Buzz pollination in eight bumblebee-pollinated Pedicularis species: does it involve vibration-induced triboelectric charging of pollen grains?

Abstract

Buzz pollination involves explosive pollen release in response to vibration, usually by bees. The mechanism of pollen release is poorly understood, and it is not clear which component of vibration (acceleration, frequency, displacement or velocity) is critical; the role of buzz frequency has been particularly controversial. This study proposes a novel hypothesis that explosive pollen release results from vibration-induced triboelectric charging. If it does, pollen release is expected to depend on achievement of a critical threshold velocity. Eight sympatric buzz-pollinated species of Pedicularis that share bumblebee pollinator species were studied, giving a rare opportunity to compare sonication behaviour of a shared pollinator on different plant species. Reconsidering previous experimental studies, it is argued that they establish the critical role of the velocity component of vibration in pollen release, and that when displacement is constrained by body size bees can achieve the critical velocity by adjusting frequency. It was shown that workers of Bombus friseanus assorted themselves among Pedicularis species by body size, and that bees adjusted their buzz/wingbeat frequency ratio, which is taken as an index of the velocity component, to a value that corresponds with the galea length and pollen grain volume of each species of Pedicularis. Sonication behaviour of B. friseanus differs among Pedicularis species, not only because worker bees assort themselves among plant species by body size, but also because bees of a given size adjust the buzz frequency to achieve a vibration velocity corresponding to the floral traits of each plant species. These findings, and the floral traits that characterize these and other buzz-pollinated species, are compatible with the hypothesis of vibration-induced triboelectric charging of pollen grains.