Come hither, says plant

Flowers: More seductive than ever

Many plants need insects to import pollen to fertilize their eggs and start making seeds. To attract these pollinators, flowers advertise with scents, colors and patterns.

Photo: Forest Wander

This honeybee has slathered herself in pollen; some goes back to the hive, and some pollenates the flowers she is visiting.

And now we hear that some plants also use electric billboards to lure their six-legged colleagues.

The plants don’t act deliberately, but their electric field nonetheless communicates with bees, says Daniel Robert, a specialist in insect sensation at the University of Bristol, in the United Kingdom.

Robert, the corresponding author of a study in tomorrow’s Science magazine, says he was intrigued that bees acquire a positive charge during flight, which holds negatively-charged pollen through electrostatic attraction. “A pollen grain needs to stick to the bee, but not too much, so it can be deposited on the next flower,” Robert says. “I thought maybe this electric field was more important than just attraction.”

Pollination is a matter of life and death for many flower species: if insects don’t truck in the pollen, the essential cross-pollination fails, and the plant makes no seeds.

Thus evolution has shaped the flower to be “an extremely manipulative organ when it comes to attracting the bee,” says Robert. Pollen is a protein-rich bee food, and nectar is a sugary delight.

Sending bumblebees to bee school

You maybe didn’t know that bumblebees are trainable. Likewise. But in a series of experiments, Robert found that bumblebees learn to associate the electric field from a fake flower with the presence of sugar.

In soil: Jem Hologram, bee: Lisa Lawley

Bumblebees live the soil, in colonies of a few dozen members. Rollover to see a portrait of the bumblebee.

Visually, the fake flowers were identical, but some emitted no electric field, others had a 10-volt or 30-volt field. Some of the flowers carried a sweet reward — sucrose — while the others delivered super-bitter quinine. “Bees do not like the taste of quinine, and they learned to go to the sucrose, but it took 40 to 50 visits to reach an 80 percent correct rate,” says Robert.
Plants are rooted in the Earth and thus have a neutral or slightly negative charge, while bees accumulate a positive charge as they fly, creating an electric field between plants and bees.
Without the 30-volt cue, the bees could not find the sucrose, “so the information from the electric field was vital.” A 30-volt field is roughly what exists in a flower 30 centimeters tall.

Adapted from Clarke et al. 2013

Fake flowers did not alter bee behavior if they emitted no voltage or 10 volts, but a 30-volt field around the flower led to 80 percent accuracy after 40 bee visits. The stronger field taught the bees which flowers had sugar, and which had bitter quinine.

“It was known before that bees would charge up as they fly through the atmosphere, and that flowers have a negative electric potential compared to the atmosphere, but we are the first to show that bees can detect the field, and can learn to discriminate between different fields,” Robert says.

Water is a great conductor of electric fields, and sharks, skates and other fish use electricity for sensory purposes. “But in terrestrial creatures, this process of recognizing electric fields is unique as far as we know, says Robert.

It’s possible that many insects use electric fields, Robert says. “There is no reason to think that any flying insect that goes through the air will not have this electric potential, because it is physically inevitable that you will accumulate a charge when flying through an ionic medium.”

The economy of flowers

The flowers seem to be an innocent bystander, not taking active charge of their charge, but why did bees evolve a receptor for electric fields, and the neural circuitry necessary to use the sensory input to change their behavior?

The answer resides in evolutionary economics, says Robert. “If a flower attracts too many bees, the nectar that they feed on will run out,” and that could spell disaster for both sides.

“If the flowers start to lie to the bees,” Robert says, “that’s not too good, as the bees are quick to learn which flowers are not good, and then they go back to the hive and say, ‘Let’s go to another place.’”

Photo: Scott Zona

If color, scent and pattern are all signals to pollinators, we don’t know how insects will respond to this beautiful petunia!

And bees can’t waste too much time visiting dry flowers.

Open for business!

But neither flower color nor scent registers the state of the nectar supply, Robert says. So how can flowers tell the bees they need some slack time to produce more nectar?

With the electric field, which gets weaker when a bee lands to gather pollen, and even more when a second bee arrives.

To the bee’s still mysterious electrical detector, Robert says, “some of the flowers look bright, and some are dim, which means they have been visited a few minutes before. And when they are out foraging, this means the bees can avoid a negative reward. It means the advertisement is honest, and it’s changing from minute to minute.”

We learn to rely on ads that accurately reflect conditions, Robert adds. “When you drive in your car, and the motel sign says ‘Vacancy,’ you might stop. When it say ‘No Vacancy, you don’t. You have learned to trust the sign.”