The electric relationship between bees and flowers!

Daniel Robert, Professor of Bionanoscience, School of Biological Sciences
The Bristol Centre for Nanoscience and Quantum Information,
University of Bristol

A reciprocally beneficial arrangement between bees and flowers has evolved: the deal is nectar versus pollination. The key thing about pollination is that a flower requires an insect to transfer the pollen from itself to another flower of the same species or vice versa. Bees learn, and if the flower has no nectar a bee will be dissuaded and visit that type of flower less. This bad news is communicated via dance to the rest of the hive, and the flower species suffers accordingly. So a flower needs to communicate with a bee that its nectar is depleted – but it can’t easily do this using colour, smell, or shape. That is where electric fields come in.

Professor Robert showed us a beautiful picture of a computational model of the charge distribution and electric field around a bee in flight. Insects generally are positively charged up to several hundred picocoulombs; this is a frictional or triboelectric effect. Flowers generally are negatively charged. The third component of the interaction is with the atmospheric electric field.

Professor Robert’s team have measured the charge on the bumblebee as on average 32 picocoulombs, using a Faraday pale. Having established that bees are charged, he introduced us to the concept of triboelectric scales; these are rankings of how good things are at transferring charge to and from each other. Bees, and rabbit paws used as bee surrogates, top his triboelectric scale. The initial work use a scale of artificial materials as the substrates, but they have now been able to develop a triboelectric series for bees against different plants – and again bees are the top dog.

Turning to atmospheric electric fields, he explained to us that we can consider the ionosphere and the surface of the earth as the plates of a capacitor, with up to 300 kV between the. In dry weather there is a strong ground-level positive vertical field gradient; in mixed weather the field can be mixed positive and negative. In dry weather the electric field could be 100 volts per metre, but in storm conditions this could be plus or minus 20 kV per metre. it’s easy to see that there could be up to 10,000 volts across a tall tree top to bottom on a dry day, and much more in stormy weather. Indeed it’s possible to see St Elmo’s Fire at the top of trees in stormy conditions.

The third component of the interaction of course is a flower. Professor Robert showed us a computational model of the distortion of local electric field around a flower; the general field distortion in the vicinity of plants is tens of volts per metre, and it reaches thousands of volts per metre very close and inside the flower.

Professor Roberts described for us a very elegant and quite simple experiment to investigate whether bees could indeed react to local electric field perturbations The arrangement is a series of platforms randomly distributed in a zero-field environment. Some of the platforms had a positive voltage of 30 volts and carried sugar, whilst the rest were at earth potential and carried quinine solution – which bees do not like. The bees were allowed to investigate and learn from this arrangement. Their ability to sense electric fields was then tested by repeating the experiment with a positive voltage of 30 volts and a control voltage of 10 volts, and allowing the bees to choose with no reward and no unpleasant quinine surprise. He was able to demonstrate that the bees were entirely capable of sensing high versus low fields. As a test as to whether this effect worked in the real world, his group did an experiment applying +/- 5 volts via battery to meadow flowers and observing bee behaviour. It was noted that when 5 V was applied to the flowers bees did not visit them, having received the message that the flower was depleted.

To further investigate whether the effect is primarily a response of the bee or the flower, Professor Robert’s group measured the charge change on actual flowers. As a bee approached a flower the potential rose, and as the bee landed on the flower it depolarised. It continued to depolarise even after the bee had left. It did recover after the bee had left, quite rapidly, but not immediately, and not to quite the same level as before the bee’s visit. Repeating the experiment with multiple bees showed that the effect was cumulative and that after each visit the repolarisation was smaller. It seems therefore that not only does a bee sense flower potentials, but the flowers react to the bees in a way that communicates their nectar availability status. The actual mechanism of the depolarisation appears to be linked to pollen exchange between stamens and anthers, the pollen having a different charge after it has been on the bee then when it was in the flower.

One interesting question is what the sensory mechanism is in the bee. This appears to be hair orientation. The bee’s hairs vibrate or move as the charge distribution on them moves, and the bee consciously or unconsciously sense the movements of its hairs and reacts accordingly.

Professor Robert then moved on to effects in other animals, and started by telling us about spider ballooning: this is a behaviour of spiders wherein they use silk threads to lift themselves into the atmosphere to travel quite long distances and quite great heights. Originally this was thought to be facilitated by air currents, but Professor Robert’s work has shown that it’s actually due quite significantly to the lifting effect on the silk threads of atmospheric electric fields. His group was able to show the effect in the laboratory and even to regulate how far the spiders levitated by changing the local electric field strength.

A less attractive example of animals using local electric fields to move is ticks. We know that ticks can “jump” from grass onto passing animals or humans, but ticks can’t actually jump. Professor Roberts showed us a computational model of a cow in the Earth’s electric field; field strengths in the thousands of volts per metre range could be seen around parts of the cow. What seems to be happening is that electrostatic forces pull the tick towards a passing animal when it releases itself from the low potential grass stalk.

Professor Roberts moved on to prey/predator interactions. His group have shown that cinnabar moth caterpillars stay curled up (which is a defence response) when exposed to the electrical signature of an approaching wasp. Wasps produce an alternating electric field because of the movement of their wings, and caterpillars are essentially uncharged. It is speculated that treehoppers may actually be specialists in electoral reception and even have 3D detection ability via a complex hair distribution over a number of nodules on their heads.

Professor Roberts finished his talk by speculating that perhaps bees even may be able to sense coming changes in the weather via changing flower potentials as the Earth’s electric field varied with approaching storm systems.

Phil Chadwick