Most bees don't bother visiting red flowers; red flowers are usually reserved for other pollinators such as hummingbirds. But no one explained this to Bombus dahlbomii bumblebees in Chile. According to Jaime Martínez-Harms from the Universidad de Chile, they are perfectly happy visiting red blooms so long as they have good quality nectar. But how do the insects locate the vivid blooms? Martínez-Harms explains that bees only have three visual receptors, tuned to ultra violet, blue and green wavelengths. They cannot see red, or at least not in the way that we see it. Curious to find out how the cunning insects have adapted to see flowers that they should be colour-blind to, Martínez-Harms and his colleague Natalia Márquez headed south to collect both the blooms and their bees to find out how bees see red (p. 564).

Carefully transporting the delicate flowers back to Santiago in coolers, Martínez-Harms and Mary Arroyo measured the wavelengths of light reflected by the flowers to see if any of them reflected colours that could be detected by the bees. But many of the flowers only reflected red wavelengths, so the bees couldn't recognise the red blooms by their colour with the standard set of bumblebee photoreceptors. They must be using another strategy.

Next, Martínez-Harms and Natalia Márquez decided to test the insects' colour vision. They travelled further south to the Chilean temperate forest to collect a nest of B. dahlbomii bees. ‘It was not easy to find a nest. The insects are rare because their numbers have been reduced by agriculture,’ says Martínez-Harms. However, after three fieldtrips, the duo eventually located a nest and drove it back to the laboratory in Santiago.

Teaming up with neurobiologists Jorge Mpodozis and Adrian Palacios, Martínez-Harms measured the sensitivity of the bees' eyes to find out if they had developed a specialised receptor to see red, but the insects had not.

Having confirmed that the bees could not detect red wavelengths, Martínez-Harms wondered whether the insects were using other non-colour cues, such as intensity differences (achromatic contrast), to find the flowers.

Simulating a red flower nestled amongst foliage with a red disc on a green background, Martínez-Harms trained bees to visit the red disc by rewarding them with a tempting sugar solution. Once the bees had learned to visit the fake flower, Martínez-Harms offered them the choice between the red ‘flower’ and a blue ‘flower’ (blue disc on a green background), to see if the bees could distinguish between the two. Sure enough, the bees ignored the blue flower and continued returning to the red flower. They could differentiate between the red and blue ‘flowers’.

Then Martínez-Harms made the bees' choice more difficult. If the insects were locating red flowers by the intensity contrast between the ‘flower’ and its surroundings, Martínez-Harms reasoned that a dark green disc on a green background would look identical to the red flower on its green background. Amazingly, when he showed the insects red and dark green discs against a green background, the insects could not distinguish between them and visited them equally. The bees use the intensity contrast between red flowers and the green background to locate them, as the blooms look like dark patches against the surrounding foliage.

So, B. dahlbomii can see red, but not the way that we do.

A. G.
M. T. K.
Can red flowers be conspicuous to bees? Bombus dahlbomii and South American temperate forest flowers as a case in point
J. Exp. Biol.