The pollination ecology of Darlingtonia californica Torr., including especially the identity of its pollinators, has remained enigmatic for more than a century. The flowers of this well-known charismatic species are unusual in form and color, and have been the subject of much speculation. Accordingly, in this study we sought to identify D. californica's floral visitors and determine their potential effectiveness as pollinators, in the context of D. californica's unusual floral morphology. We also used hand-pollinations and emasculations to determine whether plants were pollen-limited at five study sites in northwest California, and to evaluate the potential for self-pollination in natural populations of D. californica. A generalist solitary bee, Andrena nigrihirta, visited and pollinated D. californica flowers at five sites in northern CA. Despite very low visitation rates, individual flowers at all study sites were predicted to receive at least one visit by A. nigrihirta. Other regular floral visitors included thrips and several species of spiders. Plants at all five study populations were found to be pollen-limited with respect to the number of seeds produced per capsule. Fruit and seed production by emasculated flowers indicated a large degree of cross-pollination. However, emasculated flowers did not produce as many fruits and seeds as unmanipulated flowers, suggesting that self-pollination contributes to D. californica reproductive success as well. Observations of A. nigrihirta on flowers revealed that the shape and orientation of D. californica's ovary and petals promote stigma contact both when pollinators enter and exit a flower, contrary to previous thought. Our findings provide evidence that D. californica is melittophilous, and suggest a resolution of the long-standing mystery surrounding the pollination of this rare species.
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Vol. 58 • No. 1