Lilac-ash borer

Integrated pest management photos and files

Common name

Lilac-ash borer (Podosesia syringae)

Host plants

  • All adapted Fraxinus tree species, including:

    • Green ash (Fraxinus pennsyvanica)

    • White ash (Fraxinus americana)

    • Blue ash (Fraxinum quadrangulata)

    • European common ash (Fraxinus excelcior)

    • Black ash (Fraxinus nigra)

  • Lilacs (Syringa sp.)

  • Privet (Ligustrum sp.)

The lilac-ash borer is a one-inch (2.54 cm) long lepidopteran clearwing moth. However, body morphology and markings (orange and yellowish markings on a black body) are reminiscent of a paper wasp (Figures 1-3).

Lilac-Ash Borer adult
Figure 1 — Adult clearwing moth of the lilac-ash borer.
Lilac-Ash Borer larvae
Figure 2 — Pupae and tunnel gallery of a lilac-ash borer larvae.
Lilac-Ash Borer pupal casing
Figure 3 — Extruded pupal casing on an ash tree left intact in the exit hole of a lilac-ash borer moth following emergence of an adult.

Biology

The larval stage of this insect is the damaging phase. Adult females lay eggs on the cracks, crevices and damaged areas of the bark. Eggs hatch and enter the bark of the tree, foraging under the bark and damaging the phloem tissue. Injury to ash trees is not limited to the physical damage the insect does; borer holes are also entry points for turkey-tail fungus (Trametes versicolor), which can further damage plants.

Lilac-Ash Borer exit hole
Figure 4 — Typical round to slightly oval exit hole made in the spring by a lilac-ash borer larvae as it prepares to pupate and launch its mating flight.

Adult lilac-ash borer moths emerge in spring from a round exit hole bored from the inside at the top of their gallery (Figure 4). They mate, and within a few hours, lay eggs (usually at the base of major branches) and the young larvae tunnel under the bark. After a summer spent feeding under the bark tissue (cambium layer), the larvae migrate toward the heartwood where they spend the winter. In spring, fully grown larvae tunnel partially out through the bark and then complete pupation. As the adult moth emerges, the pupal shell often remains mostly intact and protrudes from the hole. Sometimes this is barely noticeable, but commonly the pupal case protrudes about one-half inch (1.27 cm). In Idaho, lilac-ash borers complete one life cycle per year.

Damage

Affected trees (Figure 5) may show a rapid or slow progression of injury symptoms, including

  • Loosened or separating bark
  • Branch dieback
  • Weakened branches that become susceptible to wind breakage
Lilac-Ash Borer damage on ash tree
Figure 5 — Dead and dying branches in the upper canopy (left) and bark cracking and uneven tissue swelling on a major structural branch (right) of an ash tree by repeated infestations of the lilac-ash borer.

Diagnostic plant symptoms caused by lilac-ash borer include

  • Extensive feeding galleries in the cambial layer underneath the bark
  • Loose or damaged bark, especially around the base of large structural branches
  • Wilting and chlorotic leaves at the ends of damaged branches
  • Dead or dying branches scattered among healthy branches of the infested tree
  • Weak branches that may result in random fall events or wind breakage

Diagnostic signs of lilac-ash borer include

  • Round larval exit holes, approximately one-quarter inch (0.64 cm) in diameter (other ash borers produce holes with differing shapes and sizes)
  • Protruding, brown, residual of the larval shell left intact in the larval exit hole
  • Tan frass and sawdust leftover after exit events that collects in bark crevices and around the base of the trunk

Management

Physical
  • For young, small trees, a thin knife or wire inserted in the borer hole will kill an individual larva or pupa.
Cultural
  • Routinely prune away damaged and dead branches in Lilac and privet. This practice is usually adequate to encourage healthy new growth and maintain plants with an attractive, healthy form.
  • Routinely prune ash trees to remove dead, weak, and unsightly branches. Time pruning in the fall or early winter to prevent the presence of open wounds during spring flight, as these are sites adult females may exploit during egg-laying. Remove and destroy all pruned branches that may serve as a refuge for maturing larvae.
  • Stressed ash trees are more prone to significant injury from lilac-ash borer than are healthy trees. Avoid planting ash trees in sites with lean soils, shallow soils or inadequate water supplies.
  • Use best irrigation and fertilization practices to maintain the overall health of the trees.
Behavioral
  • Pheromone traps are only marginally effective in removing adult males.
Biological
  • Beneficial entomopathogenic nematodes of the species Steinernema carpocaps, S. feltiae, S. glaseri, Heterorhabditis heliothidisk and H. bacteriophora have been shown to be effective against clearwing moth borers. Application can be made shortly after eggs are laid and entry holes are still open.
Chemical

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.

Photo credits

  • Figure 1. James Solomon, USDA Forest Service, Bugwood.org

  • Figure 2. David Cappaert, Bugwood.org

  • Figure 3. Whitney Cranshaw, Colorado State University, Bugwood.org

Steve Love, emeritus professor of crop science
2023