58 MISC. PUBLICATION 273, U. 8S. DEPT. OF AGRICULTURE 
inner bark usually direct their attack against trees previously 
weakened through drought, defoliation, fire, or some other cause. 
Most of the bark beetles are decidedly secondary and only attack 
trees already in a weakened or dying condition. A few species, how- 
ever, are ageressive and primary in their attacks. 
It is true that the destruction of older, mature trees in the forest 
through attack by insects is more or less of a natural process. It is 
nature’s way of disposing of the old decadent trees to make room for 
the younger, thriftier, growing individuals. Although a natural one, 
this 1s a most wasteful process from the economic standpoint, since 
the old trees carry a large volume of high-grade lumber. The pre- 
vention of this type of damage, therefore, is an important phase of 
forest protection. 
When more of our forests come under intensive management, and 
mature trees are utilized before they become decadent, much of the 
present loss in virgin. forests will be avoided. Until such a time, the 
only alternative is the application of the direct-control methods dis- 
cussed at the end of this section. 
KEY TO RECOGNITION OF INSECT INJURY TO MATURE TREES 
A. Foliage fed upon; partially or wholly stripped from trees; or turning 
yellow or red. Trees sickly or dying. No insects working on 
main <trunk: ‘branches: OF TOOUS= ses ane es ae defoliators, page 58. 
B. Terminal shoots, laterals, or tips deformed or killed. Remainder of 
treecappearing healthy = 25a" 2s ae Oreo Dees ee ee twig feeders, page 29. 
C. Entire tree, or a large part, sickly, dying, or dead; foliage fading, 
turning yellow or red. Bark and phloem of main trunk or roots 
mined-by:insects and *killeg==]28 2 we eae bark miners, page 95. 
LEAF FEEDERS AND DEFOLIATORS 
No part of a forest tree offers nourishment to such a host of insects 
- as do the leaves. There are literally thousands of insect species that 
feed on them in one way or another. Some mine within the needles, 
some skeletonize the leaves, and others eat the entire leaf tissues or 
suck the juices. Trees can withstand a great deal of such feeding 
without being seriously affected, and some such insect work is going 
on more or less constantly. If the feeding is heavy, the growth of the 
tree is retarded. If a high percentage of the leaf surface is de- 
stroyed, death of the tree may result. The damage done to the forest 
by defoliators is difficult to estimate since a large part of it involves 
only a loss of increment and not the death of trees. On the other 
hand, when epidemics of defoliators occur, their ability to destroy 
timber, especially coniferous timber, over large areas in a short time 
places them at the top of the list of destructive forest insects. 
Defoliation. affects very vital functions in a tree’s life processes. 
Without leaves a tree is unable to regulate its moisture content, to 
acquire its carbon supply from the air, or to throw off its waste gases. 
When these important functions of transpiration, assimilation, and 
respiration are greatly retarded the tree dies. Death does not take 
place at once but only after a lapse of time in which certain peculiar 
changes take place. Craighead (25) has shown that spruce bud- 
worm defoliation of fir and spruce not only results in a general 
reduction of growth but that this reduction is not evenly distributed 
(82). Growth is greatly retarded at the top of these trees, as shown 
