Bumblebee foraging at a "hummingbird" flower: reward economics and floral choice

Abstract

For a brief period in 1981 Bombus appositus queens visited Ipomopsis aggregata, a hummingbird-pollinated species with floral characteristics typical of that pollination syndrome. This behavior was not observed in other years. Despite the fact that Ipomopsis lacks floral features associated with bee visitation, bumblebees foraged on Ipomopsis at a rate only somewhat slower than that for a typical bee flower. Because of the deep corolla, bumblebees are able to probe only a limited distance into Ipomopsis flowers but in 1981 nectar was more accessible due to a higher standing crop than in other years. While foraging on Ipomopsis, bees were able to obtain a net energetic profit which was similar to that obtained from their primary forage species. These observations indicate that floral choice is governed primarily by profitability-which can overcome innate preferences for bee flowers-and that the features of hummingbird flowers pose no absolute barrier to bee visitation. Bees apparently sample a wide spectrum of available flower types and will cross pollination syndrome lines if the reward economics are favorable.