REPRESSION OF FIELD MICE. 37 



ECONOMIC STATUS OF FIELD MICE. 



The foregoing testimony as to the ravages of field mice sufficiently 

 attests their noxious character. The extent to which they are bene- 

 ficial to the farmer may be stated in few words. The upturning of 

 the soil by gophers, ground squirrels, moles, earthworms, and various 

 insects is an important part of its preparation for man's use, and 

 field mice contribute to the work. In addition, mice diminish the 

 number of Aveeds by eating the seeds and, possibly, occasionally 

 devouring the roots. 



The character of the vegetable matter eaten by field mice depends 

 entirely upon the environment. If the animals live in swamps where 

 rushes, sedges, and grasses useless to agriculture abound, their diet 

 is confined to these, and the naturalist who investigates their food 

 only in such surroundings may fall into the error of concluding that 

 they are not injurious. But field mice at all times show a decided 

 preference for cultivated plants and grains, if accessible, and in 

 winter seek diligently for succulent roots and the green bark of trees. 



REPRESSION OF FIELD MICE— NATURAL AGENCIES. 



In view of their wide distribution and the nature of their habitats, 

 the complete extermination of field mice is impossible. It is, how- 

 ever, entirely practicable to so reduce their numbers that crops shall 

 be safe from serious attack. Before describing the methods Avhich 

 the farmer ma}^ use to destroy the animals, it is important to con- 

 sider the natural agencies which aid in their repression. 



CLIMATIC INFLUENCES. 



Climatic conditions so strongly influence the natural increase of 

 field mice that an understanding of their relation to such increase is 

 important. It has been often noticed that the periods of greatest 

 abundance of voles follow one or more open, warm winters succeeded 

 by wet summers. If at. the same time food — particularly grain — is 

 abundant, the most favorable conditions for the increase of the ani- 

 mals are present, and the maximum number of young at a birth, as 

 well as the maximum number of litters, are brought forth. When 

 grain is ' abundant the young of some species are said to begin to 

 reproduce when only 2 months old. 



The continuance of favorable conditions for two or more years is 

 sure to be followed by an enormous increase of field mice, with seri- 

 ous injury to crops. 



Very cold winters without snow and summers of long droughts are 

 unfavorable to the increase of field mice. Occasionally a severe 

 frost following a heavy downpour of rain destroys nearly all field 

 mice over large areas. 



