Floral specialization for beetle pollination and its implications for pollen dispersal in Disa elegans (Orchidaceae)

Premise: Pollination by beetles is relatively rare in orchids and this has been attributed to the clumsy behavior of beetles being unsuitable for the precise pollen transfer mechanisms that characterize the orchid family. We investigated floral specialization for beetle pollination in the rare fire-dependent South African orchid Disa elegans and explored its implications for the efficiency and spatial pattern of pollen dispersal. Methods: We observed flower visitors and identified their pollen loads. We studied floral traits, including spectral reflectance patterns, nectar secretion and scent chemistry. We tracked the dispersal of colour-labelled pollen. Key results: Disa elegans was found to be pollinated by large scarab beetles. Apparent floral adaptations for beetle pollination include the platform-like corymbose inflorescence of upward-facing bowl-shaped flowers, secretion of very dilute nectar on exposed surfaces of the petals, and fruity floral scent dominated by the monoterpene alcohol R-(–)-β-linalool and benzenoid ester methyl benzoate. Beetles carry large loads of pollinaria and transfer about 13% of the pollen they remove from anthers to stigmas. We found a classic leptokurtic kernel of pollen dispersal with an average distance from donors to recipients of 6.7 m. Self-pollen made up c. 30% of all pollen deposited on stigmas by beetles. These pollen dispersal patterns are similar to those obtained for plants pollinated by other insect groups, such as bees. Conclusions: These results provide evidence for floral specialization for beetle pollination in an orchid species and show that beetles can be effective agents of pollen dispersal in orchid populations. KEYWORDS: Cape Floristic Region, enantiomer, floral scent, fruit chafer, geitonogamy, nectar, pollen, Trichostetha.