The larvae of the bronze birch borer cause extensive damage across the birch range areas of the United States and Canada. River birch trees are not affected. The larvae are inch-long and flattened. The adults are deep olive color to bronze, which has given them their common name.
Symptoms: The characteristic D-shaped holes on the bark of affected trees are the usual way to spot an infestation. After mating, females deposit their eggs in the crevices of the tree bark. In approximately two weeks, the larvae hatch and move under the bark and cause severe damage by their feeding which interferes with sap movement in the tree.
More information: University of New Hampshire Cooperative Extension and University of Wisconsin – Madison Extension