. Circular. Insects. 3 destructive to some trees in the same grove than it is to others. It is more destructive, also, to young saplings and the branches of medium- sized trees than to the larger trees. The death of a tree is caused principally by injuries to the inner bark and cambium resulting from repeated attacks. Injuries to the wood alone do not result in the death of trees, except when all of the wood is practically destroyed or sufficiently injured to cause the tree to fall or be broken down by the wind. The commercial value of the wood product is diminished or destroyed by the worm-ho
. Circular. Insects. 3 destructive to some trees in the same grove than it is to others. It is more destructive, also, to young saplings and the branches of medium- sized trees than to the larger trees. The death of a tree is caused principally by injuries to the inner bark and cambium resulting from repeated attacks. Injuries to the wood alone do not result in the death of trees, except when all of the wood is practically destroyed or sufficiently injured to cause the tree to fall or be broken down by the wind. The commercial value of the wood product is diminished or destroyed by the worm-hole defects, but for certain purposes, as, for instance, fence posts, a limited number of such defects are not detrimental, except so far as they may contribute to decay. EVIDENCES OF ATTACK. The first evidence of attack is fine brownish boring dust and wet spots on the bark, first observed in April, when the overwintered larvae begin to enter the inner bark. As soon as the larvae begin to groove the surface of the wood and enter the sapwood, their presence, in addition to the wet spots, is indicated by yellowish bor- ing dust mixed with liquids and the gum-like exudations. After all of the larvae have entered the wood their presence is plainly indicated by the quantities of yellowish boring dust lodged in the loose bark on the trunk, in the forks of the tree or branches, and around the base. At this stage, usually about the middle of May, the badly infested trees which will die are plainly indicated by the failure of the leaf buds to open or by the dwarfed or faded and sickly appearance of the foliage and, toward the last of the month until the larvae have completed their work in July, by the breaking down of the branches and small trees. FAVORABLE AND UNFAVORABLE CONDITIONS FOR DESTRUCTIVE WORK. Favorable conditions for the destructive work of the borer appear to consist in the presence of isolated trees and groves in the open, in local- ities where golden-rod is present or ab
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