The Emerald Ash Borer (EAB), Agrilus planipennis, is an invasive beetle from Asia that represents one of the most destructive forest pests in North America. This small, metallic-green insect has decimated tens of millions of ash trees (Fraxinus species) since its discovery in the U.S. in 2002. The larval stage of the borer feeds beneath the bark, effectively severing the tree’s vascular system, which transports water and nutrients.
Recognizing the Signs of Infestation
A definitive indicator of EAB is the presence of the adult beetle’s exit holes, which are distinctively D-shaped and measure approximately one-eighth of an inch in diameter. These holes are created when the mature beetle chews its way out of the bark.
Underneath the bark, the feeding larvae create characteristic serpentine or S-shaped tunnels, known as galleries, as they consume the inner tissues of the tree. This larval activity disrupts the flow of water and nutrients, leading to the first visible symptom: crown dieback. The canopy will begin to thin, usually starting at the top.
Another highly visible sign is an increase in activity from woodpeckers, which feed on the EAB larvae under the bark. This foraging leaves patches of lighter-colored wood exposed, a phenomenon often described as “blonding” on the trunk and major branches. The tree may also produce new shoots, called epicormic sprouts, from the trunk or the base of the tree.
Chemical Treatment Options for Ash Trees
Saving an ash tree from EAB relies on the timely and correct application of systemic insecticides, which move throughout the tree’s vascular system to kill the feeding larvae. Effective treatment is typically divided into preventive application for uninfested trees and remedial treatment for those with early-stage infestation. Trees that have already lost more than 20 to 35 percent of their canopy are generally not suitable candidates for treatment because the insecticide cannot be distributed effectively throughout the severely damaged vascular system.
The most effective active ingredient currently available is Emamectin Benzoate, which is a systemic insecticide applied directly into the trunk of the tree. This trunk injection method is highly effective, providing protection for two to three years with a single application, even in areas with heavy pest pressure. Because this application requires specialized equipment and precise dosage to prevent tree injury, it must be performed by a certified arborist or licensed pesticide applicator.
For trees with a diameter at breast height (DBH) of less than 15 inches, systemic products containing Imidacloprid or Dinotefuran may be applied by homeowners as a soil drench or granular application. These insecticides are absorbed by the tree’s roots and transported upward, offering protection for a single season. Soil applications must be made when the tree is actively drawing water, typically in the spring, and require removing mulch or sod around the trunk for best absorption.
Dinotefuran is also formulated for application as a basal bark spray, where the insecticide is sprayed directly onto the lower six feet of the trunk. This method is considered non-invasive, as it does not require drilling into the tree like a trunk injection, and the chemical is absorbed through the bark. Like soil treatments, basal bark sprays are most effective on smaller trees and require annual reapplication to maintain protection against the EAB.
Timing is crucial for successful chemical treatment, as the insecticide must be present when EAB larvae begin to feed. For soil-applied products like Imidacloprid, application must occur early in the spring, as it can take four to six weeks for the chemical to fully distribute throughout the canopy. Trunk injections of Emamectin Benzoate are best applied in the spring or early summer when the leaves are fully emerged and the tree’s sap flow is active, ensuring rapid uptake and distribution.
Managing Heavily Infested Trees and Preventing Spread
When an ash tree exceeds the 20 to 35 percent canopy loss threshold, chemical treatment is unlikely to succeed, and removal becomes necessary. Heavily infested trees dry out quickly, becoming brittle and structurally unsound, which poses a substantial hazard to people and property. Removal work on these compromised trees should always be carried out by professional, insured tree services due to the increased danger.
To minimize the risk of further EAB spread, the best practice is to remove infested trees during the pest’s dormant period, which typically runs from October through March. This avoids the time when adult beetles are actively emerging and flying to new host trees. Once the tree is removed, the proper disposal of the wood is a necessary step in containment.
Infested ash wood should not be moved outside the immediate area, especially if quarantines are in effect, as moving firewood is the primary way EAB spreads long distances. Processing the wood by chipping it to pieces smaller than one inch in two dimensions is sufficient to destroy any remaining larvae. If the wood is kept for on-site use, such as firewood or lumber, it should be utilized quickly or stored in a way that prevents any surviving beetles from emerging and escaping.