2: Pollen and yeast change nectar chemistry alone and in combination, but honey bee foraging behavior reflects only the avoidance of yeast in an apiary-based assay

Floral nectar often contains pollen and microorganisms, introduced via interactions with floral visitors, the local host-flower, and the wider environment. The presence of pollen and/or microbes within nectar can change its chemical composition, which in turn may impact pollinator affinity. However, these effects, particularly in combination, remain understudied. Here, in a factorial experimental design, we examine the impacts of the presence/absence of the nectar specialist yeast, Metschnikowia reukaufii, and the addition of sunflower (Hellianthus annus) pollen at three concentrations (0, 10, and 100 grains/µL) to a synthetic nectar. Changes to nectar chemistry and yeast growth were monitored, and the foraging preferences of honey bees evaluated in an apiary-based assay. Pollen increased yeast growth and generally increased amino acid content in nectar, whereas yeast depleted amino acids. Pollen, but not yeast, changed nectar sugar concentrations, converting sucrose to its monomer sugars. Both pollen and yeast introduced volatiles to nectar, though yeast volatiles were more abundant than pollen volatiles. Yeast volatile emission was positively correlated with pollen concentration and cell density and yeast depleted many pollen-derived volatiles. Honey bees avoided foraging on yeast-inoculated nectar and foraged equally among un-inoculated nectars regardless of pollen content, underscoring the importance of microbial metabolites in mediating pollinator foraging.