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

TRAPPING.

If mice are present in small numbers, as is often the case in lawns, gardens, or seed beds, they may readily be caught in strong mouse traps of the guillotine type (figs. 5 and 6). These should be baited with oatmeal or other grain, or may be set in the mouse runs without bait.

Trapping has special advantages for small areas where a limited number of mice are present, but it is also adapted to large areas whenever it is undesirable to lay out poison. It is then necessary to use many traps and continue their use for several weeks. If mice are moderately abundant, from 12 to 20 traps per acre maybe used to advantage. These should soon make decided inroads on the numbers of mice in an orchard if not practically to exterminate them. For pine mice the tunnels should be excavated sufficiently to admit the trap on a level with the bottom. A light garden trowel may be used for the necessary digging.

POISONING.

On large areas where mice are abundant, poisoning is the quickest means of destroying them, and even on small areas it has decided advantages over trapping.

The following formula is recommended:

Dry grain formula.—Mix thoroughly 1 ounce powdered strychnine (alkaloid), 1 ounce powdered bicarbonate of soda, and ¹/₈ ounce (or less) of saccharine. Put the mixture in a tin pepper box and sift it gradually over 50 pounds of crushed wheat or 40 pounds of crushed oats in a metal tub, mixing the grain constantly so that the poison will be evenly distributed.

Dry mixing, as above described, has the advantage that the grain may be kept any length of time without fermentation. If it is desired to moisten the grain to facilitate thorough mixing, it would be well to use a thin starch paste (as described below, but without strychnine) before applying the poison. The starch soon hardens and fermentation is not likely to follow.

If crushed oats or wheat can not be obtained, whole oats may be used, but they should be of good quality. As mice hull the oats before eating them, it is desirable to have the poison penetrate the kernels. A very thin starch paste is recommended as a medium for applying poison to the grain. Prepare as follows:

Wet grain formula.—Dissolve 1 ounce of strychnia sulphate in 2 quarts of boiling water. Dissolve 2 tablespoonfuls of laundry starch in ½ pint of cold water. Add the starch to the strychnine solution and boil for a few minutes until the starch is clear. A little saccharine may be added if desired, but it is not essential. Pour the hot starch over 1 bushel of oats in a metal tub and stir thoroughly. Let the grain stand overnight to absorb the poison.

The poisoned grain prepared by either of the above formulas is to be distributed over the infested area, not more than a teaspoonful at a place, care being taken to put it in mouse runs and at the entrances of burrows. To avoid destroying birds it should whenever possible be placed under such shelters as piles of weeds, straw, brush, or other litter, or under boards. Small drain tiles, 1½ inches in diameter, have sometimes been used to advantage to hold poisoned grain, but old tin cans with the edges bent nearly together will serve the same purpose.

Chopped alfalfa hay poisoned with strychnine was successfully used to destroy meadow mice in Nevada during the serious outbreak of the animals in 1907-8. One ounce of strychnia sulphate dissolved in 2 gallons of hot water was found sufficient to poison 30 pounds of chopped alfalfa previously moistened with water. This bait, distributed in small quantities at a place, was very effective against the mice, and birds were not endangered in its distribution.

For poisoning mice in small areas, as lawns, gardens, seed beds, vegetable pits, and the like, a convenient bait is ordinary rolled oats. This may be prepared as follows: Dissolve ¹/₁₆ ounce of strychnine in 1 pint of boiling water and pour it over as much oatmeal (about 2 pounds) as it will wet. Mix until all the grain is moistened. Put it out, a teaspoonful at a place, under shelter of weed and brush piles or wide boards.

The above poisons are adapted to killing pine mice, but sweet potatoes cut into small pieces have proved even more effective. They keep well in contact with soil except when there is danger of freezing, and are readily eaten by the mice. The baits should be prepared as follows:

Potato formula.—Cut sweet potatoes into pieces about as large as good-sized grapes. Place them in a metal pan or tub and wet them with water. Drain off the water and with a tin pepper box slowly sift over them powdered strychnine (alkaloid preferred), stirring constantly so that the poison is evenly distributed. An ounce of strychnine should poison a bushel of the cut bait.

The bait, whether of grain or pieces of potato, may be dropped into the pine-mouse tunnels through the natural openings or through holes made with a piece of broom handle or other stick. Bird life will not be endangered by these baits.

CULTIVATING THE LAND.

Thorough cultivation of fields and the elimination of fence rows between them is the most effective protection against field mice. Cultivation destroys weeds and all the annual growths that serve as shelter for the animals. This applies equally well to orchards and nurseries. Clean tillage and the removal from adjoining areas of the weeds and grass that provide hiding places for mice will always secure immunity to trees from attacks of the animals.

PROTECTING NATURAL ENEMIES OF MICE.

Field mice are the prey of many species of mammals, birds, and reptiles. Unfortunately, the relation that exists between the numbers of rodents and the numbers of their enemies is not generally appreciated; otherwise the public would exercise more discrimination in its warfare against carnivorous animals. It is the persistent destruction of these, the beneficial and harmful alike, that has brought about the present condition of growing scarcity of predacious mammals and birds and corresponding increase of rodent pests of the farm, especially rats and mice. The relation between effect and cause is obvious.

Among the mammalian enemies of meadow and pine mice are coyotes, wildcats, foxes, badgers, raccoons, opossums, skunks, weasels, shrews, and the domestic cat and dog. Among birds, their enemies include nearly all the hawks and owls, storks, ibises, herons, cranes, gulls, shrikes, cuckoos, and crows. Among their reptilian foes are black snakes and bull snakes. Not all these destroyers of mice are more beneficial than harmful, but the majority are, and warfare against them should be limited to the minority that are more noxious than useful.

OWLS AND FIELD MICE.

Owls as destroyers of mice are deserving of special mention. Not one of our American owls, unless it be the great horned owl, is to be classed as noxious. Especially beneficial are the short-eared, long-eared, screech, and barn owls. All these prey largely upon field mice, and seldom harm birds. Unfortunately, the short-eared and barn owls, which are the more useful species, are not plentiful in the sections most seriously infested by field mice.

The short-eared owl, while widely distributed, is not abundant, except locally, within the United States, but wherever field mice become excessively numerous these owls usually assemble in considerable numbers to prey upon them. Examinations of stomachs of these owls show that fully three-fourths of their food consists of short-tailed field mice.

The barn owl is rather common in the southern half of the United States and breeds as far north as the forty-first parallel of latitude. That mice form the chief diet of this bird has been demonstrated by Dr. A. K. Fisher, of the Biological Survey, through examination of stomachs of many barn owls and also of large numbers of pellets (castings from their stomachs) found under their roosts. In 1,247 barn-owl pellets collected in the towers of the Smithsonian Building in Washington, D. C., he found 1,991 skulls of short-tailed field mice, 656 of the house mouse, 210 of the common rat, and 147 of other small rodents and shrews. Very few remains of birds were found. Figure 7 illustrates the contents of some of these pellets.

In 360 pellets of the long-eared owl Dr. Fisher found skulls of 374 small mammals, of which 349 were meadow mice. Stomach examinations give similar testimony to the usefulness of this bird.

The common screech owl, in addition to feeding mainly upon mice, destroys also a good many English sparrows. Its habit of staying in orchards and close to farm buildings makes it especially useful to the farmer in keeping his premises free from both house and field mice.

WASHINGTON: GOVERNMENT PRINTING OFFICE: 1915