The identities of most arthropod associates of cynipid-induced oak galls in the western Palearctic are generally known. However, a comprehensive accounting of associates has been performed for only a small number of the galls induced by the estimated 700 species of cynipid gall wasp in the Nearctic. This gap in knowledge stymies many potential studies of diversity, coevolution, and community ecology, for which oak gall systems are otherwise ideal models. We report rearing records of insects and other arthropods from more than 527,306 individual galls representing 201 different oak gall types collected from 32 oak tree species in North America. Of the 201 gall types collected, 155 produced one or more animals. A total of 151,075 animals were found in association with these 155 gall types, and of these 61,044 (40.4%) were gall wasps while 90,031 (59.6%) were other arthropods. We identified all animals to superfamily, family, or, where possible, to genus. We provide raw numbers and summaries of collections, alongside notes on natural history, ecology, and previously published associations for each taxon. For eight common gall-associated genera (Synergus, Ceroptres, Euceroptres, Ormyrus, Torymus, Eurytoma, Sycophila, and Euderus), we also connect rearing records to gall wasp phylogeny, geography, and ecology - including host tree and gall location (host organ), and their co-occurrence with other insect genera. Though the diversity of gall wasps and the large size of these communities is such that many Nearctic oak gall-associated insects still remain undescribed, this large collection and identification effort should facilitate the testing of new and varied ecological and evolutionary hypotheses in Nearctic oak galls.

1. Understanding the role of divergent host-use in promoting population differentiation among herbivorous insects provides valuable insight into the origin of diversity in this species-rich group. A strong signature of divergent selection in promoting population differentiation is provided by parallel phenotypic divergence across multiple insect species adapting to the same set of divergent host plants.2. In this study, we explored whether a key phenotype of gall-forming insects-gall size-exhibits parallel patterns of divergence across a community of gall-forming insect species because of adaptation to different host plant species. A previous study found parallel divergence in gall morphology across two gall-forming insect species adapted to two sister species of oaks (Fagaceae). Here, we expand this question to four additional members of the gall-forming insect community that specialise on the same two host plant species: Quercus geminata (Qg) and Quercus virginiana (Qv).3. Consistent with the previous result, all four additional gall-forming species tended to induce larger galls on Qg than on Qv, with two of the four species exhibited statistically significant differences and one species showed marginally significant differences. Moreover, we found gall size was similar for two gall-forming species when cross-reared on natural versus planted hosts, ruling out environmental causes of parallel host-associated gall size difference across multiple species. 4. Our study documents parallel divergence in a key phenotype of gall-forming insects associated with different host environments, shedding light on the important role of divergent host use in driving the evolution of gall morphology.