5g MISC. PUBLICATION 273, U. S. 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 aggressive 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 is 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 sicklj'- or dying. No insects working on 

 main trunk, branches, or roots defoliators, page 58. 



B. Terminal shoots, laterals, or tips deformed or killed. Remainder of 



tree appearing healthy 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 killed bark miners, page 05. 



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 Avithin 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 leaved a tree is unable to regulate its moisture content, to 

 acquire its carbon supply from the air, or to throw off its waste gases. 

 AAHien 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 {^S) 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 



