The Emerald Ash Borer

What is the emerald ash borer?

The emerald ash borer (Agrilus planipennis) is an invasive wood-boring beetle from eastern Asia whose larvae feed under the bark of ash trees and kill them. It belongs to the family Buprestidae — the metallic wood-boring beetles — and it is native to eastern Russia, northern China, Japan, and Korea, where native ash species and native predators keep it in check. North American ash evolved without it and has essentially no defense.

It matters more than most invasive insects for two reasons. First, it attacks healthy trees, not just stressed or dying ones: a vigorous 40-year-old green ash in a front yard is as vulnerable as a neglected one. Second, ash was planted heavily as a street and yard tree across the Midwest and Northeast — often as the replacement for elms lost to Dutch elm disease — so entire neighborhoods were planted with a single species that this beetle can clear out. USDA and Forest Service sources describe hundreds of millions of ash trees killed in North America since 2002; USDA APHIS uses the more conservative phrase "tens of millions." Either way, the scale is unusual, and mortality among untreated ash in an established infestation is commonly reported at around 99 percent.

One important difference from the Asian Longhorned Beetle: ALB is still the target of an active federal eradication program, so the response to a sighting is capture and report. EAB is past that point. USDA APHIS removed the federal domestic EAB quarantine in a final rule effective January 14, 2021, and redirected resources toward releasing parasitoid wasps as biological control. For a homeowner that changes the practical question from "how do we stop it spreading?" to "is my tree worth treating?"

What does the emerald ash borer look like?

An adult emerald ash borer is a narrow, bullet-shaped beetle about half an inch long and a bright metallic emerald green, with a coppery-red to purplish abdomen hidden under its wing covers. It is small — roughly the length of a grain of rice laid end to end twice — and it spends most of its short adult life high in the canopy, which is why most people who have EAB in their yard never see one. The larva is what does the damage: a flattened, cream-white grub up to about an inch and a quarter long, with a distinctive series of bell-shaped body segments, found only under the bark.

In practice you will almost always identify EAB from the tree rather than the insect, and the single most useful detail is the shape and size of the exit hole. Ash trees host several other borers, and each leaves a different signature. This table is the fastest way to separate them:

The last row is worth reading carefully, because it is the one people get wrong in both directions. EAB attacks only ash — a maple with borer holes is never EAB. But the reverse does not hold: ash is on the ALB host list alongside maple, willow, elm, and birch, so on an ash tree the species alone does not settle it. The exit hole does: 1/8-inch and D-shaped is EAB, 3/8-inch and perfectly round is ALB, and the gap between those two sizes is wide enough that a coin held against the bark decides it. If you are not certain what species your tree is — ash has opposite branching and compound leaves, which is unusual — start with what's killing my tree? instead.

Why does the emerald ash borer kill ash trees so reliably?

Emerald ash borer kills ash trees so reliably because its larvae feed in exactly the tissue the tree cannot do without, and they do it in numbers. After hatching, each larva chews through the outer bark into the phloem and cambium — the thin living layer between bark and wood that moves sugars down from the leaves and grows new wood outward. As it feeds it carves a flat, winding S-shaped gallery, widening as the larva grows, and packs the tunnel behind it with fine sawdust-like frass.

One gallery is nothing to a mature tree. A few hundred, overlapping and crossing, sever enough of that conductive layer to girdle the trunk or branch — the same effect as cutting a ring of bark all the way around. Everything above the girdle is cut off from the roots. This is why EAB damage shows up first in the top of the tree: the highest branches are furthest from the roots and starve first, and why the canopy thins from the top down rather than patchily.

Two things make this worse than the arithmetic suggests. The beetle attacks vigorous trees, so keeping an ash healthy and well-watered does not protect it the way it protects a tree from opportunistic borers. And the damage is entirely hidden for the first year or more, because everything is happening under intact bark. By the time the outside of the tree looks wrong, a large share of the conductive tissue is already destroyed — which is the whole reason the treat-versus-remove decision below is so time-sensitive.

What are the signs of an emerald ash borer infestation?

The signs of an EAB infestation appear in a fairly consistent order, and knowing that order tells you roughly how far along your tree is. Woodpecker damage and canopy thinning usually come first, exit holes and bark splits later, and basal sprouting last — often when the tree is already failing.

• Woodpecker flecking, or "blonding." Woodpeckers strip the thin outer bark to reach larvae, leaving pale patches on the trunk and larger limbs. Several extension programs describe this as the earliest and most reliable outward indicator, typically appearing in the second or third year after infestation. On a bare winter tree it is visible from the sidewalk.
• Canopy dieback from the top down. The upper third of the crown thins, leafs out late, or dies back while the lower canopy still looks reasonable. Infested trees commonly lose most of their canopy within about two years.
• D-shaped exit holes. Adults chew out through the bark leaving a hole with one flat side and one rounded side, about 1/8 inch (3 mm) across. This is the one diagnostic sign — nothing else that attacks ash leaves that shape at that size. Early on the holes are high in the canopy where you cannot see them; finding them at eye level on the trunk means the infestation is well advanced.
• Vertical bark splits. As the tree tries to wall off larval galleries with callus tissue, the bark splits open in narrow vertical cracks. Peel one back and the S-shaped galleries are usually right underneath.
• Epicormic shoots and basal sprouting. The tree pushes out new growth from the trunk and root flare, below the girdled tissue — a stress response that looks like the tree fighting back but generally means it is losing.

No single sign except the D-shaped exit hole is proof on its own; drought, root damage, and ash yellows can all thin a canopy. Our signs and symptoms guide walks through each one in detail, including how to tell woodpecker flecking from ordinary bark loss and how fast symptoms progress once a tree is infested.

Where is the emerald ash borer found in the United States?

The emerald ash borer is now found across most of the eastern and central United States and continues to spread westward. As of September 2026, per USDA APHIS, EAB has been detected in 38 states and the District of Columbia. That figure moves — new states and counties are confirmed most years — so treat it as a snapshot and check the APHIS page for the current map rather than relying on any number printed on a website, including this one.

There is no longer a federal domestic quarantine to consult. APHIS removed it effective January 14, 2021, having concluded the quarantine was not stopping the spread, and shifted resources into biological control. Some states still run their own firewood or nursery-stock restrictions, so the practical rule for a homeowner is unchanged and worth repeating: do not move firewood. Long-distance jumps in the EAB range have generally been human-assisted, in firewood and untreated ash logs, not the beetle flying there.

For deciding what to do about your own tree, national spread matters less than local proximity. The widely used rule of thumb from extension entomologists is that preventive treatment starts making sense once a confirmed infestation is within roughly 15 miles of your tree — near enough that arrival is a question of when, not if.

Can an ash tree be saved from emerald ash borer?

Yes — many infested ash trees can be saved, but only if you catch the tree early and accept that treatment is ongoing rather than a one-time cure. The deciding factor is how much canopy the tree has already lost. Extension guidance generally treats an ash with up to about 30 percent canopy thinning as a good candidate for a rescue treatment with trunk-injected emamectin benzoate, and considers a tree with more than about 50 percent canopy loss unlikely to recover even with treatment; at 60–70 percent crown loss it is generally past saving. Those are guidance ranges from different state programs, not a single official cutoff, so an arborist looking at your tree may reasonably land a few points either side.

The honest framing is that you are signing up for a maintenance program, not a repair. Systemic insecticides have to be reapplied — annually for most soil-applied homeowner products, every two years for professional emamectin benzoate injections — for as long as you want the tree protected. That makes the decision partly financial: a large, healthy, well-placed ash that shades your house is often worth decades of treatment; a declining ash at the back of the lot usually is not, and the money is better spent on removal and a replacement of a different species.

Our emerald ash borer treatment guide works through that decision in full — the treat-versus-remove framework, how soil drench, trunk injection, and bark spray actually compare, what each costs, when the job needs a certified arborist, and the unproven remedies to skip.

What should you do if you suspect emerald ash borer?

If you suspect emerald ash borer, first confirm the tree is actually an ash — opposite branches, compound leaves, and diamond-patterned bark on a mature trunk — then photograph the specific signs you can see: the thinning canopy from a distance, any pale woodpecker patches, and a close-up of a suspected exit hole with something for scale beside it. Those photographs are what an arborist or a state agency will actually want.

What you do with them depends on where you live. In the large area where EAB is already established, reporting one more infested tree changes nothing; the useful next step is a treat-or-remove assessment while the canopy is still mostly intact. If EAB has not been confirmed in your state or county, the report matters — a first county detection changes what neighbors and the local program do next — so send it to your state agriculture or forestry agency.

How did the emerald ash borer reach North America?

The emerald ash borer reached North America inside solid wood packing material — the untreated pallets, crating, and dunnage used to brace freight — shipped from Asia. Larvae feeding under the bark of that wood survive the crossing easily, and until international standards for treating wood packaging were tightened, nothing about a wooden pallet flagged it as a pathway for a forest pest.

It was identified in southeastern Michigan, near Detroit, in July 2002, after local ash trees began dying in numbers no one could explain. Analysis of the damage suggested the beetle had already been established for roughly a decade, arriving in the early 1990s and spreading quietly through the region before anyone knew what to look for. That decade of invisible establishment is why eradication was never realistically on the table — and it is the same story as the Asian Longhorned Beetle's arrival, which came through the same pathway a few years earlier and was caught earlier, which is why one is still being fought and the other is being managed.