Integrated Pest Management

Lilac Ash Borer

Adult lilac ash borers. Adult lilac ash borers.
Exit holes from borers. Exit holes from borers.
Feeding injurying inside lilac stem. Feeding injury inside lilac stem. 


  • Ash
  • Lilac
  • Privet


Lilac-ash borer is a widespread pest in Utah particularly of ash. It feeds under the bark within the wood, not killing trees, but weakening them. Adults resemble paper wasps in color, size, shape, and flight habits, but this insect is actually a moth. The slender body is black in color with yellow banding on the abdomen. Full-grown larvae are about 1-inch-long with a white body and a brown head.


Depending on the location in Utah, adults may emerge from the wood of host trees as early as late March (but usually early May) and continue for about 6 weeks. Females emit a pheromone, which attracts males for mating, within 7 to 14 days after emergence. Within an hour of mating, females lay eggs in cracks, crevices, and wounds on the bark. A single female can lay about 400 eggs. Eggs hatch within 14 days, and the larvae bore into the plant and create random galleries in the wood throughout the summer. Mature larvae overwinter in the heartwood and then move toward the bark to pupate in spring. There is one generation each year in Utah.


  • Limb dieback
  • Frass (sawdust-like excrement at the base of the plant)
  • Round exit holes
  • Loose bark and wounds in bark
  • Weakened limbs leading to possible breakage


On lilac and privet, a technique known as renewal pruning can prevent damage. Renewal pruning involves removing older stems, allowing younger, more vigorous ones to grow. Renewal pruning increases the amount of flowers produced in lilac, controls the ultimate size of the shrub, and makes shearing (if desired) more manageable over time. Up to one third of branches can be removed in a given year.

On ash trees, prune out damaged branches to stop them from becoming a danger. Fresh pruning wounds are highly attractive to egg-laying moths, so it is important to avoid pruning during moth flight (April/May- early to mid-July)


Apply preventive insecticide sprays to the trunk and larger limbs during the egg-laying period to kill newly hatched larvae before they can bore into the bark. Examples of insecticides include permethrin, bifenthrin, or carbaryl. (Note that imidacloprid does NOT work against lilac-ash borer.) Repeat applications once a month through July. Larger trees may require a professional applicator due to specialized equipment needed to treat the full tree height.

Precautionary Statement: Utah State University and its employees are not responsible for the use, misuse, or damage caused by application or misapplication of products or information mentioned in this document. All pesticides are labeled with ingredients, instructions, and risks, and not all are registered for edible crops. “Restricted use” pesticides may only be applied by a licensed applicator. The pesticide applicator is legally responsible for proper use. USU makes no endorsement of the products listed in this publication.