The Emerald Ash Borer (EAB), Agrilus planipennis, is an invasive beetle from northeastern Asia that poses a severe threat to ash trees (Fraxinus species) across North America. First detected near Detroit, Michigan, in 2002, this insect has spread rapidly, causing the death of hundreds of millions of ash trees in both urban and forested settings. Native ash species lack the natural defenses required to survive an infestation. Understanding the specific biological process by which this pest kills its host is necessary to grasp the magnitude of the ecological and economic damage it causes.
The Life Cycle and Entry Point
The adult Emerald Ash Borer is a slender, metallic green beetle, typically measuring between 3/8 and 1/2-inch long, or about the size of a cooked grain of rice. Adults emerge from infested trees during late spring and early summer, where they feed minimally on ash leaves before mating. The damage caused by adult feeding is minor and does not significantly contribute to the tree’s decline.
Following mating, the female beetle lays eggs, often depositing them individually in the tiny crevices and cracks of the ash tree’s bark. A single female can lay between 40 and 200 eggs, favoring stressed or weakened trees for oviposition. The eggs are miniscule, measuring about 1 millimeter, and are extremely difficult to spot on the tree’s rough exterior.
After approximately two weeks, the eggs hatch, and the newly emerged, cream-colored larvae immediately bore through the outer bark layer. Their destination is the nutrient-rich tissue directly beneath the bark, known as the phloem and cambium layer. This entry point establishes the larvae within the tree’s vascular system, which becomes the zone of catastrophic damage. The larval stage, which can last for one to two years depending on the climate and tree health, is the sole cause of ash mortality.
Disrupting the Lifeblood: How Larval Feeding Kills the Ash
Once beneath the bark, the larva, which appears flattened and segmented, begins to feed voraciously on the phloem tissue. The phloem is the living vascular tissue responsible for transporting sugars created during photosynthesis down to the roots. As the larva grows through its four developmental stages, or instars, it tunnels through this soft tissue, creating long, winding, S-shaped patterns called galleries.
These serpentine galleries are packed tightly with frass, a mixture of sawdust and larval excrement. When multiple larvae are present, their feeding tunnels crisscross and expand, effectively severing the continuous ring of phloem tissue around the circumference of the trunk or branch. This destructive action is known as girdling, and it functions identically to completely stripping a band of bark from the tree.
The extensive damage to the phloem prevents the downward flow of sugars from the crown to the root system. The tree’s roots, deprived of the necessary energy to sustain life, begin to starve and die first. While the larvae primarily target the phloem, their feeding also often scores the outermost layer of the xylem, the tissue responsible for transporting water and minerals upward from the roots.
This dual disruption of the vascular system leads to a fatal lack of communication and resource distribution within the tree. The upper canopy can no longer send food to the roots, and the roots struggle to send water and nutrients to the leaves. The tree essentially starves to death from the bottom up, even as its canopy may temporarily remain green. Even large, healthy ash trees are unable to survive the repeated attacks of a growing EAB population.
Observable Symptoms and Progression to Mortality
The internal girdling caused by the larvae quickly manifests as external symptoms that indicate the tree is under severe stress. One of the most common early indicators is crown dieback, where the foliage in the upper third of the tree canopy begins to thin and die back. This occurs because the larval galleries are often concentrated higher up in the tree initially, blocking the flow of water and nutrients to the furthest branches first.
As the tree attempts to survive the loss of its vascular system, it often initiates a stress response known as epicormic sprouting. This involves the growth of dense, new shoots or “suckers” directly from the trunk or the base of the tree, attempting to bypass the damaged areas to access light and air. The tree also attempts to heal the internal damage by creating new layers of wood, which can cause the outer bark to split vertically over the larval galleries.
A more definitive sign of EAB presence is the appearance of D-shaped exit holes, approximately 1/8-inch wide, left behind when the adult beetles emerge from the tree. Homeowners may also notice increased activity from woodpeckers, whose feeding attempts to extract the larvae from beneath the bark often leave noticeable patches of lighter, flaked bark, sometimes called “blonding.”
Once a heavy infestation is established, the tree’s decline progresses rapidly. Small ash trees, particularly those with a small diameter, may succumb to the infestation in as little as one year. Larger, mature trees typically die within two to four years from the time of initial attack. The internal starvation and lack of defensive resources leave the tree structurally weakened and vulnerable to secondary pests, diseases, and eventual failure.