Pieris napi oleracea has suffered a range reduction in southern New England since the invasion of Pieris rapae in the 1890s. We have shown that this reduction is not due to loss of host plants, but rather a preference for P. napi by the introduced biological control agent Cotesia glomerata. This parasitoid forages in meadows, not woods, providing enemy free space for the first P. napi generation, which lives in the woods on two-leaved toothwort. Second generation P. napi, however, must reproduce in meadows, where we have demonstrated that rates of C. glomerata parasitism are high (66-100%) under realistic conditions in both western Massachusetts, where the butterfly is now virtually gone, but also in northern Vermont where it is still common. Differential commitment to diapause provides a potential explanation of this population outcome. In field studies we demonstrated that the first generation pupal diapause rate of P. napi reared on pre-flowering host plants was higher in Vermont (86%) than in Massachusetts (63%). We believe this difference may provide the most likely explanation of population persistence of P. napi in Vermont but not in Massachusetts.