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



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 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. 



Mining insects 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 they 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 suffi- 

 cient 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 bor- 

 ing dust in the crevices of the bark or pitch exuding from small 

 holes in the bark. These may or may not indicate bark-mining in- 

 sects. 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 threat- 

 ening 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 spe- 

 cies capable of attacking and killing living trees need cause any 

 concern, and fortunately the number of such species is very lim- 

 ited. 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 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, small 

 pitch tubes or both. 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 but the diverging mines are made by small, white, curled, 



legless larvae Bark beetles, p. 129 



a. Egg galleries under bark mostly packed with boring dust 

 and excrement; individual egg galleries (made by each pair 

 of beetles), single, long (more than 6 inches), longitudinal, 

 straight, crisscross, or winding, rarely transverse 



Dendroctonus, p. 131 



