The Emerald Ash Borer (EAB), Agrilus planipennis, is an invasive beetle from Asia that poses a severe threat to ash trees (Fraxinus species) across North America. Since its initial detection in Michigan in 2002, this metallic green insect has killed hundreds of millions of ash trees, with nearly all North American species proving highly susceptible. The larvae of the EAB feed on the phloem and outer xylem, the tissues responsible for transporting water and nutrients throughout the tree. This feeding creates serpentine tunnels that disrupt the tree’s vascular system, essentially cutting off its ability to sustain itself. This article explores the physical signs of an infestation, defines the point at which intervention is no longer practical, and outlines management options for both treatable and non-recoverable trees.
Recognizable Signs of EAB Infestation
Visual identification of an EAB infestation often begins with observing the upper canopy, where thinning and dieback are typically the first external symptoms to appear. Because the larvae spend most of their life tunneling beneath the bark, direct evidence of the pest is often hidden until the infestation becomes established.
Homeowners can look for specific physical signs on the trunk and branches. The adult beetles emerge by chewing distinct, D-shaped exit holes that measure approximately one-eighth of an inch wide. These small holes are a strong indicator of EAB presence, as exit holes from native borers are usually round or oval.
Beneath the bark, the feeding larvae create characteristic S-shaped or serpentine galleries that are packed with frass, a mixture of sawdust and insect waste. As the tree attempts to grow over the damage, vertical fissures may appear in the bark, sometimes revealing the larval galleries underneath.
Another common sign of a stressed ash tree is the growth of epicormic sprouts, which are new shoots emerging from the lower trunk or the base of the tree. This is a desperate attempt by the tree to produce foliage and compensate for the dying upper canopy. Increased woodpecker activity, often referred to as “flecking” or “blonding,” on the bark can indicate an EAB infestation, as the birds strip away the outer bark to feed on the larvae beneath.
Defining the Critical Threshold for Intervention
The question of when it is too late to treat an ash tree is primarily answered by assessing the percentage of canopy dieback. Once the EAB larvae have caused significant damage to the vascular tissue, the tree may lack the capacity to recover, even if the insects are eliminated.
Treatment is generally not recommended for trees that have experienced a canopy dieback of 50% or more. Many arborists and forestry professionals recommend a more conservative threshold, suggesting that trees with more than 30% dieback should not be treated. The reasoning is that a severely thinned canopy is unable to produce enough energy through photosynthesis to repair the extensive damage to the phloem.
Trees with significant canopy loss have their ability to translocate water and nutrients seriously compromised. This vascular damage also inhibits the tree’s capacity to move the insecticide throughout the entire system, making the chemical application ineffective. While some “rescue treatments” have shown success on trees with up to 30% dieback, the chances of recovery diminish rapidly as the canopy loss approaches or exceeds the 50% mark.
Effective Treatment Methods and Seasonal Timing
For ash trees with minimal dieback, a variety of systemic insecticide treatments are available to protect against EAB. Systemic products are absorbed by the tree and distributed throughout the vascular system, targeting the feeding larvae beneath the bark or the adults feeding on the leaves.
The most consistently effective treatment is the systemic trunk injection of emamectin benzoate, which professionals apply every two to three years. This method is highly effective because it delivers the insecticide directly into the tree’s xylem, providing control for multiple seasons. Other options include soil-applied systemic insecticides, such as imidacloprid and dinotefuran, which are taken up by the roots.
The timing of application is a factor in treatment success, as the insecticide must be present in the tree’s tissues when the EAB larvae are actively feeding. Treatments are best applied in the spring or early summer, shortly after the leaves have fully emerged. This timing coincides with the period of high water movement in the tree and the peak emergence of adult beetles, ensuring the chemical is distributed to the canopy and inner bark.
Basal bark sprays, which are applied to the lower trunk and absorbed through the bark, are another systemic option. For all soil and basal bark applications, sufficient soil moisture is necessary for the tree to actively move the chemical into the canopy. Systemic treatments are favored over non-systemic cover sprays, which are less effective as they only target the adult beetles and newly hatched larvae on the surface.
Managing Trees Past the Point of Recovery
When an ash tree has lost more than half of its canopy, professional arborists often recommend removal rather than investing in ineffective chemical treatments. Dead or heavily infested ash trees become brittle and pose a significant safety hazard, making their removal a priority. The wood of a dead ash tree loses moisture rapidly, becoming unpredictable and increasing the cost and complexity of removal.
If an infested tree must be removed, especially during the EAB active period (May through September), proper disposal is necessary to prevent the spread of the insect. Depending on local regulations, ash wood may need to be chipped, burned, or buried, and movement of the wood may be restricted outside of quarantine zones. The outer inch of bark and wood is where the EAB larvae reside, so controlling this material is important.
After removal, planning for a replacement tree is a necessary step to maintain the property’s canopy and aesthetic value. Arborists recommend planting a diverse selection of non-ash species to avoid future vulnerability to a single pest. This forward-looking approach ensures the continued benefits of a healthy tree canopy without the ongoing threat of EAB.