Pollination on the wing: The role of butterflies in flame azalea (Rhododendron calendulaceum) reproduction
Despite the ubiquity of flying animals as plant pollination vectors, reports of wings as the vehicle of pollen dispersal are extremely rare. Flowers which attract a diverse array of visitors present a challenge to biologists seeking to understand the nature of plant-pollinator relationships. The efficacy of all potential pollinators visiting a plant must be determined before conclusions can be made about the role each plays in the plant’s reproductive success. Here we report on the pollination of flame azalea (Rhododendron calendulaceum), a spring-flowering tree in the Appalachian Mountains. Although R. calendulaceum is visited by diverse insects, earlier observations indicated that only butterflies contacted both male and female floral reproductive parts. We sought to determine the role of butterfly wings in pollination of R. calendulaceum through field experiments addressing the following questions: 1) Which insect visitors contact both male and female reproductive structures? 2) Do butterfly wings deposit pollen on stigmas? 3) Do behavioral differences among butterflies affect their pollination efficiency?
Over two seasons of observations in 2011 and 2014, a diversity of insects were observed on R. calendulaceum inflorescences. Interspecific differences in foraging behavior were common. For example, the bee, Andrena cornelli, was a frequent visitor, yet rarely contacted stigmas while focusing almost exclusively on pollen collection. Only two insect species contacted both anthers and stigmas, the butterflies, P. glaucus and Speyeria cybele. Due to differences in size and foraging behavior, P. glaucus was twice as likely as S. cybele to contact an anther and seven times more likely to contact a stigma. All P. glaucus butterflies we collected carried flame azalea pollen, with pollen density being 3.4 times greater on wings than on bodies. Stigmas from virgin, emasculated flowers were presented to foraging butterflies, and resulted in the deposition of both butterfly scales and pollen on the stigmas. Pollinator exclusion experiments resulted in fruit set that was 12 times higher on inflorescences with butterfly access, compared to those only available to bees and smaller insects. Our results indicate that despite the diversity of insects visiting R. calendulaceum, the plant is pollinated primarily by the flapping wings of a single butterfly species.