THE LOCUST OR SHORT-HORNED GRASSHOPPER 349 



the descent of the migratory grasshopper is a " visitation " that 

 can only be controlled at a distance and over a wider territory. 



Migratory and non-migratory grasshoppers appear periodically 

 in large numbers because of the fluctuations in the abundance of 

 their enemies and in the environmental conditions which arc 

 favorable to their development. In a " plague " year it usuall / 

 happens that the enemies multiplj^ to a corresponding degree, 

 since there are many grasshoppers upon which they may feed. 

 As a result, the number of enemies is larger the next season with 

 corresponding decrease in the number of " hoppers " that can 

 reach maturity. Such a decrease in food supply in turn acts as a 



Flu. 172. — Corn ruined by locusts. 



(After Dean, from Bulletin No. 215, Kansas State Agr. Col.) 



check upon the numbers of the enemies until the number of grass- 

 hoppers again increases, as it is likely to do over a period of years 

 or in any season that presents unusually favorable conditions. 

 There are probably enough eggs laid each year to produce a plague 

 if all came to maturity, but this can only occur under exceptional 

 conditions. Thus, cold, wet weather in spring destroys countless 

 young grasshoppers, while hot and dry weather allows almost every 

 egg to hatch and almost every young grasshopper to develop. 

 Hence, it usually takes two or three favorable years for the " war- 

 riors " to reach destructive numbers, and there is little danger of 

 their appearing again in the year following a serious outbreak. 



The life history and migratory habits of the warrior grasshopper 

 are representative of other migrator}^ types. In Utah, for exam- 



