Bronze Birch Borer in the Pacific Northwest
Cody Cochran | Sept 2026
Bronze Birch Borer in the Pacific Northwest: It’s Not If Your Birch Gets Hit. It’s When.
If you have a birch tree in Western Washington, look at the top of it…
If the upper canopy is thinning, the leaves up there are yellow or undersized, or whole branch tips are bare while the lower tree still looks green, you are probably looking at bronze birch borer. We are seeing it on nearly every birch we walk up to now—Olympia, Lacey, Tumwater, the South Sound, and across the PNW. It is taking white-barked birch trees out of neighborhoods at a pace that used to feel like a few problem trees. It does not feel like that anymore.
This is not a “maybe someday” pest. For the birch species most people planted in the last few decades, it is not a matter of if the tree gets attacked. It is a matter of when—and whether we still have enough living canopy left to save it.
What bronze birch borer actually is
Bronze birch borer (Agrilus anxius) is a native North American beetle. It is not an exotic invader like emerald ash borer. It evolved here. For a long time it mostly finished off already-dying native birches. That changed.
The adult is a slender, bronze-olive beetle about half an inch long. That is not the part that kills the tree. The larvae are. After adults emerge in early summer (typically June into July here), females lay eggs in bark cracks and damaged spots. The larvae hatch, chew into the tree, and spend the rest of the year mining winding galleries through the cambium—the thin living layer that moves water and nutrients.
Those galleries girdle branches from the top of the tree downward. That is why birch decline always starts in the crown. By the time you notice a dead top, the insect has already been working under the bark for a season or more.
What to look for:
Thin, yellow, or stunted leaves in the upper crown
Branch dieback starting at the tips and working down
Raised, snake-like ridges under the bark
Small D-shaped exit holes, about 3–5 mm
Bark bleeding or wet spots
Extra woodpecker activity on the trunk and limbs
If you see D-shaped holes and a dead top, the tree is already well into the infestation. The holes are where last year’s adults left.
Why we are seeing it on almost every birch now
Three things stacked on top of each other.
First, we planted the wrong birches. The white-barked trees people love—European white birch (Betula pendula), Himalayan / Jacquemontii birch, weeping and cutleaf ornamentals—are the most susceptible species. They were planted everywhere in the 80s, 90s, and 2000s because they look striking. They were never a great fit for hot, dry summers and compacted urban soil. River birch is far more resistant. Most of the white-barked trees in local yards are not river birch.
Second, the climate shifted under those trees. Birch wants cool roots and consistent moisture. Our summers have gotten hotter and drier. A tree that used to limp through August with winter rain stored in the soil now sits drought-stressed for weeks. Stressed birch cannot wall off larvae the way a vigorous tree can. WSU’s Urban Forest Health Lab has said the borer’s impact on birch decline in the Pacific Northwest has increased exponentially because of that stress.
Third, the insect built up. One infested tree produces the next wave of adults. Those adults fly to the next stressed birch on the block. Once a neighborhood has a few declining trees, pressure on every remaining birch goes up. That is why it suddenly feels like “every birch we see.” The population is no longer rare. The host trees are aging, drought-stressed, and sitting in the open.
A healthy, well-watered birch can resist attack. Most landscape birches in Western Washington are not in that condition by midsummer.
Treatment only works if we are not already too late
This is the part people miss. You cannot spray the outside of a birch and fix an insect living under the bark. The larvae are inside the vascular system. Once more than about 40% of the canopy is dead, the tree often cannot move a systemic product well enough to save itself. At that point, the honest recommendation is removal and a better replacement—not another year of hope.
If the tree still has a usable canopy and is actively growing, a professional trunk injection can protect it. That is the service we use: a once-a-year injection that puts the product into the tree where the larvae feed. Timing matters. Injections work when the tree is still moving water—typically late spring into early summer—not after it has already shut down for winter.
Injection is not magic. It does not replace watering. Birch needs deep, consistent water through July, August, and September over the whole root zone, not a trickle at the trunk. Mulch helps keep those shallow roots cooler. Skip the water and even a treated tree stays a target.
We will tell you straight if the tree is already past the point of a good outcome. Treating a tree that has lost most of its top wastes money and delays the inevitable removal.
What to do this week
Walk the birch. Look at the top, not just the pretty white bark. If the crown is thinning, call before another summer of adults emerge and the next generation goes into the wood.
If the tree is still a candidate, we can inject it on a yearly schedule and pair that with a watering and mulch plan so the treatment has a chance to work. If it is already too far gone, we can help take it down safely and talk about a species that can actually live here long-term.
Bronze birch borer is already in the neighborhood. For white-barked birch in the PNW, waiting to “see how it looks next year” is how those trees come down. Contact us, and we will look at the tree while there is still a decision to make.
Contact us for a lawn inspection — we’re here to help.
Our crews are fully equipped, licensed, and experienced in Pacific Northwest conditions.
360-491-7711 | 360-209-4717 (text - quickest response)
Contact us today if you have questions or want a free estimate.