Lilac Borer, Podosesia syringae

Order: Lepidoptera

Family: Sesiidae

Description

Unlike most moth species, lilac borer adults are most active during the day. They resemble paper wasps (Poilstes spp.), and this mimicry helps the moths ward off predators.

Lilac borer adults have brown fore wings and clear hind wings with a dark border. Adult moths have a wingspan of 25.4 – 38.1 mm (1 – 1.5 inches).

Lilac borer larvae have a small dark head and creamy white bodies. Their legs are very reduced and called prolegs, however they do contain hook-like crochets arranged in rings at the tips. This feature can be used to help distinguish lilac borer larvae from other wood boring pests, most of which are beetle larvae that lack prolegs.

Quick Facts

• The lilac borer, also known as the “ash” borer, is a clearwing moth whose larvae bore through wood and can damage trees. Unlike most moth species, lilac borer adults fly during the daytime.
• Trees in suboptimal locations such as parking lots and streets that are likely to be stressed are more susceptible to lilac borer infestation.
• Injury is usually confined to the base of the tree and lower limbs. In Colorado, white ash trees are more susceptible than green ash trees. Lilac and privet can also serve as hosts, but they are not attacked as frequently.

Lilac borer adult
Lilac/ash borer adult
Whitney Cranshaw, Colorado State University, Bugwood.org

The lilac borer, also known as the “ash” borer, is a native insect of North America and pest of ash trees throughout Colorado. Larvae injure host trees by tunneling into the wood of trunks and lower branches. Ash trees that receive adequate water and care are less susceptible to attack.

Mating pair of lilac borers

Whitney Cranshaw, Colorado State University, Bugwood.org
Mating pair of lilac borers

lilac borer larva

David Cappaert, Bugwood.org
Lilac borer larva

lilac borer pupal case

Whitney Cranshaw, Colorado State University, Bugwood.org
Lilac borer pupal case protruding from an exit hole. Pupal skins are often pulled along by adults as they pupate and exit the tree.

damage to green ash

James Solomon, USDA Forest Service, Bugwood.org
Damage to green ash caused by lilac borer.

exit holes created by lilac borer

Steven Katovich, Bugwood.org
Exit holes created by lilac borer – note the exit holes are round rather than D-shaped.

lilac borer pheromone trap

Whitney Cranshaw, Colorado State University, Bugwood.org
Lilac borer adult approaching a pheromone trap.

Life History and Habits

After mating, females lay eggs in small groups or singly on bark of the lower trunk within a week after emerging. After about 9-13 days, eggs hatch and larvae bore into the trunk where they begin feeding.

Lilac borer larvae overwinter underneath the bark of a host tree. They resume feeding and developing in the early spring.

Pupation occurs beneath a thin layer of bark. Adults usually emerge in the spring, typically in the mornings when temperatures exceed 15.6° C (60° F). When adults exit through holes in the bark, they often pull some of the pupal cases through the exit hole. Therefore, the presence of empty pupal cases protruding from exit holes is a sign of lilac borer infestation.

Injury

Newly hatched larvae bore into the wood of trees, producing tunnels that can extend 7-32 cm. Infestations generally occur in the lower trunk, however they are rare in trees greater than about 15 cm (6 inches) in diameter. This tunneling weakens the tree’s structural integrity and often leads to breakage during storms. Borer activity also causes swelling on certain areas of stems, and limb dieback is another common symptom of lilac borer infestation.

Prevention

Eradicating lilac borer infestations is very difficult; therefore, prevention is an important part of lilac borer management. Trees at high risk of lilac borer infestation include drought-stressed trees, recent transplants, trees in urban areas surrounded by hardscape or showing signs of prior injury. Simply alleviating stress on trees (e.g., watering) makes them less susceptible to lilac borer attacks. Pruning branches near the base of the trunk removes the preferred egg laying sites as well. However, pruning should be avoided during peak moth flight, which can be monitored with pheromone traps. Degree day models can also help with monitoring. For more information on degree days, and for data from previous years in Colorado, consult the Utah State University factsheet.

Monitoring

Certain traps that emit sex pheromones can help detect the onset and length of lilac borer flights in the spring. The traps are generally hung from susceptible host trees at shoulder height. It is worth noting that there are other clearwing borer moths attracted to these pheromones, most notably the peachtree borer (Synanthedon exitiosa) and American hornet moth (Sesia tibialis). Although these species of clearwing moths typically fly later in the season than the lilac borer, they can occur in traps with lilac borer during June and early July.

Chemical Control

Insecticides can be applied to the tree bark as a preventative measure since it kills larvae before they bore into the trunk. However, sprays will not control lilac borers already inside the tree. Therefore, trunk sprays should coincide with egg hatch, which is about 10-14 days following the emergence of adults. Adult emergence usually occurs between mid-April and mid-May.

Soil applications of neonicotinoid insecticides are not recommended for controlling lilac borer since it generally has low toxicity against the larvae. For more information on which insecticides to use for managing lilac borer, refer to the full factsheet.

References

Davis, R., T. Beddes & J. Karren. 2010. Lilac-Ash Borer. Utah State University Extension – Fact Sheet ENT-135-10. Available https://extension.usu.edu/pests/uppdl/files/factsheet/lilac-ash-borer10.pdf

Iowa State University. 2022. Lilac/Ash Borer. Iowa State University: Extension and Outreach. Available https://hortnews.extension.iastate.edu/lilacash-borer

Scheufele, S. & N. Brazee. 2014. Lilac Borer (also known as Ash Borer). University of Massachusetts Amherst: Center for Agriculture, Food, and the Environment. Available https://ag.umass.edu/landscape/fact-sheets/lilac-borer-also-known-ash-borer

CSU Extension Fact Sheet

Download or view the CSU Extension’s PDF fact sheet for your reference.