Bumblebees’ flower preferences are associated with floral abundance and buzz frequency when buzz-pollinating co-flowering plants

Abstract: Buzz-pollination is used by some bees to expel pollen through vibrating flowers. Yet, little is known about the determinants influencing bee preferences among buzz-pollinated flowers. We studied five co-flowering, nectarless species of Pedicularis (Orobanchaceae) buzz-pollinated by bumblebees in an alpine meadow, to investigate bumblebees’ flower preferences in response to fluctuations of floral abundance across five years. We also recorded and analyzed the buzzing frequencies produced by the three dominant bumblebee specie. Our results indicate that Bombus friseanus and B. lepidus visited different Pedicularis flowers using similar buzz frequencies and displayed an abundance-dependent flower preference across years. These two bumblebee species had staggered phenologies with distinct timing of peak abundances across the five years. In contrast, B. festivus used lower fundamental buzz frequencies, had a constant flower preference across years, but used different buzz frequencies across Pedicularis species. Although the amount of pollen released after bumblebee visitation varied across Pedicularis species, we found that after a single visit all bumblebees deposited similar amounts of pollen on stigmas. Our study indicates that bumblebees’ flower preferences is sometimes, but not always, modulated by floral abundance, and that at least one bumblebee species was observed to produce buzzes of different frequencies in different plant species. Competition for floral resources among bumblebees and for pollination services among co-flowering Pedicularis species may structure plant-pollinator interactions and affect species coexistence.