Lilac Borer

Problem type: Insect

Name of problem: Lilac Borer

Plant name(s): Lilac, ash, privet

Symptoms / Characteristics:
stems and branches. The base of infested stems may appear swollen, and the presence of sawdust can be seen at holes made in the stem or on the ground beneath the holes. Larvae make two characteristic holes in the trunk of its host. An irregularly shaped entrance hole is made lower on the trunk where feeding commences, followed by a round exit hole about 5 mm in diameter found higher on the trunk. Scars and calluses may be present around entrance and exit holes from a previous attack. Trees and shrubs can be weakened by borer infestations and sometimes killed.

Adult borers are clear-winged moths, which resemble wasps. Adults emerge from galleries in the trunk in June and females deposit eggs in bark crevices and ridges. Eggs hatch about two weeks later and the larvae chew into the bark to feed. Partially grown larvae overwinter inside their tunnels and then complete development the following spring.

Control / Preventions:
Smaller stems or branches containing infestations should be pruned off and destroyed in the fall as larvae spend the winter in these limbs. In larger stems, a flexible wire can be pushed into holes to remove or kill tunneling larvae.

Early in the season when boring holes are noticed, an insecticide can be sprayed into excavation holes and then plugged to retain fumes, killing the larvae inside.

Pheromone traps can be used to monitor adult moth presence. Insecticides can be applied 10 days following the capture of the first moth, to kill eggs. Without the use of traps, 3 spray applications can be applied to the lower trunk at 14-day intervals starting in June.