Bronze birch borer

Adult emergence Active

Agrilus 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

Order
Coleoptera
Type
borer
Host Plants
16
GDD₃₂ Adult emergence
2,141
Indicator: Smoketree-Fringetree
Peak Activity
Early July in PNW; first emergence 400-550 GDD₅₀, active through ~800 GDD₅₀
Now: Pupation / Pre-emergenceModerate Risk

Overwintered larvae resume feeding briefly as temperatures warm, then molt into the pupal stage in late April to early May. Pupation occurs in the outer bark. Systemic insecticide uptake begins as soil warms and transpiration increases.

Next: Adult Emergence / Flight

Bronze birch borer Adult emergence typically begins around 2141 GDD₃₂. As of May 19, 2026, Puget Sound stations range from 2045.5 to 2249.7 GDD₃₂. 4 of 7 stations have reached the threshold.

Regional Season Tracker

GDD₃₂ accumulation across 7 Puget Sound stations · as of May 19, 2026
Station GDD₃₂ Current Stage Next To Go
Issaquah / East King 2,250 Adult emergence — —
Kent / Auburn 2,222 Adult emergence — —
Seattle / UW 2,189 Adult emergence — —
Olympia / Tumwater 2,151 Adult emergence — —
Bellingham / Whatcom 2,124 Pre-season Adult emergence 17
Tacoma / Puyallup 2,117 Pre-season Adult emergence 24
Sequim / Rain Shadow 2,046 Pre-season Adult emergence 96
Regional estimate

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 →

What Damage Looks Like

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.
Regional Notes

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

Data Maturity
Structured Multiple sources. Expert review underway.