Emerald ash borer identification (vs lookalikes)

The emerald ash borer (Agrilus planipennis) has killed an estimated 100 million ash trees in North America since its discovery in Michigan in 2002. It is now established in at least 35 states and continues to spread. Accurate identification matters enormously -- both because ash trees worth.

—- title: "Emerald ash borer identification (vs lookalikes)" slug: emerald-ash-borer-id hub: problems category: "Disease-by-host" description: "Emerald ash borer has killed hundreds of millions of ash trees in North America. Learn to identify the D-shaped exit holes and S-shaped galleries that distinguish it from native lookalike borers, and understand what to do if you suspect an infestation." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

The emerald ash borer (Agrilus planipennis) has killed an estimated 100 million ash trees in North America since its discovery in Michigan in 2002. It is now established in at least 35 states and continues to spread. Accurate identification matters enormously — both because ash trees worth treating deserve early diagnosis, and because misidentifying native ash borers or other common insects as EAB wastes money on unnecessary treatments and delays appropriate action.

I don't grow ash trees at my Long Island property, but the pest is established throughout the region, and I've seen EAB-killed ash trees on roadsides within a few miles. This guide draws on USDA APHIS, Penn State Extension, and Cornell Cooperative Extension research.

The pest

The emerald ash borer (Agrilus planipennis) is a jewel beetle (Buprestidae) native to Asia. Per USDA APHIS, it was first detected in North America near Detroit in 2002 and has spread to over 35 states and several Canadian provinces.

Adults:

• 7–15mm long, narrow, elongated
• Brilliant metallic emerald green
• Flat-bottomed when viewed from below (unlike the rounded body of most native metallic beetles)
• Red-purple abdominal segments visible under wing covers when resting

Larvae:

• Cream-colored, legless, S-shaped; 26–32mm at maturity
• Flattened head with bell-shaped first segment behind head
• Found in feeding galleries just under the bark

Host specificity: EAB infests only ash trees (Fraxinus spp.) with high reliability. Per Cornell Cooperative Extension, white fringe tree (Chionanthus virginicus) has also been documented as a host in rare cases.

Identification

D-shaped exit holes

Per Penn State Extension, the adult beetles emerge from under the bark by chewing through it, leaving a characteristic D-shaped (semicircular flat-bottomed) exit hole, approximately 3–5mm across. This D-shape is the most widely cited diagnostic sign.

Compare to native borer exit holes:

• Round holes: native wood-boring beetles (cerambycids, many species) leave round exit holes; EAB exit holes are distinctly D-shaped, not circular
• Oval holes: woodpeckers feeding on EAB larvae sometimes create oval or irregular excavations in the bark; these are larger and irregular, not D-shaped

S-shaped larval galleries under bark

Per USDA APHIS, peeling or cutting away a section of bark from a suspected infested ash reveals the larval feeding galleries:

• Serpentine, tightly packed galleries that fill the cambium just under the bark
• The S-shape or C-shape of the gallery is characteristic; galleries are packed with frass (fine sawdust mixed with excrement)
• Multiple galleries under a small bark section indicate heavy infestation

Per Penn State Extension, the galleries sever the phloem and cambium, girdling the tree and cutting off nutrient transport. This is why trees decline rapidly — not from direct feeding on wood, but from the disruption of the vascular layer just under the bark.

Crown dieback pattern

Per Cornell Cooperative Extension, EAB-infested ash trees show a distinctive crown dieback pattern:

• Dieback begins in the upper crown and progresses downward over 2–4 years
• The percentage of canopy remaining decreases each season
• Epicormic shoots (water sprouts) emerge from the lower trunk and major branches as the tree attempts to compensate for crown loss
• Trees typically die within 3–5 years of initial infestation without treatment

Woodpecker damage

Per USDA APHIS, woodpeckers (particularly red-bellied and downy woodpeckers) excavate bark from ash trees to access EAB larvae. Heavy woodpecker damage — large irregular patches of bark removed from the trunk and major branches — is a strong indicator of EAB infestation and sometimes the first visible sign that something is wrong.

Lookalikes and common misidentifications

Ash/lilac borer (Podosesia syringae)

Per Penn State Extension, the ash/lilac borer is a native clearwing moth that infests ash and lilac at the trunk base and branch unions. Its exit holes are round, not D-shaped, and are much larger (8–10mm). Frass is coarser and may extrude from the entry hole. Infestations are typically at the base of the trunk or at branch unions, not distributed throughout the canopy.

Banded ash clearwing borer (Podosesia aureocincta)

Per Cornell Cooperative Extension, this native clearwing moth infests ash with round exit holes and coarse frass. Less widespread than EAB.

Metallic wood-boring beetles (native Agrilus spp.)

Per USDA APHIS, several native Agrilus species also infest ash and produce D-shaped exit holes similar to EAB's. Native Agrilus on ash:

• Are generally smaller and less metallic-green than EAB
• Infest stressed, declining, or dead trees (secondary invaders); EAB kills healthy trees
• Do not cause the same rapid, progressive crown dieback as EAB

Per Penn State Extension, the key distinction: native Agrilus on ash is a secondary invader of already-stressed trees; EAB attacks healthy trees and is the primary cause of decline.

What to do if you suspect EAB

Per USDA APHIS:

1. Do not move ash firewood — EAB spreads through movement of infested wood
2. Contact your state department of agriculture or local Cooperative Extension office for confirmation
3. Do not remove the tree before confirmation — preserved evidence is useful for diagnosis
4. Photograph D-shaped exit holes, larval galleries (if bark is peeled), adult beetles, and crown dieback

Treatment

Per Penn State Extension, systemic insecticides are effective when applied before the tree has lost more than approximately 50% of its canopy:

• Emamectin benzoate (TREE-äge) — trunk injection by certified arborist; effective for 2 years
• Imidacloprid (Bayer Tree and Shrub) — soil drench; effective; requires 2–4 weeks to distribute throughout tree; apply April–May
• Dinotefuran — soil drench or bark spray; faster systemic activity than imidacloprid

Per Cornell Cooperative Extension, trees with more than 50% canopy loss are unlikely to respond to treatment and should be removed before structural failure becomes a safety hazard.

Common problems table

Frequently Asked Questions

Can an ash tree survive EAB without treatment?

Per USDA APHIS, the vast majority of untreated ash trees die within 3–5 years of initial EAB infestation. Some individual trees in areas with high natural enemy pressure (particularly in the insect's native Asian range) survive, but survival of untreated North American ash is not expected under normal infestation.

How do I know if my ash is worth treating?

Per Penn State Extension, the treatment decision should consider tree size, structural value, canopy loss percentage, and cost. Trees with less than 30% canopy loss are excellent treatment candidates; trees at 30–50% loss may respond but less reliably; trees over 50% canopy loss should generally be removed.

How far does EAB spread?

Per USDA APHIS, EAB spreads naturally 5–20 miles per year through adult flight. Long-distance spread is almost always through human movement of infested ash firewood, logs, or nursery stock. Never move ash firewood from an infested area.

Are all ash tree species susceptible?

Per Cornell Cooperative Extension, all North American ash species (Fraxinus americana, F. pennsylvanica, F. quadrangulata, etc.) are susceptible. Asian ash species (which evolved with EAB) are more tolerant but not immune. No resistant North American ash cultivar is currently commercially available.

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Sources

1. USDA APHIS — Emerald Ash Borer
2. Penn State Extension — Emerald Ash Borer
3. Cornell Cooperative Extension — Emerald Ash Borer Management