The Effects of Flower, Floral Display, and Reward Sizes on Bumblebee Foraging Behavior When Pollen Is the Reward and Plants Are Dichogamous

Abstract

Premise of research. The size of floral display, flower, and reward can influence pollinator foraging behavior. However, the importance of correlations between floral traits and rewards in affecting pollinator preferences is unclear. Moreover, in monoecious plants and plants with strong within-flower dichogamy, some flowers do not offer a pollen reward, and pollinator attraction may depend on the number of rewarding flowers rather than floral display size. We examined these questions in the Rocky Mountain columbine, where bumblebees can forage only for pollen produced in male-phase flowers.Methodology. We used dual-choice assays and experimental arrays to determine whether a correlation between pollen reward and flower size was necessary before bees could distinguish between flowers with different pollen rewards or sizes. The impact of floral display size on bee foraging was first examined using inflorescences where all flowers provided a reward, either small or large. We then varied either floral display size or number of rewarding flowers while keeping the other variable constant.Pivotal results. Bumblebees preferred flowers with a larger pollen reward or larger flowers only when the size of the pollen reward was correlated with flower size. Bees selected larger floral displays when all flowers presented a similar amount of pollen but preferred inflorescences with more pollen-rewarding flowers when only a proportion of the flowers offered a pollen reward.Conclusions. Bees could identify whether a flower offered pollen and could quantify the number of pollen-rewarding flowers. Correlations between floral traits and pollen reward affected pollinator preferences and may facilitate selection of floral traits by pollinators. Pollinators preferred inflorescences with more pollen-producing flowers rather than larger floral displays, which suggests that selection of floral display size by pollinators may differ between plants with different mating systems.