Bees Are No Bumblers

Sarina Tracy, sarina.tracy@photonics.com

When flowers start to bloom, you know it. The smell of honeysuckle, lilacs or gardenias wafts through the air and into our noses as we walk outside, drive down the road or, with any luck, wake up next to an open window. The perfume is intoxicating, and it’s no surprise that bees and other insect pollinators are drawn to these fragrant blossoms.

However, their journey has nothing to do with scent and everything to do with a very special form of sight. A sensitive area in the eyes of bees allows them to see polarization patterns in the sky and use them as a compass for foraging flights. For a long time, it was thought that navigation was the only use for this vision.

A recent discovery has altered that presumption. Bumblebees can find acceptable pollination spots by reading the polarization patterns of specific flowers when viewing their petals from below, according to researchers from the University of Bristol’s School of Biological Sciences in England and the School of Animal Biology at the University of Western Australia.

The scientists tested the bumblebees by creating two types of artificial flowers. One set of flowers, with its own distinct polarization pattern, held a rewarding sucrose solution; the other set, sporting its own pattern, contained an aversive quinine mixture. The bumblebees soon learned to discriminate between the two targets by correlating specific polarization patterns with food – or lack thereof.

“No one had previously reported a non-navigational function for polarization vision in bees,” said Dr. Julian Partridge, a zoology professor. “Numerous experiments have, of course, been conducted both on bee vision and foraging ... but none had tried to find out if bees could learn to use polarization signals of objects, which is what we did.”

Polarization patterns occur on the petals of flowers but are invisible to humans, making them an overlooked component of floral signaling. Around 53 percent of flower species face downward, presenting their biological information to specialized photoreceptors at the dorsal rim area of the bumblebee’s compound eye.

“We cannot get into the heads of other animals to experience the world as they perceive it, but we can at least look at the information available to them and test them to find out if they can use it,” Partridge said. “The latter usually requires behavioral tests, but the former depends on optoelectronics developments. Advances in that technological field continue to open up new avenues for biological research.

“Polarization imaging has moved out of the lab, and several polarization-sensitive cameras are now commercially available. These will help biologists better understand the relationship between plants and pollinators.”

Understanding those connections has never been more important. The decline in the bee population poses a severe threat to global agriculture, as one out of every three bites of food we eat exists because of pollinators, which provide approximately $20 billion worth of fertilization for American crops each year. These valuable resources have been declining in numbers, with beekeepers in the U.S. and Europe reporting annual hive losses of 30 percent or higher. In 2012, U.S. beekeepers experienced losses of more than 50 percent. The exact cause remains a mystery.

“It is difficult to overemphasize the importance of pollinators, both for natural ecosystems and food security,” Partridge said. “The more we understand about this field, the better. Sensory biology has its part to play, as does technological advancement.”

The research was published in Current Biology (doi: 10.1016/j.cub.2014.05.007).