4 MISC. PUBLICATION 278, U. S. DEPT. OF AGRICULTURE 
on fallen limbs and other debris upon the forest floor, and thus hasten 
the disintegration of dead material and make room ‘for new growth, 
Many others prey upon destructive insects and hold them in check. 
A certain proportion of the insect species, however, are distinctly 
harmful in that they attack healthy or partially weakened trees and 
impair their vitality or even cause their death. Of this group, bark 
beetles destroy more standing timber in our western forests than all 
other insects combined. Defoliators—insects which feed directly on 
the foliage of forest trees—are the next greatest destroyers of stand- 
ing timber. Other insects, such as weevils, tip moths, pitch moths, 
and cone beetles, attack various portions of the green trees, often 
with serious results, In spite of the large number of insect species 
which prey upon the forests, comparatively few cause damage of eco- 
nomic importance. 
The injurious species of insects may be roughly classed as primary 
or secondary, depending on the health of the trees which they nor- 
mally attack. Thus certain species, such as the leaf-chewing insects, 
show a decided preference for perfectly healthy trees and are consid- 
ered primary in their attack. Others, such as most bark and wood- 
boring insects, can inhabit only those trees previously weakened by 
some other agency. 
From the standpoint of control it is important to know whetner 
an insect species is primary or secondary in its attack, as it is wasted 
effort to proceed against an insect, even 1f found apparently destruc- 
tive, if its presence is conditioned by previous injury or death of the 
tree from other causes. It is the primary injury that must be discov- 
ered and dealt with. There are, however, a number of species that 
are primary under certain conditions and secondary under others. 
Every species of western forest tree has its insect enemies. West- 
erm 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 on rare occasions enter its heartwood 
through wounds, especially if the wood is beginning to decay, but no 
serious enemy is known. The cedars, cypresses, redwood, and juni- 
pers have very few injurious insect enemies and none that threaten 
the life of mature trees. Larch also is comparatively free from insect 
pests. The broad-leaved trees are the favored hosts of many leaf- 
feeding species, but since these trees can readily replace their de- 
pleted foliage such feeding rarely results in any fatal injury. Some 
species, howe ever, are much more subject to insect attack than others. 
Certain oaks are reported to be hosts for more than 1,000 species of 
insects. Pines, spruces, firs, and hemlocks suffer much, in the order 
named. 
Injurious forest insects are constantly at work, taking toll at every 
stage in the development of the stand, and even after the lumber has 
been manufactured into its final form. Some insects feed on the 
roots, others on the leaves, the terminal shoots, the branches, or the 
phloem and bark of the main trunk, Still others feed on the sap- 
wood, and even the heartwood. The fruits and the seeds also are 
subject to attack by many insect species. 
In certain types of old-growth timber stands, particularly those 
that are overmature, steady loss through insect activity is normal. 
