Insect Pollinators of Penstemon Series Graciles (Scrophulariaceae) with Notes on osmia and Other Megachilidae

Evidence is present that whole subgenera of Osmia are largely de- pendent on Penstemon for food (pollen, nectar), and that one small group of species in Osmian subgenus Nothosmia is intimately associated with Penstem on series Graciles alliance Graciles.

Abstract

Small bees of the genus Osmia are principal pollinators of Penstemon gracilis Nutt., P. pallidus Small, and apparently of the remaining species of Penstemon series Graciles alliance Graciles except P. hirsutus (L.) Willd. and its close allies P. tenuiflorus Penn. and P. oklahomensis Penn. These latter species are apparently pollinated chiefly by small bees of the genera Hoplitis and Ceratina, which were found to be secondary pollinators of P. gracilis and P. pallidus. Evidence is present that whole subgenera of Osmia are largely de- pendent on Penstemon for food (pollen, nectar), and that one small group of species in Osmia subgenus Nothosmia is intimately associated with Penstemon series Graciles alliance Graciles. Questions are raised that bear on the evolution and taxonomy of Penstemon and Osmia, as well as on methods of increasing clover pollination by planting Penstemon.