Invasive species are among the greatest threats to ecosystems, but our ability to detect species introductions varies across taxa. Parasitoid wasps, though one of the most species-rich groups of all animals, are small and have ephemeral adult stages, such that they often go unnoticed. Here, we document two separate introductions of European parasitoids of oak gall wasps into North America. Both wasps key morphologically to Bootanomyia dorsalis (Fabricus), which previous genetic data from Europe suggest comprises two distinct species, B. dorsalis sp. 1 and B. dorsalis sp. 2. We find B. dorsalis sp. 1 in oak galls from New York, USA and B. dorsalis sp. 2 in oak galls from Washington, Oregon, and British Columbia, Canada. All oak gall wasp hosts were North American natives. We detect no genetic variation at the mtCOI locus within B. dorsalis sp. 2 specimens, suggesting this introduction may have had only a small number of founder individuals. In their native ranges, both species attack several different gall wasp hosts, and we likewise reared both from galls of multiple North American gall wasp hosts, suggesting a potential for widespread impact on North American gall insect communities. These introductions were detected only because our research groups are actively sampling and identifying parasitoid communities across gall habitats. Most parasitoid communities are not regularly sampled across hosts, time and space, or are well characterized, such that many more undetected wasp introductions may be impacting native insects worldwide.