What Emerald Ash Borer Damage Looks Like and What to Do

If you own ash trees anywhere in northern Michigan, emerald ash borer isn’t a hypothetical threat — it’s very likely already in your area, and possibly already in your trees. We’ve watched this invasive beetle work through the region’s ash population over the past decade, and the trees that get saved are almost always the ones where someone caught it early.

What emerald ash borer actually is

Emerald ash borer (EAB) is a small, metallic green beetle whose larvae tunnel through the layer just beneath ash bark, cutting off the tree’s ability to move water and nutrients. By the time visible symptoms show up in the canopy, the infestation has usually been established for a year or more. Left untreated, EAB kills ash trees — not eventually, but reliably, typically within two to four years of visible symptoms starting.

The signs to look for

Canopy dieback starting at the top. This is often the first visible sign — the uppermost branches thin out and die back while the lower canopy still looks relatively normal. Homeowners frequently mistake this for drought stress or a bad year, which delays the assessment that would catch it.

D-shaped exit holes. When adult beetles emerge from the tree, they leave distinctive D-shaped holes in the bark, roughly 1/8 inch across. These are easiest to spot on smaller branches and the upper trunk, and they’re one of the more definitive visual signs once you know what to look for.

Vertical bark splitting. As larvae tunnel beneath the bark, the tree sometimes responds with vertical splits that expose the tunneling galleries underneath — winding, S-shaped tracks that are distinct from any other pest damage in ash trees.

Increased woodpecker activity. Woodpeckers feed on EAB larvae, and a sudden increase in woodpecker damage — patches of bark stripped away, especially high on the trunk — is often a sign of a significant larval population underneath, sometimes visible before other symptoms.

Epicormic sprouting. Ash trees under stress from EAB sometimes send out a burst of new shoots directly from the trunk or major limbs, a stress response that’s a fairly reliable secondary indicator once other symptoms are present.

Treat, monitor, or remove?

This is the decision that actually matters, and it depends heavily on how far the infestation has progressed. Trees caught early — canopy still mostly intact, symptoms limited to a portion of the crown — can often be saved with systemic insecticide treatment, which is applied on a recurring schedule and is genuinely effective when started before the tree is significantly compromised.

Trees with more than 30 to 50% canopy loss are generally past the point where treatment makes economic or practical sense; at that stage, the tree’s structural integrity is also declining, which turns it into both a lost cause and a safety concern. Dead and severely compromised ash trees become brittle quickly and are prone to sudden limb failure or full structural collapse, often with little warning — which is exactly the scenario our emergency line gets called out for more than we’d like.

Why catching it early matters so much

The math is straightforward: treating a healthy or lightly-infested ash tree costs a fraction of what emergency removal costs once it’s dead and hazardous, and it keeps a mature tree — often decades old — actually alive instead of becoming a stump. If you have ash trees on your property and haven’t had them assessed in the last year or two, that’s the single highest-value tree care decision available to you right now.

What an assessment involves

When we come out for a tree health assessment, we’re looking at canopy condition, checking for exit holes and bark splitting at multiple heights, and assessing overall structural risk if the tree is already declining. You’ll get a straight recommendation — treat, monitor, or remove — along with the reasoning, so you can make an informed decision rather than just taking our word for it.