Emerald Ash Borer

The emerald ash borer is, by nearly every measure, the most destructive forest insect ever to reach North America. Native to northeastern Asia — China, Korea, Japan, Mongolia, and the Russian Far East, where it is a minor pest of already-stressed ash — it was first identified in the Detroit, Michigan and Windsor, Ontario area in 2002, almost certainly having arrived years earlier hidden in solid wood packing material inside cargo shipments. With no coevolved natural resistance in North American ash and no established natural predators to check it, the beetle spread rapidly, both on its own and by hitching rides in firewood and nursery stock moved by people. It is now established across most of the eastern half of the United States and adjacent Canada, and in June 2022 it was confirmed in Forest Grove, Oregon — its first detection anywhere on the West Coast, opening an entirely new front against a species (Oregon ash) that had no prior exposure at all.

The damage is almost entirely done by the larval stage, which lives its whole feeding life hidden under the bark. After adult females lay eggs in bark crevices in early summer, larvae hatch and tunnel into the phloem and outer sapwood, carving winding, S-shaped galleries as they feed and grow through the following year. Because the phloem is the tree's only pathway for moving sugars from leaves to roots, a heavy enough tangle of galleries effectively strangles the tree from the inside — girdling it — regardless of how healthy the tree looked before infestation. Visible symptoms (canopy thinning and dieback starting at the top, water sprouts on the trunk, vertical bark splits over larval galleries, increased woodpecker activity as birds dig for larvae, and the adult's characteristic D-shaped exit holes) typically appear only after a tree is already seriously compromised, which is part of why the beetle proved so hard to contain: infestations were often invisible until it was too late to save the tree. A healthy, untreated ash usually dies within 3 to 5 years of its first infestation, and in the hardest-hit regions of the Midwest and Northeast, well over 99% of unprotected ash trees have died.

Adults are small, iridescent, metallic-green beetles about half an inch long, active from roughly late spring through summer, when they feed lightly on ash foliage (causing only cosmetic notching) before mating and laying the next generation of eggs. The USDA and university partners have released several species of parasitoid wasps native to Asia — including Tetrastichus planipennisi, Oobius agrili, and Spathius species — as biological control agents in infested areas, with early results suggesting they can meaningfully suppress borer populations over the long term, though not fast enough to save mature trees already in decline. Federal and state quarantines restricting ash wood and firewood movement, once widespread, have been relaxed in many states as the beetle's range expanded past the point where quarantines could meaningfully contain it — but moving firewood remains one of the most effective ways to accidentally start a new, isolated infestation far outside the current range, so avoiding it is still worthwhile everywhere.

Girdles and kills ash trees (Fraxinus spp.) by tunneling through the phloem under the bark, cutting off the tree's ability to move sugars from leaves to roots; kills the overwhelming majority of untreated ash within 3 to 5 years of infestation.