The ash-lilac borer, Podosesia syringae, belongs to a group of insects known as clear-winged moths. It is found throughout the United States and affects ash, privet and lilac, especially stressed plants. The caterpillar phase of this insect develops under the bark of plants, where it feeds on the nutrient-rich tissue found there, causing limb dieback.
Don’t confuse this with emerald ash borer, another borer on ash trees. Emerald ash borers make D-shaped exit holes with no sawdust-like excrement coming from them: Ash-lilac borers have larger, round to oval-shaped exit holes with sawdust excrement coming from them. A good way to judge the severity of a clearwing borer infestation is by how much sawdust excrement is located at the base of the tree.
Management of ash-lilac borer is best done through prevention. Once the larvae are inside the plant, treatments are not effective. The best prevention is achieved through a combination of minimizing tree stress and injury along with insecticide treatments to keep larvae from developing to a point where they can cause significant damage.
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Ash-Lilac Borer Treatment Option 1
Adequate water is a key factor in maintaining stress-free plants. A slow, deep watering event once every few weeks during dry conditions helps maintain soil moisture levels and minimizes the stress that invites ash-lilac borer.
Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 3 inches of wood chips or shredded bark under the drip line can have a very beneficial effect by holding moisture and promoting healthy, fibrous roots.
Application Method – Cultural Practices
DIY Equipment/Product Needed:
• Watering hose
Ash-Lilac Borer Treatment Option 2
Insecticide use is often necessary to maintain or regain tree health. Two applications of Tengard® should be applied to the trunk and upper branches. Begin applications in May or June and repeat them 4 weeks later.
Application Method – Systemic bark spray
DIY Equipment/Product Needed:
• Hand pump sprayer with wand
• Safety glasses
How Is It Spread?
Adult ash-lilac borer moths emerge from infested plants from April to July (peak is in May when crabapples are in bloom). Females emit a sex pheromone, which attracts males for mating. Hundreds of eggs are laid on rough bark of the host plant and hatch 14 days later. The newly-hatched larvae create tunnels under the bark and feed on the nutrient-rich tissue found there. Later, the larvae tunnel deeper into the plant to feed and overwinter. One generation per year.
Ash, privet, lilac, and related species
- Thinning canopy, limb dieback, and /or sprouts at the base of the tree.
- Sawdust-like shavings may be found at the base of the tree or coming out of ¼ in. round to oval holes on the bottom ten feet of the trunk.
- Larvae are whitish with a brown head and three pairs of legs. At maturity, larvae are about one inch long.
- Adult ash-lilac borers look a lot like wasps. Adults have dark forewings and narrow, transparent hind wings. The body is slender and black with yellowish to reddish banding on the abdomen.