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Abstract AimAs species' ranges shift poleward in response to anthropogenic change, they may lose antagonistic interactions if they move into less diverse communities, fail to interact with novel populations or species effectively, or if ancestral interacting populations or species fail to shift synchronously. We leveraged a poleward range expansion in a tractable insect host–enemy community to uncover mechanisms by which altered antagonistic interactions between native and recipient communities contributed to ‘high niche opportunities’ (limited biotic resistance) for a range‐expanding insect. LocationNorth America, Pacific Northwest. MethodsWe created quantitative insect host–enemy interaction networks by sampling oak gall wasps on 400 trees of a dominant oak species in the native and expanded range of a range‐expanding gall wasp species. We compared host–enemy network structure between regions. We measured traits (phenology, morphology) of galls and interacting parasitoids, predicting greater trait divergence in the expanded range. We measured function relating to host control and explored if altered interactions and traits contributed to reduced function, or biotic resistance. ResultsInteraction networks had fewer species in the expanded range and lower complementarity of parasitoid assemblages among host species. While networks were more generalized, interactions with the range‐expanding species were more specialized in the expanded range. Specialist enemies effectively tracked the range‐expanding host, and there was reduced apparent competition with co‐occurring hosts by shared generalist enemies. Phenological divergence of enemy assemblages interacting with the range‐expanding and co‐occurring hosts was greater in the expanded range, potentially contributing to weak apparent competition. Biotic resistance was lower in the expanded range, where fewer parasitoids emerged from galls of the range‐expanding host. Main ConclusionsChanges in interactions with generalist enemies created high niche opportunities, and limited biotic resistance, suggesting weak apparent competition may be a mechanism of enemy release for range‐expanding insects embedded within generalist enemy networks. 

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 toBootanomyia dorsalis(Fabricus), which previous genetic data from Europe suggest comprises two distinct species,B. dorsalissp. 1 andB. dorsalissp. 2. We findB. dorsalissp. 1 in oak galls from New York, USA andB. dorsalissp. 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 withinB. dorsalissp. 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.