Sparse gut microbiomes in solitary bees and wasps

Bees and wasps are ecologically vital, but many species are declining owing to anthropogenic stressors. Social bees harbour host-specific and dense gut microbiomes that affect their resilience to stress. However, there are tens of thousands of other bee and wasp species that vary in sociality and diet (including pollen-feeding and predatory guilds), traits known to influence host-microbe symbioses. The role of gut microbes in the biology of these species is largely unknown. Here, we measured the composition and absolute abundance of bacterial communities in adult metasomas across 61 genera and 14 families of field-collected bees, predatory wasps and pollen wasps. We found that solitary bees and both wasp guilds harbour distinct bacterial taxa and lower bacterial abundances as compared with social bees. Bacterial abundances also varied extensively among and within genera of solitary bees, with little variation explained by body size, diet breadth or nesting ecology. Furthermore, microbiome composition was only weakly differentiated among solitary bees and the two wasp groups, even when comparing herbivorous (pollen-feeding) and omnivorous taxa. We suggest that the sparse and somewhat stochastic microbiomes of solitary bees and wasps reflect weak host dependence on microbially mediated functions, a trait that may influence their responses to environmental change.