Pollination and fruit set in two rewardless slipper orchids and their hybrids (Cypripedium, Orchidaceae): large yellow flowers outperform small white flowers in the northern tall grass prairie

• Species with rewardless flowers often have low fruit to flower ratios, although wide temporal and spatial variation in fruiting success can occur. We compared floral phenotypes, insect visitors, and fruiting success in four populations of the small white (Cypripedium candidum) and yellow lady’s slipper orchids (C. parviflorum) and their hybrids near the northern extent of North America’s tall grass prairie. • Flower and fruit numbers were observed for two seasons on marked individuals (n = 1811). Floral traits were measured on 82-140 individuals per taxon, and analyzed in relation to fruiting success. All insects found inside flowers were collected, inspected for pollen smears, and measured for comparison to floral features. • Among orchid taxa, C. candidum had the smallest flowers, lowest number and variety of insect visitors, and lowest fruit to flower ratios. These measures were intermediate in hybrids and highest in C. parviflorum, despite low flower numbers in the latter. Within ...