Network structure and taxonomic composition of tritrophic communities of Fagaceae, cynipid gallwasps and parasitoids in Sichuan, China

1. A key question in insect community ecology is whether parasitoid assemblages are structured by the food plants of their herbivore hosts.
2. Tritrophic communities centred on oak-feeding cynipid gallwasps are one of the best-studied tritrophic insect communities. Previous work suggests that host plant identity is a much stronger predictor of oak-cynipid interactions than of cynipid-parasitoid interactions. However, these relationships have not been formally quantified.
3. We reason that the potential for ‘bottom-up’ effects should increase with host plant phylogenetic diversity. We therefore generated quantified interaction network data for previously unstudied tritrophic cynipid communities in Sichuan, China, where, in addition to Quercus, cynipid host plants include Castanea, Castanopsis and Lithocarpus. We characterise these communities taxonomically, and compare the extent to which host plant taxonomy predicts plant-herbivore and plant-parasitoid associations.
4. We sampled 42,620 cynipid galls of 176 morphotypes from 23 host plant species, yielding over 4500 specimens of 64 parasitoid morphospecies. Many parasitoids were identifiable to chalcidoid taxa present in other Holarctic oak cynipid communities, with the addition of Cynipencyrtus (Cynipencyrtidae). As elsewhere, Sichuan parasitoid assemblages were dominated by generalists.
5. Gallwasp-plant interaction networks were significantly more modular than parasitoid-plant association networks. Gallwasps were significantly more specialised to host plants (i.e. had higher mean d’ values) than parasitoids. Parasitoid assemblages nevertheless showed significant plant-associated beta diversity, with a dominant turnover component.
6. We summarise parallels between our study and other Fagaceae-associated cynipid communities, and discuss our findings in light of the processes thought to structure tritrophic interactions centred on endophytic insect herbivores.