How do bees see flower iridescence?

Recent studies have shown that bees can recognise iridescence. This is the rainbow pattern that you see on the shiny side of a CD and some plants use iridescence to give them their colour too.  Kate Lamble headed over to Cambridge University's Bee Laboratory to speak to PhD student, Alison Reed about her research.

Alison - I'm particularly interested in looking at flower colour and the texture of a flower surface to see how bees and insect pollinate and interact with that surface.

Kate - Why do you think your research is important?  How can it help us understand the natural world?

Alison - Well, because pollination and insect behaviour is so important and it's all around us, and it affects a lot of the things that we value including food production. That just a general understanding of how this process works is really valuable and it might feed on to other projects which might be able to benefit crop production and food production in the future.

Kate - So, how do bees see the world? Is it particularly different from what we see as humans?

Alison - Well, the colour spectrum that they see is very different to us. So, they can see UV wavelengths which we can't see and the colours that they see are quite different to what we see. And so, we need to look at things from the bee's point of view and do experiments to see if they can see colours that we can see basically. So, we're looking at iridescence in flowers because even though we can see it, bees may not be able to see it, so that's why we need to test it.

Kate - When I think of iridescence, I think of really shiny things like beetles or butterflies, but I don't necessarily think of flowers. Can you tell us a little bit more about how it works in plants?

Alison - It's very similar. A lot of the structures that animals use are also used by plants, but it wasn't really studied until very recently and that's why our lab is really interested in it. For example, we're mainly looking at flowers which have diffraction grating, so they have grooves, they're all folding at the cuticle on their petals, and that produces a diffraction grating which makes iridescence. I mean, that does happen in animals, but mostly, animals use multi-layers, but petals predominantly use diffraction gratings. It's fairly unique to flowers and we haven't found any flowers which have multi-layers on their petals.

Kate - If you want to test whether a bee can see these tiny folds on a flower, how do you go about testing that?

Alison - Well, the folds on a flower, very similar to the folds that you find on a CD. So, when you rotate a CD, you'll see the different colours and the iridescence, and that's exactly the same structure that we see on a flower. So sometimes to begin with, we wanted to see if they could see that CD iridescence because it's a perfecting grating. So, it produces a perfect iridescence. So, we first would test to see if they could see that and they could. And so then we followed that up by doing some experiments with flowers which had similar structures but not quite so perfect and they could also see those diffraction gratings in that iridescent effect.

Kate - Are there any particular flowers that you use that are particularly iridescent?

Alison - One of the first ones that we discovered was tulip flowers which have quite a good iridescence obviously and you could see that's why, one of the reasons why they're so shiny when you look at the surface of a tulip. And we've used those for most of our experiments because that's what's been used in the past, but over time, we're gradually finding more and more flowers that have got even better diffraction gratings and even better iridescence, so hopefully, we'll test them with those in the future.

Kate - If you're testing to see if a bee can see that iridescent effect, how do you physically go about it? Do you just show a bee your flower and see what happens?

Alison - Well, we can't really use real flowers because there are so many other things to consider like scent, so we make artificial flower in the lab out of plastic, and we do a differential conditioning experiment where you have two different flowers - one is iridescent and one isn't. Then with the non-iridescent flower, you provide a punishment which is a bitter quinine solution and with the iridescent one, you provide a reward which is a sugary sucrose solution. But if they can tell the difference between the two, they should learn not to go to the quinine and the horrible tasting flowers, but to go to the iridescent ones.

Kate - Could you show me a couple of those artificial flowers that you've got?

Alison - Yeah, of course. This is one made from a CD, so you have an impression of the CD and dental wax which makes a mould basically, a really fine structure mould and then you can make a plastic replica of the surface using resin.

Kate - So, it sort of looks like a small plastic coin with a hole in the middle.

Alison - It's basically like a miniature CD with a pigment in the plastic resin.

Kate - And why is there a hole in the middle? What's that used for?

Alison - That's used for putting a sucrose reward so the bees can feed from the middle, so it's basically replicating the structure of a flower.

Kate - It doesn't look particularly like a flower. I mean, if it was me, I'd make them to look like pretty petals.

Alison - The most important thing for us is that they're all the same so that they're comparable. I guess we could make them look like petals, but these bees have never seen flowers before so it wouldn't make much difference, I don't think.

Kate - If you've made that one from a CD, how would you go and make one from a flower?

Alison - It's very similar. You have to make sure that the petal that you're casting is very flat and you do the same thing basically. So, you go into the gardens and you find a flower that you're interested in testing and you make up the wax and you press the petal into the wax, and it dries within 10 minutes and then you've got your cast, your mould for making your discs.

Kate - So, what have you found out? Is there a difference between the CD ones that you've been using and the tulips?

Alison - It's very similar for CDs and for flowers, but bees move faster between the iridescent disks than they do between the non-iridescent ones. So, when you're using flower casts made from flowers with flat cells, that produces a very different result to flowers that have this diffraction grating on their surface.

Kate - So, do you think that flowers have evolved these tiny structures in order to attract insects like bees?

Alison - Sort of co-evolved I would've thought, insect vision and flower colours sort of co-evolved. I mean, it's important for the flower that it's unique in its environment so that it gets pollinated because obviously, it wants pollen from its own species to be transferred to another flower of the same species. So, it's really important that these flowers are unique and unusual. And that's one of the reasons probably why this is so helpful for flowers because it makes them unique.

Kate -   Has there anything come up that you haven't expected?

Alison - How temperamental the bees are I think. You don't realise how much they're affected by the weather and by the conditions that you keep them in, and just what they feel like on a day to day basis I guess. So, sometimes you'll get bees that come and start an experiment, and then they'll go back to the colony and they won't appear for another few days and your experiment has been ruined. So, you have to start from the beginning again, but I guess that's all part of working with animals.