The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle from Asia that threatens ash trees in North America. First identified in Michigan in 2002, this destructive insect has since killed tens of millions of ash trees across the continent. Understanding how EAB spreads is crucial for effective management and protecting remaining ash populations.
Natural Dispersal of Emerald Ash Borer
Adult emerald ash borers primarily spread through flight. While they can fly several miles, their typical annual flight range from an emergence site is often less than a mile. An adult female can fly up to 10-12 miles, but an average flight is usually about 2 miles. Factors influencing their spread include host tree density and environmental conditions like wind.
Once an adult beetle lays its eggs on an ash tree, larvae remain within the tree, tunneling under the bark. Larvae do not move between trees or locations on their own. Their movement is confined to the infested host tree, where they feed on the phloem and xylem, eventually girdling the tree and causing its decline.
Human-Driven Movement of Emerald Ash Borer
Human activities are primary drivers of EAB’s rapid, long-distance spread, often bypassing natural barriers and accelerating its reach into new areas. Infested firewood is a common and dangerous vector for EAB dispersal. EAB larvae survive in cut ash wood for extended periods, even after the tree is cut, allowing unsuspecting individuals to transport the pest across vast distances. Research indicates that EAB larvae can survive in firewood for up to two years after the tree is cut, posing a continuous risk of spread.
Transporting infested ash nursery stock directly introduces the pest to uninfected landscapes. When ash trees from infested areas are sold and planted elsewhere, they can carry hidden EAB eggs or larvae, establishing new populations far from existing infestations. This pathway allows EAB to jump significant distances, bypassing the slower natural spread.
Commercial activities involving ash logs and wood products contribute to the beetle’s spread. Movement of raw or minimally processed ash logs, lumber, and wood chips from infested regions for milling, processing, or disposal can inadvertently spread EAB. This commercial transport facilitates the accidental relocation of larvae or eggs embedded within the wood, leading to new outbreak sites.
Factors Accelerating Spread
Ash tree density within a landscape influences the rate of EAB spread. High concentrations or continuous stands of ash trees provide abundant hosts, allowing beetles to move easily and quickly from tree to tree. This facilitates faster local spread once an infestation is established.
New EAB infestations often occur adjacent to existing ones due to adult beetle dispersal. However, human-assisted movement creates new, isolated infestation points that serve as nuclei for outward spread, leapfrogging natural dispersal distances. These isolated infestations can then expand locally, much like the original infestation.
Human travel and commerce networks provide efficient pathways for accelerated EAB movement. Interstate highways, rail lines, and commercial trade routes, especially those used by the timber industry, transport infested materials over long distances quickly. These networks allow the beetle to bypass slow natural spread rates, leading to rapid colonization of new, distant territories.