At this time of year in Illinois, plants such as ash, lilac, and privet are susceptible to attack by the lilac borer, also known as the ash borer. Lilac borer adults are brown, slender clearwing moths that resemble paper wasps. The peak moth flight period occurs in early June. Females lay tan, oval eggs in cracks, crevices, or wounds at the base of plant stems. The females can live up to 1 week and lay 300 to 400 eggs.
Eggs hatch into cream-colored larvae with brown heads. The larvae cause plant injury by creating tunnels and feeding within the bark. They bore into the wood and feed within the sapwood and heartwood. Their feeding restricts the flow of water and nutrients, causing shoots to die. The lilac borer generally feeds near the base of plant canes. The larval feeding creates swollen areas or cracks at the base of plants. Evidence of larval feeding is the presence of light-colored sawdust below infected areas. Lilac borer overwinters as late-instar larvae in the tunnels of stems. There is only one generation per year.
Prevent plant stress by implementing proper cul-tural practices such as irrigation, fertility, and mulching, as stressed plants are very susceptible to lilac borer infestations. A 2- to 3-foot-wide mulched area around the base of trees and shrubs will prevent plant injury from lawn mowers and weedwhackers. In addition, avoid pruning plants in late spring and early summer when moths are present.
The insecticide chlorpyrifos (Dursban) can be ap-plied to control lilac borer larvae before they enter the plant. Pheromone traps are available that capture adult males, which indicates that females will eventually be laying eggs. This can help in timing insecticide appli-cations. Another possible management option is the use of beneficial nematodes. Nematodes are applied as a heavy spray to the larval entry points. The nematodes will attack the larvae feeding within the tunnels.