The Emerald Ash Borer (EAB), Agrilus planipennis, is a highly destructive invasive insect from Asia that has caused widespread devastation to ash trees (Fraxinus species) across North America. This pest quickly established itself without natural predators and is responsible for the death of tens of millions of ash trees, threatening the entire North American genus. The widespread impact of this small beetle is due to a specific biological process that interrupts the tree’s internal systems.
Identifying the Emerald Ash Borer
The adult Emerald Ash Borer is a small, slender beetle with a bright, metallic-green coloration on its wing covers. When the wing covers are spread, a coppery-red or purple abdomen can be seen underneath. Adults are most active during warmer months, feeding on ash leaves, which causes only minor damage to the tree’s canopy.
The larva is the stage responsible for the tree’s destruction. EAB larvae are creamy-white, flattened, and segmented, growing up to 1.5 inches in length. After hatching from an egg laid in a bark crevice, the larva immediately chews its way into the tree, beginning the fatal process. The ash tree is the sole host for this species in North America, making all native ash species susceptible.
The Fatal Mechanism: Larval Girdling and Nutrient Blockage
The EAB larva kills the ash tree by burrowing and feeding in the tissue layers immediately beneath the outer bark. This area contains the cambium, phloem, and outer sapwood. The phloem is the layer responsible for transporting sugars, produced during photosynthesis in the leaves, down to the roots and trunk for growth and storage.
As the larvae feed, they tunnel back and forth, excavating long, distinct S-shaped or serpentine galleries. This continuous tunneling effectively severs the vascular connections within the phloem and cambium layers. This process, known as girdling, prevents the downward flow of essential carbohydrates from the canopy to the roots.
The disruption starves the root system. A tree that is fully girdled can no longer sustain its basic metabolic functions. Smaller or newly infested trees may succumb to the damage in one to two years, while larger, mature trees can die within three to four years of the initial attack.
Observable Symptoms of Tree Decline
The internal damage caused by the larvae manifests as several noticeable external symptoms. One definitive sign of EAB presence is the creation of D-shaped exit holes in the bark. These holes are small, approximately 1/8 inch in diameter, and their unique shape distinguishes them from the round exit holes of native borers.
The interruption of water and nutrient flow causes the leaves to thin out, resulting in crown dieback. As a stress response, the tree often attempts to grow new, dense shoots from the trunk or the base, a phenomenon called epicormic sprouting. These sprouts are an attempt to compensate for the lost canopy.
In some cases, the tree’s natural defense mechanism attempts to wall off the larval galleries with callus tissue, causing the outer bark to split vertically and revealing the serpentine larval tunnels underneath. Increased activity from woodpeckers, who feed on the larvae beneath the bark, can lead to patches of light-colored outer bark being stripped away, creating a noticeable “blonding” effect.
Protecting Ash Trees
Managing the EAB threat requires a multi-faceted approach. For high-value ash trees, a protective measure is the application of systemic insecticide treatments. These treatments, which can include soil drench, basal bark spray, or trunk injection, are most effective when applied proactively before a heavy infestation takes hold.
Trunk injections, often utilizing chemicals like emamectin benzoate, are effective for larger trees and can provide protection for two to three years per application. Soil-applied systemic products require annual reapplication and are less effective on very large trees. Severely damaged trees with significant canopy loss should be removed by professionals to prevent them from becoming hazardous and to reduce the spread of the pest.
Preventing the human-assisted movement of EAB is an important regulatory action. Many states and provinces have implemented quarantines on transporting ash firewood outside of designated areas. Using local or certified heat-treated firewood is a simple way to avoid inadvertently spreading the beetle to new, uninfested regions.