How a small beetle found a continent of vulnerable trees
The emerald ash borer’s damage began long before anyone saw its metallic-green adults on North American trees. Native to parts of Asia, the beetle was first identified in the United States near Detroit in 2002. It likely arrived earlier in untreated wooden packing material used to ship goods; once established, it could spread to new places in firewood moved by people.
Ash trees had little protection against this unfamiliar insect. In its native range, the beetle coexists with ash species that are better able to withstand attack. North American ash trees, by contrast, often fail to contain the larvae beneath their bark.
The damage accumulates out of sight
The beetle’s life cycle turns the space between bark and wood into a feeding ground. Eggs hatch into larvae that carve winding galleries through the tree’s living tissues. These tissues carry water and sugars between roots and leaves. As the galleries multiply, they interrupt that transport; a tree may lose foliage and die even though its trunk still looks solid.
Because the larvae remain hidden, an infestation can build before its full extent is obvious. By the time a tree shows severe decline, nearby ash may already be affected. The insect can also travel farther when infested firewood is carried to campsites or homes, making ordinary transport an unintended route for the invasion.
A costly response, with no quick reset
Communities have removed dead and dying trees to reduce hazards, while restrictions on moving firewood and infested wood have aimed to slow the beetle’s spread. These measures can limit risk, but they cannot restore the ash lost from a forest or street. The lasting consequence is visible in places where ash once formed a substantial part of the canopy: gaps open, shade changes, and replacement trees must grow for years before they can fill the space.
