Springtime is abuzz with the activity of pollinating bees, travelling between flowers whose advertisements are bright, sweet-smelling and shapely. The combination of these sensory inputs help bees to discriminate between rewarding and unrewarding floral patches and thus forage more efficiently. Simultaneously, these signals ensure that plants are only visited by the right pollinators at the right time. However, these well-known sensory modalities are only part of the story. New research published in Science by Dominic Clarke and colleagues at the University of Bristol in the UK finds that bees and flowers are not only buzzing with activity, they are also literally buzzing with electricity.
Insects and plants are not electrically inert. Flying insects, it turns out, are positively charged while plants have a negative charge. It is known that these differences can facilitate pollen transfer between bees and flowers. Until now, however, it has been unclear whether this electric potential was also used as a component of a flower's sensory appeal.
To test the role of floral charge on bee foraging decisions, the team created artificial flowers, e-flowers, that were charged or uncharged and supplemented, respectively, with either a sweet or bitter ‘nectar’ reward. Strikingly, when bees were allowed to choose between these options, they rapidly learned to associate charge with the sweet reward. By contrast, when the scientists pulled the plug on the charged flower, thereby rendering it electrically equivalent to the bitter flower, the ability for bees to correctly choose e-flowers containing the sweet reward was no better than random. Bees, the team found, are even able to distinguish between e-flowers with different charge patterns, for example a flower with uniform charge and another with a charge gradient like a dart board. In short, if bees want a sugar buzz, charge matters.
But how does the ability to detect charge play out in nature? As yet, this remains unanswered. However, the team has taken two important steps forward. First, using electrostatic powder, they revealed that flowers from several plant species vary markedly in their charge pattern. Moreover, just like a heat map, some parts of flowers are charge-hot, while others are charge-cold. Second, the team showed that when charge is paired with a second sensory cue, floral hue, a bee's ability to discriminate rewarding from unrewarding flowers is enhanced. Together, these results suggest that charge patterns and perception, like colors or odors, have evolved as part of the sensory signaling occurring between plants and their pollinators.
Thus far, the story is somewhat one-sided. Recall, however, that bees are also charged. When they alight on a flower they induce nearly instantaneous charge changes in flowers that persist for more than a minute. Within this time window, do flowers withdraw their charged welcome? And do bees, in turn, modify their foraging choices? A blindfold has been lifted with this study, and in the next few years there should be exciting progress in translating the electrical chatter between insects and plants.
