Bumblebees raise their wing beat to a middle c buzz, which is just right to dislodge the pollen and a noticeably higher pitch than the humdrum wingbeat of flight. The process is imaginatively named buzz pollination, and most commercial greenhouses now employ captive bumblebees to perform this service.
Around the World in 80 Plants by Jonathan Drori
How to employ a bumblebee
My Book Joy 