Pollen–Microbe Interactions in Nectar Weakly Influence Bee Foraging Behavior

Plant-pollinator interactions are frequently affected by microbes that grow on flowers. Bacteria and yeast commonly grow within floral nectar, which is a sugar-rich floral reward often sought out by pollinators. Nectar is also commonly contaminated with protein-rich pollen. Microbes can induce this pollen to germinate or burst within the nectar, which potentially results in pollen nutrients being made available to nectar foraging pollinators. Yet whether pollen-microbe interactions in nectar impact pollinator behavior remains unknown. We therefore investigated how a common nectar yeast (Metschnikowia reukaufii) and bacteria (Acinetobacter nectaris) affected pollen germination and bursting within artificial nectar and effects on bumble bee (Bombus impatiens) foraging behavior. We found that both bacteria and yeast reduced the proportion of intact pollen in nectar, with bacteria inducing the most germination and bursting. Although microbes may thus potentially increase the quality of the nectar reward via increased access to pollen nutrients, we did not observe effects on bee flower preference. Similarly, bees did not show increased constancy (i.e., fidelity to one flower type across flower visits) to nectar contaminated with pollen and microbes. In contrast, bees were much more likely to reject flowers with nectar contaminated with pollen and yeast alone or together, relative to flowers that offered uncontaminated nectar. Altogether, our work suggests pollen-microbe interactions within nectar may have relatively minor influences on pollinator foraging behavior. We discuss possible explanations and implications of these results for plant and pollinator ecology.