Flower petals with that shiny, color-shifting effect are easier for bees to find, but if they’re perfectly iridescent, flowers risk confusing the bees. Researchers studying this delicate balance reveal that flower petals produce the “just right” signal to attract their helpful pollinators. The findings are published in Current Biology this week. 

Some beetles, birds, and fish are extremely iridescent, and researchers have wondered why floral iridescence is far less striking – at least to us. After all, animal-pollinated flowers are described as “sensory billboards,” with their otherwise showy displays. Are they just not as good at producing iridescence? (An iridescent flower is pictured below to the right.) Turns out, we’re just not their intended target. 

Based on previously-visited flowers, bees remember which colors correspond to nectar awards, and too much iridescence corrupts this flower color identity. "On each foraging trip a bee will usually retain a single search image of a particular type of flower," University of Cambridge’s Beverley Glover explains in a statement. "So if they find a blue flower that is rich in nectar, they will then visit more blue flowers on that trip rather than hopping between different colors. If you watch a bee on a lavender plant, for example, you'll see it visit lots of lavender flowers and then fly away. It won't usually move from a lavender flower to a yellow or red flower."

To test the trade-off between flower detectability and recognition, a team led by Glover and University of Bristol’s Heather Whitney created artificial flowers that varied in pigmentation and degree of iridescence: perfectly iridescent (like the back of a CD), subtle or imperfectly iridescent (like that found in nature), or not iridescent at all. And then they sent out the bumblebees (Bombus terrestris). 

The bees located the iridescent flowers faster than they did the non-iridescent ones, and it didn't make a difference whether the flowers were perfectly or imperfectly iridescent. However, when it came to finding nectar, flowers with perfect iridescence caused the bees to make many mistakes: They would visit similarly-colored flowers that didn’t contain nectar. They couldn’t distinguish between subtle color variations, and as a result, they struggled to identify and recognize flower colors that are worth visiting. 

“Floral iridescence is a trade-off that makes flower detection by bumblebees easier, but won't interfere with their ability to recognize different colors,” Glover says. "Bees are careful shoppers in the floral supermarket, and floral advertising has to tread a fine line between dazzling its customers and being recognizable," study co-author Lars Chittka from Queen Mary University of London adds.

^{Image in the text: Iridescent flower. Edwige Moyroud}