Color code tells bumblebees where to buzz.

Some flowers spurt pollen only when bees shake them just right. Some display color changes that welcome insects to the freshest blooms. A flashy North American wildflower depends on the rare combination of these specialized pollination tricks, report Brendon M.H. Larson and Spencer C.H. Barrett of the University of Toronto.

In the April AMERICAN JOURNAL OF BOTANY, they analyze the wiles of the Virginia meadow beauty, Rhexia virginica. For an East Coast flower, it "looks too gaudy, too tropical," Barrett says. Pink petals contrast with big yellow anthers, the organs that hold pollen.

Insects can grope around all they want, but the anthers release pollen only when buzzed. Bees must shiver their wing muscles, turning into live tuning forks. Vibrating anthers then shoot out streams of pollen grains. Bumblebees, but not honeybees, shake pollen from the meadow beauty, report Larson and Barrett.

Only a few buzz-pollinated species change color with age. However, on the second day a meadow beauty blooms, its anthers fade to pink. At this point, "reproductively, they're kind of eunuchs," Barrett notes. Both male and female gametes have lost most of their viability.

However, these otherwise useless blooms bulk up the display, Barrett points out. He and Larson found that big clumps of flowers, even if some are has-beens, attract more bees than skimpy displays. Once bees arrive, they use the color cue to focus on first-day flowers.

The meadow beauty's strategy avoids penalties of a plant's overselling itself. The danger of flashing a lot of flowers, Barrett says, is that pollinators just crawl from one to another, wasting the plant's pollen on its own blooms. The color change allows an alluring display but minimizes the number of blooms any insect visits. There's lots of dazzle, he says, "but the plant isn't paying the cost."

This observation fits with non-buzz flowers that shift color with age, notes Martha R. Weiss of Georgetown University in Washington, D.C. In the systems she studied, display-mass matters and pollinators avoid spent blooms (SN: 4/11/98, p. 233). She also tallied at least 214 genera that include color-shifting flowers.

About 8 percent of the world's flowers rely on buzz pollination, says Stephen Buchmann of the Department of Agriculture's Carl Hayden Bee Research Center in Tucson. Many of these grow in the tropics, but temperate-climate tomatoes, potatoes, blueberries, and cranberries need a good buzz, too.

In tests on a nightshade flower, buzzing threw out pollen with about 30 times the acceleration due to Earth's gravity, Buchmann reports. He notes that test pilots pass out at around 8 g.

Buchmann is puzzled by the honeybee's failure as a buzz pollinator. It's not that honeybees are too small. Bees half the size of a honeybee's head can successfully buzz flowers, he reports. Honeybees certainly buzz in other circumstances. Yet in blooms that need buzzes, honeybees "do crazy things like stick their tongues into the [anther] pores," he says. "It's very weird."