THE IMPACT OF APHIDIUS-RHOPALOSIPHI (HYMENOPTERA, APHIDIIDAE) ON POPULATIONS OF THE ROSE GRAIN APHID (METOPOLOPHIUM-DIRHODUM) (HEMIPTERA, APHIDIDAE) ON CEREALS IN CANTERBURY, NEW-ZEALAND

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.