Lilac Ash Borer
Adult lilac-ash borers 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. The wing span varies from 1 to 11/2 inches. Females are somewhat larger than males. Larvae are full-grown larvae are about 1 inch long, white body and a brown head.
Depending on the location in Utah, adults may emerge from infested hosts as early as late March (but usually late April) and continue until mid-July. 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 into the wood throughout the summer. Mature larvae overwinter in the heartwood. In spring, the larva cuts an emergence hole in the bark, leaving a thin flap of tissue over the hole. Pupation occurs in the burrow. There is one generation produced each year in Utah.
- Limb dieback
- Frass (sawdust-like excrement)
- Round exit holes
- Empty pupal cases
- Loose bark and wounds
- Weakened limbs leading to possible breakage
Lilac-ash borer does not typically kill plants, but causes weak wood, leading to breakage and hazards. Because they prefer to attack larger, older branches on lilacs and privet, a technique known as renewal pruning is a great preventive method. 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 1/3 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 from March to late April.
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. Pheromone traps can be purchased online and hung from infested trees in early April. They are not a control measure, but will indicate when adults have begun to fly, and thus, when to apply the first spray. Primary insecticides include permethrin, bifenthrin, or carbaryl. Keep in mind that imidacloprid, does NOT work against lilac-ash borer. Repeat application once a month through July. Larger trees may require a professional applicator due to specialized equipment needed to treat the full canopy.