Is UV just another color to a bee? Nectar guide preferences of stingless bees

The foraging behavior of bees depends on innate visual perception and preferences as well as learned behavior. Flowering plants that rely on bee pollination have evolved multiple strategies to attract pollinators and improve the efficiency of pollination. One such strategy is the presentation of nectar guides, commonly radial lines in colors that contrast with the surrounding petals. Nectar guides are often perceptible in the visible light spectrum, but unlike humans, bees can also see in ultraviolet (UV). Both UV-reflecting and UV-absorbing nectar guides are found in nature and can be attractive to bees, but their attractiveness has not been compared directly. Is UV just another color, or are bees attracted to flowers with UV-reflecting nectar guides? Using artificial flowers in a field setting, we surveyed Trigona (Hymenoptera: Apidae) stingless bees in Panama to assess their responses to different nectar guide colors. Specifically, we tested the landing and feeding preferences of four Trigona species for nectar guides that contrasted in color and in UV-reflectance. Stingless bees chose to land and feed upon flowers with guides that were blue and UV-reflective more often than those with blue, non-UV-reflective (χ² = 6.62, df = 2, P = 0.04) or yellow (χ² = 7.13, df = 2, P = 0.03) guides. Contrary to expectations, these results suggest that UV reflectance may be especially attractive for some types of bees.