The essential is invisible to the (human) eyes: UV patterns explain the increased visit rate of pollinators to the yellow flowers of the Cytisus scoparius bush

Abstract

Flower colors are frequently used by pollinators as indirect signals of reward status. However, flower visitors are often UV-perceptive, a non-visible chromatic spectrum for the human eye. Cytisus scoparius (scotch broom) is an abundant shrub of NW Patagonia that shows intra-specific variation in the color of their flowers. We evaluated whether the previously documented higher visitation rate of pollinators towards yellow flowers is consequence of a more attractive UV pattern in this floral morph. We counted the visitation rate every 4 days along 3 weeks in yellow and red flowers with sunscreens in their petals (which cancel the UV spectrum), control flowers (without sunscreen), and with sunscreen in their pedicels (to control undesirable sunscreen effects like odor). The addition of sunscreen in the petals reduced the visitation rate in the yellow floral morph since the first sampling day. At the last sampling day, yellow flowers with sunscreen in their petals were visited 7 times less than control ones. However, the effect of sunscreen in the petals of red flowers was only evident at the last sampling day, reducing the visitation rate at the end of sampling solely 3 times. These results suggest that both floral morphs show UV patterns and that these patterns attract pollinators, but also that UV patterns are particularly relevant for pollinators in yellow flowers. This may be an effect of a more contrasting UV pattern in yellow than in red background color. Finally, both flower color morphs with sunscreen in their petals showed similar visitation rate, suggesting that pollinators are attracted towards yellow flowers because of the existence of UV patterns rather than being attracted by the yellow color per se. This work emphasizes the importance of analyzing the chromatic patterns invisible to the human eye to better understand the relationship between flower signals and the visitation rate of pollinators.