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Bronze Birch Borer
This insect attacks paper birch, yellow birch, gray birch, poplar, cottonwood and willow. The European white birch is the tree most commonly damaged in Wisconsin.
The adult is a slender, iridescent olive-bronze to black beetle. It is normally less than ½-inch in length. The larvae is a flat-headed borer that is cream color and about 3/4 to 1 inch long. Adult beetles emerge from holes in the branches and trunks during June and July and lay their eggs in cracks in the bark. The adults feed on the leaves of birch, aspen, poplar and willow trees but do not cause significant damage. The larvae make crooked, crisscrossing galleries in the inner bark. The galleries are packed with dark brown sawdust-like excrement. Frequently callous tissue will form around the winding larval galleries and cause ridges to appear, like varicose veins, on the bark. When the larvae are full-grown, they are about 1 inch long, ribbon-like, with a flattened, enlarged head area. In the fall, they bore into the sapwood and excavate overwintering cells. In the spring, the larvae pupate and later the adults chew D-shaped holes through the bark and emerge.
Tunneling by the larvae often girdles branches, cutting off the flow of sap. This causes the tips of the branches to die back to the point of girdling. Infested trees die from the top downward. Girdled branches retain dead leaves after they die.
Heavily infested trees should be cut down and destroyed. To reduce infestations that only affect a branch or two, prune below the visible infestation and destroy branches showing symptoms of borer attack by early May. This should destroy the insects before they emerge to lay more eggs.