As they travel through the air, bumblebees lose electrons, accumulating a small positive electrical charge. Flowers, meanwhile, are generally negatively charged at the top, thanks to a slight positive charge in the air around them. As a bee approaches a flower, a tiny electric field is created between plant and pollinator. In the past, scientists have suggested that these differing charges encourage the transfer of pollen between flower and bee, helping the tiny pollen grains “jump” onto the pollinator.
However, [a] new study showed that the bee’s landing actually influences the flower’s electrical charge—increasing it slightly—for a short period of time. The study’s authors hypothesize that this change may signal to the next bee that the flower has just been visited and that its nectar stash is depleted. Other cues, such as a flower’s shape or color, sometimes change in response to a bee’s visit, but these changes can take hours. The electrical field, on the other hand, changes almost instantaneously, providing a nearly immediate signal to incoming bees.
In order for this process to work, bees must be able to sense the electrical fields of flowers. To test this ability, the researchers created a field of fake flowers that they could manipulate. Half the flowers were positively charged, and these flowers held a tiny bit of sugar solution as a reward for the bees. The remaining flowers had no charge and held a bitter quinine drink. After just 40 visits, the bees had learned that the positively charged flowers were rewarding, and they visited them more than 80 percent of the time. Once the charges were turned off, the visitation rate to the sugar-laden flowers decreased to random chance, since the bees no longer could use the electric field as a cue.
In a similar test, the researchers found that the bees could even distinguish between flowers that differ in the geometry of their electric field. The bees learned quickly that flowers with a “bullseye” electrical pattern—with a negatively charged center ring and a positively charged outer ring—were rewarding, while flowers with an even positive charge were not.