The **Emerald Ash Borer** (EAB) (*Agrilus planipennis*) is a devastating, highly invasive beetle native to Asia. It is the most destructive forest insect ever introduced to North America, causing the death of hundreds of millions of ash trees (*Fraxinus* species). The larvae feed on the inner bark and phloem layer of the tree, effectively **girdling** the tree and cutting off its ability to transport nutrients and water. Infested ash trees usually die within 2 to 4 years of infestation, making EAB a major economic and ecological threat.
Taxonomy and Classification
EAB belongs to the family Buprestidae (metallic wood-boring beetles) in the order Coleoptera. It undergoes complete metamorphosis. It was first discovered in North America in 2002 near Detroit, Michigan, and is believed to have arrived in solid wood packing material from Asia. EAB is a secondary pest in its native range but is highly successful and lethal against all species of ash native to North America.
Physical Description
Adult EAB beetles are small, slender, and bullet-shaped, about $1/2$ inch long. They are a distinctive, **metallic emerald-green** color, though they may appear dark in flight. The head is flat and the ends of the wing covers are pointed.
The **larvae** are the damaging stage: flat, white, legless grubs that are deeply segmented (giving them a “stacked coin” appearance) and grow up to $1 \frac{1}{4}$ inches long. When the adult emerges, it leaves a characteristic **D-shaped exit hole** in the bark.
Distribution and Habitat
EAB is widespread across the Eastern, Midwestern, and parts of the Western United States and Canada. Its habitat is exclusively the cambium layer of ash trees. The beetle overwinters as a prepupa in the tree and emerges in late spring/early summer, leaving the host tree to mate and search for new trees to infest.
Behavior and Life Cycle
The life cycle typically takes one to two years. Adults feed briefly on ash leaves, causing little damage, and then lay eggs in bark crevices. The larvae bore into the tree and spend most of their lives tunneling through the phloem and cambium layer, creating long, winding, S-shaped tunnels called **galleries**. These galleries are packed with **frass** (sawdust and excrement).
The S-shaped galleries are the key diagnostic sign, severely disrupting the tree’s vascular system. Other signs of infestation include: **canopy dieback** (thinning leaves starting at the top), **epicormic sprouting** (suckering from the base or trunk), and increased **woodpecker activity** (woodpecker holes searching for larvae).
Feeding and Damage
The EAB larval feeding causes the rapid death of the host tree due to the interruption of water and nutrient flow. The insect is considered an **extreme pest** because it attacks healthy, vigorous trees and can kill them in large, contiguous numbers, leading to mass ash mortality in affected areas. This loss dramatically impacts urban canopies and forest ecosystems.
Management and Prevention
Control involves a combination of regulatory measures, removal, and chemical treatments.
- **Systemic Insecticides:** High-value ash trees can be protected via chemical trunk injections or soil drenches of systemic insecticides, often requiring annual or biennial application. This treatment must be applied preventatively or in the early stages of infestation.
- **Quarantine:** State and federal quarantines restrict the movement of ash wood and firewood to prevent the beetle’s spread.
- **Removal:** Heavily infested trees must be removed and properly disposed of (e.g., chipped or processed) to eliminate breeding sites.
- **Biological Control:** Specific parasitic wasps native to Asia are being released in affected areas to establish long-term, natural controls on EAB populations.
Conservation and Research
EAB is the subject of intense research and conservation efforts focused on protecting the remaining ash gene pool and integrating biological controls into large-scale management plans to ensure long-term ecosystem viability.