Emerald Ash Borer in Monroe County: How to Spot and Stop It

By Linda Marsh, Pests & Diseases. Last updated: June 13, 2026.

If your ash tree is showing trouble, watch for these warning signs:

• Thinning or dieback starting at the very top of the canopy
• Small D-shaped holes, about 1/8 inch wide, in the bark
• Pale "blonding" patches where woodpeckers have stripped bark to reach larvae
• Leafy sprouts (epicormic shoots) erupting along the trunk and base

What is emerald ash borer, and why does it matter here?

Emerald ash borer (scientific name Agrilus planipennis, often shortened to EAB) is a small, metallic-green beetle native to Asia, first detected in North America near Detroit in 2002. The adult beetle is barely the size of a grain of rice, but the real damage comes from its larvae, which tunnel under the bark and cut off the tree's ability to move water and nutrients.

EAB attacks ash trees in the genus Fraxinus: green ash, white ash, and black ash are all common targets across Upstate New York. The insect is now established throughout New York State, including the Rochester area and the surrounding Finger Lakes region. For Monroe County homeowners in communities like Fairport, Pittsford, Penfield, Webster, Brighton, and Greece, the practical takeaway is blunt: if you have a mature ash and you do nothing, EAB will almost certainly kill it.

Ash was planted heavily as a street and yard tree for decades because it grows fast and tolerates tough sites, which is exactly why the loss has been so visible. If you are still working out what species you have or why it looks stressed, our guide on why tree leaves curl, yellow, or die can help you read the early symptoms before you assume the worst.

What are the signs of an emerald ash borer infestation?

EAB is sneaky because the damage happens out of sight, beneath the bark, before the canopy visibly suffers. By the time most people notice, the tree has often been infested for two or three years. Here is what to look for, roughly in the order it tends to appear:

• Canopy dieback from the top down. Ash decline almost always starts at the crown. You will see thin, sparse foliage and bare branch tips in the upper third of the tree while lower limbs still look reasonably full.
• D-shaped exit holes. When adult beetles emerge in early summer, they chew a distinctive capital-D-shaped hole about 1/8 inch wide. That flat-sided D is one of the clearest fingerprints of EAB and helps rule out other borers, which leave round or oval holes.
• S-shaped galleries under the bark. If you peel back a piece of loose or dying bark, EAB larvae leave winding, S-shaped feeding tunnels packed with fine sawdust. These serpentine galleries girdle the tree from the inside.
• Bark splitting. Vertical cracks open in the bark as the tree reacts to larval tunneling, sometimes revealing the galleries underneath.
• Woodpecker "blonding." Woodpeckers feast on EAB larvae and flake off the outer bark to reach them, leaving pale, blond patches on the trunk and limbs. Heavy woodpecker activity on an ash is a strong red flag.
• Epicormic sprouts. Stressed ash trees push out leafy shoots directly from the trunk and root flare, a last-ditch survival response that signals the canopy above is failing.

One or two of these signs can have other causes, so it pays to confirm with a professional. Some symptoms overlap with other local tree problems; for example, top-down decline can superficially resemble the canopy thinning seen with beech leaf disease in the Rochester area, even though the cause and host trees are completely different.

Can an infested ash tree actually be treated?

Yes, but timing is everything. A valued ash caught early, while it still has most of its canopy, can often be protected with insecticide treatments. The most effective approach for mature, high-value trees is a trunk injection of a systemic insecticide such as emamectin benzoate, applied by a licensed professional. The product moves up into the canopy and kills larvae as they feed. These injections are not a one-time fix: they are repeated every two to three years for as long as EAB pressure remains in the area, which in Monroe County means indefinitely.

Treatment makes the most sense when:

• The tree is structurally sound and has lost less than roughly a third to a half of its canopy.
• It is a desirable, well-placed shade tree worth a long-term investment.
• You are committed to the recurring cost of re-treatment.

Treatment is generally not worth it for trees that are already in steep decline, poorly placed, or otherwise in bad health. Once an ash has lost more than about half its canopy, the odds of a meaningful recovery drop sharply, and the money is usually better spent on removal and replanting. A certified arborist can measure canopy loss accurately and give you an honest treat-versus-remove recommendation rather than a guess.

Why is removing a dead ash so dangerous?

Dead and dying ash are among the most hazardous trees to remove, and EAB makes the problem worse fast. As the larvae destroy the living tissue, the wood dries out and becomes brittle far more quickly than other dead hardwoods. Limbs that would normally flex instead snap without warning, and large sections can fail unpredictably during climbing or rigging.

This is not a do-it-yourself project once a tree is dead or declining. Brittle, compromised ash near a house, driveway, power line, or play area is exactly the kind of removal that injures homeowners every year. It calls for crews trained in tree-risk assessment, equipped with the right rigging, and most importantly fully insured. Hiring certified pros is not just about a neat job; it is about getting it done without anyone getting hurt and without damaging your property. If you are weighing your options, our guide on how to choose the best tree service in Rochester explains the credentials and insurance to verify before anyone climbs.

How do you decide whether to treat or remove?

Start by getting an honest assessment from a certified arborist, ideally before the tree's decline forces your hand. A good evaluation looks at the percentage of canopy already lost, the tree's overall structure and health, its location and the targets it could hit if it failed, and your long-term goals for the landscape. From there the decision usually sorts itself out:

• Treat if the tree is healthy, valuable, well-placed, and still has most of its canopy, and you are willing to commit to repeat injections.
• Remove and replant if the tree is in significant decline, hazardous, or simply not worth the recurring cost. Replacing it with a diverse mix of species protects you against the next pest that targets a single genus.

Whatever you decide, do not wait. EAB does not pause, and every season of delay narrows your options and raises the risk that a once-treatable tree becomes a removal-only liability.

FAQ

How can I tell if my tree is even an ash?

Ash trees have compound leaves with 5 to 9 leaflets, branches and buds arranged directly opposite each other, and a diamond-patterned bark on mature trunks. Because EAB only attacks ash, confirming the species is the first step; a certified arborist can identify it on sight if you are unsure.

Is it too late to treat my ash if I already see D-shaped holes?

Not necessarily, but it depends on how much canopy remains. If the tree has lost less than roughly a third of its crown, trunk-injection treatment can still be effective. Once more than half the canopy is gone, treatment rarely succeeds and removal is usually the safer, smarter investment.

How often do EAB treatments need to be repeated?

Systemic trunk injections such as emamectin benzoate are typically repeated every two to three years for as long as emerald ash borer remains active in the area. In Monroe County that pressure is ongoing, so treatment is best viewed as a long-term commitment, not a one-time cure.

Can I just leave a dead ash standing in my yard?

It is strongly discouraged. Dead ash dry out and turn brittle quickly, and limbs or whole sections can fail without warning onto homes, cars, walkways, or people. The longer a dead ash stands, the more dangerous and expensive it becomes to remove safely.

Sources

• Monster Tree Service of Rochester
• New York State Department of Environmental Conservation: Emerald Ash Borer
• Cornell University / Cornell Cooperative Extension
• USDA APHIS: Emerald Ash Borer