The long spur of Impatiens macrovexilla may reflect adaptation to diurnal hawkmoth pollinators despite diversity of floral visitors

Abstract

Many tropical and subtropical Impatiens species possess conspicuously long and narrow spurs and have been assigned to the butterfly pollination syndrome in investigations of floral evolution of this genus. However, this contradicts some previous studies documenting abundant hawkmoth visitors rather than butterflies. In this study, we explored the pollination ecology of Impatiens macrovexilla, a long-spurred species in Asia. We investigated a series of floral traits, tested the breeding system and contributions of different floral visitors to reproductive success, observed floral visitors and compared the spur length with the proboscis length of the nectar-collecting floral visitors. I. macrovexilla shows floral traits that cannot clearly be associated with a certain group of long-proboscid insects. Results showed that the species was visited by large nectar-collecting bees, diurnal hawkmoths, small pollen-collecting insects and rarely butterflies at different sites and in different years. Pollination treatments indicated that I. macrovexilla is self-compatible (Self-Incompatibility Index = 0.02) but completely relies on floral visitors for pollination. A treatment which excluded hawkmoths but permitted pollen-collecting insects to visit flowers revealed that diurnal hawkmoths contributed considerably to reproductive success of this species but that pollen-collecting insects acted as pollen thieves. Large bees firmly contacted anthers and stigmas when visiting flowers so probably also act as pollinators. The proboscises of large bees were shorter than half of the spur of I. macrovexilla in length, suggesting that they could only access the nectar accumulated near the mouth of the spur. However, hawkmoths can access most or all of the nectar because their proboscides match the length of the floral spur. Thus, the long nectar spur presumably reflects adaptation to diurnal hawkmoths. Using evidence from multiple lines of investigation, our study is the first to confirm the role of diurnal hawkmoths as pollinators in Impatiens.