The pollination ecology of five species of Penstemon (Scrophulariaceae) in the tallgrass prairie

Abstract

The floral ecology of Penstemon cobaea Nutt. vat. cobaea, P. cobaea var. purpureus Pennell, P. digitalis Nutt. ex Sims, P. grandiflorus Nutt., P. pallidus Small, and P. tubaeflorus Nutt. was studied by sampling populations at nine prairie sites in Illinois, Kansas, and Missouri. All five species show protandry, but the receptive stigma lies only 2 mm away from the two pairs of fertile stamens. Bagging experiments on three Peustemon spp. show that only P. digitalis sets seed when insect visitors are excluded. Flowers of all five species exhibit a horizontal presentation of the corolla and emit a slightly discernible scent. However, the presumed pollination systems of Penstemon spp. correlate with corolla form and the size of the floral sinus. The tubular, white flowers of P. tubaeflorus appear to be pollinated by a combination of diurnal Lepidoptera and some native bees favoring a dorsal deposition of pollen on mouthparts and upper thoraces. The four remaining species have gullet- or bell-shaped corollas ornamented with violet-purple blotches or lines. These species appear to be pollinated primarily by polylectic/polyphagic bees (including six Bombus spp.), with P. digitalis and P. cobaea visited infrequently by the rare Penstemon wasp, Pseudomasaris occidentalis. Queens of Bombus pennsylvanicus subsp. pennsylvanicus forage primarily on the large, gullet flowers of P. grandiflorus and both varieties of P. cobaea. Queens of Bombus nevadensis subsp. auricomus prefer those P. digitalis and P. pallidus showing a reduced gullet or bell form. Bee pollination in four Penstemon spp. may operate within a two-tiered system. Large-bodied Bombus spp., Megachile brevis, and anthophorids (Synhalonia hamata and Anthophora terminalis) carry dorsal depositions of Penstemon pollen because they contact anthers and stigmas while they forage exclusively for nectar. In contrast, small-bodied members of the Anthophoridae (Ceratina), Collelidae (Hylaeus spp.), Halictidae (Augochlorella, Halictus, Lasioglossum), and Megachilidae (Hoplitis and Osmia) forage actively for Penstemon pollen encouraging repeated, ventral contact with the sexual organs of the flowers. Bombus queens were more prevalent at large Penstemon populations (especially P. cobaea). Bombus workers were collected primarily on restored sites. The importance of small bees as pollinators appeared to vary indirectly with Penstemon population size.