Life history pattern, host plants, and habitat as determinants of population survival of Pieris napi oleracea interacting with an introduced braconid parasitoid

Abstract

Abstract Pieris napi oleracea Harris is a native pierid butterfly whose range in New England has decreased since the 1869 invasion of an exotic congener, Pieris rapae (L.). Populations have disappeared from all but a few locations in western Massachusetts, but persist more widely in northern Vermont. Parasitism by the introduced braconid wasp Cotesia glomerata (L.) has been postulated as responsible for this decline. Field exposures of cohorts of P. napi larvae in 2002 showed that, on the summer host plant Sisymbrium officinale (L.) Scoop., parasitism rates of second generation butterfly larvae were very high (100% in Massachsusetts and 66% in Vermont). Direct observations of host plant and habitat use patterns by P. napi females in northern Vermont in 2000 showed that this species changes habitats between the first and second generation, with over 95% of eggs of the first generation of P. napi being laid in wooded habitats on the crucifer Cardamine (formerly Dentaria) diphylla (Michx.) Wood. Second generation butterflies fly in meadows and oviposit on S. officinale and other crucifers in full-sun habitats. The second flight of P. napi is, however, only a partial generation, with many first generation pupae entering diapause. The proportion of butterflies entering diapause increases if eggs are laid on C. diphylla plants at or past the flower bud stage. Of first instar P. napi reared outdoors on pre-flower-bud C. diphylla, 63% (n=64) entered diapause in Massachusetts, but 86% (n=119) did so in northern Vermont. Higher rates of diapause in early larval groups in northern Vermont, plus lower rates of parasitism of second generation larvae in meadows, as compared to Massachusetts, indicate that the impact of C. glomerata on total population survival is lower in Vermont and is likely the reason P. napi has persisted in northern Vermont, but not in western Massachusetts. This case illustrates how non-target effects of parasitoids may be mediated by habitat and life history features of populations of affected insects, which may differ geographically.