Bumblebee foraging and floral scent dimorphism: Bombus kirbyellus Curtis (Hymenoptera: Apidae) and Polemonium viscosum Nutt. (Polemoniaceae)

Abstract

Plants of Polemonium viscosum have either sweet- or skunky-scented flowers. In this paper we examine the relationship between floral scent and visitation by the principal pollinator, Bombus kirbyellus. Field observations of nectar collecting bumblebee queens showed that bees visited plants with sweet-scented flowers significantly more often than those with skunky scent and tended to visit more flowers per inflorescence of sweet than of skunky scent. These observations agree with results from preference tests with captive bees foraging under controlled conditions on enclosed arrays of the two scent morphs. In experimental trials, bee orientation on the initial visit to the array and on visits between plants within the array favored sweet-scented inflorescences. Bees moved on the grid in such a way that skunky-scented nearest neighbors were undervisited, while sweet-scented flowers were visited more often than expected. In one set of trials, sucrose solution was added to flowers in order to compensate for natural standing crop variation. This treatment did not reduce bumblebee preference for the sweet scent morph nor alter the movement patterns of bees on the grid. Hence, in this system, floral visitation pattern appears to be influenced more strongly by scent than by nectar level.