The Emerald Ash Borer (EAB), Agrilus planipennis, is an invasive, metallic-green beetle native to Asia. Since its discovery in 2002, the EAB has caused the death of tens of millions of North American ash trees. The beetle’s larvae feed on the phloem layer, effectively girdling the tree and cutting off the flow of water and nutrients. Taking protective measures is necessary due to the high mortality rate.
Evaluating Tree Health and Infestation Status
Before investing in treatments, an assessment of the ash tree’s health and infestation level is necessary. The most definitive sign of an active infestation is the presence of distinct D-shaped exit holes, approximately one-eighth of an inch wide, left by emerging adult beetles. These D-shaped holes indicate EAB, as native borers typically leave round or oval exit holes.
Beneath the bark, the larvae create serpentine or S-shaped galleries as they feed, which can often be seen when the bark splits vertically. This feeding disrupts the tree’s vascular system, leading to symptoms of decline, such as canopy thinning and dieback, usually starting at the top. A common symptom is the formation of epicormic sprouts, or “water sprouts,” which are dense clusters of new growth emerging from the trunk or base in response to severe stress.
The viability of a tree for treatment is linked to the extent of its canopy decline. Ash trees showing less than 30% canopy thinning are excellent candidates for insecticide treatment and have a high chance of recovery. However, if the tree exhibits canopy dieback exceeding 50%, the chances of successful recovery are significantly lower, and removal may be necessary.
Selecting the Appropriate Insecticide Treatment
The choice of insecticide is based on the size of the tree and the severity of the infestation. Systemic insecticides are the only effective method because they are absorbed by the tree and circulate through the vascular system to kill the feeding larvae. Treatments fall into two categories: those available for homeowner application and those requiring a certified arborist.
For smaller trees, typically under six inches in diameter, systemic neonicotinoids like imidacloprid are available over-the-counter. These are applied as a soil drench or injection around the base of the trunk and must be applied annually for protection. While convenient, their efficacy decreases on larger ash trees, as the chemical may not translocate effectively throughout the canopy.
The most effective treatment is emamectin benzoate, a systemic insecticide applied via trunk injection by a licensed professional. This chemical is acutely toxic to EAB larvae and provides a longer period of control compared to neonicotinoids. A single professional application of emamectin benzoate can reliably protect an ash tree for two years, and sometimes up to three years, even in areas with high pest pressure. This makes professional trunk injection the preferred long-term strategy for high-value landscape trees.
Application Methods and Timing
The application method determines how quickly and effectively the chemical protects the tree. Homeowners often utilize the soil drench or soil injection method, where the solution is poured onto the soil near the trunk or injected just below the surface. This method relies on the tree’s roots to absorb the chemical, which can take longer to reach the canopy.
Professional treatments typically involve either a basal bark spray or a trunk injection. Basal bark sprays, often using dinotefuran, are sprayed directly onto the lower bark and absorbed through the tissue. Trunk injection involves drilling small holes into the trunk and injecting the insecticide, which minimizes environmental exposure and ensures rapid distribution throughout the tree.
The timing of the application is important, as the systemic insecticide must be present when the larvae begin feeding. Treatments work best when the ash tree is actively transpiring and moving water and nutrients, typically in the late spring or early summer after the tree has fully leafed out. Applying the insecticide during this period ensures the chemical reaches the canopy before the EAB larvae hatch and begin tunneling under the bark. For example, emamectin benzoate application is generally required every two years, whereas soil-applied products often require annual re-treatment.
Safe Removal and Disposal of Infested Trees
When an ash tree’s decline is too advanced or treatment has failed, safe removal is required. Ash wood killed by the EAB quickly loses its structural integrity and is prone to sudden limb and trunk failure. Due to this increased hazard, removal should be carried out by professional arborists who can manage unstable wood.
The disposal of infested wood must adhere to local and federal quarantine regulations to prevent the beetle’s spread. Agencies like the USDA Animal and Plant Health Inspection Service (APHIS) often restrict the movement of ash wood, logs, and firewood outside of designated quarantined areas. Moving infested wood allows the EAB to travel long distances, establishing new outbreaks.
To comply with these guidelines, ash trees should ideally be removed during the EAB dormant period, which typically spans from October 1 to March 31. Wood removed during the active period should be processed on-site, such as by chipping the outer inch of bark and wood where the larvae reside. All ash wood material, including firewood, must remain within the quarantine boundary unless it is processed or heat-treated.