The arthropod associates of 155 North American cynipid oak galls

Abstract

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.

Latitudinal gradient in species diversity provides high niche opportunities for a range‐expanding phytophagous insect

“…There are ~700 described oak cynipid species in North America, with western oak ecosystems being a hotspot of diversity (Burks, 1979; Russo, 2006; Weld, 1957). Cynipid wasps are attacked by a suite of parasitoids wasps, largely in the Superfamily Chalcidoidea, that specialize on one or more cynipid species (Askew et al, 2013; Bailey et al, 2009; Forbes et al, 2016; Hayward & Stone, 2005; Ward, Busbee, et al, 2022).…”

“…In its expanded range, it occurs at higher abundances on a higher proportion of trees compared with its native range (Prior & Hellmann, 2013). Q. garryana also hosts a community of cynipid wasps that co‐occur with N. saltatorius that are attacked by a community of parasitoid wasps (Prior & Hellmann, 2013; Smith, 1995; Ward, Busbee, et al, 2022).…”

Galls on galls: A hypergall‐inducing midge and its parasitoid community

“…A single gall can host up to 102 individual arthropods, which equates to 194.8 arthropods per cubic centimeter. Most community studies of galls discover not only new species but new types of species interactions (Brandão‐Dias et al, 2022; Ward et al, 2020, 2022; Weinersmith et al, 2017). Our results show that close observations can uncover previously unknown interactions and add to the rich biodiversity that is associated with galling insects.…”

Host-enemy interactions provide limited biotic resistance for a range-expanding species via reduced apparent competition

“…Given that many direct associations of parasitoids are undescribed, we were conservative and included all morphospecies in our analysis from families with known parasitoids, assuming that most caused deaths (be it direct or indirect) of gall wasp hosts. Over 99% of parasitoid individuals reared were from taxonomic groups that are known to directly associate with gall wasps (see S1) (64,67).…”

“…Oak gall wasps support a rich community of natural enemies, predominantly parasitoid wasps in the superfamily Chalcidoidea. These wasps are often solitary ectoparasites that attack one to a few hosts (specialists) to multiple hosts (generalists), and largely only attack oak gall wasps or parasitoids attacking gall wasps (24,63,64). Parasitoid wasps that emerge out of galls are parasitoids that directly attack gall wasps or inquilines (other organisms that live inside galls or gall tissue) or are hyperparasitoids of parasitoids attacking gall wasps or inquilines (64).…”