It is time to be on the lookout for the lilac borer, Podosesia syringae, also referred to as the ash borer. Plants susceptible to attack from the lilac borer include ash, lilac, and privet. Pheromone traps are available that capture adult males, which indicates that females will be laying eggs within a short time. This can be helpful in timing insecticide applications, which should be conducted 7 to 14 days after males have been captured in the pheromone traps.
Adults are brown, slender clearwing moths that actually resemble paper wasps. The peak moth flight typically occurs from late May through early June. Females lay oval, tan eggs in cracks, or wounds (caused by mowers, weed-whackers, storm damage, or pruning), at the base of plant stems. A female can live for about a week and lay up to 400 eggs.
Eggs hatch into cream-colored larvae that are 1.0 to 1.5 in. long when full-grown, with brown heads. Larvae, like most wood-boring insects, cause plant injury by creating tunnels and feeding within the bark. Lilac borer larvae tunnel and feed initially in the bark region (cambium). They eventually bore further into the wood and feed within the sapwood and heartwood. Feeding by the larvae restricts the flow of water and nutrients, causing shoot dieback. Lilac borer generally feeds near the base of plant canes—especially those that are wounded. 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 residing in the tunnels of stems.
Lilac borer partially tunnels out through the bark before pupating. The moth emerging from the pupa is not able to chew, so it simply pushes out the thin layer of remaining bark. When the moth emerges, the brown shell of the pupa usually remains and protrudes from the hole. Sometimes this is barely noticeable, but commonly the pupal case sticks out about 1/2 inch. Male moths emerge first, females several days later. Adult moths are 1.0 in. long and appear wasplike, with a brown-colored body. They are very active fliers. In Illinois, there is generally one generation per year.
As with most wood-boring insects, the primary way to alleviate problems is the implementation of proper cultural practices such as watering, fertilizing, mulching, and pruning—as stressed plants are extremely susceptible to lilac borer. For example, a 2-to-3-foot-wide mulched area around the base of trees and shrubs prevents plant injury from lawn mowers and weed-whackers. On lilac, borers attack larger, older stems, so pruning out large trunks and leaving young suckers prevents borer problems. Avoid pruning lilacs or ash 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. As mentioned above, the use of pheromone traps, which contain a lure that attracts males, are very helpful in timing applications of permethrin (Astro). Once male numbers peak in the pheromone traps, it is prudent to make applications 7 to 14 days later to make sure there is an insecticide barrier on plants when the eggs hatch. Pheromone lures for lilac borer can be purchased from Great Lakes IPM, Inc., (Vestaburg, MI; email: firstname.lastname@example.org).