Emerald Ash Borer: Signs, Damage & Treatment Options

What Is the Emerald Ash Borer?

The emerald ash borer (EAB), scientifically known as Agrilus planipennis, is a metallic green beetle native to eastern Asia. Adults are approximately half an inch long and one-eighth inch wide with a distinctive bright emerald green color. While the adult beetles feed on ash leaves, causing minor damage, it is the larval stage that kills trees. Female beetles lay eggs in bark crevices, and the hatching larvae bore into the cambium layer where they create winding, S-shaped galleries as they feed on the tissue responsible for transporting water and nutrients.

EAB was likely introduced to North America in the 1990s through solid wood packing material from Asia. It was first identified in southeastern Michigan in 2002 and has since spread to 36 states and five Canadian provinces. All North American ash species are susceptible, including green ash, white ash, black ash, and blue ash. An estimated 8.7 billion ash trees are at risk across the continent, and the pest has already killed hundreds of millions.

Signs of Emerald Ash Borer Infestation

D-Shaped Exit Holes

When adult beetles emerge from beneath the bark in late May through July, they leave distinctive D-shaped exit holes approximately one-eighth inch (3 mm) wide. These holes are unique to EAB and are one of the most reliable identification features. Round exit holes on ash trees indicate other borers, not EAB. Exit holes may be found on the trunk and larger branches but are often high in the canopy where they are difficult to spot from the ground.

Canopy Thinning

As EAB larvae destroy the cambium layer, the tree progressively loses its ability to transport water to the canopy. Thinning typically begins at the top of the tree (where water transport is most difficult) and works downward over two to four years. By the time top-down thinning is noticeable, the infestation is well established. Compare your ash tree's canopy density to how it looked in previous years.

Woodpecker Damage (Flecking)

Woodpeckers feed heavily on EAB larvae beneath the bark. Increased woodpecker activity on an ash tree is often the first visible sign of infestation. Woodpeckers strip outer bark to reach larvae, creating light patches called flecking. If an ash tree suddenly becomes a favorite woodpecker target, EAB is the likely reason.

Bark Splitting and S-Shaped Galleries

As larval galleries weaken the cambium, the bark may crack and split vertically, revealing the serpentine (S-shaped) feeding tunnels packed with sawdust-like frass. These galleries are typically visible on the inner surface of bark that has been pulled away. Each gallery represents the feeding path of a single larva over one to two years.

Epicormic Sprouting

As the canopy dies back, many ash trees produce vigorous new shoots (epicormic sprouts or suckers) directly from the trunk or major branches. This is the tree's attempt to replace lost foliage but indicates significant stress. Epicormic sprouting combined with canopy thinning is a strong indicator of advanced EAB infestation.

Treatment Options

Emamectin Benzoate Trunk Injection (Recommended)

Emamectin benzoate (marketed as TREE-age and other brand names) is the gold standard for EAB treatment. It is injected directly into the base of the trunk through small, drilled ports. The insecticide is translocated upward through the vascular system, killing larvae feeding in the cambium and adult beetles feeding on leaves. Studies consistently show 95 to 99 percent efficacy. A single treatment lasts two to three years, making it cost-effective at $100 to $300 per application depending on tree diameter.

Imidacloprid Soil Drench

Imidacloprid-based products can be applied as a soil drench around the base of the tree, where roots absorb the insecticide and translocate it upward. This method is available as a homeowner product and is less expensive ($20 to $50 per application) but is less effective than trunk injection, particularly for trees larger than 20 inches in diameter. Soil drench requires annual application and takes four to six weeks to reach the canopy, so it must be applied in early spring before beetles emerge.

Dinotefuran Bark Spray

Dinotefuran can be sprayed directly on the trunk bark and is absorbed through the bark into the cambium. This provides a supplemental treatment layer and is sometimes used in combination with soil drench or injection. It is less proven as a standalone treatment but adds protection against adult beetles that land on the trunk to lay eggs.

When to Treat vs. When to Remove

Learn more about the treatment vs. removal decision in our treatment vs. removal guide.

Cost: Treatment vs. Removal

Current Spread

As of 2025, emerald ash borer has been confirmed in 36 states and five Canadian provinces. The beetle spreads naturally at approximately 10 to 20 miles per year through adult flight but can jump hundreds of miles through human transport of infested firewood, nursery stock, and wood products. Federal and state quarantines restricting the movement of ash wood products have slowed but not stopped the spread.

In heavily infested areas like the upper Midwest, estimates suggest that 99 percent of untreated ash trees will eventually die from EAB. Communities that once relied on ash as a dominant street tree (ash made up 20 to 30 percent of the urban canopy in many Midwestern cities) have faced massive removal and replanting costs. Proactive treatment of high-value trees is far more cost-effective than reactive removal.

Frequently Asked Questions