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-iourth 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 sap wood 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 

 Avhite 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 w^eakened, 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 is 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. 



