Drivers, Diversity, and Functions of the Solitary-Bee Microbiota

Accumulating reports of global bee declines have drawn much attention to the bee microbiota and its importance. Most research has focused on social bees, while solitary species have received scant attention despite their enormous biodiversity, ecological importance, and agroeconomic value. We review insights from several recent studies on diversity, function, and drivers of the solitary-bee microbiota, and compare these factors with those relevant to the social-bee microbiota. Despite basic similarities, the social-bee model, with host-specific core microbiota and social transmission, is not representative of the vast majority of bee species. The solitary-bee microbiota exhibits greater variability and biodiversity, with a strong impact of environmental acquisition routes. Our synthesis identifies outstanding questions that will build understanding of these interactions, responses to environmental threats, and consequences for health. Social bees are tightly linked to host-specific microbes which are actively transmitted within and between generations through social contact. For solitary bees, the more direct exposure to the environment and lack of active offspring care, are factors leading to a more diverse and environmentally associated microbiota. This limits the applicability of the social-bee model.Known social-bee-associated bacteria defend the hive against microbial pathogens, activate the host’s immune system, and ferment complex carbohydrates. In solitary bees, bacteria may assist with pollen fermentation, nutrition, spoilage inhibition, and defending the nest against harmful microbes.Honey bee guts are colonized during the first days of adult life and have a relatively stable microbiota. In solitary bee species, likely environmental or nest bacteria drive bacterial community shifts during development.