Signs and Symptoms of Emerald Ash Borer

Canopy dieback and thinning

Canopy dieback from emerald ash borer starts at the top of the tree and works down — that top-down direction is the detail worth remembering, because it is unusual and it follows directly from how the beetle does its damage. Larvae girdle the conductive tissue in the trunk and larger limbs; everything above the girdled point is cut off from the roots; and the highest branches, being furthest from the roots, starve first.

In practice you notice it as a crown that is thinner than the neighbours', leafs out later in spring, holds smaller or paler leaves, or shows bare twigs against the sky in midsummer. An infested ash commonly loses most of its canopy within about two years of the point where decline becomes visible. Estimating that loss matters: it is the number the whole treat-versus-remove decision runs on, with roughly 30 percent thinning as the outer edge of a good rescue candidate and roughly 50 percent as the point where treatment stops being recommended.

On its own, though, a thinning canopy is not proof of anything. Drought, root severance from construction, girdling roots, compacted soil, and ash yellows all produce a thin crown. Canopy dieback tells you to investigate the trunk; it does not tell you what you will find there.

D-shaped exit holes

A D-shaped exit hole about 1/8 inch across is the one sign that identifies emerald ash borer on its own. Nothing else that attacks ash leaves that shape at that size, which is why extension programs treat D-shaped holes on an ash as diagnostic where every other symptom is merely suggestive.

Picture a capital D pressed into the bark: one side is a flat straight edge, the other is a smooth curve, and the whole thing is roughly 3 millimetres wide — about the width of a pencil lead, small enough that people routinely walk past it. The shape comes from the beetle's flat-backed, rounded-bellied cross-section as it chews its way out in early summer. Look for them on the smooth upper trunk and the undersides of larger limbs, in raking light, and put a coin or a pencil tip beside one before you photograph it, because scale is exactly what makes the photograph useful.

Two cautions. First, timing: exit holes appear only when adults emerge, so a tree can be badly infested with none yet visible. Second, height: for the first couple of years the beetles are concentrated in the upper canopy, and holes appear there first. Finding exit holes at eye level means the population has worked down the tree and the infestation is well advanced — which is useful information, just not good news. If the hole you are looking at is round rather than D-shaped, or noticeably larger, see the comparison table on the emerald ash borer overview, or work through what's killing my tree?

Bark splitting and S-shaped larval galleries

Vertical splits in the bark are the tree's own response to the tunnels underneath, which makes them a useful place to look for confirmation. As the tree tries to wall off larval galleries with callus tissue, the bark over them is pushed outward and cracks open along the grain in narrow vertical splits, typically a few inches long.

Ease the bark back at one of those splits and you should see the diagnostic pattern beneath: flat, winding S-shaped galleries — sometimes described as serpentine — cut into the surface of the wood and the inner bark. They are about 1/8 inch wide, widen along their length as the larva grew, and are packed with fine, sawdust-like frass. Several overlapping galleries crossing one another over the same patch of trunk is what girdling actually looks like.

This is worth contrasting with the other borers of ash. Clearwing borers tunnel deeper into the wood and push coarse frass out of an oval hole; ash bark beetles cut short egg galleries across the grain, perpendicular to the branch, rather than winding along it. Shallow, S-shaped, frass-packed and just under the bark means EAB.

Increased woodpecker activity

A sudden interest from woodpeckers is one of the earliest outward signs of EAB, and it is genuinely distinctive — several extension programs describe it as the first and most reliable indicator a homeowner is likely to spot. The birds are not damaging the tree; they are finding a food supply that was not there before.

What you see is called flecking or blonding: woodpeckers chip away the thin dark outer bark to reach larvae underneath, exposing the pale layer below and leaving the trunk and larger limbs mottled with light-coloured patches. On a mature ash the effect is striking once you know to look — a trunk that appears to have been sanded in irregular blotches — and it is most visible in winter, with the leaves down and low sun raking across the bark. Reported timing puts flecking at roughly the second or third year after infestation, before most owners have noticed anything in the canopy.

The reason this sign is so useful is that it is visible from the ground on a tall tree, at the stage when exit holes are still 30 feet up and invisible. Blonding on an ash in an area where EAB is present is worth acting on straight away.

Epicormic shoots and basal sprouting

Shoots erupting from the trunk and the base of an ash are a late-stage stress signal, and they usually mean the tree is losing rather than fighting back. Ash trees carry dormant buds under the bark; when the canopy above can no longer feed the tree, those buds are released and push out new growth low on the trunk, at the root flare, or as a thicket of water sprouts around branch stubs.

It looks encouraging — fresh green growth on a struggling tree — but the position is the tell. This growth appears below the girdled tissue, which is precisely where the tree can still move resources. Meanwhile the crown above continues to die. By the time an ash is sprouting from the base, it has usually lost enough canopy that treatment is no longer recommended, and the practical question has shifted from saving the tree to removing it safely.

Basal sprouting has other causes too — severe pruning, sun scald, trunk wounds, transplant stress — so read it together with the rest. Sprouts plus a thin top plus blonding is a strong EAB picture; sprouts alone on an otherwise full-crowned tree is not.

How fast symptoms progress once a tree is infested

An infested ash typically dies within about three to four years, and reported timelines run from three to five years depending on tree size, vigour, and how many beetles arrive. The progression is consistent enough to place a tree roughly on the timeline:

• Year 1 — nothing visible. Eggs are laid in bark crevices and larvae begin feeding under intact bark. From outside, the tree looks healthy. This silent first year is why an infestation is almost never caught at the start.
• Years 2–3 — the first outward signs. Woodpecker flecking appears; the upper canopy begins to thin; the earliest exit holes are chewed, high in the tree. This is the window in which treatment has the best chance.
• Years 3–4 — obvious decline. Substantial canopy loss, exit holes and bark splits working down to eye level, epicormic shoots at the base. Most trees are past the point where treatment is recommended.
• Year 4 onward — death and brittleness. Untreated ash mortality in an established infestation is commonly reported at around 99 percent. A dead ash then becomes unusually brittle and hazardous to fell, which is a removal-cost argument for deciding early.

Those brackets are approximate and overlap in the field — a small, stressed tree can be killed faster and a large, vigorous one held longer. The useful takeaway is directional: the gap between "looks fine" and "cannot be saved" is short, and most of it is spent invisible.

What to do next

If two or more of these signs line up on a tree you have confirmed is an ash, the next step is a treat-or-remove assessment while the canopy is still mostly intact — the decision gets easier and cheaper the earlier it is made. Photograph the crown from a distance, any blonding on the trunk, and a close-up of a suspected exit hole with something for scale, then take those to a certified arborist or your extension office.

Read our emerald ash borer treatment guide for the canopy-loss thresholds, how the treatment methods compare, and when the job needs a professional. If you are not certain the tree is an ash, or the exit holes you have found are round rather than D-shaped, start instead with what's killing my tree?, which works from the symptom to the culprit. And for the biology behind all of it, go back to the emerald ash borer overview.