Nectar yeast scent additions fail to impact overall bouquet composition and bumble bee visitation in a montane herb

Premise of research The factors that mediate how foragers locate food supplies are of vital importance in understanding their energy acquisition and survival. Microbes that inhabit floral nectar can play outsized roles in altering nectar chemistry and nutrition, thereby affecting floral visitors. Methodology Here, we consider one such nectar microbe, the cosmopolitan, specialist nectar yeast, Metschnikowia reukaufii (Metschnikowiaceae), by inoculating a nectar analog of the subalpine wildflower, Corydalis caseana ssp. brandegeei (Fumariaceae), and then characterizing the yeast’s impacts on floral scent composition and the foraging behavior of its main pollinator, Bombus appositus (Apidae). We assessed foraging behavior of B. appositus in a flower array near the Rocky Mountain Biological Laboratory (Colorado, USA) to test the hypothesis that foragers preferentially visit yeast-inoculated flowers over sterile controls. Additionally, we assessed whether bees spent more time at, and fed more quickly on, yeast-inoculated flowers. In a separate experiment, we tested whether Corydalis inflorescences inoculated with M. reukaufii had different scent bouquets than sterile inflorescences. Pivotal results We found that bumble bee pollinators showed no preference for the focal yeast species, with inoculated nectar having no effect on number of flowers visited, time spent on individual flowers, or time spent accessing nectar. Further, the overall scent bouquet compositions of yeast-inoculated Corydalis inflorescences were not statistically significantly different than those of sterile inflorescences, despite increased emissions of several volatiles that are known to be produced by M. reukaufii . Conclusions Our findings suggest that B. appositus does not respond to the presence of M. reukaufii in the nectar of Corydalis, and instead, yeast-associated volatile emissions may serve as a reliable cue of a nectar reward that is unused by these pollinators. These findings suggest a few avenues for future research, particularly how morphologically complex, highly scented flowers interact with VOCs produced by nectar-inhabiting microbes, and how floral visitors interpret these signals.