This time of year, plants such as lilac, ash, and privet are susceptible to attack by the lilac borer, also known as the ash borer. Male moths have been present in pheromone traps at The Morton Arboretum for over a week, so we should be approaching peak emergence in northern Illinois.

Lilac borer adults are brown, slender, clearwing moths that resemble paper wasps. Peak moth flight occurs in early June. Females lay tan, oval eggs in cracks and crevices or wounds at the base of plant stems. Females live for approximately one week and can lay 300 to 400 eggs. Eggs hatch into cream-colored larvae with brown heads.

Larvae cause plant injury by creating tunnels and feeding within the bark. They also bore deeper into the wood to feed within the sapwood and heartwood. Their feeding restricts the flow of water and nutrients, causing shoots to die. The lilac borer generally feeds at the base of plant canes and at branch crotches, creating swollen areas or cracks in these areas. Larval feeding is evident by the light-colored sawdust below infected areas. Lilac borer overwinters as late instar larvae in the tunnels of stems. There is one generation per year.

Stressed plants are very susceptible to lilac borer infestations. Maintain proper cultural practices such as watering and fertilization. Avoid pruning plants in late spring and early summer when moths are present. A 2- to 3-foot-wide mulched area around the base of trees and shrubs will prevent plant injury from lawn mowers and weed whackers.

Chlorpyrifos (Dursban) can be applied to control lilac borer larvae before they enter the plant. Pheromone traps that capture adult males are available. Spray about 2 weeks after male moth catch in pheromone traps peaks. This should be close to when eggs are hatching. Another management option is the use of beneficial nematodes. Nematodes are applied as a heavy spray to the larval entry points. They will attack the larvae feeding within the tunnels.