Researchers at the Adolphe Merkle Institute, the University of Cambridge, and Kew Royal Botanical Gardens, have demonstrated that many types of flowers produce a so-called ‘blue halo’ that produces a blue shine allowing bees to identify them more easily. This color is produced by the nanostructure of a flower’s petal, which scatters light in the blue to ultraviolet spectrum.

By manufacturing artificial surfaces that replicated the phenomenon, the scientists were able to test the effect on pollinators, in this case foraging bumblebees. Their findings, published in the journal Nature, demonstrate that bees can see the blue halo, and use it as a signal to locate flowers more efficiently and more quickly.

Replacing pigments
All flowering plants, including the Oenothera stricta (a type of Evening Primrose), Ursinia speciosa (a member of the Daisy family) and Hibiscus trionum (known as Flower-of-the-hour) considered by the study’s authors, belong to the angiosperm lineage. The researchers found several examples of halo-producing petals among the two major flower groups (monocots and eudicots) that emerged during the Cretaceous period over 100 million years ago – coinciding with the early evolution of flower-visiting insects, in particular nectar-sucking bees.

Previous studies have shown that many species of bee have an innate preference for colors in the violet-blue range. However, plants do not always have the means to produce blue pigments. «Many flowers lack the genetic and biochemical capability to manipulate pigment chemistry in the blue to ultraviolet spectrum,» says Adolphe Merkle Institute Professor of Soft Matter Physics Ullrich Steiner. «The presence of these disordered photonic structures on their petals provides an alternative way to produce signals that attract insects.»

Structured disorder
On a flower petal surface, nanoscale ridges and grooves line up next to each other «like a packet of dry spaghetti», but when analyzing different species the researchers discovered they vary greatly in height, width and spacing. In fact, even on a single petal these light-manipulating structures were found to be surprisingly irregular. This is a phenomenon physicists describe as ‘disorder’. Despite this, the flowers all produce a similar ‘blue halo’ effect.

The researchers conclude that these “messy” petal nanostructures likely evolved independently many times across flowering plants, but reached the same luminous outcome that increases visibility to pollinators – an example of what’s known as ‘convergent evolution’.

To test the halo theory, the scientists artificially recreated nanostructures and used them as surfaces for artificial flowers. In a “flight arena”, they tested how bumblebees responded to surfaces with and without halos. Their experiments showed that bees can perceive the difference, finding the surfaces with halos more quickly – even when both types of surfaces were colored with the same black or yellow pigment. Using rewarding sugar solution in one type of artificial flower, and bitter quinine solution in the other, the scientists also found that bees could use the blue halo to learn which type of surface had the reward.

The team says the findings open up new opportunities for the development of surfaces that are highly visible to pollinators, as well as exploring just how living plants control the levels of disorder on their petal surfaces.

• Read in Nature