As a bee forages for pollen and nectar, it acquires beneficial and pathogenic microbes from flowers. In human-influenced landscapes, variation in vegetative and floral diversity may therefore influence bee-microbe interactions. This study asks how variation in the resources available to bees in urban settings influences the composition of the microbiome and infection by fungal parasites in the Blue Orchard Bee, Osmia lignaria. Bees were installed and collected in 17 urban garden field sites in along the central coast of California. At each garden we measured local vegetation using random sampling in 1x1m plots at the center of each garden. We determined landcover composition with ArcGIS. To determine if bee diversity influences microbiome composition, we measured bee diversity at each site using visual transects. At the time of bee installation, we also reared a subset of bees in sterile petri dishes as controls to determine if the environment is important for microbiome composition. In the lab, we used 16S amplicon Illumina sequencing an average of 19 bees/garden and 10 control bees. We clustered sequences into OTUs and analyzed alpha and beta diversity using Qiime. We tested each bee for fungal infections by: Crithidia spp., Ascosphaera spp., Aspergillus spp., and Apicystis spp.
We found 42,104 distinct OTUs across all bees. To compare the community of microbes in bees from controls and bees sorted into experimental sites, we used NMDS ordination to plot differences in community composition (p< 0.001), finding that the environment does confer a unique community of microbes to bees. When we categorized urban gardens into “urban” or “natural” landscapes we found that 40 bacterial OTUs were higher in abundance in bees from sites with urban landscapes (p<0.05). Bees from sites with urban landscapes also had a greater range of OTU alpha diversity (200-800 OTU) in comparison to bees form sites with predominately natural landscapes (200-450 OTU). We also found that urbanization also positively drives bee species richness (p=0.051), and that this bee diversity is positively correlated with OTU richness in O. lignia (p=0.007), suggesting that urbanization-mediated increases in bee diversity influences increases in microbiome diversity. In understanding the implications of this finding, it is important to note that while the microbiome is important for bee health, a diversity of microbiomes is not necessarily “better”: some microbes are pathogenic, microbes can compete for resources, and diversity has not been experimentally addressed.