Reproductive biology of the rare orchid, Spiranthes diluvialis: breeding system, pollination, and implications for conservation

We examined the breeding system and pollination of Ute lady's tresses, Spiranthes diluvialis Sheviak, a rare orchid from Colorado and Utah. S. diluvialis exhibits a mixed-mating system: the degree of selfing depends in part upon the abundance of pollinators visiting the flowers. Outcrossing is promoted by protandrous flowers and by acropetal movement of long-tongued bees on inflorescences. Male and female phases overlap, however, and flowers are fully self-compatible. No autogamous or agamospermous fruit set was observed, indicating that a pollen vector is required for reproduction. Observations indicate that bumblebees (Bombus spp.) are the most important pollinators of S. diluvialis. Land managers must include pollinators and pollen-producing plants in their plans to preserve this rare orchid. The effects of pest management programs on bumblebees and the availability of suitable bee nesting habitat should be considered. Management should strive to maintain floral diversity because ther flowering species may facilitate S. diluvialis pollination.