The pollination ecology of orchid (Orchidaceae) has attracted the attention of many scientists including Charles Darwin. Although the majority of pollination network between animals and plants involves food rewards such as nectar and pollen to the pollinators, orchids are exceptional in that as many as one third of the 30,000 or so species achieve pollination by deception; for example, they may lure pollinators to the flower by using a food deceptive, advertising the presence of food by displaying bright colors and sweet scents (Jersakova et al. 2006; Ledford 2007). In addition, they can use sexual deception, as more than 600 species of orchids have evolved adaptations for pollination by male euglossines (Dodson et al. 1969). Euglossine-pollinated orchids produce attractive volatile compounds (terpenes and aromatics), exhibit intricate mechanisms for the attachment of pollinia (pollen masses) on male bees, and lack additional floral rewards such as nectar, pollen, or pseudopollen.
In non-autogamous orchids, a single season pollinator limitation to fruit setting is strongly indicated by a low natural fruit set rate reflected by an extraordinarily high proportion of flowering individuals that do not set fruit, presumably because of a lack of pollinators (Darwin 1877). For example, Tolumnia variegate, a small twig epiphyte in the West Indies has an extremely low natural fruit set rate, often below 5% (Ackerman and Oliver 1985). However, hand pollination results in a marked increase in fruit set, to over 70% (Calvo 1993). Similarly, Disa tenuifolia in South Africa depends on the exploitation of instinctive foraging and territorial behavior of male megachilid bees and its fruiting success is pollinator dependent (Johnson and Steiner 1994).
Over 100 species of Cleisostoma have been reported in tropical and subtropical Asia and Australia (Wood 2014). Cleisostoma scolopendrifolium, an epiphyte orchid that grows on rocks, has been recorded on subtropical and temperate trees of Northeast Asia, including Korea, Japan, and China. The extent of occurrence of this species in Korea has been estimated to be 2000 km2 (National Institute of Biological Resources 2014). Each year, C. scolopendrifolium plants produce a new rhizome that ends in a vegetative shoot composed of 6–10-mm-long leaves. The inflorescence is a 1- or 2-flowered raceme, located laterally, and often shorter than leaves. In Korea, most C. scolopendrifolium flower late in July. This orchid species is self-incompatible and insect pollinated. The pollinator species of C. scolopendrifolium in Mokpo, South Korea, is assumed to be male megachilid bees, based on the observation that only male megachilid bees have visited the orchid and the occurrence of pollinia attached to the head of flower-visiting megachilid bees (Son et al. 2017).
There is a large body of literature describing the pollination biology of orchids worldwide. However, there is little information on the pollination ecology of orchid species in Korea. This study aimed to confirm that male megachilid bees are the unique pollinators of C. scolopendrifolium in southern Korea and to dertermine whether pollinator visitation affects the fruiting of the orchid.