Emerald Ash Borer: Identification and Treatment

title: "Emerald Ash Borer: Identification and Treatment"

—- title: "Emerald Ash Borer: Identification and Treatment" slug: emerald-ash-borer hub: care category: Tree care description: "How to identify emerald ash borer infestation, understand treatment options and their limits, and decide whether to treat or remove an ash tree." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 9 —-

Agrilus planipennis, the emerald ash borer, has killed hundreds of millions of ash trees in North America since its detection in Michigan in 2002. It kills all North American ash species — green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana), black ash (Fraxinus nigra), and blue ash (Fraxinus quadrangulata) — in 3 to 5 years if untreated.

This is not a pest where you wait and see. By the time symptoms are obvious, treatment is less effective and sometimes too late.

Table of Contents

1. Identification: EAB Damage Signs
2. EAB Distribution and Spread
3. Treatment Options and Effectiveness
4. Treatment Decision: Treat or Remove?
5. Timing Your Treatment
6. Frequently Asked Questions

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Identification: EAB Damage Signs {#identification}

EAB damage signs from most to least obvious:

Crown dieback (most visible)

Canopy death begins at the top and progresses downward. This is the last thing to appear — it means larvae have been feeding for 2 to 3 years. Per Michigan State Extension, by the time 30% of the canopy is dead, the infestation has been active for several years.

D-shaped exit holes

Adult beetles emerge in May-June and leave D-shaped exit holes in the bark, approximately 1/8 inch wide. These are the most diagnostic sign. The flat side of the D aligns horizontally. Woodpecker holes are round; beetle holes are D-shaped.

Serpentine galleries under bark

If you peel back a section of bark on a declining ash, the inner surface will show S-shaped or serpentine tunnels in the cambium layer. These are the feeding galleries of EAB larvae. Per Cornell Cooperative Extension, a heavily infested tree has galleries so dense they interrupt the flow of water and nutrients between roots and canopy.

Bark splits

As larvae feed and galleries expand, the outer bark splits vertically, sometimes exposing the galleries beneath. This usually appears 2 to 4 years into an infestation.

Epicormic sprouting

Stressed ash trees push epicormic sprouts (similar to water sprouts) from the trunk in response to crown dieback. Vigorous sprouts from the lower trunk of an otherwise declining ash are a diagnostic warning sign.

Woodpecker activity

Woodpeckers excavate heavily infested ash bark looking for EAB larvae. Bark flecking and excavation pits concentrated on a particular tree are a useful early indicator, often appearing before the D-shaped exit holes are abundant.

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EAB Distribution and Spread {#distribution}

As of 2024, EAB is present in 36 U.S. states and 5 Canadian provinces, per USDA Animal and Plant Health Inspection Service (APHIS). If you live in eastern North America and have ash trees, assume EAB is in your area or will arrive.

Movement is primarily through firewood transport — a single piece of infested firewood can carry hundreds of larvae. Do not move ash firewood.

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Treatment Options and Effectiveness {#treatment-options}

Four active ingredients are approved for homeowner use in the U.S. for EAB treatment:

Per Purdue Extension, imidacloprid soil drench is the most common homeowner treatment and is effective when applied before significant canopy loss. It is a systemic insecticide — taken up through roots and distributed in the tree's vascular system, killing larvae feeding on treated tissue.

How to apply imidacloprid soil drench (homeowner):

1. Mix product per label instructions (concentration varies by product).
2. Apply around the base of the tree within the dripline.
3. Apply when soil is moist and rain is expected within 48 hours, or water in after application.
4. Timing: apply in late April through May, when EAB adults are beginning to emerge and trees are taking up water.
5. For trees over 47 inches DBH, a single drench may not distribute adequately — trunk injection or a professional arborist is recommended.

Rutgers NJAES notes that imidacloprid is a neonicotinoid with potential effects on pollinators — it moves into pollen and nectar. Ash trees are wind-pollinated and produce insignificant amounts of nectar, reducing (but not eliminating) this concern. Still, avoid treating if the tree is near significant bee forage.

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Treatment Decision: Treat or Remove? {#treat-or-remove}

The 50% canopy rule is the general guide: if a tree has lost 50% or more of its canopy, treatment success rates drop significantly and removal becomes the economically rational choice.

Per Michigan State Extension, trees with significant canopy dieback also have compromised structural integrity. Dead limbs drop without warning. A tree that is 60% dead poses a serious safety risk and should be removed promptly regardless of treatment potential.

Also consider the tree's location and value. A large ash in a prominent location where removal would cost $2,000-5,000 is a candidate for treatment. A 4-inch-caliper ash in a row of six similar trees is not.

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Timing Your Treatment {#timing}

Apply soil drench treatments in April-May when soil temperatures at 4 inches depth reach 50°F and adults are beginning to emerge. Applications in late summer or fall are less effective because the tree is reducing uptake heading into dormancy.

For trees already infested but being treated preventatively going forward, treat every year. For uninfested trees in areas where EAB has been confirmed within 15 miles, begin preventative treatment — per Purdue Extension, preventative treatment is cost-effective for ash trees with more than 15 inches trunk diameter (about 47-inch circumference).

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Frequently Asked Questions {#faq}

How do I know if my ash tree is an ash?

Ash trees have opposite branching (branches and buds emerge in pairs directly across from each other), compound leaves with 5-9 leaflets, and distinctive diamond-patterned bark on mature trees. The leaf scars are distinctive. Per University of Minnesota Extension, opposite branching is the fastest diagnostic — very few common landscape trees have truly opposite branching. Use the mnemonic "MAD CAP Horse" for opposite-branching trees: Maple, Ash, Dogwood, Catalpa, Ash, Paulownia, Horse chestnut.

Can I buy trunk injection kits at hardware stores?

Arborjet-style injection kits marketed to homeowners (Mauget capsules, TreeTech) are available and work for small trees. Per Purdue Extension, homeowner injection kits are most effective on trees under 10 inches DBH. Larger trees benefit from professional-grade pressure injection equipment that distributes the chemical more thoroughly.

My neighbor's ash is dying — will my tree get infested?

Yes, eventually. EAB adults can fly up to 5 miles, though most spread is local. A heavily infested tree within 100 yards of your ash substantially increases your tree's risk. Begin preventative treatment now rather than waiting for symptoms.

What do I do with the removed wood?

Ash logs and chips from EAB-infested trees can be chipped, burned on-site, or buried. Do not move infested logs. Do not take firewood off the property. Per USDA APHIS, many states have regulations restricting the movement of ash wood and bark to prevent further spread.

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Recommended gear: Best dogwood cultivars (Cornus) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Sources

1. Michigan State Extension &mdash; <a href="https://www.canr.msu.edu/news/emerald_ash_borer_biology_and_identification">Emerald Ash Borer Biology and Identification</a>.
2. Cornell Cooperative Extension &mdash; <a href="https://www.gardening.cornell.edu/homegardening/">Home Gardening</a>.
3. USDA APHIS &mdash; <a href="https://www.aphis.usda.gov/aphis/resources/pests-diseases/emerald-ash-borer">Emerald Ash Borer</a>.
4. Purdue Extension &mdash; <a href="https://extension.entm.purdue.edu/publications/E-series/E-440.html">Emerald Ash Borer Management</a>.
5. Rutgers NJAES &mdash; <a href="https://njaes.rutgers.edu/plant-disease/plant-disease-fact-sheets/pdfs/FS1135.pdf">Emerald Ash Borer in New Jersey</a>.
6. University of Minnesota Extension &mdash; <a href="https://extension.umn.edu/tree-care/tree-identification">Tree Identification</a>.