Lilac borer
Podosesia syringae
12 host plants
Last updated
Lilac borer larvae tunnel within the trunks and larger branches of lilac trees, creating exit holes in the bark with frass (insect droppings) visible around them. You notice the holes and sawdust-like material on the bark surface during summer and fall months. Internal tunneling weakens branches and can eventually girdle trunks of heavily infested lilacs. Damage becomes apparent by the appearance of emergence holes.
Prune out heavily infested branches below visible damage and burn or bag the material. Make clean cuts to encourage wound healing. Monitor lilac trunks during summer for evidence of fresh exit holes or frass. If borers are confirmed, treat trunk crevices with spinosad or pyrethrin-based insecticides in late June and early July.
Quick Reference
Lilac borer Adult emergence typically begins around 1550 GDD₃₂. As of May 13, 2026, all seven Puget Sound stations have passed this threshold (1906.2–2098.2 GDD₃₂), so Adult emergence is likely underway across the lowlands.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of May 13, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,098 | Adult emergence | — | — |
| Kent / Auburn | 2,089 | Adult emergence | — | — |
| Seattle / UW | 2,063 | Adult emergence | — | — |
| Olympia / Tumwater | 2,025 | Adult emergence | — | — |
| Tacoma / Puyallup | 1,993 | Adult emergence | — | — |
| Bellingham / Whatcom | 1,972 | Adult emergence | — | — |
| Sequim / Rain Shadow | 1,906 | Adult emergence | — | — |
Source: Herms (OSU) phenological tables: 324 GDD₅₀ adult emergence, Dow Gardens MI 1985-1989 (Table 3); 330 GDD₅₀ Secrest Arboretum OH 1997-2001 (Table 4). MSU IPM: 325-350 GDD₅₀ corroborates. UMass Extension: 10% male emergence at 500 GDD₅₀. Profile range 305-500 spans first flight to 10% cumulative emergence. Updated 2026-04-03. UC Davis IPM: UC Davis IPM (base 50°F trunk spray timing 507.4 DD). Rutgers: plant-pest-advisory.rutgers.edu (148-299 GDD₅₀). Wisconsin Extension: Wisconsin Extension (landscape pest table). About GDD₃₂ →
Cultural Controls
- Since eggs are almost always laid in or near wounds, avoid pruning when moths are present.
- There is little that can be done after trees have been attacked.