Landscapers and nursery personnel should watch for the lilac borer (Podosesia syringae), also called the ash borer. Traps at The Morton Arboretum caught many males in late May. The lilac borer is a clearwing moth that resembles a wasp. Borers overwinter as late instar larvae and emerge as adults from pupal cases in spring to mate and lay eggs. Eggs hatch in late May or early June, depending on their location in Illinois and the earliness of the spring. Larvae burrow into plants and feed on phloem tissue. Unlike beetle borers (such as bronze birch borer), clearwing moth larvae leave an entrance hole to the outside where sap and sawdust collect.
Like most boring insects, lilac borers take advantage of plants that are environmentally stressed. Newly transplanted materials are extremely susceptible to this moth. Lilac borers injure lilac, ash, privet, and other members of the olive family. In addition to cultural methods such as watering and mulching to increase plant vitality, well-timed insecticidal treatments can control lilac borer.
Pheromone traps contain an analogue of the sex pheromone produced by the female moth and catch males searching for unmated females. Traps do not cure the problemóbecause some males avoid traps and succeed in inseminating females--but are useful for an accurate estimate of infestation levels and optimal time for treatment. Current recommendations are to treat branches and trunks with chlorpyrifos (Dursban) one week after peak trap catch.