Associative learning of flowers by generalist bumble bees can be mediated by microbes on the petals

Communication is often vital to the maintenance of mutualisms. In plant-pollinator mutualisms, plants signal pollinators via floral displays, composed of olfactory, visual, and other plant-derived cues. While plants are understood to be associated with microbes, only recently has the role of microbial (yeast and bacteria) inhabitants of flowers as intermediaries of plant-pollinator communication been recognized. Animals frequently use microbial cues to find resources, yet no study has examined whether microbes directly mediate learned and innate pollinator responses. Here, we asked whether microbes on the flower surface, independent of their modification of floral rewards, can mediate these key components of pollinator preference. In the field, we characterized flower and bumble bee microbial abundance, and in laboratory assays we tested whether bumble bees (Bombus impatiens) discriminated flowers on the basis of an experimental floral microbial community on the petals and whether microbe-derived chemicals were effective cues. Learning of microbial community cues was associative and reward context-dependent and mediated by microbial chemicals. Deconstructing the experimental microbial community showed bees innately avoided flowers with bacteria, but were undeterred by yeast. Microbial cues thus potentially facilitate dynamic communication between plants and pollinators such as bumble bees, especially as pollinator visitation can change flower microbiota. We suggest that the study of communication in mutualism generally would benefit by considering not only the multicellular eukaryote partners, but their microbial associates.