Pikes/Pines | The Bronze Birch Borer Beetle is also leaving its calling card across Capitol Hill

(Image: US Forest Service)

Dead tops in some streetside birches on E John (Image: Brendan McGarry)

Just like living as a birder and naturalist, being an arborist means that I can’t help but look at trees.

I see poor pruning, beautiful forms, and signs of stress — there’s no way for me to filter them out of my attention span.

Sometimes it gets me in trouble with my partner — like when she’s telling me about her day and I am staring at the tops of a line of birches in a grocery store parking lot.

Recently I’ve been noticing a lot of birches with dead tops. This isn’t necessarily a new thing, but they have been popping up all over the Pacific Northwest with increasing regularity. The culprit isn’t a mystery either. We’ve known why ornamental birches have been dying from the top down, across the United States for over 100 years.

Bronze Birch Borer Beetles, Agrilus anxius, are good at what they do, and that is eating birches.

Unlike so many stories about “killer” insects infesting our trees, this one is a little more nuanced because Bronze Birch Borer Beetles are actually a native insect across North America. In a wild setting they live their lives associated with birch trees that are already stressed, contributing to the natural decline of older trees, and stratifying ecosystems with a beautiful jumble of decay and life and everything in between. In an urban space, with non-native birches, the story changes considerably.

A particularly poorly birch (there’s a scale for the levels of infestation, this is a mid-stage) – according to a good friend who regularly runs by this tree, it’s been losing branches and tops for years. It’s by the Kaiser Permanente Parking Lot on Thomas and 17th. (Image: Brendan McGarry)

A wide variety of beetles live a majority of their lives buried in wood and the term “borer beetle,” while not taxonomic, or indicative of what our level of interest in them should be, does a good job of describing their general habits. In some species, adults chew up wood, and in others larvae grow towards adulthood living off the innards of a tree. Bronze Birch Borers are of the latter variety. A female beetle lays her eggs in a bark furrow or in a wound, and the hatching larvae chew their way into living layers of the tree just below the bark and keep chewing until they are ready to pupate and hatch in April or May.

Bronze Birch Borers are so destructive because of where they focus their attention: the cambium layer, between the bark and wood of the tree. Larvae feed exclusively on the layers of the tree that transport water and nutrients. Eventually the galleries that the beetles create can cut off that transportation, killing whole limbs and eventually an entire tree over time.

Not all birch trees are as seriously susceptible to the borer, but unfortunately the ones most prized for their white bark are typically the worst hit.

The various species of birches used as ornamental trees, from Silver Birch, Betula pendula, to Himalayan Birch, Betula utilis, don’t fare well even in the best of circumstances. The beautiful River Birch, Betula nigra, is the most resistant species across North America (native to much of the territory east of the Rockies). The Washington native, Paper Birch, Betula papyrifera, only dips into Washington in northern parts of the state and is unfortunately only somewhat resistant. (Resistance usually means the trees are able to create calluses that cut off larval beetles from traveling throughout the rest of the tree.) However, resistance sorta misses the point. A major reason we’re seeing dead tops on so many birch trees is that those trees are stressed.

Paper Birch, Betula papyrifera, looking lovely (with their dead and dying tops cropped out). (Image: Brendan McGarry)

Birches are trees that occupy a variety of niches, but broadly they like moisture and cold. Most birch species are native to temperate and boreal locales. Many are pioneer species that rush into openings, live fast, and die young (much like the related Hill local, Red Alder). I understand why people want to plant birches. They are truly striking and beautiful throughout the year – but I wish that folks would put a little more thought into how they choose trees.

Trees are a long-term investment and most birches are not resilient enough to face the rapidly changing climate in most urban situations we expose them to across Seattle. They have shallow roots, which are easily damaged, and don’t enjoy clay soils. They like water, which is notoriously difficult in planting strips or in little patches of dirt in parking lots. Our regular droughts are hard on them.

I love birches as much as most people. Maybe even more so. I love to carve their beautiful buttery wood. I enjoy crafting various things with their papery bark. And I collect extra bark unsuitable for crafts as fire starter throughout the year. I know to watch them visiting winter finches when they put out millions of seeds every year. I also see their overall grace, their showy bark – I know that’s what people are gravitating towards along with their moderate mature height and upright form. And despite all this I believe they aren’t a good choice for new plantings in the vast majority of settings on the Hill. But back to the perceived culprit.

Bark from birches has been used to make a variety of objects across the Northern Hemisphere since time immemorial. It’s a magical, highly rot resistant material. Our climate doesn’t create bark that’s quite so useful, but it’s still obviously beautiful. (Image: Brendan McGarry)

Almost certainly the offspring of the “poorly birch” pictured above, this young Silver Birch, Betula pendula, is growing opportunistically by the drain outflow in a rain garden of apartments on East Thomas. This is where birches gravitate towards in wild spaces (I also suspect that the other reason this sapling made it is because the retaining wall protected it from bunnies). If this tree is allowed to grow large it might have enough water to be happy but it’s root system will likely struggle in the confined space and would eventually succumb to birch borers. (Image: Brendan McGarry)

Bronze Birch Borer Beetles can be controlled, of course, with chemicals. There are injections and foliar sprays that variously kill larvae and adults. I don’t hesitate suggesting that people with infested wood remove the dead and dying tops of their trees and sign up for regular injections – if they want to try to keep their birches alive.

Most trees die because of a variety of causes and borers are just a symptom of other potential issues. Injections, avoiding root damage, mulching, and watering during heat spells can extend an infested birch’s life but probably won’t save it entirely. Death comes for us all.

If you happen to steward a birch tree, I highly recommend keeping a close eye on their canopy, especially this time of year (when beetles are actively laying eggs and larvae are feeding). If you start seeing the tips of upper branches yellowing and dying back, it could be hosting the beetle. There are telltale marks of adults emerging – “D” shaped exit marks in the bark. Lumpy, particularly loose bark can also be a tell. If this is a beloved tree, you might want to figure out a plan of action. (I’ll also add that as a person who sometimes has to climb birches with dead tops, I wouldn’t procrastinate because the job gets much more dangerous and challenging in a mature tree with a persistent infestation.)

Funnily enough, despite having spent a good amount of time in and near birches with Bronze Birch Borer Beetles, I’ve never seen one, adult or larvae. My causal census of tree people (not the trees themselves, but people who work with trees in some fashion), most haven’t either. For having such an impact on our urban birches, they are a surprisingly cryptic species. But I always know they are there, because their calling card is so glaring.

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