Coping with a Capricious Environment: A Population Study of a Rare Pierid Butterfly

Found in small, scattered colonies, the West Virginia white butterfly (Pieris virginiensis) is absent from many areas where its food plant, Dentaria diphylla, is common. To better understand the demographic processes contributing to this butterfly's rarity, we studied the natural history and population ecology of a colony of Pieris virginiensis in Connecticut. Survival to instar IV for P. virginiensis is comparable to that observed for its superabundant congener, P. rapae. However, for P. virginiensis problems begin with instar IV and V larvae, which appear to be victimized by unpredictable host phenology. In 1982, early senescence of Dentaria stranded most fourth and fifth instar caterpillars; stranded caterpillars were unsuccessful at locating secondary host plants. P. virginiensis adults also encounter difficulties because they attempt to oviposit in the spring, when weather conditions are rarely suitable for flight or oviposition. Furthermore, when the females do fly, they are not very effective at laying eggs rapidly. Buildup of granulosis virus through soil contamination may also contribute to the butterfly's rarity. By coating Dentaria plants with water—and—soil slurries from woodland areas with and without P. virginiensis populations, we documented significant vertical transmission of granulosis virus through the soil (i.e., higher virus mortality on plants coated by slurries from P. virginiensis locales). Finally, because P. virginiensis appears reluctant to fly across open fields, recolonization of extinct colonies is an unlikely event. We speculate that P. virginiensis may have been more abundant in the past, when butterflies would have had less trouble dispersing among Dentaria stands because fewer woodlands were dissected by open habitats.