INSEC^T ENEMIES OF WESTEKN FORESTS 



57 



looper outbreaks, where heav_y defoliations ceased when stands of 

 young growth were reached. 



Since the damage done by defoliators to mature forest trees is 

 usually of greater importance than that done to young trees, this 

 group of insects will be discussed in the following section (p. 58). 



INSECTS INJURIOUS TO MATURE FOREST TREES 



The insects that prey upon young forest trees and that may, during 

 the formative years, cause serious injury througli stunting, deform- 

 ing, or halting growth are of little importance after the trees have 



Figure 26. — Ponderosa pines severely defoliated by the pine butterfly. 



reached maturity. The mature trees may still be fed upon by root- 

 feeding or terminal-feeding insects, but such damage within reason- 

 able limits can be borne without fatal consequences, and the small 

 loss in growth increment on mature trees is of minor importance. 

 In general, there are only two large groups of insects that may bring 

 about the death of mature trees, (1) those that feed on the leaves and 

 cause severe defoliation and (2) those that bore into the inner bark of 

 the main trunk and cut off the supply of moisture and food. 



In many w^ays the defoliators are the most dangerous. They are 

 primary and aggressive in their activities and attack healthy, vigor- 

 ous trees as quickly as undernourished, weakened ones (fig. 26). The 

 injury they cause does not always result in the immediate death of 

 the tree but often so greatly weakens it that it becomes readily sus- 

 ceptible to bark-beetle attack. On the other hand the miners of the 



