GEARS Internet Classroom Topic: Buzzz pollination

Why is that bee giving me the "raspberry?"

If you are ever lucky enough to be standing alongside the road in the southwestern U.S. deserts by a patch of deadly nightshade flowers (Solanum spp.) you may experience a marvelous yet mostly unnoticed form of pollination. All around you female "digger bees" are noisily flying about and landing on the nightshade blossoms. As they grasp the bright yellow pollen-containing anthers, they produce a comical, almost rude sound, somewhat reminiscent of one person giving another "the raspberry" or the "Bronx Cheer" as they work the flowers. What you hear are the high-pitched intense buzzes produced by rapid fire contractions of their flight muscles, sonications which are transmitted inside the pollen-containing hollow anthers. These strong bee buzzes, giving rise to what Dr. Stephen Buchmann of our laboratory has termed buzz pollination, are used as a physical pollen-harvesting technique by female bees. The protein-rich pollen (up to 50% protein) will be carried back on their hind legs and later mixed with nectar to provision their underground brood cells. The bees turn themselves into living tuning forks and the pollen is sonically discharged as an explosive cloud. The bees may use a single buzz on a flower if the blossom is nearly empty or a long train of multiple buzzes on a previously unvisited flower. Some bees, like the familiar honey bees, never use buzz pollination to harvest pollen. A buzzing bumble bee can extract pollen from nightshade or tomato flowers hundreds of times faster than honey bees. Most buzz blossoms also produce no floral nectar as a reward for pollinators thus honey bees aren't impressed with this sort of flower and typically avoid them.

What types of flowers require buzz pollination to set fruit? About 8% of the world's 250,000 species of flowering plants (angiosperms) have anthers with apical pores and require buzz pollination. In addition to the small pores in the anthers (like a salt shaker), these flowers have microscopic pollen grains which are extremely smooth and not oily. This makes them easier to discharge from the tiny pores by the hard-working bees.Common examples of crop plants which require floral sonication by certain bees include; blueberries, cranberries, chile peppers, eggplants, kiwi fruits and tomatoes. Worldwide, about 20,000 species of flowering plants have these unifying (due to convergent evolution) fetures and require floral sonication by pollen-harvesting female bees.

In this drawing we illustrate a diversity of blossom classes that are united by the fact that they possess apically-pored anthers. About 8% of the worlds' flowering plants fall into a natural grouping, a floral syndrome, such that they have two small apical pores at the anther tips, usually provide only pollen as a floral reward and have small, smooth pollen grains which are effectively expelled from the anthers by bee-produced or mechanical vibrations, such as by using a musical tuning fork.

This is an enlarged stamen from a deadly nightshade (Solanum sp.) flower showing the paired apical pores and other labeled parts. Within the large number of flower types that exploit bees as "living buzzers" for pollination, all are related by having prominent pores at the anther tips through which pollen must pass to either feed bees or be transported to anther flower ensuring pollination and subsequent fruit and seed set. Another interesting feature of these blossoms is that they are practicising a form of "deceit" on the unsuspecting bee visitors. As in Solanum, the bright yellow anthers remain turgid and erect even when they have been long-emptied by numerous female bees collecting pollen for their offspring. This is an excellent strategy on the part of the flowers since it maximizes the amount of outcrossing (pollen movement to distant genetically-unrelated plants) produced.

Here we see a small andrenid bee (Protandrena mexicanorum) perched atop a stamen of Buffalo Burr (Solanum rostratrum) follwing a vigorous bout of floral sonication. Bees of various sizes are able to turn themselves into a vibration machine and exploit the pollen riches of wild and cultivated crop plants. Until a few years ago the pollination of all the greenhouse tomatoes grown in Europe was performed by expensive labor using human workers and electrical vibrators to hand pollinate the entire crop. Today, insectary-reared bumblebee (Bombus spp.) colonies are placed within large glass greenhouses and effectively sonicate and set the crop. It is only with bee pollination that the largest, best-formed and highest value tomatoes can be produced. Small experiments in Arizona and other regions of the U.S. with this tomato/bumblebee technology are being tried by various commercial growers.

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Above, we see a colorful analyis of the alternating wingbeat frequency (lower pitch) and the higher frequency (the "raspberry") sonication buzzes made by a female Bombus sonorus, a giant black and yellow bumblebee in the Arizona desert. The display presents the sound dissected to show frequency up the vertical axis and time in seconds across the horizontal axis. Note that most of the sound energy is located at the lower frequencies.

By pressing this button you can hear the actual pollen-harvesting buzzing sounds made by a female desert bumblebee. The entire sound is 8 seconds long and presents 2 portions of lower frequency wingbeat sounds and 5 higher frequency buzzes (in between the wingbeat sounds) used to release the pollen from a native deadly nightshade (Solanum sp.) blossum. Have you ever heard bees making this sound on the tomato flowers in your garden?

By Dr. Stephen Buchmann

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