Watrous, K. M. | Search bioRxiv

bioRxiv2026

Sparse gut microbiomes in solitary bees and wasps

Bees and wasps are ecologically vital, but many species are declining due 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 abdomens across 61 genera and 14 families of field-collected bees, predatory wasps, and pollen wasps. We found that solitary bees and both wasp guilds harbor 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. Further, microbiome composition was only weakly differentiated among solitary bees and the two wasp groups, even comparing herbivorous (pollen-feeding) and carnivorous 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.

ecology↗

bioRxiv2026

Pervasive and dynamic gut dysbiosis in wild bumble bees is linked to the host life cycle

Stressors can shift the microbiome into an altered, "dysbiotic" state that reduces host fitness. While well-studied in humans and laboratory models, the prevalence, predictability, and drivers of dysbiosis in wildlife remain unclear. We addressed these questions by monitoring gut microbiomes of wild bumble bees in Southern California, focusing on Bombus vosnesenskii, a major pollinator in western North America. More than a third of all B. vosnesenskii bees exhibited dysbiosis, when defined as a >50% replacement of host-specialized core bacteria by environmental bacteria. This replacement covaried with increased alpha and beta diversity, an enrichment of oxygen-tolerant taxa, and pathogen infection--all common hallmarks of dysbiosis in other hosts. Other co-occurring Bombus, including two at-risk species, also exhibited dysbiosis. In B. vosnesenskii, dysbiosis was not correlated with certain stressors, such as heat or resource limitation, although other, unmeasured stressors cannot be ruled out. We next examined how dysbiosis varied over two years of sampling. In the first year, dysbiosis emerged only late in the season, when bumble bee colonies normally reproduce and then senesce. Two years later, following an intervening year with historic rainfall and elevated resources, dysbiosis was entrenched throughout the season. These findings show that dysbiosis is both pervasive and highly dynamic in wild bumble bees. The dynamics coincide with host life cycle transitions and environmental change, but the underlying causality remains uncertain. Given that dysbiosis may harm host health, we argue that long-term microbiome monitoring should be considered both for bumble bees and for other wildlife of conservation concern.

microbiology↗