Lilac borer, also known as ash borer, moths are flying throughout the state. Moths have been seen in northern Illinois, and empty pupal cases have been seen sticking out of borer holes in ash in central Illinois. One useful method to scout for this borer is to check infested trees once or twice a week at this time of year for protruding pupal cases.
Lilac/ash borer larvae tunnel out to and partially through the bark before pupating. The moth that emerges from the pupa is unable to chew so it just pushes out the thin layer of bark remaining. As the moth emerges, the brown shell of the pupa is usually left behind and protrudes from the hole. Sometimes this barely shows but commonly it sticks out about 1/2 inch.
Male moths emerge first and then females several days later. Eggs laid by the females will take several days to hatch. If a pheromone trap is used, we suggest treatment 2 weeks after peak moth catch in the pheromone trap. Because the pheromone trap catches only males and males emerge before females, we assume that females emerge when males are most numerous. Allowing time for mating, egg laying, and egg hatch, we thus suggest treatment about 2 weeks later. An estimation of treatment time based on first moth catch or observation of the first pupal cases sticking out of trees would be 3 weeks after the first moth or pupal case observation.
Putting it all together, insecticide applications should be applied now in southern Illinois and near the end of the month in central and northern Illinois.