Introduced braconid parasitoids and range reduction of a native butterfly in New England

Pieris napi oleracea Harris is a native pierid butterfly that has suffered a range reduction in New England that began after the invasion of its range by the non-native congener Pieris rapae L. and one of its braconid parasitoids, Cotesia glomerata (L.). P. napi has nearly disappeared from Massachusetts, but remains common in northern Vermont. We investigated food plant abundance and Cotesia spp. larval parasitism as possible factors to explain the historical changes in P. napi s distribution. We found that the current range of P. napi was not explained by the abundance of its key first generation food plant (two-leafed toothwort, Cardamine diphylla [Michx.]). We also found that levels of Cotesia spp. parasitism in meadows in the second generation were similar in Vermont and Massachusetts. Further, we found that both C. glomerata and the related introduced Pieris spp. parasitoid Cotesia rubecula (Marshall) forage for hosts predominantly in sunny meadows and not in woods, where the first generation of P. napi occurs. We found that under field conditions in meadow habitats, C. glomerata parasitizes P. napi at higher rates than P. rapae. We postulate that the persistence of P. napi in Vermont and its disappearance in Massachusetts is caused by high parasitism of the second generation by C. glomerata in meadow habitats, coupled with a north–south cline in the rate of commitment of first generation P. napi pupae to diapause, such that northern populations act functionally as univoltine species developing in a parasitoid free habitat (woods), while southern populations acted as a bivoltine species and went extinct due to low survival in the second generation in meadows due to C. glomerata parasitism. 2003 Elsevier Science (USA). All rights reserved.