INSECT ENEMIES OF WESTERN FORESTS 95 
Northwest as the cottonwood leaf beetle, which it closely resembles 
in appearance and habits. The adults are reddish, one-fourth inch 
in length, and spotted with black. 
MINERS IN THE INNER BARK AND PHLOEM — 
Many different species and families of insects are represented 
among those that select the cambium region of the main trunk of 
trees as a suitable place to feed. All of these are chewing insects 
that bore under the bark and feed in the soft layers of bark and 
wood. As feeding progresses the channels may penetrate deeply 
into the sapwood or be extended into the outer bark. 
The insects of this habit that are capable of attacking living, 
healthy trees are among the most destructive species with which the 
forester must deal. By far the greatest number of the cambium 
feeders, however, are capable of attacking only unhealthy, weakened, 
dying, or felled trees and cannot resist the copious flow of sap or 
resin which in the normal tree serves as a defense against attacking 
bark borers. At times, when a tree’s resistance is low, even these 
normally secondary species may kill trees, if they attack in sufficient 
numbers. 
It is easy to recognize the work of bark-feeding insects. Usually 
a close inspection of the trunk of an infested tree will reveal boring 
dust in the crevices of the bark or pitch exuding from small holes 
in the bark. These may or may not indicate bark-mining insects. 
Positive evidence of infestation can be obtained only by removing a 
small chip of bark and determining whether the phloem is fresh and 
white or discolored with the mines of some boring insects. If such 
mines are found, a larger piece of bark can be removed, and the 
species responsible for the damage usually can be identified by the 
character of its work. 
A few species of inner-bark miners, such as the pitch moths, may 
work in the phloem from the edge of wounds without threatening 
the life of the tree, and no attempt need be made to control such 
species under forest conditions. Nor is. it necessary to attempt 
any control of the vast number of inner-bark-feeding insects that 
confine their attack to weakened, sickly, or felled trees. Only those 
species that are capable of attacking and killing living trees need 
cause any concern, and fortunately the number of ‘such species 1s very 
limited. It is not difficult for the forester to learn to recognize the 
comparatively few phloem-mining insects that are aggressive killers 
of the trees in his region. Such species are discussed in detail in the 
following pages. 
KEY TO THE DIAGNOSIS OF INSECT INJURY TO THE INNER BARK 
A. Entire tree or a large part sickly, dying, or dead; foliage fading, 
turning yellow or red. Inner bark of main trunk and sometimes 
roots attacked and killed. 
1. Outside of bark showing boring dust collected in crevices, or 
small pitch tubes. Small egg tunnels under bark usually 
of uniform width, from which extend diverging tunnels 
usually packed with fine borings. The egg tunnels are 
made by small beetles while the diverging mines are made 
by small, white, curled, legless larvae______ bark beetles, page 96. 
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