Pollen viability determines influences bumblebee visitation rates to Mimulus guttatus flowers

Background/Question/Methods

The effect of variation in nectar rewards on pollinator visitation to flowers has been well documented.  Variation in the quality of pollen rewards has received much less attention.  We explored the effects of pollen viability in experimental populations of the yellow monkeyflower, Mimulus guttatus, on pollinator visitation using captive Bombus impatiens hives.

Results/Conclusions

We found that bumblebees significantly discriminated against flowers of inbred plants, with visitation rates reduced by as much as 35% relative to outbred plants.  Inbred plants also had significantly lower pollen viability, with only 23% the viability of outbred plants.  Using a multiple regression, we were able to determine that pollen viability explained a small (5%) but significant amount of variation in the number of visits by bumblebees after controlling for the effect of overall display size.  Experiments with artificial flowers suggested that bumblebees had the ability to respond to differences in pollen viability in isolation from other potential floral cues.  Although olfactometer experiments conclusively demonstrated the ability of bumblebees to visit artificial flowers based on the intensity of olfactory cues from pollen, the results of tests for their ability to discriminate between high and low viability pollen on the basis of olfactory cues were equivocal.