The effect of sunflower pollen on the gut microbiome of the common eastern bumble bee Bombus impatiens

Diet can influence the gut microbiome and gut microbes can influence host pathogen resistance, however these links are poorly understood. Consuming sunflower pollen reduces a gut pathogen, Crithidia bombi, in the common eastern bumble bee, Bombus impatiens, but the mechanism is unknown. We asked if the gut microbiome mediates this effect. We conducted a lab experiment where bees were infected with Crithidia or not and fed either sunflower pollen or a wildflower pollen mix for 7 days. We additionally collected B. impatiens workers from eight farms that varied in their relative amount of sunflower blooms (none, low, medium, and high). We characterized the gut bacterial communities from both lab and field bees by sequencing the 16S rRNA genes. In the lab, diet did not affect gut bacterial community composition or diversity; however, infected bees had wider community variation than uninfected bees, regardless of diet. In field-collected bees, microbiomes significantly differed between farms and sunflower categories, but bacterial diversity was not correlated with sunflower acreage or floral diversity. Understanding how sunflower monocultures affect other aspects of bee health is an important next step before recommending sunflower plantings to support their populations. Our results suggest that a sunflower pollen diet and sunflower-dominated landscapes with low floral diversity provided enough nutrition for bees to maintain diverse gut microbiomes. Furthermore, we found that bee gut microbiomes are influenced by landscape but not pollen diet, and that the gut microbiome does not mediate sunflower’s medicinal effect in B. impatiens.