The emerald ash borer (EAB), Agrilus planipennis, is a highly destructive, non-native beetle from Asia that poses a severe threat to ash trees (Fraxinus species) across North America. First detected in the United States in 2002, EAB has since killed hundreds of millions of ash trees. EAB larvae feed on the inner bark layers (phloem and cambium), disrupting the tree’s ability to transport water and nutrients. This effectively girdles the tree, causing death often within a few years of infestation. Homeowners must take swift action to protect valuable ash trees or safely remove infested ones.
Identifying an Emerald Ash Borer Infestation
Correctly identifying the signs of an active infestation is the first step in control. Adult EAB beetles are metallic, iridescent green, but they are difficult to spot since they emerge only briefly in the summer. The most telling sign is the characteristic D-shaped exit hole, about one-eighth of an inch across, left by the adult beetle as it emerges from the bark.
Beneath the bark, the feeding larvae create serpentine or S-shaped tunnels, called galleries, which disrupt the tree’s vascular system. This damage causes the canopy to thin out, often starting at the top. The tree may attempt to compensate by producing dense clusters of new growth, called epicormic sprouts, near the base of the trunk or on lower branches. Increased activity from woodpeckers, which strip away bark while feeding on EAB larvae, also indicates an infestation.
Treatment Feasibility and Timing
The decision to treat or remove depends on the extent of existing damage. Chemical treatments are only effective if the tree is healthy enough to absorb and distribute the insecticide. The “30% rule” suggests that trees with more than 30% to 50% canopy dieback are typically beyond saving because the vascular damage is too severe for recovery. Treatment is a viable option only for trees with minimal or no canopy thinning.
Timing is important for maximizing effectiveness. Systemic insecticides work best when applied in the spring or early summer, after the tree has fully leafed out, allowing for efficient chemical transport. Since EAB remains established indefinitely, successful chemical control requires a long-term commitment. Treatment is a multi-year strategy that must be repeated for the tree’s lifetime to maintain protection.
Chemical Control Options for Healthy Trees
Systemic insecticides offer the most effective control for healthy or lightly infested, high-value ash trees. These products are absorbed and moved through the vascular system, killing EAB larvae beneath the bark and adults feeding on the leaves. The application method often depends on the tree’s size and whether a professional service is used.
Trunk Injection
The most effective treatment is a systemic trunk injection of Emamectin Benzoate, typically administered by a licensed professional. This involves injecting the insecticide directly into the sapwood, providing protection for two to three years per application. Trunk injections are recommended even for trees with up to 30% canopy dieback, as they ensure rapid transport throughout the canopy while avoiding non-target exposure.
Soil Application
A soil drench or granular application uses an insecticide like Imidacloprid, often available as a do-it-yourself product. Soil-applied products are absorbed by the roots and translocated upward. They are generally less effective for very large trees and require annual application. Adequate soil moisture is necessary for the roots to successfully take up the insecticide.
Basal Bark Spray
The basal bark spray uses a systemic insecticide like Dinotefuran. This product is sprayed directly onto the lower five to six feet of the trunk and penetrates the bark to enter the vascular system. Basal bark sprays are non-invasive and effective on moderately sized trees, but they also require annual application. Homeowners must strictly follow all label instructions and local regulations regarding pesticide use.
Managing and Disposing of Infested Trees
Trees showing more than 50% canopy loss must be removed to prevent safety hazards and reduce pest spread. EAB-killed ash trees become brittle quickly, making them dangerous. It is strongly recommended to hire a professional arborist experienced in removing structurally compromised ash trees.
Timing the removal is important. Cutting down an infested tree during the EAB active period (typically April to September) can release adult beetles. If possible, removal should be scheduled during the dormant season (October 1 to March 31), when insects are less likely to emerge. Proper disposal is mandated by state and federal quarantines designed to limit the beetle’s spread.
The primary disposal rule is to avoid moving infested wood, including firewood, logs, and chips, out of a quarantined area. EAB larvae survive in untreated wood with bark attached, which is the main way the pest is moved long distances. If wood must be moved within a quarantined area, transport it as little as possible. Approved disposal methods include:
- Chipping the wood into pieces less than one inch thick.
- Burying the wood.
- Burning the wood on-site, provided local ordinances allow.
Preventing Future Infestation and Spread
The long-term strategy involves vigilance and landscape diversification. After removing an ash tree, replace it with a non-ash species, such as maple, oak, or elm. This diversifies the local tree population, making the area less susceptible to widespread damage from a single pest.
Preventing spread requires adhering to the “Don’t Move Firewood” campaign, which discourages transporting firewood across county or state lines. Since all hardwood firewood can harbor EAB, it should be sourced locally and burned close to where it was harvested. This simple step reduces the human-assisted movement of the beetle.
For chemically protected trees, continuous monitoring ensures treatment effectiveness. On a broader scale, biological controls, such as releasing non-native parasitic wasps from Asia, are used as a long-term, self-sustaining management tool. These wasps target and kill EAB eggs and larvae, offering natural population control, but they are not an immediate solution for individual homeowners.