Swallowtail butterflies and hawkmoths contribute equally to the pollination of Habenaria dentata (Orchidaceae)

Abstract

Mixed pollination system with both diurnal and nocturnal visitors contributing to pollen transfer and seed set have been documented in many angiosperm species. The flowers of Habenaria species are usually green or white, with a long nectar secreting spur, and are usually interpreted as specialized for pollination by nocturnal moths. With a few exceptions, nocturnal moths and diurnal butterflies are both reported as pollinators of Habenaria species. However, the relative contributions made by moths versus butterflies to the reproductive success in the same orchid species has rarely been studied. We investigated the floral morphology, pollinators, pollination efficiency, and breeding system of Habenaria dentata in southern China. We showed that two species of diurnal swallowtail butterflies (Papilio helenus and Papilio memnon) and a species of crepuscular hawkmoth (Eupanacra mydon) were this orchid's effective pollinators. The pollinaria were attached to the eyes of these three insect species. Pollinator exclusion experiments showed that pollinarium removal and pollinium deposition rates on stigmas by butterflies (47.51% and 38.64%, respectively) were not significantly different from those made by hawkmoths (50.70% and 34.47%, respectively), indicating that diurnal and crepuscular visits contributed equally to reproductive success. As natural rates of insect-mediated pollination of our population of H. dentata resulted in fruit set rates of 93.33%, rejected the prospect that this population was pollinator limited. Hand pollination experiments indicated that H. dentata was self-compatible but without autonomous self-pollination. The proportion of well-developed seeds with large embryos in self-pollinated fruits was significantly lower than in cross-pollinated fruits, suggesting natural selection favors xenogamy. Our results highlighted the importance of both butterflies and hawkmoths contributing to the reproductive success of a rare orchid with a mixed pollination system.