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Species are expanding their ranges poleward as a consequence of global climate change. However, natural enemies may lag behind range-expanding hosts, facilitating “ecological release.” A species that has experienced a recent poleward range expansion is Neuroterus saltatorius, an oak gall wasp (Hymenoptera: Cynipidae) that expanded its range from mainland western North America to Vancouver Island, BC. In its expanded range, it is threatening oak savanna ecosystems. This species induces gall structures on its host plant, Quercus garryana. Parasitoid wasps are attracted to galls and feed on developing insects in galls, acting as population control agents. We examine how interactions with enemies and competitors change over the range of Q. garryana and in the native and expanded range of N. saltatorius, and if changes in interactions relate to ecological release. Parasitoid wasps were reared from cynipid species collected from Q. garryana at 18 sites from Northern California to Vancouver Island, BC and identified to morphospecies using taxonomic keys. We found 21 cynipid morphospecies that co-occur with N. saltatorius on Q. garryana throughout its range, with a subset of cynipids (9 morphospecies) in the expanded range. We have identified 16 species of parasitoids from N. saltatorius. Preliminary results suggest a different composition of parasitoid enemies in its expanded range compared to its native range. This is likely due to a change in the composition of known generalist parasitoids that attack the range-expanding host that may not be effectively switching other from other competitors, rather than from a loss of known specialist parasitoids. We have also identified parasitoid morphospecies from a co-occurring competitor cynipid, Andricus quercuscalifornicus, and found no overlap in parasitoids with N. saltatorius. This suggests that A. quercuscalifornicus is not a competitor and does not contribute to the release of N. saltatorius. Uncovering how species interactions with range-expanding hosts are altered is important to predict outcomes of anthropogenic range expansions.



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Biogeographical Patterns in Oak Gall Wasp-Parasitoid Communities Associated with Oregon White Oak, Quercus Garryana, Under Anthropogenic Change