The Locust Borer (Megacyllene robiniae) is so-called because its host tree is Black Locust (Robinia pseudoacacia). Locust Borer larvae tunnel into a tree’s trunk and branches, weakening the tree and making it susceptible to wind breakage. In addition, these tunnels serve as a primary infection site for the wind-borne spores of the fungus Phellinus robiniae, which causes a damaging heart rot disease in Robinia species. If you see a Black Locust with many dead and broken limbs, and/or knotty swellings on the trunk, chances are great that it has been attacked by Locust Borers.
The conspicuous, brightly-colored adults appear when goldenrod is in bloom. Adults are commonly seen feeding on goldenrod pollen. Mating takes place now, in the fall, and eggs are laid in cracks, wounds and under loose bark of a Black Locust tree. The eggs hatch in about a week, the larvae bore into the inner bark of the tree and each larva makes a small hibernation cell and overwinters there. In the spring the larvae begin to bore into the woody part of the tree, enlarging their tunnels as they grow. By mid-summer they pupate and adult beetles emerge in late summer. Previously confined to the native range of Black Locust in the Northeast, Locust Borers have spread with the trees throughout the U.S. and parts of Canada.