INSECT ENEMIES OF WESTERN FORESTS 57 
looper outbreaks, where heavy defoliations ceased when stands of 
young growth were reached. 
Since the damage done by defoliators to mature forest trees is 
usually of oreater 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 through 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 ways 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 
