212 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



Figure 102. — The California tortoise-shell butterfly (Nymphalis cdlifomica) : 

 A, Adults; B, caterpillar; and C, pendulous chrysalids. About natural size. 

 (Drawings by Edmonston.) 



are introduced and become established where food and climatic 

 conditions are favorable. So far, no introduced forest insects of 

 major importance have found their way into the forests of the 

 West. The destructive species were already here and widely 

 established when the forests were first examined. Given favorable 

 conditions for their increase, they can suddenly build up their 

 numbers from the few parents which are normally present and 

 develop epidemics. 



In the insect world a constant struggle is going on for survival. 

 On the one hand, the insects themselves are provided with po- 

 tentialities for tremendous increase. Some females lay hundreds 

 of eggs and some species produce many generations a year, so if 

 all individuals survived the world would soon be overrun with 

 the progeny. On the other hand, the insects must contend with 

 many adverse conditions that serve to hold them in check. 

 Weather conditions, food supply, prevalence of natural enemies, 

 and many other environmental factors influence their abundance. 

 Some of the more important of these factors will be considered. 



CLIMATIC AND ENVIRONMENTAL INFLUENCES 



Climatic factors, such as temperature, moisture, and weather 

 conditions in general, have an important bearing on the abun- 

 dance, activity, and distribution of insects. 



