none The parasitoid Aphidius rhopalosiphi De Stefani-Perez was imported into New Zealand as a biological control agent against the rose-grain aphid (Metopolophium dirhodum (Walker) in 1985. In 1988-89, the rate of parasitism by A. rhopalosiphi was studied by rearing fourth instar M. dirhodum collected from barley at Lincoln and other sites in Canterbury. The rate of parasitism at the time of peak aphid numbers at Lincoln was approximately 50% in winter barley and 100% in spring barley. In spring barley, peak numbers did not exceed five aphids per tiller, and regulation of aphid population density below this level was attributed to parasitism following a large immigration of adult A. rhopalosiphi . Suction trap catches of A. rhopalosiphi increased between 1984, prior to release, and 1989, while catches of M. dirhodum showed a corresponding decline. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships; Land and freshwater zones; Australasian region Aphidius rhopalosiphi (Braconidae); Hemipteran hosts; Metopolophium dirhodum; Prevalence; Hemipteran host, impact on host abundance on cereals; New Zealand; Canterbury; Prevalence impact on hemipteran host abundance on cereal plants Metopolophium dirhodum (Aphidoidea); Hymenopteran parasites; Aphidius rhopalosiphi; Plant hosts; Hordeum vulgare (barley), effect of hymenopteran parasite; Prevalence; Hymenopteran parasite, impact on abundance on plant hosts; New Zealand; Canterbury; Hymenopteran parasite impact on prevalence on plant hosts