Comparative pollination ecology between two populations and two varieties of Cypripedium parviflorum (Orchidaceae) in Missouri, United States of America - does size matter?

Abstract

The pollination ecology of two sympatric varieties of Cypripedium parviflorum was compared to determine whether floral dimensions had an impact upon potential pollinator diversity. We observed, collected and compared 235 insects that entered the labella of sympatric populations of C. parviflorum vars. parviflorum and pubescens and compared the insects between two allopatric populations in Missouri. In all cases, insects most likely to carry the pollen masses of this orchid were small- to medium-sized bees (6.75 mm length, 2.28 mm width, 1.82 mm depth) native to the central region of the USA. The dominant carriers of pollen masses were polylectic females in the family Halictidae. We found that 40 bee species visited the flowers of both varieties, although floral dimensions of C. parviflorum var. pubescens were significantly greater than those of var. parviflorum. Cypripedium parviflorum var. parviflorum bloomed earlier than var. pubescens. We observed bees taking refuge in flowers of C. parviflorum var. pubescens on cold, overcast days. The two isolated populations of var. pubescens shared eight species of pollen vectors. Results suggest that C. parviflorum exploits a diverse range of bees based on its local distributions and seasonal fluctuations. There is overlap between bee species shared by both varieties and overlap between flowering periods. However, based on floral morphometrics, evidence did not suggest intergradation between the two sympatric variants. Intraspecific isolation in C. parviflorum at one site appeared to be the result of a combination of differing phenology, floral presentation and regional distribution. Bee size matters for successful pollination events to occur in C. parviflorum.