Bronze birch borer in Washington and Oregon

Where it is in Washington and Oregon

Bronze birch borer (Agrilus anxius) is a native North American beetle, in the same genus as emerald ash borer. WSU’s Urban Forest Health Lab describes it as native to Washington and says its role in birch decline in the Pacific Northwest has “increased exponentially” with hotter, drier summers. Seattle’s urban forestry pest list confirms it is present in the state.

In Oregon it was long an east-side insect. OSU Extension’s homeowner guide (EM 9189, revised July 2023) records the first detection west of the Cascades in Portland (Multnomah County) in 2003, and the first in Klamath Falls (Klamath County) in 2017, when six adults were caught; larvae had been there since at least 2015. The City of Portland now calls it “an established pest in western Oregon.”

There is no quarantine for this beetle in either state.

Which birches die, and which do not

The trees at risk are the ones most yards have. OSU names Jacquemontii birch (Betula jacquemontii), European white or weeping birch (B. pendula) and the cutleaf ornamental forms as the most susceptible; Portland calls European birch “very susceptible.” Native birches are the most resistant.

WSU’s Hortsense fact sheet lists sweet birch, river birch including ‘Heritage’, and paper birch as resistant, with gray birch cultivars such as ‘Whitespire’ moderately resistant. Seattle’s list, though, names paper birch among the trees attacked, so treat any stressed birch as a candidate. Portland’s Urban Forestry plants river birch ‘Heritage’ and ‘Dura-Heat’ in its free tree giveaways, and OSU suggests ‘Heritage’ river birch and ‘Whitespire Senior’ gray birch as replacements.

How birch compares with aspen as a yard tree is on aspen vs birch.

Signs on your tree

The larvae feed under the bark at the top of the tree first, so the damage starts where you cannot see it. “If you don’t know you have an infestation, it’s usually too late,” OSU horticulturist Nicole Sanchez told OSU News. Look for:

• Thin, yellow or wilted leaves at the very top. OSU calls wilting near the top the earliest sign; Seattle says top leaves turn brown and drop from June through September.
• Branch dieback from the top down, one limb at a time.
• Lumpy, ridged bark over healed larval galleries, which WSU describes as serpentine ridges on branches.
• D-shaped exit holes, 3–5 mm across according to WSU, with yellow-to-brown staining or bleeding on the bark (OSU, WSU).
• The adult beetle: olive to bronze, about half an inch long (Hortsense, Portland).

When the beetles fly

Timing drives both treatment and pruning. WSU says larvae overwinter under the bark, pupate in late April or early May, and adults emerge in early June in Washington. In the Klamath Basin, OSU puts emergence at about the first week of July, with adults active for a 2–3 week window that shifts year to year. Larval development takes one to two years depending on climate.

Treat or remove?

OSU’s guide rates damage on a five-level scale. Levels 1–3 have a good chance of recovery; at level 4, “saving the tree is unlikely, but possible under ideal conditions”; level 5 is a dead tree that needs removal. Treatment is worth discussing only while less than 40 percent of the canopy is affected; once more than 50 percent is affected, EM 9189 says no treatment will save the tree, and OSU News advises removal at 40 percent or more.

Have a certified arborist confirm the diagnosis and rate the hazard, especially when the tree stands over a driveway or roof; see tree risk assessment.

Modeled estimate, not quotes (method): a 25–50 ft birch models at $850–$1,700 in Washington and $900–$1,800 in Oregon with chipping and haul-away but no stump; under 25 ft models at $400–$900. See tree removal cost, and for standing-dead trees, dead tree removal.

Permits for a dying birch

• Portland: private trees 12 in DBH and larger are regulated, but dead, dying or dangerous trees qualify for a Type A permit with no public notice (PCC 11.40).
• Seattle: hazardous trees can be removed with replacement under SMC 25.11, and any regulated removal must be done by an SDCI-registered tree service provider. Seattle’s urban forestry line is 206-684-8733.
• Spokane: no permit for private trees; street and planting-strip trees need a city permit (SMC 12.02.960).
• Klamath Falls: no private-tree permit, but the code bars dead or hazardous trees on private property (KFC 5.632).

Other cities: Oregon tree removal permits and Washington tree removal permits.

What to do this year

1. Look at the top of every white-barked birch in midsummer with binoculars.
2. Water deeply through the dry season on any birch you want to keep, to about 10 inches (OSU).
3. Decide per tree using the 40 percent line: treat early, or schedule removal in the dormant season.
4. Replant with a resistant birch or another species; Portland’s own choice is ‘Heritage’ river birch.
5. Check the crew with L&I’s Verify tool in Washington or the CCB and LCB lookups in Oregon.

Related pests are on pests and diseases; emerald ash borer, the same genus on ash, is on emerald ash borer in Oregon.