Why Do Ohio Ash Trees Succumb To Emerald Ash Borer?

Emerald ash borer (EAB) has reshaped Ohio’s forests and urban tree canopy over the last two decades. The relentless mortality of ash trees in many communities raises the question: why are Ohio ash trees so vulnerable to this insect? The answer combines the insect’s biology, the evolutionary naivety of North American ash, landscape and human factors in Ohio, and specific limitations in management. This article explains the mechanisms of decline, identifies early warning signs, and provides practical, actionable guidance for landowners, arborists, and municipal managers facing EAB in Ohio.

The insect and its biology: why EAB is such a potent killer

Emerald ash borer (Agrilus planipennis) is a small, metallic green beetle native to parts of Asia. In its native range, ash species have evolved defenses and co-exist with the insect, but North American ash species lack these adaptations. Several biological traits make EAB particularly destructive:

• Extremely effective larval feeding. EAB larvae feed in the phloem and outer sapwood (cambium), producing serpentine galleries that interrupt the tree’s ability to transport water and sugars.
• Multiyear population build-up. Local infestations can remain subtle for several years while beetle numbers increase, then rapidly escalate into widespread tree mortality.
• High reproductive output. Adult females can lay hundreds of eggs over their lifespan, creating exponential population growth when conditions are favorable.
• Long-distance spread via human movement. The insect can hitchhike in firewood, nursery stock, or other ash wood products, enabling jumps beyond the slow natural spread by flight.

These traits mean that a single introduction can lead, within a few years, to a deadly infestation across a community if not detected and managed early.

Why Ohio ash species are especially at risk

Three biological realities make Ohio ash trees particularly susceptible:

• Lack of co-evolved defenses. North American ash species (including white ash, green ash, black ash, and others) did not evolve with EAB and therefore lack effective chemical and structural defenses against larval feeding.
• Abundance and distribution. Ash was a common component of Ohio forests and a preferred choice for street and yard plantings, creating large contiguous host resources for EAB populations to exploit.
• Tree physiology and EAB feeding location. EAB larvae feed in the phloem and cambium–critical tissues for nutrient and water transport. Once galleries extensively girdle the trunk and larger branches, trees decline rapidly.

Together these factors create a scenario where EAB can establish, build high local densities, and cause widespread mortality across multiple ash species in Ohio.

How EAB kills a tree: the mechanism of decline

Understanding the specific way EAB kills ash clarifies why tree decline is often rapid and difficult to reverse.

• Initial infestation. Female beetles lay eggs in bark crevices and under bark flaps. Eggs hatch into larvae that bore into the bark and begin feeding.
• Gallery formation. Larvae feed on the inner bark (phloem) and outer sapwood, creating winding galleries that sever the tissues that move carbohydrates and water.
• Progressive canopy decline. As phloem and cambial tissues are destroyed, the tree cannot support new leaf growth and water transport becomes impaired. Symptoms often appear as thinning in the upper canopy first.
• Epicormic sprouting and stress response. Stressed trees produce shoots from the trunk and lower branches. These sprouts are a sign of decline and indicate that the tree is diverting limited resources in a last effort to survive.
• Mortality. When sufficient phloem/cambium is disrupted (girdling), the tree dies. Time from first infestation to death varies by tree size and infestation intensity, typically from 2 to 10 years.

Signs and symptoms to detect EAB early

Early detection improves management options. Look for these characteristic signs:

• Thinning or dieback in the upper canopy, often starting at branch tips.
• Epicormic shoots (sucker growth) on the trunk and main branches.
• D-shaped exit holes, roughly 3-4 mm wide, where adults have emerged.
• Serpentine, S-shaped larval galleries under the bark visible if bark is peeled.
• Increased woodpecker activity; woodpeckers feeding on EAB larvae can produce flaking or stripped bark patches.
• Bark splitting and loose, flaky bark on the trunk and large branches.

If these signs are found, rapid assessment and action are warranted because EAB populations can intensify quickly.

Ohio-specific landscape and human factors

Several landscape and human behaviors in Ohio have amplified EAB impacts:

• Urban plantings. Many Ohio towns historically used ash as a primary street and park tree because of its form and adaptability. This monoculture-style planting created large, continuous ash resources for EAB.
• Forest composition. Ash is a common riparian and upland species in Ohio forests, providing abundant wild hosts outside urban areas.
• Movement of infested material. Transport of firewood and untreated ash wood for logs, pallets, and nursery stock can carry EAB beyond infested zones.
• Recognition lag. Early infestations can be cryptic; communities without active monitoring discovered EAB only after trees showed advanced decline.

These factors explain why EAB can sweep rapidly through both city streets and rural woodlands in Ohio.

Management options: what works, what to expect

Management can be grouped into prevention, treatment, and removal/replacement. Choice depends on tree value, infestation level, and resources.

Prevention and monitoring

• Monitor susceptible trees annually for the signs listed above, focusing on high-value or prominent trees.
• Avoid moving ash wood (firewood, logs, mulch) out of infested areas. Use local wood or certified heat-treated products.
• Promote species diversity in public plantings to reduce future risk of any single pest decimating the canopy.

Chemical controls

Systemic insecticides can protect individual high-value trees when applied properly. Common active ingredients and general characteristics:

• Emamectin benzoate (injection). Highly effective and typically provides two years of protection per trunk injection. Best for high-value trees. Requires certified applicator.
• Dinotefuran (trunk injection or soil application). Rapid uptake and fast-acting, useful for trees already under EAB pressure, but protection is usually shorter (often a single season).
• Imidacloprid (soil drench/injection). Long-used systemic with variable results; soil applications depend on soil type, moisture, and tree health; often annual or biennial treatment.

Key practical notes:

1. Timing matters. Apply treatments as recommended for the product–often in late spring to early summer when sap flow and active root uptake are adequate.
2. Treatment interval. Choose an insecticide based on desired duration of protection and tree value; emamectin offers longer control per application while others may require annual application.
3. Professional application. For trunk injections and complex decisions, hire a certified arborist. Incorrect application wastes money and may harm trees.

Biological control and long-term restoration

• Natural enemies imported and released (parasitic wasps) have established in parts of North America and help reduce EAB population locally, but they do not eliminate the pest and are a long-term, slow-maturing control.
• Breeding and selection. Researchers are working on identifying resistant ash genotypes and developing resistant stock for restoration. This is a multi-decade endeavor.

Removal and replacement

When treatment is not feasible or the tree is too heavily infested, removal is necessary to eliminate hazard and reduce local EAB reproduction:

• Remove and properly dispose of infested trees and avoid moving untreated wood.
• Replant with a diversity of non-ash species to rebuild canopy and reduce future pest vulnerability. Prefer native species well adapted to Ohio conditions.

Practical decision pathway for homeowners

1. Inspect: Annually check any ash on your property for thinning, D-shaped exit holes, and epicormic sprouts.
2. Value assessment: If the tree is large, healthy, and of high landscape value, consider treatment. If small, already severely declining, or hazardous, schedule removal.
3. Treatment options: For high-value trees, consult a certified arborist about emamectin trunk injection (multi-year control) or a dinotefuran treatment for quicker knockdown if infestation is advanced.
4. Timing and follow-up: Begin treatments early in the season and plan for re-treatment per product instructions. Maintain records of applications and monitor tree condition.
5. Replacement: If removal is performed, plant a diverse mix of native, pest-resilient species suited to the site.

Long-term landscape and policy lessons for Ohio

The EAB invasion exposed vulnerabilities in Ohio’s urban and forest management: reliance on single genera for street trees, delayed detection, and challenges coordinating community-wide responses. Long-term resilience requires:

• Diversifying tree species in urban and riparian plantings.
• Investing in active monitoring programs and rapid response capacity.
• Community coordination to prevent movement of infested wood and to share treatment or removal costs.
• Supporting research and restoration efforts aimed at resistant ash genotypes and effective biological control.

Final takeaways

• Emerald ash borer kills by destroying the ash tree’s phloem and cambium; North American ash species lack evolved defenses, making them highly vulnerable.
• Ohio’s prior abundance of ash in forests and urban plantings created ideal conditions for rapid EAB spread and heavy mortality.
• Early detection and timely, appropriate treatment can protect individual high-value trees; large-scale landscape preservation is unlikely without broad, coordinated effort.
• Practical steps for homeowners: inspect annually, consult a certified arborist for treatment versus removal decisions, avoid moving ash wood, and replant with diverse, site-appropriate species after tree removal.

Understanding why Ohio ash succumb to EAB clarifies how to respond: vigilance, informed treatment choices, and intentional planting decisions will reduce future losses and help communities rebuild resilient canopies.