The Emerald Ash Borer (EAB), a destructive, metallic-green beetle, poses a significant threat to ash trees across North America. The larvae feed beneath the bark, severing the tree’s vascular system and preventing the transport of water and nutrients. Untreated ash trees in infested areas face near-certain mortality, requiring homeowners to act quickly to protect their trees. Protection involves assessing the tree’s condition, committing to chemical intervention, and providing ongoing care.
Evaluating Ash Tree Health
Before committing to treatment, thoroughly examine the ash tree’s health to determine if saving it is feasible. The most visible early symptom of an EAB attack is a thinning crown, where leaves become sparse, often starting at the top of the tree. This occurs because the larvae disrupt the flow of sap to the canopy. Another symptom is the appearance of epicormic sprouts, which are clusters of vigorous shoots that grow directly from the trunk or larger branches as a stress response.
Physical evidence of the borer includes small, D-shaped exit holes, about one-eighth of an inch wide, left in the bark by emerging adult beetles. If bark is removed, serpentine or S-shaped larval galleries packed with digested wood material, called frass, are visible underneath. Increased woodpecker activity, often referred to as “blonding” due to the light-colored patches of outer bark removed during foraging, is a strong indicator that larvae are present.
The extent of canopy dieback is the most reliable factor for deciding whether to proceed with treatment. Trees showing less than 30% crown dieback are good candidates for successful recovery with insecticide treatment. Ash trees that have already lost 50% or more of their canopy are typically too far gone, and the likelihood of recovery is low. Consulting a certified arborist provides the most accurate diagnosis, especially for trees with advanced symptoms, as they can assess internal damage and structural integrity.
Applying Preventative and Remedial Insecticide Treatments
Chemical intervention using systemic insecticides is the only effective method to save ash trees from the Emerald Ash Borer. These treatments are absorbed by the tree and distributed throughout the vascular system, poisoning the feeding larvae beneath the bark. The choice of insecticide and application method depends on the tree’s size and the severity of the infestation.
One effective active ingredient is emamectin benzoate, applied via professional trunk injection, which provides protection for two to three years per application. This concentrated treatment is ideal for larger trees and those already showing signs of infestation. It is delivered directly into the xylem tissue, ensuring rapid distribution throughout the canopy. Trunk injections must be performed by a licensed pesticide applicator due to the specialized equipment and chemicals required.
For smaller, healthy trees or as a preventative measure, systemic insecticides containing imidacloprid or dinotefuran can be applied using a soil drench or soil injection method. The product is mixed with water and poured around the base of the tree, where it is taken up by the roots and translocated upward. This application is easier for a homeowner to perform, but it is less effective on very large trees due to difficulty achieving adequate uptake and transport to the upper canopy.
Dinotefuran is also formulated as a basal trunk spray, where the insecticide is sprayed onto the lower five to six feet of the trunk and absorbed directly through the bark. Because this active ingredient is highly mobile, it provides a quicker response than soil-applied products. This quick response is necessary when an infestation has already begun. Soil drenches and basal sprays need to be reapplied annually to maintain protection against the borer.
Optimal timing for all systemic treatments is in the spring after the leaves have fully emerged. At this time, the tree is actively moving water and nutrients upward, maximizing insecticide uptake and circulation. Regardless of the chosen method, follow all label instructions concerning dosage, safety, and local regulations regarding pesticide use. A long-term commitment to re-treatment is required, as the protection provided by the insecticide is temporary.
Ongoing Tree Care and Replacement Planning
Treating an ash tree for EAB is most successful when combined with good general care to improve the plant’s natural defense mechanisms and overall vigor. Proper watering is important, especially during drought, as a well-hydrated tree can better absorb and circulate the systemic insecticide. Applying mulch around the base of the tree, keeping it a few inches away from the trunk, helps retain soil moisture and prevent mechanical damage from lawn equipment.
Even with the best care, some ash trees will be too damaged to save, making removal a necessity for safety. Dead or severely compromised ash trees become brittle quickly, increasing the risk of limbs or the entire tree failing unexpectedly, especially near a home or structure. Removing a dead, brittle tree is substantially more hazardous and costly than removing a healthy or recently infected one.
If removal is necessary, hire an insured, qualified professional, as the compromised wood makes the process unpredictable and dangerous. The final step in managing the EAB crisis is to plan for replacement by planting a non-ash species to restore lost canopy cover. Selecting replacement trees like oaks, maples, hackberry, or buckeye increases species diversity, making the landscape more resilient to future pest outbreaks.