Stu Goldstein

We have a nonindustrial private tree farm in northern Idaho, on the southeast rim of the Moscow basin, between Paradise Ridge and Tomer Butte, at the eastern edge of the Palouse. We've planted close to 40,000 seedlings on our hilly terrain, in a Conservation Reserve Program.

Voles-genus Microtus, a.k.a. meadow mice, have been our major cause of seedling mortality. While in a typical year there are only a few voles per acre, populations cycle unpredictably; the number of voles can increase by 100 times or more during a population explosion, and they can devastate a young plantation.

We've tried several vole-control methods, with varying degrees of success (or, more accurately, varying degrees of failure). A summary of our experiences follows.

TREE SHELTERS

Tree protector tubes at least eight inches tall prevent vole damage when the tubes fit snugly to the ground. We've experimented with different kinds and sizes.

We have given up using tree tubes because:

1) On hillsides, tubes often leave a vole-sized gap at the base on the downhill side.

2) Frost heave can lift staked tree tubes as much as a couple of inches in winter.

3) Unstaked tubes are susceptible to snow and windthrow, creating a gap that admits voles as well as putting the seedlings at risk of deformity.

4) Tubes seem to encourage height growth at the expense of caliper.

5) Tubes can crowd and shade low laterals, reducing valuable photosynthetic surface area.

FOIL

We tried wrapping the stems of the seedlings with aluminum foil, an experiment we won't perform again. You can only wrap as high as the first lateral, and unless that's more than eight inches (which it rarely is) voles can chew off the seedlings just above the foil. And we've observed that foil tends to abrade bark and retard stem lignification.

CLEARINGS

Voles are said to be reluctant to cross clear areas. We applied hexazinone (Pronone®) granules over the tops of ponderosa pine seedlings at planting time to promote thrift by controlling vegetative competition. The hexazinone produced clear areas about five feet in diameter. We've observed that voles readily traverse the cleared areas and eat the seedlings. Some growers install plastic sheeting around seedlings to inhibit competition; they tell us that voles cross those also.

ANI-PEL

We placed Ani-Pel tablets, marketed as a bitter systemic repellent, in the planting holes of several hundred seedlings, per the manufacturer's directions. Voles devoured Ani-Pel-treated seedlings and untreated seedlings with equal relish.

RAPTOR POLES

Small rodents make up 90 percent of the diet of the adult hawk, which eats about five mice a day. Perch poles attract birds of prey to areas without tall trees. We erected poles, the hawks perched on them, and we found mouse bones at their bases. The trouble is that during vole population explosions there may be upward of 500 voles per acre, and each adult hawk patrols an area of many acres. Raptors, along with other small-rodent eaters, can't begin to severely diminish the vole population.

ZINC PHOSPHIDE

We've read reports that zinc phosphide rodenticide has not generally proved effective. Poisoned voles can be fatal to small rodent eaters-birds of prey, cats, dogs and raccoons, for example. And since it happens that our spring is at the bottom of our drainage, we felt that there were persuasive reasons not to use rodenticide.

PLUG+1s

Our most successful seedlings have been plug+1s (seedlings that spend their first nursery year in a tubular container and their second nursery year in the ground). We attribute this to the fact that plug+1s have the greatest caliper and most mature lignification of all classes of seedlings that can be planted efficiently with a planting hoe. Greater caliper and lignification generally correlate with decreased depredation. We haven't been through a major vole population explosion since we began planting plug+1s in large numbers, but given our discouraging experiences with tree tubes, foil wraps, localized clearings, Ani-Pel, raptor poles and rodenticide, we're inclined to bet on plug+1s.

Stu Goldstein is a member of the Idaho Forest Owners Association. Comments can be directed to him at bluedawg@aol.com.

This article appeared in the Northwest Woodlands Magazine, Spring 2001- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.