Pollination ecology of Cypripedium regina Walter (Orchidaceae): Size matters

Abstract

As in most pollinator-limited orchids lacking edible rewards, a population of C. reginae in southern Missouri showed a low conversion ratio of flowers into fruits (0.046-0.23) over two seasons. There was no relationship between the length of the secondary flowering stem, the number of foliage leaves on the same stem and the number of flowers (one or two) produced at the terminus of the stem. However, the size mattered based on the physical dimensions of pollinia-carrying insects vs. parameters of floral architecture. While a diverse range of floral visitors (Coleoptera, Diptera and Hymenoptera) to C. reginae were observed over three seasons, only six medium-sized bees (Anthophora, Apis and Megachile spp.) carried segments of massulate pollinia after three seasons of observation and collection. Pollinia were always deposited dorsally on the thorax. These bees had a mean width of 4.44 mm and depth of 3.41 mm whereas the rear exit length and width of the orchid measured 6.53 mm and 3.41 mm. respectively. In contrast, the more numerous but smaller bees (2.66 mm width and 2.16 mm depth) in the genera Augochlorella, Augochlora, Ceratina, Lasioglossum spp. etc., exited the flower via the same rear orifices without pressing against the dehiscent anthers. Larger bees (gynes of Bombus spp.) measuring 9.06 mm in width and 6.25 mm in depth, were too large to escape via the rear exits so they left the flower via the large, dorsal entrance (through which they first entered the labellum) never contacting either anther. As in the small-flowered C. plectrochilum, the larger flowered C. reginae receives many floral visitors but selects for pollinia-vectors of a discrete body size. © 2011 Royal Botanic Gardens and Domain Trust.