The large white butterfly (Pieris brassicae L) first invaded northernmost Japan from Siberia around 1994, and after a few years, began to expand its range. The wasp, Cotesia glomerata (L) parasitizes larvae of the small white butterfly (Pieris rapae crucivora Boisduval), a usual host in the same geographic area. Some Pieris brassicae larvae in Hokkaido have been parasitized by Cotesia glomerata, but the parasitism rate of Pieris brassicae larvae tends to be lower than that of Pieris rapae. To examine the process of parasitizing Pieris brassicae larvae, we observed how the parasitoid wasp responded to the host larvae on damaged leaves. Cotesia glomerata females tended to avoid Pieris brassicae larvae, and even when female wasps inserted their ovipositors into Pieris brassicae larvae, none laid eggs. The parasitoids obtained from Pieris rapae larvae failed to parasitize Pieris brassicae during the host-acceptance step. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land zones; Palaearctic region; Eurasia; Asia Pieris brassicae (Pieridae); Hymenopteran parasites; Cotesia glomerata; Host acceptability, prevalence & reaction of parasite to host; Japan; Hokkaido; Iwanai & Sapporo; Hymenopteran parasites prevalence, host acceptability & reaction of parasite to host Cotesia glomerata (Braconidae); Lepidopteran hosts; Pieris brassicae; Host acceptability; Japan; Hokkaido; Iwanai & Sapporo; Lepidopteran host acceptability, prevalence & reaction of parasite to host none