Microbial mediation of pollinator foraging behavior

Background/Question/Methods - Floral nectar is a key trait that mediates interactions between plants and pollinators. Comprised primarily of sugars and amino acids, nectar is prone to microbial colonization, with recent studies finding a significant number of plants surveyed host nectar-inhabiting microorganisms. It has been hypothesized that microorganisms in floral nectar, such as yeasts, may affect pollinator foraging behavior through changes in nectar chemistry. In this study, we tested the hypothesis that nectar yeasts affect pollinator foraging behavior by manipulating the presence of the common nectar yeast Metschnikowia reukaufii and measuring its effects on the foraging behavior of both naïve and experienced Bombus impatiens workers foraging on artificial flowers in the laboratory. Naïve foragers were exposed to a monochromatic array with randomly distributed artificial flowers containing either yeast-inoculated or sterile sucrose solutions. We examined the effect of yeasts on foraging behavior, including the number of visits per treatment, visit length, and foraging rate. In a separate experiment, foraging workers were trained to associate similar nectar treatments with flower color and then were subjected to repeated choices to determine preference between treatments.

Results/Conclusions - We found that nectar yeasts influenced the behavior of both naïve and experienced B. impatiens foragers. In the first experiment, preliminary analyses suggest that nectar yeasts may serve as an honest signal for the presence of nectar in artificial flowers, as bees foraged on yeast-inoculated flowers more frequently during initial visits to the array. This trend however decreased with increased experience as foragers discovered that all monochromatic flowers contained a reward and switched to foraging between treatments. In the second experiment, preliminary analyses show that a greater proportion of choices made by individual foragers is for yeast-inoculated flowers compared to controls, suggesting a preference for sucrose solutions containing yeast. Taken together, our results suggest that yeasts may serve as honest signals for the presence of floral nectar. Effects on foraging behavior as a result of yeast presence and metabolic activity may have important consequences for components of plant fitness. Additional experiments will test the mechanism underlying these trends, specifically testing the role of floral yeasts as modifiers of nectar scent.