Lilac borers are brown-headed creamy white caterpillars about 3/4 inch long. They overwinter in their tunnels in lilac stems. Adults are clear-winged moths that emerge from the stems in spring and lay their eggs on roughened or wounded places on the bark. Rough knot-like swellings on the trunk and older limbs indicate lilac borers. Leopard moth borers also attack lilacs. These caterpillars are pale yellowish or cream-colored, with numerous black spots.
Borer infested lilac branches often wilt, and are frequently so weakened by the boring that they break. Sawdust hangs at the openings made by the borers. These opening also open the way for wood-destroying fungi and other diseases to attack shrub tissues. Roughened scars showing the old borer holes may occur on larger stems at places where the borers have worked for several seasons.