Bronze birch borer

Bronze birch borer

Name: Agrilus anxius

Alias: Bronze birch borer. As the name suggests, the adult version of these beetles are a shiny bronze color with a blunt head used for boring through wood. The creamy white larvae are a long and soft worm-like grub that feed on the cambium layer between the bark and the trunk. Larvae overwinter in the bark, and resume activity in the early spring as temperatures rise. Adults emerge in late spring, and lay their eggs on the bark surface. Hatched eggs bore immediately into the bark.

Crimes: The larvae feed on the most important part of the tree under the bark, which stresses the tree. It is not uncommon for branches to become girdled. Adults emerge in late spring, leaving a distinct D shaped hole. Through the winter and early spring woodpeckers are looking for the newly-active larvae and boring holes into the trees. Both borers and woodpeckers cause significant physical damage that can stress the tree.

Redeeming qualities: None.

Sentence: Birch trees naturally grow in shady, moist environments such as the draw between ridges on a mountain. When they are in dryer environments or they are drought stressed they are usually more susceptible to borer infestation. A borer cannot survive in a healthy tree, so usually borers are a symptom of a larger problem. Permethrin or carbaryl based products can be effective if they contact the insect, so the best time to apply is in late spring when adults have emerged or use a systemic insecticide.

For more information on dangerous and beneficial bugs, call UI Extension educator Joseph Sagers at 208-745-6685 or email