Microbiome assessment of two Osmia rufa nesting sites including larvae

Microbial activity is known to have profound impact on bee ecology and physiology, both by beneficial and pathogenic effects. Active manipulation of microbial communities towards the beneficial organisms has been observed for several Hymenoptera. This is achieved by inoculation with specific bacteria or treatment with antimicrobial substances. Most information about such associations is available for hive building organisms, and especially the honey-bee. There, manipulations especially result in a very restricted diversity of microbes present within the direct environment of their offspring. Currently, solitary bee nests have not been regarded according to microbial diversity, although their larvae face a very different situation from those of social bees and differences are to be expected. With this study, we assessed the bacterial biodiversity present in nests of Osmia rufa, the red mason bee, by culture-independent pyrosequencing. We found a high diversity not comparable to that in well tended hives of honey-bees. Although we identified a variety of potentially interacting organisms, positvely as well as negatively, most of the bacteria seem to originate from environmental sources through incorporated nest building material. Despite this untreated components of nest chambers, we also have indications that active pollen treatment to manipulate bacterial communities by mother bees may have similar importance as in hive-bees.