THE EFFECT OF STAMINODE REMOVAL ON FEMALE REPRODUCTIVE SUCCESS IN A WYOMING POPULATION OF THE ENDANGERED BLOWOUT PENSTEMON, PENSTEMON HAYDENII (SCROPHULARIACEAE)

Abstract

We examined the effect of removing the staminode of the rare Blowout penstemon, Penstemon haydenii, on female reproductive success in a south-central Wyoming population. We found no significant difference in fruit set or in seeds per fruit between flowers with and without staminodes. The most frequent flower visitor, the megachilid bee, Osmia brevis, modified its behavior when visiting flowers without staminodes. In intact flowers, O. brevis collected pollen by straddling the staminode and rubbing its head and thorax across the anthers. In flowers without staminodes, O. brevis, appeared to compensate for the increased distance to the anthers from the corolla floor by simply stretching its legs to make contact with the anthers. The smaller, native generalist sweat bee (Dialictus pruinosus), which commonly collected pollen in intact flowers, appeared disoriented in flowers without staminodes, and frequently left without collecting pollen. Other taxa (e.g., bumblebees, Bombus spp., the masarid wasp Pseudomasaris vespoides), appeared unaffected and plied the flowers without incident. We speculate that the staminode of Blowout penstemon is intermittently vestigial: where O. brevis, bumblebees, and P. vespoides are common, the staminode is of little consequence; where D. pruinosus is the primary pollinator, the staminode is essential to sexual reproduction by the plant.