality material and source). Remove any buds that grow from the scion as soon as they start. Successful bridge grafts can grow and support the tree for the life of the orchard.
A few growers have reported that white latex trunk paints, with or without copper added, deter the mice from trunk feeding. This may be worth a try, particularly where trees are young and mice are an ongoing problem.