Answer: Unfortunately, it sounds like your birch tree (Betula) has been attacked by the Bronze Birch Borer. Initially, this insect causes the tree to defoliate and die back from the top downward, started with a few leaves, progressing to smaller branches and ultimately to larger branches. During the larval stage, the borer chews its way into the tree and overwinters under the bark. From late spring to early summer the adults chew their way out, leaving signature D shaped holes in the trunk and large branches. Early control is to prune out and destroy infected branches before late spring when the adults emerge, and to attempt control with several timed applications of an appropriately labeled insecticide. It is important to cover not only the foliage but also the trunk and larger branches with the spray, so the job usually requires professional equipment. It is also possible to use an injectable insecticide during the summer when the larvae have reentered the tree, but this involves the use of restricted chemicals and must be done by a professional. This procedure further weakens the vascular system of the tree and is not usually recommended for a tree seriously weakened by a heavy infestation. Most often, severely damaged trees are removed. If this is done, it is a good idea to remove the wood promptly so it won't continue to serve as a source of infection for remaining birches in the neighborhood.
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