This time of year in Illinois, plants such as ash, lilac, and privet are susceptible to attack from lilac borer (Podosesia syringae), also known as ash borer. Adults are brown, slender clearwing moths that resemble paper wasps. Peak moth flight occurs in late May to early June. Females lay oval, tan eggs in cracks, crevices, or wounds at the base of plant stems. A female lives about a week, laying 300 to 400 eggs.
Eggs hatch into cream-colored larvae, about 1-1/2 inches long when full-grown, with brown heads. Larvae cause plant injury by creating tunnels and feeding within the bark. They tunnel into the wood and feed within the sapwood and heartwood. Feeding restricts the flow of water and nutrients, causing shoots to die. This 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.
Lilac borer partially tunnels out through the bark before pupating. The moth emerging from the pupa is unable to chew, so it simply pushes out the thin layer of bark remaining. When the moth emerges, the brown shell of the pupa is usually left behind and protrudes from the hole. Sometimes this barely shows, but commonly the pupal case sticks out about 1/2 inch. Male moths emerge first; females emerge several days later. The moths are 1 inch long, with a brown-colored body. They are very active fliers. There is one generation per year in Illinois.
Prevent plant stress by proper cultural practices such as watering, fertilizing, and mulching; stressed plants are very susceptible to lilac borer. A 2-to-3-foot-wide mulched area around the base of trees and shrubs prevents plant injury from lawn mowers and weed-trimmers. In addition, avoid pruning plants in late spring and early summer when moths are present.
The insecticide permethrin (Astro) can be applied 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 applications. Another option is to use beneficial nematodes. Nematodes are applied as a heavy spray to the larval entry points; nematodes attack the larvae feeding within the tunnels.