The role of visual and olfactory floral cues in twilight foraging by Ptiloglossa and Xylocopa bees

Bees of Ptiloglossa and Xylocopa explore the chiropterophilous flowers of Pseudobombax longiflorum at twilight, but how the bees find the flowers in low light is unclear. In field experiments, we investigated if visual and olfactory floral cues are used by these bees to find P . longiflorum flowers, and which behaviors are triggered by these cues. While the crepuscular Ptiloglossa bees were more attracted to flowers with a combination of visual and olfactory cues than to isolated cues, the diurnal Xylocopa bees were equally attracted to the combination of visual and olfactory cues and to flowers with visual cues alone. Ptiloglossa bees visit the flowers under lower light intensity than Xylocopa bees. This indicates that the synergy between visual-olfactory cues facilitates flower detection in crepuscular bees. However, in higher light intensities, the large size of flowers with their broad spectrum reflectance may be enough to produce a reliable visual signal for the Xylocopa bees. Olfactory stimuli alone trigger only floral approaches in bees, while visual ones frequently trigger approaches followed by landings on flowers. This suggests that olfactory cues guide the bees to the flowers in twilight, but the presence of a visual cue is necessary to trigger landings and collection of floral resources. Significance statement Crepuscular and some large diurnal bees fly in the twilight and collect pollen and nectar from flowers with nocturnal anthesis. However, finding food in a dimly lit environment is not an easy task! In this study, we used a combination of visual and chemical approaches to describe, for the first time, how bees do it. We showed that although bees use olfactory and visual floral stimuli as cues, they have different strategies for finding flowers. Furthermore, olfactory and visual cues play different roles during bee foraging. Floral odors are responsible for guiding bees toward flowers in the dark, and the visual cues are responsible not only for guiding bees but also for triggering landings and floral resource collection.