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In 2025, conservationists in Oregon released more than 22,000 tiny parasitoid wasps into the woodlands surrounding the Portland metro area. Although it may sound unusual to introduce an insect swarm into a forest, the same strategy has been implemented across New York, Minnesota, Vermont, and over 30 other states for the past decade.
The released wasps are almost invisible and harmless to humans, yet they play a crucial role in suppressing one of the world’s most destructive invasive pests—the emerald ash borer (Agrilus planipennis). This half‑inch green beetle, native to northeast Asia, was first detected in the U.S. in 2002 near Detroit, Michigan. Since then, it has spread to 37 states and has killed hundreds of millions of ash trees in North America, rendering quarantine efforts ineffective.
In the beetle’s native range, populations are kept in check by several species of parasitoid wasps. Parasitoidism is a highly specialized form of parasitism in which the wasp lays eggs on or near its host; the hatching larvae consume the host from within. Because these wasps are host‑specific, they pose minimal risk to non‑target species when introduced to North America.
Four parasitoid species have received USDA approval for release: Oobius agrili, Spathius agrili, Tetrastichus planipennisi, and Spathius galinae. The first three originate from China, while S. galinae comes from Russia and is well suited to colder climates such as the Great Lakes region and Canada. In China, S. agrili can kill up to 90 % of emerald ash borer larvae near Beijing. The USDA’s breeding facilities have produced more than half a million parasitoids in 2020 alone.
Three years after introduction, S. galinae parasitized up to 78 % of emerald ash borer populations in Massachusetts, Connecticut, and New York—an encouraging result. However, the pest continues to spread, necessitating complementary approaches.
Woodpeckers—17 species across the U.S.—have naturally adopted the emerald ash borer as a food source. Their drilling activity not only removes larvae from infested trees but also provides a visual cue for scientists to identify attack sites.
Long‑term solutions involve breeding ash trees with inherent resistance. Only about 1 % of current ash trees exhibit resistance to the beetle’s larvae. Selective breeding programs led by organizations such as the Nature Conservancy aim to develop robust, resistant trees, though results will take years to materialize. Until then, the tiny wasps continue to guard our forests.
