Nano scale ridges on petals give off a ‘blue halo’ helping bees locate them more easily

The research has shown that the disordered surfaces, found in most major lines of angiosperms such as daisies, produce visual signals that are salient to bees. The optical trick is not always visible to humans, but it can be spotted by bees, suggesting that it may have evolved to attract pollinators.

Previous research has shown many species of bee prefer petals in the blue-to-ultra violet range but plants can’t always produce these shades in their pigmentation. Many flowers lack the genetic and biochemical capability to manipulate pigment chemistry in the blue to ultraviolet spectrum. So breaking up the light spectrum to attract bees by displaying the colour they prefer is an alternative strategy.

A paper recently published in the journal ‘Nature’ explains the researchers artificially recreated ''blue halo'' nanostructures and used them as surfaces for artificial flowers. They then tested how bumblebees responded to surfaces with or without halos. The experiments revealed the bees can identify surfaces with halos more quickly regardless of the colour of the surface and the order in which they are presented.

The team offered the bees a sugar solution in one type of flower and a bitter solution in the other. Researchers found that the bees could track down the reward using the blue halo. The study’s lead writer, Edwige Moyroud, from Cambridge''s Department of Plant Sciences, has previously been supported by the EU-funded NANOPETALS project. He explains that insect visual systems are different to human ones: bees have enhanced photoreceptor activity in the blue-UV parts of the spectrum.

Moreover, the bees were about a third quicker at travelling between squares with a blue halo than smooth squares, suggesting the optical trick improved their foraging efficiency, putting it on a par with speeds for blue-pigmented flowers.

All flowering plants belong to the ''angiosperm'' lineage. Researchers analysed some of the earliest diverging plants from this group, and found no halo-producing petal ridges but they did find 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.

Species which the team found to have halo-producing petals included Oenothera stricta, Ursinia speciosa and Hibiscus trionum. The researchers say that presence of tiny ridges on the petals of species from across the family tree of flowering plants suggests these nanostructures evolved independently.

The findings of the research raise as many questions as they answer, for example, how do plants control the level of disorder on their petal surfaces. The team describes the developmental biology of these structures as a real mystery, so the research opens up new avenues for investigation. It also opens up new opportunities for the development of surfaces that are highly visible to pollinators.

The EU-funded NANOPETALS (Molecular mechanisms of petal iridescence: how do structural colours arise in flowers?) project ended in 2014. Support to two researchers during that project have fed into the recent work set out in the paper.

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