Quick facts…
- Lilac/ash borer is an insect native to North America that is common in ash trees, particularly if trees are stressed due to drought, injury, or recent transplantation.
- Damage is done by the larva; a type of caterpillar that tunnels into the trunk and lower branches of ash trees.
- Lilac/ash borer prefer to attack stressed trees. Addressing the root cause of stress should be prioritized in the tree’s management plan.
- This pest usually isn’t deadly to mature ash trees. If necessary, lilac/ash borer can be easily controlled by spraying the trunk and lower branches in spring during the time when adult females lay eggs on the trunk and the newly hatched caterpillars begin to tunnel into the wood.
- Lilac/ash borer should not be mistaken for the emerald ash borer, an introduced insect of Asian origin that was recently found in a part of NE Colorado and is far more seriously damaging to ash trees.
Introduction
Lilac/ash borer (Podosesia syringae) is a common wood borer associated with ash throughout Colorado and a species that is native to North America. This insect is a clearwing moth (Family: Sesiidae).
Damage is caused by the larvae which tunnel into the trunks and lower branches of ash trees. These feeding injuries produce irregular gouging wounds under the bark and tunneling frequently extends deeply into the heartwood (Figure 1). Almost all larval feeding activity occurs in the lower trunk, particularly around the soil line (Figure 2). Lower scaffold limbs may also be attacked and infestations may extend about 10 feet up the trunk. Feeding damage can often be found in the mid and upper crown of trees above areas that were treated with contact insecticides.
under the bark. Photograph by David Leatherman.
transplanted “Autumn Purple” white ash by the lilac/ash borer.
External evidence of lilac/ash borer activity in trees can include irregularly round exit holes of about 1/4-inch diameter on trunks (Figure 3). As larvae near full development, some sawdust may be expelled from these holes and, when adults emerge, the pupal skin often remains extruded from the hole (Figure 4). Ash trees heavily damaged by lilac/ash borer often show some disfiguring of the trunk, with areas of irregular, gnarled growth, and often development of excessive branching. Extensive tunneling of the lower trunk may seriously weaken plants and cause them to break. Trees may be killed by this insect, although that is rare.
Serious problems almost always involve ash grown in suboptimal locations (e.g., parking lots, street trees) or recently transplanted trees. Damage is usually insignificant to well-established trees that receive adequate water and care. In Colorado, more problems have been associated with white ash (e.g. ‘Autumn Purple’) than green ash. Lilac (Syringa) and privet (Ligustrum) are other hosts for this insect but damage to these plants is much less frequently observed in Colorado. On lilac and privet, damage is confined to the base of the plant. On smaller diameter branches, such as is common with lilac and privet, larval injuries may girdle and kill stems.
adult lilac/ash borer emerges from the trunk in spring.
bark produced by lilac/ash borer wounding. Two pupal skins of the insect extruding from the trunk are visible on the left.
The larvae are creamy white grubs with a small dark head (Figure 5). Prolegs on the underside of the abdomen are highly reduced but elliptical rings of small hook-like crochets are readily visible at the tip of the prolegs. The presence of abdominal prolegs allows separation from lilac/ash borer larvae from those of the various other wood borers found in ash, all of which are larvae of beetles.
Adult lilac/ash borers are mimics of Polistes paper wasps, quite similar in both size and coloration (Figure 6). The wings are dull black with chestnut brown tones and wingspan typically ranges from 28-35 mm (ca 1-inch). The body is generally dark brown with reddish markings and narrow yellow bands occur on the abdomen. Long hind legs also reinforce the superficial similarity with paper wasps.
Lilac/Ash Borer Life History and Habits
The lilac/ash borer spends the winter as a partially grown larva within tunnels under the bark. It resumes feeding and larval development in early spring, pupating just under a thin cover of the bark. Adults emerge during the morning on warm (above 60°F) days in spring. Depending on location in the state and seasonal temperatures first emergence of the adults begins within a range of early April to early May and typically lasts for about 4-6 weeks. During emergence the pupal skin is often partially pulled through the emergence hole and visibly extrudes from the trunks of infested trees. Lilac/ash borer is one of only a few kinds of moths that fly during the daytime.
of David Cappaert/Michigan State University and BugWood.org.
Shortly after mating the females begin to lay eggs. These are deposited singly or in small groups in bark crevices on the lower trunk. Adults are short-lived and typically deposit all eggs within a week after emergence. Eggs hatch about 9-13 days after they have been laid.
The larvae hatching from the eggs enter the trunk and initially feed in cambium and phloem. During this feeding phase they will excavate an irregular, shallow gallery that may be 1-3 cm wide and 2-5 cm long. They then move into the trunk and tunnel upwards for an extended period, producing a tunnel that may extend 7-32 cm. When full-grown in late winter, they move back to the cambium and produce a pupal chamber just under the bark, with a paper thin covering that the adult can later push through (Figure 8). One generation is produced annually.
Control of Lilac/Ash Borer
Mature ash trees usually do not warrant treatment from lilac/ash borer. Since lilac/ash borer prefers to infest trees experiencing stress, addressing the root cause of stress can be a beneficial strategy for the long-term health of the tree. Activities that can relieve tree stresses, such as provision of supplementary watering, should be prioritized and considered part of any program for lilac/ash borer control.
High-risk ash trees may benefit from control of lilac/ash borer. These could include recently transplanted trees, trees in poor sites, trees receiving limited water, or trees that show significant effects of previous damage. Before transplanting or treating a severely stressed ash tree for lilac ash borer, consider the threat of emerald ash borer. Since EAB is deadly to all ash trees, eventually, all trees will need preventative treatment to survive. Due to systemic properties of EAB insecticides, only healthy trees will be candidates for EAB insecticide treatment. EAB management should be a consideration in transplanting or maintaining ash trees. Contact your local county Extension office for alternatives and replacements for ash trees.
Treatments for lilac/ash borer involve use of insecticides applied to the bark in a preventive manner to kill larvae before they enter the trunk. Certain pyrethroid insecticides used to control wood boring insects (e.g., permethrin, bifenthrin) have long been standards for this application, and some permethrin formulations useful for control of this insect are commonly available through many nurseries and hardware store outlets. More recently an alternative insecticide, chlorantraniprole (Acelypryn), has become available to commercial applicators. Trunk sprays should be applied shortly before or coincide with the anticipated time of egg hatch. This occurs about 10-14 days after adults begin to emerge from trees, typically sometime between mid-April and mid-May.
Emamectin benzoate, a recommended insecticide for use in managing emerald ash borer may also be effective against the larvae of lilac/ash borer. Emamectin benzoate is applied through trunk injections by a certified pesticide applicator using specialized equipment. The benefits of trunk injections outweigh the risks since a healthy tree will recover from the trunk injection wounds. Emamectin benzoate is a systemic insecticide meaning the tree will uptake the product and move it throughout the tissues of the plant. Keeping the tree well-watered after application can help the tree distribute the product. It is best applied in the spring for effectiveness against lilac/ash borer and emerald ash borer.
Soil-applied systemic insecticide applications of imidacloprid are not recommended for control of this species of borer, largely because imidacloprid has poor activity against larvae of moths.
Traps containing lures with the ‘clearwing borer’ sex pheromone can be very useful for detecting the onset and length of lilac/borer flight activity in the spring (Figures 9, 10). These traps also capture several other types of clearwing borer moths, notably the peach tree borer and cottonwood crown borer/American hornet moth (Sesia tibialis). These moths fly a bit later in the season than does lilac/ash borer but may co-occur in traps during June and early July. The traps attract male moths which can help establish when the females will lay eggs, an important component for the timing of trunk sprays.
Note on Emerald Ash Borer
The most serious pest of ash in North America is the emerald ash borer, Agrilus planipennis. The emerald ash borer is an Asian insect that is not native to North America but was accidentally introduced into the US and first found in the Detroit area in 2002. Since then, emerald ash borer has devastated all North American species of ash (Fraxinus spp.)—including the green ash and white ash grown commonly in Colorado—killing tens of millions of trees. EAB was first discovered in Boulder, Colorado in 2013. Emerald ash borer slowly continues to spread in Colorado. For updated information on confirmed locations of EAB, contact your local county Extension office or visit the Colorado State Forest Service website for a current map.
Emerald ash borer is a very different insect than lilac/ash borer and is far more damaging and deadly to ash trees. The distinctions between EAB and lilac/ash borer are important to note.
EAB characteristics include:
- Meandering tunnels to areas of the tree immediately under the back.
- Attacks concentrated to the upper crown of the tree that may not be visible to homeowners.
- D-shaped exit holes, approximately 1/8th of an inch in the bark from emergence, a trait of metallic wood borer (Coleoptera: Buprestidae).
Lilac/ash borer characteristics include:
- Irregular tunnels confined to the lower trunk of the tree.
- In extreme cases, the injuries could cause internal riddling.
- Irregular, round-shaped exit holes approximately ¼ of an inch.
References
Cranshaw, W.S. (2020). Insecticides Used to Control Emerald Ash Borer on Residential Shade Trees Fact Sheet #5.626. Colorado State University Extension. https://csfs.colostate.edu/wp-content/uploads/2020/06/InsecticidesEAB-Control_05626.pdf
Davis, R.S., Beddes, T., & Karren, J.B. (2010). Lilac-Ash Borer. Utah Pests Fact Sheet. Utah University Extension. https://extension.usu.edu/planthealth/uppdl/files/factsheet/lilac-ash-borer10.pdf
Engelhardt, G. P. (1946). The North American clear-wing moths of the family Aegeriidae. Bulletin of the United States National Museum, 190, 1–222. https://doi.org/10.5479/si.03629236.190.1
Herms, D. A., McCullough, D. G., Smitley, D. R., Sadof, C. S., Miller, F. D., & Cranshaw, W. (2019). Insecticide Options for Protecting Ash Trees from Emerald Ash Borer. North Central IPM Center Bulletin. 3rd Edition. 16 pp. https://www.emeraldashborer.info/files/96967b8d-940b-4959-8102-a210590c48bd-82l4wt.pdf
Meyer, W., & Cranshaw, W. (1994). Capture of Clearwing Borers (Lepidoptera, Sesiidae) with 3 Synthetic Attractants in Colorado. The Southwestern Entomologist, 19(1), 71-76.
Pest Notes: Clearwing Moths. (2013). UC ANR Publication 7477. https://ipm.ucanr.edu/PMG/PESTNOTES/pn7477.html
Sadof, C. S., McCullough, D. G., & Ginzel, M. D. (2023). Urban ash management and emerald ash borer (Coleoptera: Buprestidae): facts, myths, and an operational synthesis. Journal of Integrated Pest Management, 14(1), 14. https://doi.org/10.1093/jipm/pmad012
Reference: CSU Fact Sheet 5.614