Bronze birch borer
Adult emergence ActiveAgrilus anxius
16 host plants
Last updated
Look for D-shaped emergence holes in birch bark with nearby sawdust trails; branch dieback progresses from upper canopy downward on affected trees. Beneath bark you will find white flattened larvae about 1 inch long creating winding galleries along cambium. Stressed, drought-weakened, or older birch trees are most vulnerable.
Maintain tree vigor through consistent deep watering during dry summers; this is single most important control. Prune out dead and dying branches during winter dormancy to reduce beetle reproduction. Preventive trunk sprays of permethrin in late May to early June protect specimen trees from newly emerged adults. Once larvae establish galleries, no insecticide reaches them.
Quick Reference
Bronze birch borer Adult emergence typically begins around 2141 GDD₃₂. As of June 3, 2026, all seven Puget Sound stations have passed this threshold (2435.5–2672.8 GDD₃₂), so Adult emergence is likely underway across the lowlands.
Regional Season Tracker
GDD₃₂ accumulation across 7 Puget Sound stations · as of Jun 3, 2026| Station | GDD₃₂ | Current Stage | Next | To Go |
|---|---|---|---|---|
| Issaquah / East King | 2,673 | Adult emergence | — | — |
| Kent / Auburn | 2,665 | Adult emergence | — | — |
| Seattle / UW | 2,610 | Adult emergence | — | — |
| Olympia / Tumwater | 2,570 | Adult emergence | — | — |
| Tacoma / Puyallup | 2,535 | Adult emergence | — | — |
| Bellingham / Whatcom | 2,533 | Adult emergence | — | — |
| Sequim / Rain Shadow | 2,436 | Adult emergence | — | — |
Source: Herms (OSU) phenological tables: 550 GDD₅₀ adult emergence, Dow Gardens MI 1985-1989 (Table 3); 547 GDD₅₀ Secrest Arboretum OH 1997-2001 (Table 4). UMD IPMnet Pest Predictive Calendar (Gill & Klick): 547 GDD₅₀ adult emergence (confirms Herms). UMass Extension: 400-550 GDD₅₀ adult emergence window. USA-NPN forecast model: 450 GDD₅₀. Range 400-550 reflects early scouts to peak emergence. Rutgers: plant-pest-advisory.rutgers.edu (400-880 GDD₅₀ range). Wisconsin Extension: Wisconsin Extension (landscape pest table). About GDD₃₂ →
Larvae mine serpentine galleries through the cambial layer, severing vascular tissue. Damage progresses from upper canopy downward (top-down dieback). Early symptoms include sparse, yellowing, or undersized leaves in the upper crown and branch dieback starting at tips. Heavy infestations girdle main scaffold branches and the trunk, killing the tree over 2-4 years.
Cultural Controls
- Maintain tree vigor through irrigation: 1-1.5 inches per week during July-September dry season.
- Mulch root zone with 2-4 inches arborist wood chips; keep mulch away from trunk.
- Avoid soil compaction in root zone.
- Prune only during dormancy or after mid-July; avoid pruning late April through mid-July when adults are attracted to fresh wounds.
- Plant resistant species: B. nigra (river birch), B. lenta (sweet birch), B. davurica (Dahurian birch), B. occidentalis (water birch).
- Site birch on east-facing exposures or with afternoon shade to reduce heat stress on bark.
First detected in Oregon in 2003, initially in Portland; has spread throughout the Willamette Valley and urban areas since. European white birch and Jacquemontii birch most heavily impacted in urban landscapes. In dry areas such as eastern Oregon, birch species are no longer recommended for most landscape uses. OSU Extension recommends proactive replanting with resistant species (B. nigra 'Heritage') rather than treating heavily infested trees.
Host Plants (16)
Well-established throughout urban landscapes. Maritime moisture provides some buffering against drought stress that triggers attacks, but summer dry periods (July-September) still create vulnerability. B. pendula and B. jacquemontii are the most common ornamental birch in the region and the most frequently killed.
Sources & References
Primary: PNW Insect Management Handbook
- Bronze birch borer threatens Oregon's beloved birch trees
- Herms (OSU) phenological tables — GDD emergence data
- UMD IPMnet Pest Predictive Calendar (Gill & Klick)
- UMass Extension: Bronze birch borer
- WSU HortSense: Birch — Bronze birch borer