Discrimination of flower colours in natural settings by the bumblebee species bombus terrestris (Hymenoptera:Apidae)

Free-flying bumblebees (Boinbus terrestris Linnaeus 1758) were trained to visit homogeneously coloured plastic stimuli in a controlled illumination laboratory, and then their ability to discriminate this target colour from differently coloured stimuli was evaluated in non-rewarded tests. Bees did not discriminate between similar colours until the colour distance was well above discrimination threshold, but then discrimination fitted a sigmoidal-type function. The results for this absolute conditioning function and a previously determined function for differential conditioning were compared, and it is shown that the absolute conditioning function best explains the discrimination and flower constancy behaviour of bees that has previously been observed in natural settings. Modeling of the empirically determined absolute discrimination function allows for predictions of how bees would discriminate between flowers in complex foraging conditions that might include changes in the background and/or illumination colour. The absolute conditioning function should thus serve as an important guide for the evaluation of pollinator discriminations between flowers in natural settings, and for the understanding of why there may have been considerable evolutionary pressure on plants to evolve distinctive flower colours so as to promote flower constancy in important pollinators.