BioEd Online

As they zero in on their nectary reward, bumblebee follow an invisible clue: electric fields. Although animals including sharks have been known to have an electric sense, this is the first time the ability has been documented in insects.

Pollinating insects take in a large number of sensory cues buzz between flowers to minimise the energy they expend while foraging, from colour hues and fragrances to petal textures and air humidity.

It has long been known that bumblebees build up positive electrical charge with their rapidly flapping wings. When they land on electrically grounded flowers this charge helps pollen to stick to their hairs. Biologist Daniel Robert at the University of Bristol knew that such electrical interactions would temporarily change the electrical status of flowers, but what he did not know was whether bumblebees were picking up on this.

Keen to find out, he and a team of colleagues measured the net charge of Bombus terrestris, a common species of bumblebees, by luring them with sucrose into a Faraday pail — an electrically shielded bucket that reacts to the charge of anything inside it. As expected, most bumblebees were carrying a positive charge.

Next, the team placed the insects into an arena with petunias (Petunia integrifolia) and measured the flowers' electrical potential. Sure enough, when bumblebees landed the flowers' electric potential went more toward positive.

Finally, the team released bumblebees into an arena with artificial flowers, half of which were positively charged and carried a sucrose reward while the other half were grounded and carried a bitter solution. Over time, the bumblebees increasingly visited the rewarding charged flowers.

In contrast, when the researchers turned off the electrical charge on the flowers and re-released the trained bumblebees, they visited rewarding flowers only 54% of the time, results that were statistically no different from random choice, suggesting that the bees were detecting the and using the fields to guide their activities, rather than, say, using their sense of smell.

“We think bumblebees are using this ability to perceive electrical fields to determine if flowers were recently visited by other bumblebees and are therefore worth visiting” says Robert.

“We had no idea that  this sense even existed. Assuming we can replicate the findings, this is going to open up a whole new window on insect sensory systems for us to study” says behavioural biologist Thomas Seeley at Cornell University.

Some experts suggest that the new work has implications for insects other than bees. “If you think about it, these discoveries could also apply to hover flies and moths. We don’t know if they can perceive charge differentials, but they burn a lot of energy while hovering around looking for pollen or nectar, so it would make sense for them to attend to such cues” says chemical ecologist Robert Raguso of Cornell University in Ithaca, New York.