Wild bees preferentially visitRudbeckiaflower heads with exaggerated ultraviolet absorbing floral guides

Abstract

Here, we report on the results of an experimental study that assessed the visitation frequency of wild bees to conspecific flowers with different sized floral guides. UV absorbent floral guides are ubiquitous in Angiosperms, yet surprisingly little is known about conspecific variation in these guides and very few studies have evaluated pollinator response to UV guide manipulation. This is true despite our rich understanding about learning and color preferences in bees. Historical dogma indicates that flower color serves as an important long-range visual signal allowing pollinators to detect the flowers, while floral guides function as close-range signals that direct pollinators to a reward. We initiated the work presented here by first assessing the population level variation in UV absorbent floral guides for conspecific flowers. We assessed two species, Rudbeckia hirta and R. fulgida. We then used several petal cut-and-paste experiments to test whether UV floral guides can also function to attract visitors. We manipulated floral guide size and evaluated visitation frequency. In all experiments, pollinator visitation rates were clearly associated with floral guide size. Diminished floral guides recruited relatively few insect visitors. Exaggerated floral guides recruited more visitors than smaller or average sized guides. Thus, UV floral guides play an important role in pollinator recruitment and in determining the relative attractiveness of conspecific flower heads. Consideration of floral guides is therefore important when evaluating the overall conspicuousness of flower heads relative to background coloration. This work raises the issue of whether floral guides serve as honest indicators of reward, since guide size varies in nature for conspecific flowers at the same developmental stage and since preferences for larger guides were found. To our knowledge, these are the first cut-and-paste experiments conducted to examine whether UV absorbent floral guides affect visitation rates and pollinator preference.

Sex‐specific floral attraction traits in a sequentially hermaphroditic species

“…The positive effect of UV nectar guide area on pollen receipt observed here corroborates previous studies that have reported a positive effect of UV floral guide presence, size, or proportion on pollinator visitation rate (Horth et al, ; Leonard & Papaj, ; Rae & Vamosi, ). UV bullseyes or UV nectar guides are often interpreted as guides that direct floral visitors toward a floral reward (Leonard & Papaj, ; Orban & Plowright, ).…”

“…Several studies have shown that the spatial patterns in which flowers reflect and absorb UV light specifically affect the behavior of potential pollinators. A variety of floral UV-absorption patterns (e.g., "nectar guides" or "bullseyes") may attract and guide insects, promoting efficient pollination (Horth, Campbell, & Bray, 2014;Leonard & Papaj, 2011). In addition, the size of the UV floral guide has been shown to positively influence pollinator visitation rate and pollen transfer efficiency (Horth et al, 2014;Leonard & Papaj, 2011).…”

Natural selection on floral traits of Caltha scaposa (Ranunculaceae), an alpine perennial with generalized pollination system from Northwest Yunnan

“…UV patterns on petals can be perceived by bees ( Chittka, 1992 ), birds ( Bennett and Cuthill, 1994 ), moths ( Sheehan et al., 2016 ) and flies ( Koski and Ashman, 2015a ). Furthermore, both bees and flies have been shown to prefer flowers with large UV bulls-eye patterns ( Horth et al., 2014 , Koski and Ashman, 2015a , Brock et al., 2016 ). Therefore, we predicted that natural selection would drive the evolution of flowers with larger bulls-eye patterns in C. scaposa .…”

“…For example, flowers of most species in Potentilla are yellow, but some of them show quite different UV absorption patterns ( Naruhashi and Ikeda, 1999 , Koski and Ashman, 2016 ). Despite the fact that this kind of UV absorbing floral type is generally correlated with bioclimatic factors ( Koski and Ashman, 2015b , Koski and Ashman, 2016 ), it has been suggested that flowers with larger UV absorption patterns may be perceived ( Chittka et al., 1994 , Briscoe and Chittka, 2001 , Kevan et al., 2001 , Koski and Ashman, 2014 , Sheehan et al., 2016 ) and preferred ( Horth et al., 2014 , Brock et al., 2016 ) by insects as the bulls-eye patterns are thought to act as nectar guides. Accordingly, we cannot judge pollination syndrome of a flower color based on human eyes only, and measurements of phenotypic selection on floral traits should include these “cryptic” floral traits, like the UV bulls-eye.…”

Getting to the start line: how bumblebees and honeybees are visually guided towards their first floral contact

“…Ultraviolet absorbing ''floral guides'' that are invisible to humans are perceptible to pollinators. Not only do they serve to orient bees at close range towards the source of reward, but they also affect visitation rates (Horth et al, 2014). The appearance of flowers revealed by UV photography is illustrated in Fig.…”