Interspecific Differences in the Relative Importance of Pollen and Nectar to Bee Species Foraging on Sunflowers

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Abstract
We examined predictions of the theory of energy maximization for a field population of several species of bees foraging for nectar and pollen on a planting of six male-sterile and one male-fertile sunflower, Helianthus annus, cultivar. Honey bee and bumble bee workers gathered only nectar from sunflowers and mostly visited the two male-sterile cultivars that supplied the greatest caloric rewards; pollen was collected from other sources. Individuals of nonsocial species such as Melissodes agilis preferred the male-fertile cultivar (the only one supplying pollen), although it was lowest in caloric reward, because of their requirement for pollen for brood provisioning. Specialized workers of social species typically collect either nectar or pollen and presumably maximize caloric or nutrient acquisition. In contrast, individuals of nonsocial bee species forage for a mixed currency (energy and nutrients) and should not be expected to behave as predicted by the theory of energy maximization.