Can red flowers be conspicuous to bees? Bombus dahlbomii and South American temperate forest flowers as a case in point.

It has been argued that trichromatic bees with photoreceptor spectral sensitivity peaks in the ultraviolet (UV), blue and green areas of the spectrum are blind to long wavelengths (red to humans). South American temperate forests (SATF) contain a large number of human red-looking flowers that are reported to be visited by the bumblebee Bombus dahlbomii. In the present study, B. dahlbomii's spectral sensitivity was measured through electroretinogram (ERG) recordings. No extended sensitivity to long wavelengths was found in B. dahlbomii. The spectral reflectance curves from eight plant species with red flowers were measured. The color loci occupied by these flowers in the bee color space was evaluated using the receptor noise-limited model. Four of the plant species have pure red flowers with low levels of chromatic contrast but high levels of negative L-receptor contrast. Finally, training experiments were performed in order to assess the role of achromatic cues in the detection and discrimination of red targets by B. dahlbomii. The results of the training experiments suggest that the bumblebee relies on achromatic contrast provided by the L-receptor to detect and discriminate red targets. These findings are discussed in the context of the evolutionary background under which the relationship between SATF species and their flower visitors may have evolved.