Birch leafminer
Fenusa pusilla
16 host plants
Last updated
This profile contains verified pest data from extension databases. Regional field notes and expert review are in progress.
On birch leaves you will notice pale or brownish blotch mines starting in early summer, with translucent larva visible inside if held to light. Infested leaves may drop by midsummer if mining is severe, creating patchy defoliation. Look for this damage on paper birch and other Betula species beginning late May.
Rake and destroy fallen leaves in autumn to eliminate pupae before spring emergence; this breaks lifecycle and reduces next year's population significantly. Prune back new growth in June if miners appear, encouraging replacement foliage. Parasitoid wasps naturally suppress populations; avoid insecticides that kill beneficials. Cosmetic damage; trees recover and produce new foliage by late summer.
Quick Reference
Birch leafminer Adult emergence typically begins around 1102 GDD₃₂. As of April 23, 2026, all seven Puget Sound stations have passed this threshold (1434.4–1592.7 GDD₃₂), so Adult emergence is likely underway across the lowlands.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Apr 23, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 1,593 | Adult emergence | — | — |
| Seattle / UW | 1,554 | Adult emergence | — | — |
| Kent / Auburn | 1,537 | Adult emergence | — | — |
| Olympia / Tumwater | 1,505 | Adult emergence | — | — |
| Bellingham / Whatcom | 1,483 | Adult emergence | — | — |
| Tacoma / Puyallup | 1,472 | Adult emergence | — | — |
| Sequim / Rain Shadow | 1,434 | Adult emergence | — | — |
Source: Herms (OSU) phenological tables: 215 GDD₅₀ adult emergence, Secrest Arboretum OH 1997-2001 (Table 4); 189 GDD₅₀ Dow Gardens MI 1985-1989 (Table 3). Profile value 219 near-exact match with OH data. Rutgers: 123-290 GDD₅₀ activity window corroborates. Updated 2026-04-03. About GDD₃₂ →
Cultural Controls
- Plant resistant species and cultivars, including Betula davurica, B.
- jacquemontii, B.
- nigra, and the variety 'Crimson Frost'.
- Pinch leaves to kill larvae in minor infestations on small trees.
- Reported as 95% resistant (B.
- maximowicziana, B.