Herbivore specialization and host plant genotypes interactively alter herbivore community assembly

Evolutionary community ecology seeks to understand how the dynamic interplay between ecology and evolution shapes community organization and ecosystem functioning. Although numerous studies have investigated the consequences of genetic variation within a single plant species, it remains poorly understood whether genetic variation in interacting organisms across multiple trophic levels jointly influences the assembly of associated communities. Because individual specialization is common in consumer populations, genetic variation in both consumer specialization and genetic variation in plants can interactively influence community assembly. In this study, we focused on genetic differentiation in feeding specialization in terms of leaf‐age type in close proximity to two populations of the leaf beetle Plagiodera versicolora in the willow species Salix eriocarpa . To investigate the effects of genetic interactions between leaf beetles and host willow plants (i.e. genotype by genotype [G × G] interactions) on the subsequent insect herbivore communities, we conducted a common garden experiment using five clones of willows and two leaf beetle populations with different specializations for leaf‐age types. Without prior leaf beetle herbivory, no significant genetic effects of host plants were detected on the overall abundance, species richness, community composition of herbivores, or number of dominant herbivore species. In contrast, in the presence of prior leaf beetle herbivory, willow genotype and beetle genetic background interactively affected species richness, relative abundance, the abundance of dominant herbivore taxa, and community composition. Our results indicate that genetic variation in specialization within herbivore species, coupled with host plant genetic variation, has the potential to produce divergent trajectories in the assembly of plant‐associated insects.