Some flowers release pollen only when the anthers are vibrated within a certain range of frequencies. Bumble bees and a few other bees can vibrate the anthers at these frequencies.

A bee accomplishes this by vibrating its body while gripping the flower. The vibration is produced by rapidly contracting the flight muscles while the wings are folded over the back. Thus the bee is vibrating but is not flying. This vibration is a higher frequency (therefore pitch) than the frequency produced when the bee is flying. This method of obtaining pollen is referred to as buzz pollination or sonication. (Scientists can gather pollen from such plants using a tuning fork to vibrate the flower.)

On a sunny day near the Obstruction Point Road, about 5,700 feet elevation, we watched, photographed and video recorded Fuzzy-horned Bumble Bees, Bombus mixtus, buzz pollinating Broadleaf Lupine, Lupinus latifolius, a native of the high ridges of Olympic National Park.

We could hear the buzz as a bee flew from flower to flower on a lupine plant. We noticed that when the bee paused at a flower (presumably to gather pollen) the buzz changed to a higher pitch. When the bee was flying to the next flower the buzz was at the original lower pitch. Because lupine flowers only release a small amount of pollen at a time, a bumble bee can visit the same flower more than once, sometimes circling around the flower stalk.

Bombus mixtus packs pollen into pollen baskets on its hind legs—the reddish blobs visible in photos and videos.