302 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



related to the species which have caused such widespread destruction 

 in the Old World. 



THE NEVADA OUTBREAK. 



In 1907-8 an outbreak of field mice in Nevada, Utah, and north- 

 east California threatened to develop into a plague as great as any 

 recorded. The greatest loss occurred in the rich fields of alfalfa bor- 

 dering Humboldt River for the last 10 or 12 miles of its course to the 

 Sink (Humboldt Lake, Nevada). Noticeable here through gradu- 

 ally increasing damage during 1906, the field mice appeared early 

 in the summer of 1907 in alarming numbers. By November they had 

 overrun a large part of the cultivated area, and on many large 

 ranches were estimated by one of the assistants of the Biological 

 Survey to number from 8,000 to 12,000 to the acre. Fields were 

 literally honeycombed by their holes, which numbered about 24,000 

 to the acre. During the summer they ruined one-third of the alfalfa, 

 destroyed three- fourths of the potatoes and badly damaged the re- 

 mainder, and severely injured root crops, as beets and carrots. Upon 

 the disappearance of green food in fall they attacked the roots of 

 alfalfa and trees, causing far more serious damage. They ate so 

 large a percentage of the plants as to render many alfalfa fields a 

 total loss. They girdled and killed most of the young shade trees 

 planted along ditches and about the borders of fields. Even such 

 hardy trees as large Lombardy and silver poplars were killed, while 

 small orchards suffered severely. (PI. XXL) By January, 1908, the 

 ravages had extended over considerably more of the district, and the 

 main body of mice was gradually progressing to fresh fields. From 

 this time, however, the abatement of the plague was rapid. By March 

 15 the invasion of fresh lands had ceased, though mice continued con- 

 siderably in excess of normal abundance until May. By August 

 they had practically disappeared from the valley. This scourge 

 left a dismal scene of destruction over four-fifths of the cultivated 

 area in the district. Of 20,000 acres in alfalfa, 15,000 were so com- 

 pletely destroyed as to require replanting. (PI. XXII.) Consider- 

 ing the actual losses in crops and the cost of restoring the alfalfa 

 fields, and allowing for the value of the wheat which replaced alfalfa 

 in most of the ruined fields for the season of 1908, a conservative 

 estimate of the losses in this district is $250,000. 



CONDITIONS FAVORING MOUSE PLAGUES IN THE UNITED STATES. 



While the Nevada plague is the most serious recorded in the 

 United States, frequent milder outbreaks in many parts of the 

 country indicate that practically all our species of short-tailed field 



