UV bullseye contrast of Hemerocallis flowers attracts hawkmoths but not swallowtail butterflies

Abstract

The color and patterns of animal‐pollinated flowers are known to have effects on pollinator attraction. In this study, the relative importance of flower color and color contrast patterns on pollinator attraction was examined in two pollinator groups, swallowtail butterflies and hawkmoths using two Hemerocallis species; butterfly‐pollinated H. fulva and hawkmoth‐pollinated H. citrina, having reddish and yellowish flowers in human vision, respectively. Flowers of both species have UV bullseye patterns, composed of UV‐absorbing centers and UV‐reflecting peripheries, known to function as a typical nectar guide, but UV reflectance was significantly more intense in the peripheries of H. citrina flowers than in those of H. fulva flowers. Comparison based on the visual systems of butterflies and hawkmoths showed that the color contrast of the bullseye pattern in H. citrina was more intense than that in H. fulva. To evaluate the relative importance of flower color and the color contrast of bullseye pattern on pollinator attraction, we performed a series of observations using experimental arrays consisting of Hemerocallis species and their hybrids. As a result, swallowtail butterflies and crepuscular/nocturnal hawkmoths showed contrasting preferences for flower color and patterns: butterflies preferred H. fulva‐like colored flower whereas the preference of hawkmoths was affected by the color contrast of the bullseye pattern rather than flower color. Both crepuscular and nocturnal hawkmoths consistently preferred flowers with stronger contrast of the UV bullseye pattern, whereas the preference of hawkmoths for flower color was incoherent. Our finding suggests that hawkmoths can use UV‐absorbing/reflecting bullseye patterns for foraging under light‐limited environments and that the intensified bullseye contrast of H. citrina evolved as an adaptation to hawkmoths. Our results also showed the difference of visual systems between pollinators, which may have promoted floral divergence.

Light environment drives evolution of color vision genes in butterflies and moths

“…UV light sensitivity is prevalent in nocturnal animals, even dichromats such as rodents, owls and deep-sea fishes 23,67,68 . UV contrast is a foraging cue for moths, for example nectar guides in many-night blooming flowers 69 , and UV light is commonly used as an attractant to moth traps [70][71][72] . Because short wavelengths increase around twilight 73 , UV light is a possible signal for pupil responses, (anecdotally more prevalent across nocturnal moths than butterflies), which could explain the lower selection rates in nocturnal species.…”

Floral UV Features of Plant Species From a Neotropical Savanna

“…In this context, one could expect that the presence of floral colour patterns, regardless of the wavelength, would be hard to distinguish by nocturnal insect pollinators. However, there is evidence that hawkmoths can perceive UV bullseye patterns in dim light (Hirota et al, 2019). Bats use different sensory cues for perceiving the surrounding environment and may constantly integrate information obtained through echolocation and vision (Boonman et al, 2013).…”

Macroevolution of Flower Color Patterning: Biased Transition Rates and Correlated Evolution with Flower Size

“…However, one cannot rule out that the observed pattern could be explained by pollinator mediated selection. If smaller flowers are less attractive to pollinators than larger flowers ( Johnson et al., 1995 ; Conner and Rush, 1996 ) they may be under selection to evolve additional visual signals to attract pollinators, like UV-reflective patterns (e.g., Hirota et al., 2019 ). Additionally, if smaller flowers are pollinated by small insects with low-resolution compound eyes, floral guides may still be important for orienting pollinators once they are very close to flowers and are able to distinguish the patterns ( Hempel de Ibarra et al., 2015 ).…”