Turnover of nest bacterial microbiome in nests of deceased red mason bees

Solitary bees are subject to a variety of pressures that cause severe population declines. Currently habitat loss, temperature shifts, agrochemical exposure, and new parasites have been identified as major threats. Knowledge about detrimental bacteria is however scarce, although they may disturb natural microbiomes, nest environments or harm the larvae directly. To address this gap, we investigated 12 Osmia bicornis nests with deceased and 31 nests with healthy larvae from the same localities in a 16S rDNA metabarcoding study. We sampled larvae, pollen provisions, and nest material and then contrasted bacterial community composition and diversity in healthy and deceased nests. We identified three bacterial taxa assigned to Paenibacillus spp. (closely related to P. pabuli/amylolyticus/xylanexedens), Sporosarcina spp. and Bacillus spp. as indicative for bacterial communities of deceased larvae. Furthermore, we isolated the Bacillus sp. strain from a deceased larva and used it to inoculate the pollen provision of healthy larvae. Comparing the larval bacterial microbiota of treated individuals with control individuals, we observed strong microbiota changes and enhanced mortality. This underlines the need for further investigation of detrimental bacterial in solitary bees, to understand their impact on bee population dynamics and the importance of their natural microbiome for controlling pathogenic bacteria.