Scientists Identify Two Parasitic Wasps Consuming Hosts From Inside, Spreading Across U.S.

Researchers have identified two previously unrecorded species of parasitic wasps that are spreading across the United States and feeding on native oak gall wasps from the inside out, raising concerns about potential ecological impacts.

Entomologists said the tiny parasitoid wasps, likely originating in Europe, have been detected in multiple states from New York to the Pacific coast. The insects deposit eggs into oak galls, where their larvae consume the bodies of the native oak gall wasp larvae over days to weeks before emerging as adults.

Scientists described the wasps’ life cycle as typical of parasitoids: an adult female injects one or more eggs into or onto a host insect — in this case the larvae of oak gall wasps sheltered inside plant galls. After hatching, the parasitoid larvae feed on nonessential tissues first, which prolongs the host’s life and provides a fresh food source, then progressively consume the host’s tissues until the host dies and the parasitoid pupates and later emerges as an adult.

The newly identified species were not previously recorded in U.S. faunal surveys, and researchers believe they were introduced from Europe. Potential pathways of introduction under consideration include the importation of oak trees and plant material and transport via aircraft. Scientists said there is no evidence that the wasps pose any direct threat to humans.

The primary concern among researchers is the potential ecological ripple effects. Oak gall wasps are native insects that induce characteristic growths — galls — on oak trees. Those gall wasps and the complex communities associated with the galls, including native parasitoids, are components of local food webs and biodiversity. Introducing new parasitoids could reduce populations of gall-forming wasps and may also lead to competition with, displacement of, or hybridization with native parasitic wasp species that perform important roles in controlling insect populations.

Researchers emphasized the need for systematic monitoring to determine the distribution, population dynamics and host range of the newly recorded species. Determining whether the parasitoids will remain specialized on oak gall wasps or expand to additional hosts will be important for assessing long-term ecological effects.

Past introductions of nonnative insects and parasitoids have produced a range of outcomes, from negligible effects to substantial changes in host population dynamics and community composition. Experts caution that early detection and careful study are critical to understanding whether management actions are warranted.

Field surveys and specimen analyses are ongoing. Scientists urged arborists, naturalists and the public to report unusual gall activity or unexpected parasitoid emergence from oak galls to local extension services and entomology departments to help map the spread of the species and better assess potential impacts on native insect communities and oak ecosystems.