Print PageInbreeding in Mimulus guttatus lowers pollen production and pollen viability. Pollinators also strongly discriminate against inbred plants. In this species, pollen represents the primary reward to pollinators. In a series of experiments using live and artificial plants, we addressed the following questions: 1) Does variation in pollen production and viability account for variation in visitation by bumblebees (Bombus impatiens)? and 2) Do bumblebees make foraging decisions based on differences in the olfactory cues produced by anthers and pollen of varying quality?
Bumblebees from a commercial hive foraged freely on greenhouse populations of M. guttatus. The plants were either fully outbred individuals or individuals derived from a single generation of self-fertilization. The pollen viability of each plant was evaluated by staining with lactophenol blue, and pollen production was measured with an HIAC/Royco particle counter. After several weeks of experience on live plants, bees were allowed to forage on artificial plants that had been provisioned with either fertile or sterile anthers. Finally we measured bee foraging preference on artificial flowers provisioned only with olfactory cues from either fertile or sterile anthers. Samples of fertile and sterile anthers were analyzed with a gas chromatograph to examine differences in volatile profiles.
Results/Conclusions
Using a multiple regression approach, we determined that pollen viability and total pollen production per flower explained a small but significant amount of variation in the number of visits by bumblebees to live M. guttatus after controlling for the effect of overall display size. Experiments with artificial flowers suggested that bumblebees had the ability to respond to differences in anthers when isolated from variation in other potential floral cues. Bumblebee visitation to artificial flowers that were provisioned with sterile anthers received 24% fewer visits than flowers provisioned with fertile anthers. In olfactometer experiments, bumblebees were 2.7 times more likely to visit artificial flowers provisioned with the scent of fertile anthers than those provided with the scent of sterile anthers. Gas chromatograms revealed a prominent peak (putatively nonadecane) in all samples of fertile anthers that was absent in all samples of sterile anthers. This and possibly other differences in the volatile profiles of infertile anthers could serve as foraging cues for pollinators. The results of these experiments suggest that B. impatiens prefers to forage on M. guttatus offering the highest quality pollen rewards and that foraging decisions can be guided in part by olfactory cues contained within the pollen or anthers.
