Specialized pollination by honeybees in Cymbidium dayanum, a fall-winter flowering orchid

摘要

Floral biology and pollination mechanisms of Cymbidium dayanum, an endangered epiphytic orchid, were investigated in south Kyushu, Japan. The flowering period spanned approximately 4 months from fall to winter, with the inflorescences often blooming asynchronously on each plant and individual flowers commonly lasting for a month. The nectarless flowers are self-compatible but cannot autonomously self-pollinate; the orchid needs to attract pollinators by deceit for capsule production. The field observations showed that the flowers were pollinated exclusively by the Japanese honeybee Apis cerana japonica, despite its long flowering period. The worker bees pollinated flowers and/or received the pollinarium on the thorax, while escaping from the labellum chamber. The natural fruit-set ratios at the population level, an estimate of pollination success, varied interannually but were generally low, as the result of infrequent flower visits by honeybee workers owing to the cold winter climate and the lack of a nectar reward. Because most flowers that were artificially pollinated in the winter successfully developed into capsules, the coldness was not considered a direct cause of the low fruit-set ratios. Our results explicitly indicate that the bee pollination niche could be available to temperate plant species even during mid-winter. We inferred advantages for the winter flowering of C. dayanum.