Ash Borers is a general term applied to several species of wood-boring insects that attack trees in the genus Fraxinus (Ash trees). The most significant and destructive species globally is the Emerald Ash Borer (EAB), Agrilus planipennis. This highly invasive beetle, native to Asia, has caused the death of tens of millions of ash trees across North America since its accidental introduction, resulting in catastrophic ecological and economic losses. The larvae of borers tunnel and feed on the vital tissues just beneath the bark, eventually girdling the tree.
Taxonomy and Classification
Most destructive ash borers, including the Emerald Ash Borer (EAB), belong to the family Buprestidae (metallic wood-boring beetles) in the order Coleoptera. Other borers, like the Banded Ash Clearwing Borer (*Podosesia syringae*), are moths in the family Sesiidae. EAB is distinct in that it attacks healthy, stressed, and young ash trees, unlike many native borers which typically only infest trees already weakened by disease or injury. This aggressive behavior makes EAB a category-defining invasive pest.
Physical Description
The adult Emerald Ash Borer is small and slender, measuring about 8.5–14 millimeters (1/3 to 1/2 inch) in length. It is easily recognized by its metallic, iridescent emerald-green coloration, a feature unique among North American beetles. Adults leave behind characteristic **D-shaped** exit holes in the bark, measuring about 3-4 mm wide.
The larval stage, which causes all the damage, is a flat, creamy white, legless grub. It is easily identified by its segmented body, with bell-shaped, distinct, flattened segments. These larvae can reach 30 millimeters in length and are found feeding in the cambium layer between the bark and the sapwood.
Distribution and Habitat
The Emerald Ash Borer is native to eastern Russia, northern China, Japan, and Korea. In North America, it was first discovered in Michigan in 2002 and has since spread to over 35 U.S. states and several Canadian provinces. Its habitat is defined by the presence of its host trees, which include all native ash species (green, white, black, and blue ash) in both urban forests and natural woodland settings. The movement of infested firewood is the primary method of long-distance spread.
Behavior and Life Cycle
The EAB typically completes its life cycle in one to two years. Adults emerge from the tree in late spring and summer, feed on ash leaves (causing minor damage), and mate. Females lay tiny eggs in bark crevices. Upon hatching, the larva bores directly into the tree and begins feeding on the phloem and cambium.
The larva tunnels extensively in a characteristic serpentine or S-shaped pattern. These galleries are what eventually girdle the tree, blocking the flow of water and nutrients. The larva overwinters in the inner bark or outer sapwood, and pupates in the spring before emerging as an adult through the D-shaped hole.
Feeding and Damage
The tunneling of the larval stage is responsible for the destructive nature of EAB. The larval feeding galleries effectively cut off the tree’s circulatory system. External signs of infestation include severe dieback of the tree crown (beginning at the top), sprouting of new shoots or “suckers” from the trunk base or roots (epicormic growth), vertical splits in the bark covering the underlying galleries, and increased feeding by woodpeckers, which chip away the bark to access the larvae.
Infested trees typically die within 2 to 4 years of infestation. The economic impact includes the cost of removing dead trees, loss of property value, and the loss of ash timber in commercial forestry.
Management and Prevention
Management strategies for EAB focus on quarantine, removal, and chemical protection. Quarantines restrict the movement of ash wood and firewood to prevent spread. Chemical treatment, involving systemic insecticides injected into the tree trunk or applied as a soil drench, can effectively protect high-value, non-infested ash trees for up to two years per application.
Infested trees must be removed and properly disposed of (chipped, burned, or debarked) according to local regulations to eliminate the beetles. Research and federal programs have also introduced specific species of parasitoid wasps, native to Asia, that prey on EAB eggs and larvae as a form of biological control.
Conservation and Research
EAB is a target of elimination and control. Research continues to seek more effective biological control agents, develop resistant ash cultivars, and improve detection methods, such as pheromone lures and specialized monitoring traps. The long-term goal is to establish a sustainable population of natural enemies that can keep the EAB population at a level that allows native ash trees to survive and regenerate.