NC State Extension Publications

Description and Biology

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The adult lilac borer, Podosesia syringae, is a clearwing moth with brown forewings and clear hind wings that closely resembles a paper wasp in form and behavior although it cannot sting. Adults emerge in April and May to mate, and then females lay their eggs on the rough bark or wounds of ash, lilac, and privet. As soon as they hatch, the tiny worms bore into the bark. The borers eventually grown to about inch-long white caterpillars with brown heads. Contact is maintained with the outside, and frass is expelled through the opening. Lilac borers overwinter as larvae in the wood, usually near the ground. The following spring and summer, the borers tunnel under the bark and into the wood. The larvae pupate close to the surface. There is one generation per year. We have relatively few complaints about lilac borers mainly because relatively few lilacs are used in our landscapes. Older literature was confusing about lilac borer moth emergence period. Pheromone trials showed that there is a second species of clearwing moth borer that emerges in late Aug-Sept. This second one is called the Banded ash clearwing, Podesia aureocinta, that has a narrow gold band around the fourth abdominal segment. The banded ash clearwing is thought to infest lilac, too.

lilac borer mines

The lilac borer mines under the bark and then tunnels into the wood.

Lilacsh clearwing borer entrance wound.

Lilacsh clearwing borer entrance wound.

Lilac borer pupal skin

Lilac borer pupal skins are a sure sign of this pest.

Lilac borer moth

Lilac borer moths resemble paper wasps.

lilac borers mating

A pair of lilac borers mating on a lilac shrub.

Lilacsh clearwing borer exit hole.

Lilacsh clearwing borer exit hole.

Host Plants

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The lilac borer is probably the main reason lilacs are not more widely grown in North Carolina. This insect is also called the ash borer and only recently have we had complaints on ash. Landscape architects sometimes specify 'Marshall Seedless' and other named varieties of ash trees for landscapes because these cultivars produce fewer seeds, a nuisance in landscapes. Unfortunately, ash borers infest 'Marshall seedless' ash trees with a vengence. Moutain ash, osmanthus, privet, and other trees in the olive family are also victims of the lilac borer.

Lilac borers tunnel into the main stems, causing the plant to wilt during hot weather. Infested shrubs appear unhealthy, and infested stems breaking off easily. Infested areas appear swollen and cracked because the sapwood is also destroyed. Numerous holes are visible in heavily infested stock, and frass is usually abundant. Moreover, the wounds caused by the lilac borer allow a wood-destroying fungus, Polyporus vericolor, to enter, producing additional damage.

Residential Recommendation

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Forest and landscape ash trees are a source of lilac/ash clearwing borers in the landscape.Heavily infested stems should be pruned out and destroyed. Consider using permethrin or some other pyrethroid be used for protecting lilacs and Marshall seedless ash trees from these borers. It should be applied around the first and third weeks of May although applications can be more accurately applied based on pheromone trap counts. The banded ash clearwing is thought to infest lilac, too. Protective sprays for the banded ash clearwing would need to be applied in mid August. Pheromone traps are commercially available to monitor lilac borer moth flights. Because traps attract moths from a distance, place traps in locations that will be convenient to check once a week for moths.

Pheromone traps

Pheromone traps are commercially available for monitoring lilac borer flights.

References

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For assistance with a specific problem, contact your local Cooperative Extension Center.

This Factsheet has not been peer reviewed.

Author

Professor Emeritus
Entomology and Plant Pathology

Publication date: April 4, 2013
Revised: Oct. 1, 2019

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