Why has a population of the Massachusetts state-listed butterfly Pieris oleracea prospered using the invasive form of cuckoo flower, Cardamine pratensis?

The native pieirid butterfly, Pieris oleracea, underwent a large range reduction in New England in the 20^th century, likely due to the introduction the invasive butterfly Pieris rapae (Lep.: Pieridae) to North America in 1860, and later the oliphagous parasitoid Cotesia glomerata (Hymen: Braconidae) in 1884. Thought extirpated from the state by the 1970s, one large dense population of the butterfly was found in the mid 1980s in a flood plain meadow along the Housatonic River in Lenox, Berkshire Co., Massachusetts. We examined how this native pierid was able to maintain a relatively dense local population by feeding on a novel, invasive host plant, Cardamine pratensis (cuckoo flower) in a meadow habitat despite known parasitoid presence. We approached this question in three ways. First, we deployed trap host plants (cuckoo flower and collards) stocked host larvae (first and second instars of either P. rapae or P. oleracea) at the Lenox site and other locations to determine current rates of C. glomerata attack, for comparison with historical information. Second, we used olfactometer experiments to determine if C. glomerata females could detect the cuckoo flower volatiles released during P. oleracea larval feeding. Third, we used field-cage experiments to determine if the plant architecture found in the flood plain meadow inhibited the ability of C. glomerata females to locate and parasitize hosts. Specifically, we asked if overtopping vegetation prevented or reduced parasitism of P. oleracea larvae feeding on the covered basal rosettes of C. pratensis, which the physical for of the plant for 3 of the 4 butterfly generations at the site.