Sparks fly between bees and flowers

21 February 2013

Interview with

Professor Daniel Robert, University of Bristol

We've long known that many animal such as hammerhead sharks and platypus can read the electric field of their prey.  But new research out this week suggests that bees might also be able to sense electric fields around flowers. We spoke to Professor Daniel Robert from Bristol University to find out more.

Chris -   So first of all, what did you do this for?  Why were you going looking at bees and electric fields?

Honeybee (Apis mellifera) landing on a milk thistle flower (Silybum marianum).

Daniel - The idea was to first look at adhesion in pollen grains because we were interested in the travel of a pollen grain from the flower to a bee and the fact that it has to stick to a bee. And then when the bee visits another flower, the pollen grain has to jump from that bee to the next flower for pollination to take place. So, we started with that and soon realised that the electricity or electric fields, electrostatics was actually very important in the life of that pollen, in the transport of that pollen. They soon became apparent that this electricity was not just because the pollen had a charge on it, but that was also because the bee, as it approaches a flower carries a charge.

Chris -   So then you say, right, the bees got a charge. So, the flower also has a charge. So, is the bee going to be sensitive to that?

Daniel - So, as a sensory biologist, at first, my question was immediately, can the bee pick that up? Does the bee know that there is a charge difference or a potential difference between herself and the flower?  o, it was known from a literature that bees carry a positive charge as they move across the air.

Chris - Is that just because the bees are rubbing against their molecules as they fly?

Daniel - That's correct, because the air is ionised very often, except for when there's a lot of water, that cancels out the ionisation. When the air is sufficiently dry, airplanes or helicopters also have the same effect, they charge up as they move through the air.

Chris - Do bees get an electric shock when they land?

Daniel - No, they don't because they accumulate that much. They can accumulate a charge equivalent to 100 or 200 volts or so, not thousands of volts as airplanes or even ourselves can accumulate when we walk on static carpets for instance.

Chris - So, what happens when this bee with its net positive charge approaches a flower then?

Daniel - Well, the important bit in that is that the bee is in the air, so it's not earthed to anything, so it carries a charge by itself. So, it's kind of a little bundle of positive charges and it approaches the flower which is itself then linked to the ground. It's earthed. And because flowers are mostly made of water and water is a conductor, flowers will tend to be at a potential or the type of level of electricity if you want, that is on the Earth. So, as the bee approaches with a positive charge, the flower is relatively negative to it. So there wouldn't be an effect where plus and minus will generate an electric field in between these two objects. So, as the bee approaches even without touching the flower, there is a force being developed there.

Chris - Does it matter whether it's a flower or other bits of the plant because after all, petals are just modified leaves, aren't they?

Daniel - That's correct. We haven't looked at petals necessarily so far, but your question is right. There is no reason to think that petals might not be of the same effect.

Chris - So, what effect do you think this has on the bee?

Daniel - So, what we could show is that bees can learn the presence or the absence of these charge difference in the flower. So, we designed a learning experiment for bees in the lab for bumble bees in which we presented artificial flowers which where all of the same colour and all had little pads underneath, electric pads in order to test whether the bee could tell or whether there was a charge or not. And what we did, we repeated experiments that people did before in studying, learning about bees is that we produce some flowers that have sugar and some flowers that have quinine. So, bees like sugar and they don't like quinine. So, on the flowers that had sugar, we put 30 volts of voltage bias which is very reasonable. We find that in nature and the others were at 0 volts, and we soon discovered that after 40 or 50 visits, individual visits by individual bees, that we know, we marked. Bees will tend to choose the flowers with the sugar with 80% accuracy.

Chris -   By presumably recognising that there's that field, so they must be sensitive to it. How do you think they're using that in nature, if at all?

Daniel -   Well, when we remove that field after the learning experiment, we took the same bees that were then educated to find the sugar associated with the electrical charge. When we removed that, bees were unable to find the flowers with sugar. So, they have used that field to orient themselves.

Chris -   How far away from a flower are they sensitive to, do you think? Do you think this is actually a useful thing for them in nature to orientate themselves, locate food sources, and maybe also avoid visiting flowers more often than they should before the nectar is replenished?

Daniel -   That's right.  We think that the bees can detect that at 4 or 5 cm or so from say, a petunia flower and the functionality of that, as you suggest, is about the nectar and the pollen. What flowers want to do is advertise themselves as much as they can to bees. They have nice scents, they have nice colour, they have a texture, they have asymmetry. They look beautiful to us, but they look fantastic to bees of course. Now, what we show is that when a bee visits a flower and we could measure that with electrodes that we implanted into flowers, the potential of the flower changes. And when a bee visits that potential changes and when the second bee comes along, and happens to land for instance, that potential changes even more. So, it would be in the interest of a bee to understand that perhaps that flower has been visited and is depleted from nectar. Therefore, not incurring the risk of predation by spiders, but not wasting time in foraging about.

Chris -   Calling at a shop that's already been visited.

Daniel -   That's right.

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