Bacterial community structure and succession in nests of two megachilid bee genera.

Studies on honeybees have revealed bacterial taxa which adopt key functions in the hive, in terms of nutrient uptake and immune responses. Despite solitary bees providing invaluable ecological services, the contribution of their microbial communities to larval health and the development and fitness of adults is mainly unknown. To address this gap, we conducted a 16S rDNA meta-barcoding study including larvae and stored pollen in nest chambers from two different megachilid solitary bee genera. We tested how host taxonomy, environmental context and the developmental stage of larvae determined richness and composition of associated bacterial communities. A total of 198 specimens from Osmia bicornis, Osmia caerulescens, Megachile rotundataandMegachile versicolor nests were investigated. Solitary bee bacterial microbiota in the nesting environment were mostly homogeneous within species, and not significantly affected by landscape. For each bee species, we identified bacterial taxa that showed consistent occurrence in the larvae and stored pollen. For the pollen provision, we also described a community shift with progressing larval development, suggesting a reduction of imported floral bacteria.