INSECT ENEMIES OF WESTERN FORESTS 167 



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

 peratures either above or beloAv this optimum range limit activity 

 and extremes cause death (^^, 60). Few insects can withstand tem- 

 peratures 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 (4«) (S6a). 

 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 upon the insects and indirectly through its 

 influence upon the host. Some insects require very moist conditions 

 under which to work to best advantage and are killed by dryness; 

 others require very dry conditions and are killed by moisture. More- 

 over, moisture through precipitation has an important 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. 



FOOD SUPPLY 



The abundance or scarcity of the food supply is an important 

 consideration in determining the distribution and successful develop- 

 ment of insect progeny. Most of the insects which prey upon living 

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

 host, whereas those which feed upon 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 a sufficient quantity 

 of their food material in a suitable condition for attack is available. 

 Thus the development of certain destructive bark-beetle outbreaks 

 is dependent to a large degree upon a supply of overmature or deca- 

 dent trees, fire-weakened trees, slash, windfalls (^i), snow-broken 

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

 smoke, disease, or other causes. Under favorable conditions, 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 resistance 

 to beetle attack is lessened in other ways during periods of drought. 

 Such conditions present a highty favorable field for bark-beetle 

 activity, and the heavy losses that have been sustained in the pine 

 regions during the past few years indicate very clearly that the bark 

 beetles have not neglected this opportunity. 



