Pollinators face a complex floral marketplace in which rewards and advertisements vary dramatically among plant species. For example, nearly all bee species collect both pollen (largely a source of protein) and nectar (largely a source of carbohydrates), from flowers that attract pollinators with a diversity of displays. As plants differ in reward composition, most bees must eventually visit multiple species to meet their nutritional needs. However, on shorter timescales bees are often constant, restricting visits to a single species. Though nectar traits are known to influence floral constancy, whether multiple rewards shape this long-studied behavior is unknown. We asked how reward composition (whether a flower offered nectar, pollen, or both resources) shaped bumblebee color constancy, and whether bees showed stronger constancy in relation to aspects of floral display, reward, or both aspects of floral phenotype. We allowed 53 foragers from 5 captive colonies of the Eastern Bumblebee (Bombus impatiens), to forage on ‘dual-species’ arrays of artificial flowers that differed in color, but which all offered either 4ul of sucrose solution (Nectar Treatment, N=21), 2mg of pollen (Pollen Treatment, N=18), or both (Dual-reward Treatment, N=14). We calculated foragers’ constancy across reward treatments, and foragers’ relative constancy to color and reward in the Dual-reward Treatment.
Bees were significantly color constant (Bateman’s index= 0.162, p<0.01), but color constancy did not depend upon the composition of floral rewards. In the dual reward treatment, preliminary analyses suggest that bees were also reward constant (Bateman’s index= 0.194, p<0.01), but that when given the chance bees were more constant to color than floral reward. Though bumblebees tend to be generalists, visiting many plant species and collecting both pollen and nectar, we believe these are the first data that reveal differential patterns of short-term specialization on colors and rewards simultaneously. Further, the possibility that bees may show differing constancy to rewards and colors raises intriguing questions about pollinator foraging in real plant communities where bees encounter variation across many aspects of floral phenotype. These results are a first step in understanding how differences in display and reward may shape bee foraging, and subsequently pollen movement among and between plant species.