Pollination is necessary for plants to reproduce, and so many species of plants depend on bees and other insects as pollinators. Flowers employ a number of strategies to attracts pollinators such as bright colors, patterns and alluring fragrances. Flowers however also have a secret weapon—electric fields (e-fields)—and pollinators such as bumblebees are able to detect and distinguish between different e-fields given out by flowers. These electric signals work in concert with other attractive signals (e.g., bright colors and enticing fragrances) to enhance flower detectability and recognition.
What makes bumblebees electro-sensitive? You walk across a carpet or rug, reach for a metal doorknob and…ZAP! You get an instant shock. Why? This is because the friction between our feet and the carpet as we walk leads to a build up of electric charge on the surface of our bodies, and we only notice it when it momentarily stimulates our pain-sensitive neurons upon discharge. This build up of charge through friction is known as the triboelectric effect, and bumblebees use this phenomenon to enhance pollen transfer and fertilization. As bees fly through the air, friction with air molecules such as dust particles knocks electron from their surface, leaving them with a surplus of positive charge, particularly on their hairs and antennae.
Similarly, flowers hold electric charge—they usually have a negative charge, especially in open regions with fair weather. The charge usually creates a pattern on the flower’s petals, and the shape of the pattern depends on the shape of the petals and how well they conduct electricity. How do flower hold electric charge? The air around flowers is positively charged, and the charge increases by 100V with every meter above the ground. In response to the positive potential of the air, negative charge builds up on the surface of the flowers through a process known as electrostatic induction i.e., the process by which a charged object near a conductor creates or generates an opposite charge on the conductor’s surface. Thus, as the bee approaches the flower, the floral potential becomes more positive and the electrostatic forces between the two bodies increase rapidly, causing some pollen grains to “jump” from the flower to bee’s hairs and antennae. The change in floral potential persists even after the bee leaves, thus other bees are able to tell whether a given flower has been recently visited or not.
Are bees aware that they are electro-sensitive? To answer this question, researchers created two groups of e-flowers (artificial flowers with electric fields). One group had a sweet reward while the other one had a bitter reward. The bees soon learned to visit the flowers with the sweet reward with about 81% accuracy. They however couldn’t pinpoint the flowers with the sweet reward when none of the flowers were charged.
Bees are also able to distinguish between e-fields of different patterns. To demonstrate this, researchers created two groups of e-flowers with different e-field patterns. The first group had a sweet reward while the second group had a bitter one. Also, the first group (sweet reward) had +20V on the outside and -10V on the inside, whereas the second group (bitter reward) had a uniform voltage at +20V. The bees soon learned to visit the flowers with the sweet reward with about 70% accuracy. They however lost the ability to distinguish between the two groups when both groups were set to a homogenous charge at +20V.
Finally, to demonstrate that bees use electric signals in concert with other attractive signals to enhance flower detectability and recognition, researchers trained the bees to discriminate between two groups of e-flowers with the same e-field pattern, but different shades of green. The bees quickly learned to discriminate between the flowers with 80% accuracy in 35 visits. However, when the researchers introduced different e-field patterns to the flowers, the bees were able to reach the same level of accuracy in just 24 visits!
- Clarke, Whitney, Sutton & Robert. Detection and Learning of Floral Electric Fields by Bumblebees. Science http:/dx.doi.org/10.1126/science.1230883
- Clarke, D., Morley, E. & Robert, D. J Comp Physiol A (2017) 203: 737. https://doi.org/10.1007/s00359-017-1176-6