[FIELD MICE AS FARM AND ORCHARD PESTS.]

By D. E. Lantz, Assistant Biologist.

Note.—This bulletin describes the habits, geographic distribution, and methods of destroying meadow mice and pine mice, and discusses the value of protecting their natural enemies among mammals, birds, and reptiles. It is for general distribution.

[INTRODUCTION.]

The ravages of short-tailed field mice in many parts of the United States result in serious losses to farmers, orchardists, and those concerned with the conservation of our forests, and the problem of controlling the animals is one of considerable importance.

Short-tailed field mice are commonly known as meadow mice, pine mice, and voles; locally as bear mice, buck-tailed mice, or black mice. The term includes a large number of closely related species widely distributed in the Northern Hemisphere. Over 50 species and races occur within the United States and nearly 40 other forms have been described from North America. Old World forms are fully as numerous. For the purposes of this paper no attempt at classification is required, but two general groups will be considered under the names meadow mice and pine mice. These two groups have well-marked differences in habits, and both are serious pests wherever they inhabit regions of cultivated crops. Under the term "meadow mice"[1] are included the many species of voles that live chiefly in surface runways and build both subterranean and surface nests. Under the term "pine mice"[2] are included a few forms that, like moles, live almost wholly in underground burrows. Pine mice may readily be distinguished from meadow mice by their shorter and smoother fur, their red-brown color, and their molelike habits. (See fig. 1.)

[1] Genus Microtus.

[2] Genus Pitymys.

Fig. 1.—Field mice: a, Meadow mouse; b, pine mouse.

[MEADOW MICE.]

Meadow mice inhabit practically the whole of the Northern Hemisphere—America, north of the Tropics; all of Europe, except Ireland; and Asia, except the southern part. In North America there are few wide areas except arid deserts free from meadow mice, and in most of the United States they have at times been numerous and harmful. The animals are very prolific, breeding several times a season and producing litters of from 6 to 10. Under favoring circumstances, not well understood, they sometimes produce abnormally and become a menace to all growing crops. Plagues of meadow mice have often been mentioned in the history of the Old World, and even within the United States many instances are recorded of their extraordinary abundance with accompanying destruction of vegetation.

The runs of meadow mice are mainly on the surface of the ground under grass, leaves, weeds, brush, boards, snow, or other sheltering litter. They are hollowed out by the animals' claws, and worn hard and smooth by being frequently traversed. They are extensive, much branched, and may readily be found by parting the grass or removing the litter. The runs lead to shallow burrows where large nests of dead grass furnish winter retreats for the mice. Summer nests are large balls of the same material hidden in the grass and often elevated on small hummocks in the meadows and marshes where the animals abound. The young are brought forth in either underground or surface nests.

Meadow mice are injurious to most crops. They destroy grass in meadows and pastures; cut down grain, clover, and alfalfa; eat grain left standing in shocks; injure seeds, bulbs, flowers, and garden vegetables; and are especially harmful to trees and shrubbery. The extent of their depredations is usually in proportion to their numbers. Thus, in the lower Humboldt Valley, Nevada, during two winters (1906-8) these mice were abnormally abundant, and totally ruined the alfalfa, destroying both stems and roots on about 18,000 acres and entailing a loss estimated at fully $250,000.

When present even in ordinary numbers meadow mice cause serious injury to orchards and nurseries. Their attacks on trees are often made in winter under cover of snow, but they may occur at any season under shelter of growing vegetation or dry litter. The animals have been known almost totally to destroy large nurseries of young apple trees. It was stated that during the winter of 1901-2 nurserymen near Rochester, N. Y., sustained losses from these mice amounting to fully $100,000.

Older orchard trees sometimes are killed by meadow mice. In Kansas in 1903 the writer saw hundreds of apple trees, 8 to 10 years planted, and 4 to 6 inches in diameter, completely girdled by these pests. (Fig. 2.) The list of cultivated trees and shrubs injured by these animals includes nearly all those grown by the horticulturist. The Biological Survey has received complaints of the destruction of apple, pear, peach, plum, quince, cherry, and crab-apple trees, of blackberry, raspberry, rose, currant, and barberry bushes, and of grape vines; also of the injury of sugar maple, black locust, Osage orange, sassafras, pine, alder, white ash, mountain ash, oak, cottonwood, willow, wild cherry, and other forest trees.

Fig. 2.—Apple tree killed by meadow mice.

In the Arnold Arboretum, near Boston, Mass., during the winter of 1903-4, meadow mice destroyed thousands of trees and shrubs, including apple, juniper, blueberry, sumac, maple, barberry, buckthorn, dwarf cherry, snowball, bush honeysuckle, dogwood, beech, and larch. Plants in nursery beds and acorns and cuttings in boxes were especial objects of attack.

The injury to trees and shrubs consists in the destruction of the bark just at the surface of the ground and in some instances for several inches above or below. When the girdling is complete and the cambium entirely eaten through, the action of sun and wind soon completes the destruction of the tree. If the injury is not too extensive prompt covering of the wounds will usually save the tree. In any case of girdling heaping up fresh soil about the trunk so as to cover the wounds and prevent evaporation is recommended as the simplest remedy. To save large and valuable trees bridge grafting may be employed.

[PINE MICE.]

Pine mice occur over the greater part of eastern United States from the Hudson River Valley to eastern Kansas and Nebraska and from the Great Lakes to the Gulf of Mexico. Inhabitants chiefly of forested regions, they are unknown on the open plains. Ordinarily they live in the woods, but are partial also to old pastures or lands not frequently cultivated. From woods, hedges, and fence rows they spread into gardens, lawns, and cultivated fields through their own underground tunnels or those of the garden mole. The tunnels made by pine mice can be distinguished from those made by moles only by their smaller diameter and the frequent holes that open to the surface.

While the mole feeds almost wholly upon insects and earthworms, and seldom eats vegetable substances, pine mice are true rodents and live upon seeds, roots, and leaves. Their harmful activities include the destruction of potatoes, sweet potatoes, ginseng roots, bulbs in lawns, shrubbery, and trees. They destroy many fruit trees in upland orchards and nurseries (fig. 3). The mischief they do is not usually discovered until later, when harvest reveals the rifled potato hills or when leaves of plants or trees suddenly wither. In many instances the injury is wrongly attributed to moles whose tunnels invade the place or extend from hill to hill of potatoes. The mole is seeking earthworms or white grubs that feed upon the tubers, but mice that follow in the runs eat the potatoes themselves.

Fig. 3.—Root and trunk of apple tree from Laurel, Md., gnawed by pine mice.

Pine mice feed to some extent outside their burrows, reaching the surface through the small openings made at frequent intervals in the roofs of the tunnels. In their forays they rarely go more than a few feet from these holes. Most of their food is carried under ground, where much is stored for future consumption. While they differ little from meadow mice in general food habits, their surroundings afford them a larger proportion of mast. They are less prolific than meadow mice, but this is more than made up for in the fact that in their underground life they are less exposed to their enemies among birds and mammals. Like meadow mice, they sometimes become abnormally abundant.

In the eastern part of the United States pine mice do more damage to orchards than do meadow mice, partly because their work is undiscovered until trees begin to die. The runs of meadow mice under grass or leaves are easily found and the injury they do to trees is always visible. On the other hand, depredations by pine mice can be found only after digging about the tree and exposing the trunk below the surface. The roots of small trees are often entirely eaten off by pine mice, and pine trees as well as deciduous forest trees, when young, are frequently killed by these animals (fig. 4).

Fig. 4.—Pine tree killed by pine mice.

[DESTROYING FIELD MICE.]

Methods of destroying field mice or holding them in check by trapping and poisoning are equally applicable to meadow mice and pine mice.