The role of predators and parasites in the natural regulation of lucerne aphids in eastern Australia.
The impact of invertebrate predators on lucerne aphids (Acyrthosiphon kondoi, Therioaphis trifolii and A. pisum) was studied in the Hunter Valley, New South Wales. Aphid and predator numbers were monitored weekly over 2 years in unsprayed plots and plots treated weekly with carbaryl at 1.1 kg a.i./ha to suppress predators. The major predator groups were the hemerobiid Micromus sp., Coccinella repanda [C. transversalis], bdellid mites and syrphid larvae. In 1980-81, predators had a significant effect on aphid numbers in late spring, summer and early autumn but not in late autumn. Thre was no build-up of aphids in spring or summer of the 2nd year (1981-82). When aphid numbers did increase, in autumn 1982, no significant impact of predators was demonstrated. Parasitism by the braconid Aphidius ervi 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. 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. It was concluded that predators and parasitoids are capable of making a significant contribution to lucerne aphid control in a pest management programme.