Butterfly pollination of Bonatea cassidea (Orchidaceae): Solving a puzzle from the Darwin era

Abstract

Although moths and butterflies are closely related, they tend to respond to different floral cues for foraging and thus select for different suites of floral traits. Shifts between moth and butterfly pollination in the same plant clade provide an opportunity to assess the traits that are required to impart a shift between these pollination systems. The orchid genus Bonatea consists mostly of night-scented moth-pollinated taxa, but here we document a butterfly pollination system in Bonatea cassidea, a century after Mansel Weale's initial observations of butterflies carrying pollinaria of this taxon in South Africa were communicated to the Linnean Society by Charles Darwin. We recorded visits by a number of species of butterflies from the family Hesperiidae and Pieridae, but contrary to Weale, we found that the majority of pollinaria were firmly affixed between the palpi and not on the sternum. Removal of pollinaria from flowers during the day, but not at night, suggested that moths do not contribute to pollination, despite the white flower colouration. Floral traits found in B. cassidea but not its moth-pollinated congeners include diurnal anthesis, scent emission which is weak in the day and virtually absent at night, and the production of small amounts of relatively dilute sucrose-rich nectar in short spurs. These features are consistent with those expected for butterfly-pollinated plants in general. This study represents the first detailed report of butterfly-pollination in Bonatea and one of very few cases of orchid pollination by pierid butterflies.