b MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



Certain species, such as the leaf-chewing insects, show a decided 

 preference for perfectly healthy trees and are considered primary 

 in their attack. Others, such as most bark and wood-boring insects, 

 can inhabit only trees previously weakened by some other agent. 



In insect-control (fig. 1) work it is important to know whether 

 a species is primary or secondary in its attack, as it is wasted 

 effort to proceed against an insect if its presence is conditioned by 

 previous injury or death of the tree from other causes. It is the 



Figure 1. — Insect control is one of a forest ranger's many routine tasks. 

 Marking an infested tree. 



primary injury that must be discovered and dealt with. However, 

 a number of species are primary under certain conditions and 

 secondary under others. 



Every kind of western forest tree has its insect enemies. West- 

 ern yew is probably as nearly free from insect attack as any other 

 forest tree in the West. An occasional scale or caterpillar may be 

 found on its foliage and beetles occasionally enter its heartwood 

 through wounds, especially if the wood is beginning to decay, but 

 no serious enemy is known. The cedars, cypresses, redwood, and 



