MOUSE PLAGUES, THEIR CONTROL AND PREVENTION. 303 



mice periodically tend toward enormous multiplication. That this 

 tendency is inherent can scarcely be doubted. Agricultural develop- 

 ment, however, distinctly increases the danger of plagues by further- 

 ing the destruction of their natural enemies, by furnishing a great 

 abundance of food, and by increasing the area in which they find 

 favorable homes. The reclamation of arid lands affords most suit- 

 able conditions in large areas which were formerly uninhabitable. 

 In these new lands, restricted by surrounding desert conditions, and 

 stimulated by the rich food and dense shelter furnished by alfalfa, 

 these mice are especially dangerous. 



NATURE AND DURATION OF MOUSE PLAGUES. 



Accounts of mouse plagues agree that the mice increase in numbers 

 for a season or two preceding serious outbreaks, that the final pro- 

 duction of hordes is comparatively sudden, and that the period 

 during which mice swarm over the land is rarely longer than a year. 

 The total duration of a plague may thus cover three or four years. 

 Natural control invariably asserts itself by somewhat sudden and 

 decisive destruction of the abnormal numbers. Usually plagues sub- 

 side during the winter and spring following their maximum, dis- 

 ease and predaceous enemies being the most apparent causes. The 

 subsidence of a plague is usually followed by a long period of de- 

 pression. It takes the mice several years to regain normal abundance, 

 and several years more before the danger of producing a plague 

 again 'becomes imminent ; hence in no locality have plagues been 

 recorded oftener than once in eight or ten years. Field mice are very 

 prolific, and in the absence of natural checks might produce a plague 

 every four or five years. From two to six litters of young are pro- 

 duced annually. The average number of young at a birth is 

 about 6, though frequently 8 to 10 are produced, and occasionally 

 12 or 13. Even the young born early in the season are said to breed 

 before fall. 



These mice are always present in or near the districts which they 

 occasionally overrun, but ordinarily live in small colonies in favorable 

 locations, particularly in damp areas bordering swamps, streams, or 

 irrigation ditches. In alfalfa and other cultivated lands where the 

 food supply is abnormally plentiful, and parti cularh^ if the natural 

 enemies of mice are destroyed, the animals may increase greatly 

 in one or two seasons, and the first breeding in the second or third 

 season may produce great numbers. Then the mice spread over a 

 greater area, and as the food at hand is consumed they may move 

 on in troops. The final production of hordes requires but a few 

 months — in fact, a plague may be well established by fall. 



