What are the first signs of emerald ash borer?

The earliest visible sign is thinning of the upper crown — the top quarter to third of the canopy begins losing leaves before lower branches are affected. This is easy to miss in a dense stand. As infestation progresses, epicormic sprouting appears — clusters of leafy shoots erupting directly from the trunk and major branches below the dying crown. These are the tree's stress response to interrupted nutrient flow and are one of the clearest field indicators of EAB infestation.

What do emerald ash borer holes look like?

Adult emerald ash borers emerge from the bark through D-shaped exit holes approximately 1/8 inch (3-4mm) wide — about the size of a pencil eraser. The flat side of the D faces upward. These holes are distinct from the round holes left by other wood-boring beetles. Finding D-shaped exit holes on ash bark is a strong confirming indicator of EAB even without seeing the insect itself.

How do I check for EAB galleries under the bark?

S-shaped larval galleries are located in the cambium layer — just beneath the outer bark. To check, use a knife or hatchet to remove a section of bark from a suspicious area and examine the inner surface. EAB galleries are serpentine feeding channels packed with frass (a mixture of excrement and wood powder) that wind in an S or Z pattern through the cambium. A high density of overlapping galleries effectively girdles the tree, cutting off nutrient transport.

Do woodpeckers indicate emerald ash borer?

Yes — heavy woodpecker activity on ash trees is a secondary indicator of EAB. Woodpeckers forage for EAB larvae under the bark, creating vertical strips of exposed wood called blonding. A tree with significant blonding typically has a high larval density and is in moderate to advanced infestation. Blonding is often visible from a distance and is one of the easiest field indicators to spot during a winter woodland walk.

What ash species does EAB affect?

Emerald ash borer affects all North American ash species. In Sullivan, Orange, and Ulster counties, the primary affected species are white ash (Fraxinus americana) and green ash (Fraxinus pennsylvanica). Both are common in mixed hardwood stands throughout the Hudson Valley and Catskills. Mountain ash (Sorbus) is not a true ash and is not affected by EAB.

Emerald Ash Borer Symptoms: How to Identify EAB on Your Ash Trees

Identifying emerald ash borer early gives you more options — for treatment, for timber recovery, and for protecting the rest of your woodland.

This guide covers the specific visual symptoms you can look for on foot, without laboratory equipment, on your own property.

Have ash trees and want a professional assessment? Call Henry Kowalec at (845) 754-8242 for a free woodland walk.

Step 1: Identify Your Ash Trees

Before looking for symptoms, confirm you are looking at ash trees.

Ash (Fraxinus spp.) has compound leaves with 5 to 11 leaflets arranged in opposite pairs along a central stem, with one leaflet at the tip. Ash bark on mature trees has a distinctive interlocking diamond pattern. Ash produces paddle-shaped samaras (seeds) that hang in clusters and persist through winter.

White ash and green ash are the most common species in Sullivan, Orange, and Ulster counties. Both are susceptible to EAB.

The 5 Visual Symptoms of EAB

Crown Dieback Starting at the Top

The first symptom most landowners notice is thinning in the upper crown — the top 25% to 50% of the canopy loses leaves and fine branches while lower branches remain full. This pattern of top-down dieback is characteristic of EAB and reflects where larval feeding is most concentrated in the early stages of infestation.

A single ash showing upper crown dieback in a stand where other trees appear healthy is a strong reason to investigate further.

Epicormic Sprouting on the Trunk and Branches

As EAB larvae disrupt nutrient transport in the cambium, ash trees respond by pushing out epicormic shoots — dense clusters of leafy growth erupting directly from the bark of the trunk and major branches. These sprouts appear below the dying crown and are the tree’s attempt to compensate for the loss of canopy photosynthesis.

Epicormic sprouting on an ash tree is a clear stress indicator. Combined with upper crown dieback, it is a reliable field sign of active EAB infestation.

D-Shaped Exit Holes in the Bark

Adult EAB beetles emerge from the bark in late spring through a D-shaped hole approximately 1/8 inch (3-4mm) wide. The straight edge of the D is at the top.

These holes are distinctive — no other common New York forest insect produces the same D-shaped emergence hole on ash bark. They may be small enough to overlook on a casual inspection, so look carefully at the mid-trunk section of suspicious trees, particularly in areas where the bark has a smooth texture.

Exit holes can appear anywhere on the trunk but are often concentrated in areas where bark thickness is moderate — not the roughest bark at the base, not the thinnest bark at the top, but the mid-trunk zone.

S-Shaped Larval Galleries Under the Bark

EAB larvae feed in serpentine S or Z-shaped channels through the cambium — the living tissue just under the outer bark. These galleries are packed with fine frass and wind back and forth across the grain of the wood, effectively girdling the tree when gallery density is high enough.

To check for galleries, use a knife or hatchet to peel back a section of bark on a suspected tree. The inner surface of the bark and the exposed cambium will show the serpentine channels clearly if EAB is present. A tree with overlapping galleries across a significant portion of its trunk circumference is effectively doomed regardless of treatment.

Woodpecker Blonding on the Trunk

Woodpeckers — particularly downy, hairy, and pileated woodpeckers — forage aggressively for EAB larvae. On heavily infested trees, they strip bark in vertical channels, exposing the pale sapwood beneath. This creates a characteristic blonded appearance on the trunk that is visible from a distance during winter and early spring.

A stand of ash with multiple blonded trees indicates a significant EAB population. This level of infestation typically means the affected trees have passed the window for chemical treatment.

What to Do If You Find These Symptoms

Finding EAB symptoms on your ash trees does not require an immediate emergency response — but it does require a decision.

The key question is how far the infestation has progressed, and that determines your options: chemical treatment, timber salvage harvest, or a combination of both depending on individual tree condition and the economics of each option.

Henry Kowalec walks EAB-affected properties throughout Sullivan, Orange, and Ulster counties and advises landowners on exactly those decisions — based on what he sees on the ground, not a generic recommendation.

Call (845) 754-8242 to schedule a free woodland assessment.

Henry Kowalec — Certified Consulting Forester — Environmental Forest Products, Westbrookville, NY