INSECT ENEMIES OF WESTERN FORESTS 213 



TEMPERATURE 



Insects as a rule have a very small range of temperatures (50°- 

 95° F.) within which they are the most active; and the optimum 

 for many of the Temperate Zone species appears to be about 77°. 

 Temperatures either above or below this optimum range limit 

 activity and extremes cause death (119, 120). Few insects can 

 withstand temperatures above 120° and this makes possible the 

 control of many species of bark- and wood-boring insects by 

 raising the temperature of their environment to fatal heights. 

 Low temperatures also are fatal. It has been found that larvae of 

 the western pine beetle are killed when exposed to a temperature 

 of -10° and extremely cold winters with air temperatures below 

 -20° have proved fatal to a high percentage of this and other 

 bark-beetle broods (8, 9U, 162). Moreover, elevation and exposure 

 modify temperature and limit the distribution of insect species, 

 regulate their activity, and govern the number of generations 

 per year. 



MOISTURE 



Moisture has an important bearing on insect abundance, both 

 through its direct effect on the insects and indirectly through its 

 influence upon the host. Some insects require very moist condi- 

 tions under which to work to best advantage and are killed by 

 dryness; others require very dry conditions and are killed by 

 moisture. Moreover, moisture through precipitation has an im- 

 portant influence upon the growth and resistance of trees to 

 bark-beetle attack ; and periods of drought are frequently followed 

 by serious consequences in supplying insects with an abundance 

 of weakened host material {11, 92). 



FOOD SUPPLY 



The abundance or scarcity of the food supply is an important 

 consideration in determining the distribution and successful 

 development of insect progeny. Most of the insects that prey upon 

 living forest trees are limited in their distribution to that of their 

 favorite host, whereas those that feed on dying or dead trees 

 are apt to be widely distributed throughout forest regions. 



Insects, such as the defoliators, that attack healthy forest trees 

 always have an abundant food supply at their disposal, and their 

 numbers are controlled primarily by biological and climatic 

 factors. On the other hand, a great many insects, such as most of 

 the bark beetles, can develop in large numbers only when enough 

 of their food material in a suitable condition for attack is avail- 

 able. Thus the development of certain destructive bark-beetle 

 outbreaks is dependent largely on a supply of overmature or 

 decadent trees, fire-weakened trees, slash, windfalls (118), snow- 

 broken or lightning-struck trees, or trees weakened by drought, 

 smelter smoke, disease, or other causes. Under favorable condi- 

 tions, however, a few species of bark beetles can develop in 

 epidemic numbers without such contributing factors. 



In the virgin pine forests of the West, a high percentage of the 

 trees are mature or decadent. They grow more slowly, and 



