Electric fields of flowers stimulate the sensory hairs of bumble bees.

Most of us have been shocked after walking across a carpet and touching a metal doorknob. The build-up of charge—“static” electricity—on the surface of some nonconductors because of friction is called triboelectricity. We are unaware of the build-up of charge on our bodies as wewalk and only notice it upon discharge when it briefly stimulates our pain-sensing neurons; it is essentially an epiphenomenon for us. That positive charge builds up on flying insects, such as bees, has been appreciated for decades (1, 2). Similarly, flowers hold electric charge and their negatively charged pollens are attracted to the positive charge on the bodies of alighting bees (3). So at the very least, a bee’s accumulation of charge is harnessed to aid in pollination. But, do bees sense the charge on their bodies or on flowers, and use this information to guide their behavior or, like us, are they unaware of it? If bees sense electric fields, then how? A recent set of experiments by Robert and colleagues demonstrated that bumble bees (Bombus terrestris) indeed sense a flower’s electric fields, that these convey important information to them (4) and, in an article in PNAS (5), Sutton et al. show that these electric fields are sensed by electrostatic movements of the many mechanosensory filiform hairs over their bodies. In their first study, Clarke et al. (4) showed that flowers have distinct patterns of electric charge over their surface, and that bees learn to discriminate charged and uncharged artificial flowers. Adding electric patterns to visual patterns on these flowers enhanced the bees’ rate of discrimination learning. One other interesting point is that when bumble bees land on flowers, some of the positive charge from their bodies moves to the flower and cancels some of the flower’s negative charge; this lasts for 1 to 2 min (Fig. 1). The authors hypothesized that a beemight use the net charge of a flower to judge if the flower has been recently visited by another bee and, therefore, has diminished offerings of nectar and pollen. In PNAS, Sutton et al. (5) test the sensitivity of two candidate structures for sensing electric charge: the many tiny filiform hairs distributed over the head and body, and the antennae, both of which are deflected by electric charge and innervated. The hypothesis is that the movement of either or both structures by an electric charge is detectable by the mechanosensory neurons that innervate the filiform hairs and the base of the antenna. In other words, there is no dedicated electroreceptor per se, as found in sharks or electric fish (6, 7), but rather the electrically forced movement of a mechanosensory structure. Using laser Doppler vibrometer, Sutton et al. (5) measured the movements of these structures to applied electric fields, finding that the filiform hairs move with an order-of-magnitude greater velocity than the antennae to the same applied fields. The key experiment was recording from the mechanosensory neurons emanating from the Fig. 1. A bumble bee can detect the electric fields of flowers via the deflections of many tiny mechanosensory filiform hairs on its head and body. (A) Bees accumulate positive charge on their bodies as they fly. Flowers have negative charges. The interaction of these charges when a bumble bee alights on a flower mechanically moves the bee’s antennae and filiform hairs. (B) Stimulation of antennae or filiform sensory hairs with electric chargemoves them. The electro-mechanical movements of the bumble bee antenna (red arrows) do not activate antennal sensory neurons, whereas movements of the filiform hairs (blue arrows) do.