Floral arrangement influences components in the transmission of a bumble bee (Hymenoptera: Apidae) gut pathogen

In disease ecology, identifying traits that influence pathogen transmission at shared resources is a key goal. Pollinators can deposit feces containing pathogens while foraging on flowers. Floral traits may shape where feces land, whether pathogens survive, and if they are contacted by other visitors. Using two morphologically similar species (Securigera varia and Lotus corniculatus), we examined how floral arrangement (horizontal or vertical) and plant part (flowers or leaves) affect Bombus impatiens fecal deposition patterns, gut pathogen Crithidia bombi survival, and pathogen acquisition. First, we allowed infected bees to forage in cages with either horizontal or vertical floral arrangements, after which the number of fecal drops and fecal locations were recorded. We then mimicked deposition by applying C. bombi inoculum inside flowers, outside flowers, or on leaves to test pathogen survival. Finally, we exposed uninfected bees to C. bombi inoculum placed on either flowers or leaves in vertical or horizontal floral arrangements to evaluate pathogen acquisition. Floral arrangement influenced the distribution of feces across plant parts: in horizontal cages, 88% of feces were on flowers, compared to only 40% in vertical cages. Pathogen survival was significantly higher on flowers than on leaves, and bees exposed to inoculated flowers were more likely to acquire C. bombi than those exposed to inoculated leaves. Overall, horizontal floral arrangements resulted in more fecal deposits on flowers, where both pathogen survival and acquisition were elevated. These findings suggest that floral architecture may play an important but understudied role in mediating pathogen transmission risk to pollinators.