Guided by scent and vision, foraging bumblebees are on a single-minded mission to collect nectar. However, Sarah Arnold and Lars Chittka from Queen Mary University of London, UK, explain that distinguishing the colours of succulent blooms can be challenging when the transition from broad daylight to leafy shade can dramatically alter a flower's hue. ‘Fortunately bees have colour constancy. They can correctly recognise colours even if the illumination changes’, explains Arnold, although she adds, ‘but there were hints that this wasn't completely perfect and under some lighting conditions they make more mistakes than under others’. Curious to find out how foraging bumblebees cope with shady conditions, the duo decided to test the insect's abilities to distinguish similarly coloured flowers in clear and shady circumstances (p. 2173).

Using a flight arena where they could control the lighting conditions, the duo installed a well-stocked nectar feeder and trained bees that had never seen a flower to visit it. Once they were sure that a bee was a keen forager, they introduced sweet-tasting purple simulated flowers and bitter-flavoured mauve ‘flowers’ for the bee to explore, hoping that the bees would learn to visit the sweet purple flowers. ‘We knew the bees could tell the two colours apart, but it was a reasonably difficult task’, says Arnold. Then, after 100 training sessions under simulated daylight, Arnold removed the simulated flowers' flavours, tested how well the bees recognised the purple flowers and was pleased to see that they were able to successfully pick them out 86% of the time. However, when she mimicked the effect of leaf shade – by filtering the daylight through green filters and tracing paper – the bumblebees found the discrimination task much trickier. They took longer to learn to distinguish between the colours and struggled to pick out the purple ones, only successfully identifying them on 75% of their visits. The bees found it more difficult to distinguish colours under leaf shade than daylight.

Next, Arnold tested how the bees behaved when they had a choice of lighting conditions to forage under. Simulating the patchy shade encountered in a wood – by filtering the light illuminating two of the arena's quadrants with the green simulated-leaf filters while leaving the other two quadrants exposed to full light – Arnold monitored the foraging bees' preferences. However, instead of preferring to forage under the daylight quadrants where their colour discrimination was best, the bees' preference was determined by their life experience. If they had previously foraged under daylight, they initially explored the quadrants that were illuminated by daylight, whereas bees that had been pre-trained under simulated leaf shade had no preference, and were equally happy foraging under daylight or shade. Arnold also recalled that when she first introduced bees into the leaf-shaded arena at the start of training, they seemed fazed, while the bees that were introduced to the arena under daylight took to the new setting much better.

The duo realised that in addition to finding colours more difficult to distinguish under leaf shade, the insects did not like change. However, the novelty-averse bees eventually got over their misgivings and began foraging equally under broad daylight and shade. Arnold also adds that the bees that had previously encountered both daylight and leaf shade were more at ease under patchy lighting, contentedly foraging under both. So, bumblebees don't like change and prefer foraging in light environments with which they are familiar. However, they have the ability to adapt rapidly, allowing them to move on to new environments when flowers no longer provide fertile foraging grounds.

S. E. J.
Illumination preference, illumination constancy and colour discrimination by bumblebees in an environment with patchy light
J. Exp. Biol.