plants that create blue light districts for bees

Plants have a problem when it comes to reproduction.  In most cases they need genetic material from another partner, but they can’t just walk around and find one.  About one hundred and thirty million years ago Nature came a neat solution to the problem - flowers.  Flowers attract insects in a variety of ways .. color, patterns, scent and so on.  Bees are an important intermediary and are attracted by color and shape. Apparently how they use color has been a bit of a mystery.

Dogs, ferrets and most carnivores have color receptors (cones) that are sensitive to two colors.  Humans and insects like bees are sensitive to three. Bee color vision peaks in the blue - their receptors are ultraviolet,  blue and a rather weak green. Not that many plants produce blue and even fewer drop down in the ultraviolet.  There’s shape, but when you control for it bees still go to blue-free flowers.  Something’s going on!

As a teenager I worked on the grounds crew at a hospital.  I spent most of my break time getting bored and just looking at things.  I noticed a daisy-like flower … yellow petals and a brown interior - that would form a faint blue circular halo coming out at about a twenty to twenty five degree angle.  I managed to convince myself that it was an effect like a rainbow where most of the action is at about a twenty three degree angle.  I’m sure many others had the same idea.  I never asked myself ‘why blue>?’  (it’s quite beautiful and difficult to focus on…  the human eye has real problems focusing on blue, fascinating, but another topic)

Now a bit of a side track.  You’ve seen fancy sunglasses with that produce iridescent colors or select for a certain color in sunlight.   And the colors that form on a thin film of oil on water,.  It's just light reflecting off surfaces and interfering in such a way that different wavelengths of light are made more or less  intense.  Butterfly wings and some plants do it, but usually it isn’t strong enough to attract bees.

There’s another trick.  The rainbow effect you get when you hold a CD or DVD up to white light is caused the geometric pattern of tiny dots.  You get the same thing with lines and spaces.  It’s called a diffraction grating and is a cheap way to make something like a prism.  (another aside - aske me how I managed draw in rainbows that floated on a piece of quartz)  All you need are microstructures roughly the size and spacing of a wavelength of light…  about a hundredth the diameter of thin human hair (blonde for example).

It turns out a lot of plants form microstructures like this…  linear ridges around a half micron wide.  They’d produce a rainbow, but often they’re flawed with a bit of disorder … their spacing might change or parts of them are missing.  The neat thing is they tend to scatter short wavelengths of light - blue light.


Here’s the amazing thing.  In a beautiful paper published in Nature, researchers  looked at a dozen flowers that are distant relatives — like they have common ancestors about fifty million years ago.¹  The disorder in their microstructures are mathematically similar,  It strongly scatters blue light and at about a twenty five degree angle.  Many of these plants form blue halos.  An early ancestor found it useful for bees and it was too useful to go away,

Armed with this knowledge researchers built fake plastic flowers with a variety of microstructures.  Bees will try and pollinate flowers that form these blue halos  and it’s been going on for tens of millions of years,.

So the next time you’re in a garden try looking carefully at some of the flowers.  I had luck with common daisies.  There is some neat physics, material science, genetics and animal behavior going on in your garden.  

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¹ Disorder in convergent floral nanostructures enhances signalling to bees. Edwige Moyroud et. al.  26 OCTOBER 2017 | VOL 550 | NATURE | 469

The image is from the paper.