The Role of Predators and Parasites in the Natural Regulation of Lucerne Aphids in Eastern Australia

Abstract

(1) The impact of invertebrate predators on lucerne aphids (Acyrthosiphon kondoi (Shinji), Therioaphis trifolii (Monell) and Acyrthosiphon pisum (Harris) was studied in the Hunter Valley, New South Wales, Australia, using a selective insecticide to suppress predators. Aphid and predator numbers were monitored weekly over 2 years in unsprayed and carbaryl-treated plots. (2) The major predator groups were hemerobiids (Micromus sp.), coccinellids (Coccinella repanda Thunberg), predatory mites (Bdellidae) and syrphid larvae. (3) In 1980-81, predators had a significant effect on aphid numbers in late spring, summer and early autumn but not in late autumn. (4) There was no build-up of aphids in spring or summer of the second year (1981-82). When aphid numbers did increase, in autumn 1982, no significant impact of predators was demonstrated. (5) Parasitism by the braconid parasite Aphidius ervi Haliday became an increasingly important mortality factor for A. kondoi during the 2-year period. The high incidence of parasitism in the winter and spring in 1981 and 1982 was significant in preventing the spring build-up that had characterized populations of A. kondoi since its arrival in the Hunter Valley. (6) A. kondoi appeared to be sensitive to the stage of growth of the host plant, alate production increasing as the plant approached maturity, even when there were few aphids. (7) It was concluded that predators and parasites are capable of making a significant contribution to lucerne aphid control in a pest management programme.