Emerald Ash Borer Signs: 4 Things to Look For on Your Ash Tree

Emerald Ash Borer kills ash trees, and if you live in southern Ontario, your ash is at risk. EAB has been spreading through the GTA for over a decade, and most untreated ash trees end up infested eventually. The good news is that the emerald ash borer signs are visible from the ground if you know what to look for. The better news is that not every infested tree has to come down. This walks through the four reliable signs you can confirm yourself, how to score what you find, and the real options once you confirm it.

D-shaped exit holes on the bark

When adult EAB beetles emerge from the tree in late spring or early summer, they chew their way out through the outer bark. The exit hole they leave is the most diagnostic of the visible emerald ash borer signs. Look for D-shaped holes, roughly 3 to 4 mm wide, on the trunk and major scaffold limbs. Not round. Not large. Specifically D-shaped, with the flat side often facing the ground.

Carry a small flashlight if the tree is shaded. The holes are easier to spot on smoother bark sections, which on most ash means younger or middle-aged trees more than mature trees with deep furrowed bark. If you find one hole, you almost certainly have many. EAB rarely produces a single beetle. The bark itself may also have a slight bulge or cracking around the holes from the larvae feeding underneath.

Worth knowing: if you see round exit holes instead of D-shaped ones, that is a different insect. EAB exits make a specific shape because the beetle has a flat back. No other Ontario borer leaves this signature.

S-shaped galleries under the bark

This is the strongest of the four signs. EAB larvae spend their working life under the bark, in the cambium layer that carries sap up and down the tree. They feed in serpentine, S-shaped tunnels through that tissue, and those tunnels are how the tree actually dies. The galleries cut the vascular connection between the roots and the canopy.

To check, find a section of trunk where the bark is loose, cracked, or already peeling, and lift a small piece off with a flathead screwdriver. If you see winding, sawdust-packed tunnels carved into the wood underneath, that is EAB. One mature infestation will produce hundreds of these galleries through a single trunk. You do not need to strip the tree to confirm. One peeling patch is enough.

If you are not sure whether what you are looking at counts as S-shaped, take a photo. Send it our way. EAB galleries are distinct enough that a single clear photo is usually all we need to call it.

Crown dieback that starts at the top and works down

Healthy ash trees push new leaves uniformly across the canopy. An EAB-infested ash starts losing leaves from the top of the tree first, not from the edges in. That is the opposite of drought stress (which works outside-in) and root damage (which is patchy and asymmetric). Stand back 30 feet, squint at the silhouette, and look for thinning at the crown.

By year two or three of infestation, the upper third of the canopy will be visibly bare. By year four to five, most of the tree is dead branches with leaves only on the lower limbs. The lower trunk often pushes out epicormic shoots in a kind of panic response. Short vertical sprouts coming straight off the trunk where there were no branches before.

Those sprouts are not a sign of recovery. They are the tree trying to photosynthesize anywhere it still can, because its top is gone. Once you see them, the tree is in late-stage infestation.

Woodpecker damage that exposes blonde patches

EAB larvae are calorie-dense, and Downy, Hairy, and Pileated woodpeckers figure out infested trees fast. They strip the outer bark off in chunks to get at the grubs underneath. Look for patches where the rough grey-brown outer bark has been peeled away, exposing a paler, almost blond inner layer. This is called blonding.

The patches concentrate on the upper trunk and the larger scaffold limbs, because that is where the larval density is highest. If you see broad bark-stripping with no other obvious cause, that alone is a strong indicator. Other woodpecker activity, like rectangular nest cavities or deep excavations, means something else. Blonding is specifically the shallow, wide bark-removal pattern.

Quick check before you panic: blonding combined with one of the other three signs is essentially confirmatory. Blonding alone in a tree with a full healthy canopy can sometimes be other things, but in an Ontario ash, the odds favor EAB.

How confident can you be?

One sign on its own is suggestive but not definitive. Two signs make EAB the leading suspect. Three or four emerald ash borer signs together, no other Ontario pest produces this combination, and you can treat the diagnosis as confirmed.

The two most diagnostic individual signs are the S-shaped galleries (basically pathognomonic in southern Ontario) and the D-shaped exit holes (no other native borer makes these). Crown dieback and woodpecker blonding can have other causes when isolated, but combined with the bark and gallery signs the case is closed. If the tree has all four, you are not looking at a maybe.

Once you confirm EAB, you have three options

Option one: TreeAzin injection, if the tree is still healthy enough to save. TreeAzin is a systemic insecticide injected into the trunk every two years. It works, but only if you start before the tree has lost more than about 30 percent of its canopy. Past that, the tree cannot transport the chemical effectively through what is left of its vascular system, and you end up paying to slow a decline you cannot reverse.

Option two: monitored decline. If the ash is over a target that does not matter (no house under it, no power line, no patio, no driveway, no kids play area), you can let the tree go down on its own schedule and take it out once it becomes hazardous. This is the lowest-cost path, but it requires being honest about whether the tree is actually over a target you care about.

Option three: removal. Take the tree down while it is still safe and inexpensive to take down. This is where most ash end up, because most ash on suburban properties are over a target. Tree removal is cheaper when the wood is still structurally sound. It gets more expensive every year that passes after the tree dies.

When removal becomes urgent

EAB-killed ash gets brittle fast. Once the tree has been dead for a year or more, the wood loses its tensile strength. Branches snap unpredictably under their own weight, even on still days. The trunk itself becomes harder to rig because it cannot be trusted to hold the loads a controlled takedown puts on it.

Crews can still take these trees down, but the work requires more setup. Sometimes a crane. Sometimes a chunking approach instead of standard rigging. Costs go up, sometimes substantially, on trees that have been dead for several years. If your ash has been dead for more than 18 months, get a quote in the next month or two. If it has been dead for 3 or more years, treat it as urgent.

The longer the tree sits, the higher the chance one of those upper limbs ends up through a roof, a fence, a car, or a power line. We respond across Toronto, Mississauga, Oakville, Burlington, and the rest of the south Ontario service area, with fastest response inside the GTA.

What to do this week

Pick a clear afternoon and walk around the trunk of your ash tree, slowly. Look for D-shaped holes on the bark at eye level and below. Lift any loose bark patches with a flathead screwdriver and check for galleries underneath. Step back 30 feet and look at the silhouette for top-down thinning. Note any blond patches on the upper trunk.

Take photos of anything you find, even if you are not sure what you are looking at. A few angles of the trunk plus the full-tree silhouette from 30 feet back are enough for us to give you a preliminary read over the phone.

If you see anything that looks like S-shaped galleries or extensive blonding, do not wait. The injection option stays open only for the first one or two seasons of visible damage. After that, the only honest options are monitor or remove. The emerald ash borer signs are subtle in year one and unmissable by year three, and the difference between those two windows is the difference between a manageable treatment plan and a more expensive removal.