THE POLLINATION ECOLOGY OF 5 SPECIES OF PENSTEMON (SCROPHULARIACEAE) IN THE TALLGRASS PRAIRIE

The floral ecology of Penstemon cobaea Nutt. var. cobaea, P. cabaea va r. 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. rill five species show protandry, but the receptive stigma lies only 2 mm away from the two pairs of fertile stamens. Bagging experiments on three Penstemon spp. show that only P. digitalis sets seed when insec t visitors are excluded. Flowers of all five species exhibit a horizon tal 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, whit e flowers of P. tubaeflorus appear to be pollinated by a combination o f diurnal Lepidoptera and some native bees favoring a dorsal depositio n of pollen on mouthparts and upper thoraces. The four remaining speci es 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 sis Rombus spp.). with P. digita lis and P. cobaea visited infrequently by the rare Penstemon wasp, Pse udomasaris occidentalis. Queens of Bombus pennsylvanicus subsp. pennsy lvanicus forage primarily on the large, gullet flowers of P. grandiflo rus and both varieties of P. cobaea. Queens of Bombus nevadensis subsp . auricomus prefer those P. digitalis and P. pallidus shoring a reduce d gullet or bell form. Ree pollination in four Penstemon spp. may oper ate within a two-tiered system. Large-bodied Bombus spp., Megachile br evis, and anthophorids (Synhalonia hamata and Anthophora terminalis) c arry dorsal depositions of Penstemon pollen because they contact anthe rs and stigmas while they forage exclusively for nectar. In contrast, small-bodied members of the Anthophoridae (Ceratina), Colletidae (Hyla eus spp.), Halictidae (Augochlorella, Halictus, Lasioglossum), and Meg achilidae (Hoplitis and Osmia) forage actively for Penstemon pollen en couraging repeated, ventral contact with the sexual organs of the flow ers. Bombus queens were more prevalent at large Penstemon populations (especially P. cobaea). Bombus workers were collected primarily on res tored sites. The importance of small bees as pollinators appeared to v ary indirectly with Penstemon population size.