Investigating the potential mechanism behind bumble bee preference for Corydalis flowers inhabited by nectar specialist yeast

Abstract

Flowering plants are in an evolutionary battle for the attention of pollinators to increase their fitness. However, microbes are increasingly recognized as key players in mediating interactions between plants and pollinators. Most studies of floral microbes have focused on the role of obligate nectar yeasts (e.g., Metschnikowia reukaufii), which increase flower preference in bumble bees through their ability to alter nectar chemistry and add fermentation volatiles to floral scent. However, these studies were conducted on a limited number of bumble bee species, so it is not known to what extent preference for yeast-colonized flowers is exhibited across a wider array bumble bee species. Furthermore, the chemistry of Metschnikowia rekaufii could alter the dynamics of mutualism by influencing the behavior of floral visitors such as legitimate visitors and nectar robbers. Therefore, we investigated how bumble bees with different foraging tactics reacted to yeast-associated volatile organic compounds (VOCs). By performing choice assays on wild-caught bumble bees with different foraging strategies using a Y-tube olfactometer, we found that facultative secondary robbers display a preference for yeast- inoculated Corydalis caseana flowers, whereas bumble bee pollinators did not. This suggests that yeast VOCs may only be utilized as a cue for the presence of nectar by secondary robbers. Further choice assays with the secondary robber bees showed that a synthetic, simplified blend of yeast-associated VOCs did not elicit the same preference and that they only preferred yeast volatiles when combined with floral volatiles. Ultimately, these results inform our understanding of how ecological context mediates bumble bee preference for yeast-colonized flowers.

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