The Emerald Ash Borer (EAB), Agrilus planipennis, is a non-native, highly destructive beetle that poses a severe threat to ash trees (Fraxinus species) across North America. Since its initial discovery, this invasive pest has caused the death of tens of millions of trees in forests and urban landscapes. Identifying the specific marks this insect leaves on the tree is the first step in managing its spread and impact. The damage inflicted by the EAB follows a predictable pattern, moving from subtle signs in the upper reaches of the canopy to definitive physical markings on the trunk.
The Culprit: Identifying the Emerald Ash Borer
The adult Emerald Ash Borer is a slender, metallic-green beetle, typically measuring between 3/8 and 1/2 inch in length, which is roughly the size of a cooked grain of rice. When viewed closely, the beetle has a distinct, slightly flat back and a narrow body that tapers toward the rear. The insect’s wings cover a coppery-red or purplish-magenta abdomen, a color visible only when the wings are lifted.
The larval stage is responsible for the destructive damage inside the tree. Larvae are creamy white, legless, and can reach up to 1 to 1.5 inches in length when fully grown. Their bodies are noticeably flattened and segmented, giving them a bell-shaped appearance. These larvae spend most of their life tunneling just beneath the bark, where they disrupt the tree’s vascular system.
Initial Indicators: Signs of Stress in the Canopy
The first noticeable evidence of an EAB infestation often appears high in the tree’s crown, rather than on the trunk. Larval feeding damages the tissues that transport water and nutrients, leading to a visible thinning of the foliage in the upper third of the canopy. This canopy dieback typically begins subtly and progresses downward over several years, causing branches to appear increasingly bare.
As the tree attempts to survive the disruption of its vascular system, it often redirects its energy into growth lower down the trunk. This manifests as “water sprouts” or epicormic shoots, which are dense clusters of small, vigorous branches emerging from the main trunk or near the base. These sprouts are a clear sign of severe stress, as the tree tries to produce foliage wherever the vascular tissue remains functional. The appearance of these shoots often indicates that the main canopy is already compromised by the larval feeding.
Physical Hallmarks: Unique Damage on the Trunk
The most definitive evidence of EAB presence requires a closer inspection of the bark surface and the area immediately beneath it. Adult beetles, upon emerging from the tree, leave behind small, distinctly D-shaped exit holes. These holes measure approximately 1/8 inch wide. The unique shape is a near-unmistakable signature of EAB, as most native borers create rounder exit holes.
The true damage is revealed just under the bark in the phloem tissue, where the larvae feed in a characteristic pattern. Their tunnels, known as galleries, wind back and forth in a serpentine or “S” shape. These S-shaped galleries are packed tightly with a fine, sawdust-like material called frass, which is a mixture of wood dust and insect excrement. The larvae’s feeding completely girdles the tree by destroying the phloem, effectively cutting off the flow of food from the leaves to the roots.
The tree’s internal struggle to wall off the destructive galleries can cause vertical cracks or fissures to appear on the trunk and branches. In some cases, the bark may peel away vertically, and the S-shaped larval galleries become directly visible on the wood underneath. Finding these winding tunnels beneath the bark is the most conclusive evidence of an active EAB infestation.
Confusing the Signs: EAB Damage Versus Other Issues
A common sight on infested ash trees is irregular patches of missing outer bark, often referred to as “blonding” or “flecking.” This is a secondary sign created by woodpeckers feeding heavily on the EAB larvae beneath the bark. Woodpeckers strip away the outer bark to access the insects, which can be an early indicator of an infestation. The holes they create are larger and more irregular than the EAB’s small exit holes.
Distinguishing EAB damage from native borers often comes down to the shape and size of the exit holes. Native ash borers, such as the lilac/ash borer, typically leave exit holes that are round or oval and generally larger, sometimes measuring 1/4 inch or more in diameter. Furthermore, the larvae of native clearwing borers tunnel deeper into the sapwood, while EAB galleries are confined to the tissue just under the bark. Unlike EAB, some native borers expel their frass, leaving sawdust visible outside the tree.