Floral electric fields could join color and fragrance as cues to pollinators
Slight electric fields that form around flowers may lure pollinators much as floral colors and fragrances do.
In lab setups, bumblebees learned to distinguish fake flowers by their electrical fields, says sensory biologist Daniel Robert at the University of Bristol in England. Combining an electrical charge with a color helped the bees learn faster, Robert and his colleagues report online February 21 in Science.
Plants, a bit like lightning rods, tend to conduct electrical charges to the ground, Robert says. And bees pick up a positive charge from the atmosphere’s invisible rain of charged particles.
“Anything flying through the air, whether it’s a baseball, 767 jumbo jet, or a bee, acquires a strong positive electrostatic charge due to interaction with air molecules,” says Stephen Buchmann of the University of Arizona in Tucson.
Robert and his colleagues checked whether bees could choose flowers based solely on the electric fields the plants produce. Purple metal disks (encased in plastic so as not to shock bees) stood in for flowers. Half of them, wired for 30 volts, held sips of sugar water. The unwired ones offered a bitter quinine solution that bees don’t like.
Bombus terrestris bumblebees learned to chose sweet, wired disks more than 80 percent of the time. When researchers unplugged the wired disks, the bees bumbled, scoring sugar only by chance.
“The big question is how bees do this,” says Lars Chittka of Queen Mary University of London. Bees bristle with hairs, and he speculates that a charged insect nearing an oppositely charged flower feels the hairs bend.
Electric charges of bees and flowers do interact, Robert confirmed after studying bees visiting real petunias. When a bee landed, and sometimes even before, flower stems registered an electrical surge that didn’t fade until after the bee had buzzed onward.
This surge in electric potential might tip off another passing bee that the flower had just lost nectar to a different visitor, a change that scent or color would not reveal. Robert says, channeling a petunia: “I’m still pretty and smell nice, but my potential tells you to come back later.”
Variations within a flower’s electric field may even hold clues to where bees should probe for nectar, Robert speculates. Researchers found striking electrical patterns when they wafted positively charged colored aerosol particles over blooms.
“I am blown away,” said Anne Leonard of the University of Nevada, Reno. “I imagine that we’ll all be desperate to spray our flowers down with the aerosol they describe.”
Bumblebees can distinguish at least between simple electrical patterns, Robert says. They learned to fly to disks with a negatively charged bulls-eye ringed with a positive charge instead of to disks producing a uniform field.
Charge patterns can also reinforce other flower advertisements, the researchers found. Bees challenged to learn to distinguish two slightly different shades of green learned faster when researchers paired each shade with a distinctive electric charge.
In the real world, bees face a marketing barrage of combined scents, colors and other cues from flowers. “One open question in behavior is when are such cues used, and when are they ignored,” says Robert Raguso of Cornell University. Pollinators need to find their next meal before they run out of fuel, and if a cue such as humidity or electrical charge can let them make snap judgments to save resources, he predicts bees would pay attention.
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