Lilac borer (also known as ash borer), Podosesia syringae (Harris), is classified as a clear-wing moth but adult females mimic the appearance of wasps. Borer larvae cause damage to lilac (Syringa spp.), ash (Fraxinus spp.), and privet (Ligustrum spp.) by feeding on the phloem and outer sapwood of infested trees. The larvae are quite large, growing up to 2.5 cm in length, and their galleries can cause extensive damage even when only a few larvae are present. This insect pest primarily colonizes the main trunk but will also attack larger branches. The insect is widely distributed across the United States.
Lilac (Syringa spp.), ash (Fraxinus spp.), mountain ash (Sorbus) and privet (Ligustrum spp.) are the preferred hosts but other members of the olive family (Oleaceae) may also serve as occasional hosts.
Symptoms and Life Cycle
The lilac borer overwinters as mature larvae in the heartwood of the host. Adult emergence from overwintering tunnels peaks in late-May to early-June in Massachusetts but can occur over a large window of time depending on local climate. Adults mate within two weeks of emergence and females lay tan, elliptical eggs singly or in clusters on cracks or wounds in host bark. Each female lays about 400 eggs and immature larvae hatch within 14 days and bore into the tree. Larvae are white with amber-colored head capsules. Larvae tunnel through the sapwood as they feed, leaving behind tunnels that can be over 7 cm in length and up to 1 cm wide. Symptoms of the pest include irregularly-shaped entrance holes, often in cracks or crevices. Larvae keep their tunnels clean, so frass is often visible building up out of the entrance hole. Circular exit wounds are also visible above the entrance holes. Trees can often heal these wounds with callus tissue, leaving a scar as evidence of attack. Damage to trees occurs when the base of infested branches or the main trunk becomes swollen and the bark cracks and breaks away from the wood. Adult moths are about 2.5 cm in length with a wingspan of nearly 4 cm. They are wasp-like with long hind legs that are yellow and black banded and long antennae, but have amber-colored abdomens that discriminates them from true wasps.
Keeping susceptible trees stress-free is critical because once the lilac borer is established, it is particularly difficult to eradicate. Pruning older limbs near the base of potential hosts may help prevent lilac borer attack, since these are preferred sites for egg laying. Yet, avoid any pruning during the period of peak moth flight. The flight of adults can be monitored using pheromone baited traps which attract the male moths, or flight can be predicted using growing degree days (GDD). Once ~500 GDD have been accumulated since January 1 (at base 50°F), 10% of overwintered males should have emerged and 90% emergence occurs at ~1370 GDD. Once adult moths are trapped or indicators suggest the flight may have begun, sprays should be initiated and should continue at the labeled interval until flight ends, usually by the end of July or when the appropriate number of GDD have accumulated. The timing of sprays is important because once larvae enter the tree they will be protected. Effective insecticides include the permethrins (group 3A), bifenthrin (group 3A) and endosulfan (restricted use; group 2A). Note that, while it does control other trunk borers, imidacloprid does not have any effect on lilac borer.
Written by: Susan Scheufele and Nicholas Brazee