Adult lilac borer moths resemble (mimic) paper wasps and are approximately 1/2 inch in length when their wings are folded. The abdomen and front wings of the adult insect are a metallic brownish-black. Adult moths fly during the day, unlike most other moth species. Females lay eggs that are flattened, oval, tan, and approximately 0.7 mm in size. Eggs are laid singly or in clusters in bark crevices, occasionally on smooth bark, and almost always in or near wounds. Each female lives for approximately 1 week and is capable of laying up to 395 eggs. Eggs hatch and young larvae are able to chew into the bark and feed laterally and vertically in the phloem tissue (innermost layer of the bark, the vascular tissue where the nutrients made during photosynthesis are transported). Entrance holes may at first only look like sap flow with a small amount of frass mixed in. Once mature, larvae are approximately an inch in length and white with brown heads. Larvae possess crochets (small hooks) on the bottom of their abdominal prolegs. Larvae injure their hosts by tunneling and feeding eventually in the sapwood of stems, branches, and trunks. Light colored frass (sawdust-like) accumulates at entrance holes and the ground nearby. Larvae actively push debris and frass out of their tunnels. Larvae overwinter beneath the bark, in their tunnels, in the final instar. Pupation occurs in the spring, near the surface of the bark. Shed pupal skins (cases) are often left behind when the insect emerges as an adult. The lilac borer is a native North American moth with a large geographic range. One generation occurs per year throughout most of the insect's range, however in parts of Canada the life cycle may take 2 years to complete. In northern or northeastern states, peak moth flight activity may occur in June. Adult activity is typically completed by August 1. Other, closely related species may have adult activity that extends later in the season.
Older lilacs are typically more susceptible. Damage occurs to the bark of the trunk and larger branches on lilac. Dead canes in the center of a lilac may indicate lilac borer activity on that host. Heavily infested trees that are utilized by these insects during multiple seasons may experience branch dieback and eventually, mortality. Visible holes and scars are also signs of the presence of this insect. Open grown ash and street trees are particularly susceptible to lilac borer infestation. Wounds caused by the activity of this insect may predispose its hosts to certain fungal infections as well, allowing spores to enter the plant easier.
Visual observation of lilac can detect this wood boring insect. Look for dead canes or signs of cracking bark on living canes. Circular exit holes that are approximately 1/4 inch in diameter may also be a sign of lilac borer activity. Entrance holes are irregular, and often seen pushing out frass (sawdust-like excrement). Shed pupal skins can also be detected while scouting. Monitoring for adult emergence to time chemical management decisions can be done with pheromone traps. Traps can be hung at shoulder height with the appropriate lures (commercially available) in or near trees that need to be monitored. Applications can be made 7-10 days following the capture of the first adult moth. Some sources suggest a second application may be needed if moths continue to be captured in traps.
First and foremost, avoid plant stress that lilac borers find attractive. Avoid mower injury to the trunk and other wounds that attract this pest. Prune out all dead and dying lilac canes as close to the ground as possible and destroy, but avoid doing this prior to or during the period of moth flight. "Renewal" pruning can also be done outside of lilac borer moth flight season in lilac. This involves removing the older branches near the shrub base, which removes the favored locations for lilac borer activity while possibly increasing flower production in the lilac. Up to roughly 1/3 of these branches can be pruned in a single season. Supplemental watering during periods of drought can also help protect hosts from this insect.
Woodpeckers are important predators of lilac borers. Research in Mississippi indicates that woodpeckers feeding on lilac borer in a green ash stand were able to capture 67-81% of the larvae (Solomon, 1975). Braconid wasp parasitoids have also been noted for the lilac borer. Macrocentrus marginator and Apanteles spp. wasps were noted as larval parasitoids in Dix et al. 1978. They also note that natural outbreaks of Beauveria bassiana may occur in the galleries of this insect and kill the larvae.
Entomopathogenic nematodes can be applied by injecting them into lilac borer entrance holes. Those in the genus Steinernema have been noted to be at least effective against some species of clearwing moth larvae.
Abamectin (NL)
Acephate (NL)
Azadirachtin (NL)
Bifenthrin (NL)
Chlorantraniliprole (NL)
Chlorpyrifos (N)
Cyantraniliprole (NL)
Emamectin benzoate (L)
Entomopathogenic nematodes (L)
Flonicamid+cyclaniliprole (N)
Neem oil (NL)
Permethrin (L)
Spinosad (NL)
Timing chemical management options to target the newly hatched larvae before they enter the host may be most effective. Lilac borer larvae can be difficult to manage with insecticides once in the host plant.
Active ingredients that may be applied systemically include: azadirachtin (injection, soil drench), cyantraniliprole (soil drench, soil injection), and neem oil (soil drench).
When used in a nursery setting, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
