Fertilizers disrupt the electric feel of flowers

Abstract

Electricity is literally all around us. As humans, we generate weak electric fields whenever we move our muscles; spiders use static electricity in the air to fly across great distances; and geckos use static in their sticky feet to scale vertical walls with ease. Since everything has an electric field, we can imagine that this fundamental force of nature plays an important role in many ecological interactions. Among insects, bumblebees are known to alter the static electricity of flowers to communicate with members of their hive; the positive charge that collects on their tiny hairs alters the electric field around a visited flower for ∼100 s, communicating to the next bee that buzzes by that the nectar and pollen rewards from that flower have already been exhausted. Ellard Hunting and colleagues from University of Bristol, UK, and the Woods Hole Oceanographic Institution, USA, set out to test whether the application of agricultural sprays alters the electric field of flowers and whether bumblebees can detect these disruptions.The pervasive use of chemicals in agriculture is a substantial source of pollution. But besides chemical toxicity, little is known about how agrochemicals otherwise affect plant–pollinator interactions. Hunting and colleagues observed that bumblebees tend to hover around chemically treated flowers and decide not to land. To investigate the cause of this avoidance behaviour, the researchers developed a computational model for how the application of agricultural sprays may alter floral electric fields up to 4 cm from the flower. Their model considered how the presence of droplets on the flower's surface and localized increases in humidity from a spray application might impact the flower's electric field.Next, the researchers studied the effect of common agricultural fertilizers on flower features that bees recognize. To see if fertilizer sprays change the way bees see flowers, the researchers determined what wavelengths of light are reflected from droplets with and without fertilizers added. To study whether fertilizer sprays change the way flowers smell, they measured how much effort the bees put into feeding when offered sugar water laced with fertilizer compared with just sugar water. Lastly, to visualize the electric fields surrounding various blooms, the scientists sprayed charged coloured powder particles onto the centre of ragwort flowers and observed how the powder distributed across the surfaces. Although the fertilizers did not affect how bees see or smell flowers, the chemicals did alter the electric field around each bloom for several minutes.Next, Hunting and colleagues tested whether the adverse effects of spray applications persist over time. To do this, they planted electrodes into the cut stems of potted lavender and Texas bluebell plants and recorded the electrical current streaming down the stems in response to different spray applications. While a water spray caused a change in current that only lasted 30–60 s, a spray containing the fertilizer caused a consistent change in current that lasted up to 16 min. A spray with another common agricultural chemical – the pesticide Imidacloprid – induced a change in current that lasted up to 25 mins. Even subsequent water sprays, simulating rain, produced prolonged changes in the plant stem's electrical signature, suggesting that this effect can endure even after a single application of fertilizer.Finally, the researchers observed wild bumblebees foraging on cut flowers with artificially applied electric fields to mimic the effect of fertilizers. They pierced lavender stems with metal electrodes, placed them in glass jars of tap water, and attached a 13 V battery to produce an electric current in the plant. By recording 2 h videos and counting the number of bumblebees that approached versus those that landed on the lavender, the team showed that bumblebees somehow sense weird electricity as they approach a flower and land less as a result, serving as a healthy reminder that as humans we affect our environments in ways we can't even sense.