The clearest signs of emerald ash borer are small D-shaped exit holes in the bark, about an eighth of an inch across, together with thinning in the top of the canopy, light-colored patches where woodpeckers have stripped the outer bark, and clusters of sprouts on the trunk. Under loose bark you will find winding, S-shaped tunnels packed with fine sawdust. If your ash tree shows these signs but still has most of its canopy, it can often be saved with an insecticide treatment. If more than about half the canopy is gone, removal is usually the safer and cheaper long-term choice, because dead ash becomes brittle quickly. This guide shows how to confirm an ash tree, how to read the damage, and how to make the treat-or-remove call.
In this guide
Key Takeaways
- Emerald ash borer attacks true ash trees (Fraxinus), not mountain ash.
- Look for D-shaped exit holes, top-down canopy thinning, woodpecker blonding, bark splits and trunk sprouts.
- By the time you see symptoms, the tree has usually been infested for two or more years.
- Trees with less than roughly a third of their canopy lost are good treatment candidates; heavily declined trees should be removed.
- Do not move ash firewood; it is the main way the beetle spreads to new areas.
First, Confirm It Is an Ash Tree
Emerald ash borer (EAB) attacks green, white, black and blue ash and their cultivars, as well as white fringetree. It does not attack mountain ash, which is a different genus despite the name. It is one of the most destructive tree pests covered in our pest control guide. Before you worry about the insect, confirm the tree:
- Opposite branching: Buds and branches grow in pairs directly across from each other. Only a few common trees do this: maples, ash, dogwoods and horse chestnut.
- Compound leaves: Each leaf has 5 to 11 leaflets along a central stem. Maples have single lobed leaves, which rules them out.
- Bark: Mature ash bark has interlacing ridges forming a diamond pattern.
- Seeds: Ash produces single, paddle-shaped winged seeds in clusters, unlike the paired helicopters of maples.
The Main Signs of Emerald Ash Borer
1. D-shaped exit holes
Adult beetles chew their way out of the tree in late spring and early summer, leaving holes about 1/8 inch wide with one flat side, like a capital D. They are the most specific sign of EAB, but they are small and often start high in the canopy where you cannot see them. Look on the trunk and larger limbs in good light, especially on the sunny side. Other borers make round or oval holes. Native clearwing moth borers, for example, leave larger round holes, often with a papery pupal case sticking out.
2. Canopy thinning and dieback
Larvae feed in the layer just under the bark that carries water and nutrients. As tunnels accumulate, the upper canopy is starved first. You will see sparse leaves at the top, smaller and yellowing leaves, and bare twigs at branch tips, while lower branches still look normal. Thinning usually progresses over two to four years before the tree dies. Compare the tree’s crown with healthy ash trees nearby in midsummer; a thinner crown is often the first symptom homeowners notice.
3. Woodpecker damage, or blonding
Woodpeckers hunt EAB larvae through the winter by flaking off the outer bark. This leaves light tan or cream patches against the gray bark, a pattern foresters call blonding. Heavy blonding high on the trunk and main limbs is a strong indicator of a well-established infestation. It is often easier to spot than exit holes, especially in winter and early spring.
4. S-shaped galleries under the bark
If bark is loose or split, peel a small section back. EAB larvae carve serpentine, S-shaped tunnels that wind back and forth and are packed with fine, light brown frass. The creamy white larvae are flat with bell-shaped body segments and grow to about an inch long. These galleries are diagnostic, but do not strip bark from a healthy-looking tree just to check; that wounds it.
5. Bark splits
Callus tissue forms around larval galleries and can cause vertical splits in the bark a few inches long. Through the split you may see the gallery underneath.
6. Epicormic sprouts
A stressed ash tree often pushes out clusters of leafy shoots along the trunk and at the base. These water sprouts are the tree trying to replace the canopy it is losing. Sprouts alone can follow other stresses such as drought, root damage or construction, but sprouts plus top dieback on an ash tree are a red flag.
7. Adult beetles
Adults are bright, metallic emerald green, about half an inch long and slender, with a coppery red upper abdomen visible when the wings are open. They are active from roughly May through July. Several native metallic green insects, such as six-spotted tiger beetles, are often mistaken for EAB. Tiger beetles are shorter, broader, run on the ground and have white spots.
Look-Alike Problems to Rule Out
Not every thin ash canopy means EAB. Ash anthracnose, a fungal leaf disease, causes brown blotches and heavy leaf drop in cool, wet springs, but the tree usually refoliates by midsummer and has no exit holes. Ash yellows, a disease spread by leafhoppers, causes slow decline, small pale leaves and witches’ brooms of tufted shoots. Drought, compacted soil, girdling roots and construction damage near the root zone can also cause top dieback and trunk sprouts. The difference is in the bark: without D-shaped holes, blonding or serpentine galleries, look for one of these other causes first. When in doubt, many state extension services and departments of agriculture will confirm a suspected EAB sample or photo for free.
How EAB Damage Progresses
| Stage | What you see | Typical recommendation |
|---|---|---|
| Early | Canopy looks full; few or no visible symptoms; EAB confirmed nearby | Start preventive treatment on high-value trees |
| Light | Slight thinning at the top; some woodpecker activity | Treat promptly; good chance of recovery |
| Moderate | Noticeable dieback up to roughly a third of the canopy; exit holes; sprouts | Treatment can still work; get an arborist assessment |
| Severe | Half or more of the canopy dead; heavy blonding; bark splitting | Plan removal; treatment rarely recovers the tree |
| Dead | No leaves, bark falling off | Remove soon; dead ash becomes brittle and hazardous |
Treat or Remove: How to Decide
The main questions are how healthy the tree is now, how much you value it and how big it is. University extension programs commonly use a canopy loss threshold of about 30 percent: below that, treatment has a good chance of keeping the tree alive; well above it, the tree is unlikely to recover even if the insects are killed. Treatment is a long-term commitment, since EAB pressure continues for years in an infested area.
Treatment makes sense if:
- The tree has lost less than about a third of its canopy.
- It is structurally sound, without major trunk decay.
- It provides significant shade, screening or value, and removing it would be costly or disruptive.
- You are willing to continue treatment on a regular cycle.
Removal makes sense if:
- Half or more of the canopy is dead or thinning.
- The tree is near a house, driveway, power lines or play area, where a failing limb is a real risk.
- It is small, poorly placed or not particularly valuable to you.
- You do not want an ongoing treatment schedule.
Do not wait too long once a tree is declining. Ash killed by EAB dries out and becomes brittle faster than many other dead trees, so climbing it becomes unsafe and removal often requires a bucket truck or crane, which raises the cost. See our guide on tree removal cost for the factors that drive pricing, and on choosing a tree removal service for what to ask before hiring.
Treatment Options
Several systemic insecticides protect ash trees. They are absorbed and move through the tree’s vascular system to kill feeding larvae and adults. Always follow the product label, which is the law for pesticide use.
- Trunk injection with emamectin benzoate: Applied by licensed professionals, typically in spring after leaf-out. Labeled protection lasts up to two or three years per application, and it is widely regarded as the most effective option, especially for larger trees.
- Soil drench or injection with imidacloprid: Homeowner products are available and applied yearly in spring around the base of the tree. Labels limit homeowner use by trunk size, and results are less reliable on large trees. Avoid near ponds or streams.
- Basal bark spray with dinotefuran: Sprayed on the lower trunk by a professional, absorbed through the bark, and repeated yearly.
Systemic insecticides can affect pollinators that visit flowers, so many applicators prefer trunk injections, which keep the product inside the tree. Ash is wind-pollinated and not a major bee plant, which reduces but does not eliminate concern.
Preventing Spread
- Do not move firewood: Buy or gather firewood where you will burn it. Larvae survive in cut logs and emerge later.
- Check state rules: The federal EAB quarantine ended in 2021, but many states still regulate moving ash wood and firewood.
- Dispose of removed ash locally: Chip it on site or follow your municipality’s guidance for ash debris.
- Replant with diversity: Replace lost ash with a mix of species such as oaks, Kentucky coffeetree, hackberry, linden or serviceberry, so a single pest cannot wipe out your shade again.
When to Call a Pro
A certified arborist can confirm EAB, estimate canopy loss accurately from the ground or a lift, check for structural weaknesses and recommend whether treatment or removal makes more sense. Large trees and anything close to structures should be treated or removed by professionals. If you need an assessment, injection or removal, you can get quotes from local tree service pros.
Frequently Asked Questions
What does an emerald ash borer exit hole look like?
It is a small hole about 1/8 inch wide with one flat side, shaped like a capital letter D. Round or oval holes are usually made by other insects.
Can an ash tree with emerald ash borer be saved?
Often, yes, if it has lost less than about a third of its canopy and is otherwise sound. Systemic treatments, especially professional trunk injections, can protect it, but they must be repeated on a regular cycle.
When is the best time to treat for emerald ash borer?
Most treatments are applied in spring, after the tree leafs out and is actively moving water, so the insecticide spreads through the canopy before larvae begin feeding.
Does emerald ash borer attack mountain ash?
No. Mountain ash is not a true ash and is not a host. EAB attacks Fraxinus species such as green, white, black and blue ash, plus white fringetree.
How fast does emerald ash borer kill a tree?
Once symptoms are visible, trees often die within a few years, though large trees with light infestations can take longer. By the time thinning appears, the tree has usually been infested for at least two years.