APHIDS ON CABBAGE - TRITROPHIC AND SELECTIVE INSECTICIDE INTERACTIONS

Laboratory-based experiments are presented involving two aphid species (Myzus persicae Sulzer, a generalist and Brevicoryne brassicae Linnae us, a crucifer specialist), and the predatory gall midge, Aphidoletes aphidimyza Rondani, on three cultivars of common cabbage Brassica oler acea var, capitata cv. Derby Day (green-leaved), Minicole (green-leave d) and Ruby Ball (red-leaved). In a laboratory-based tritrophic system including both species of aphid, the three cabbage cultivars and A. a phidimyza, predator larvae grew most rapidly when feeding on M. persic ae or B. brassicae on cv. Derby Day, while growth was slowest on cv. R uby Ball, although these differences were not always significant. In a separate experiment, A. aphidimyza larvae feeding on B. brassicae on each of the three cultivars were significantly smaller and consumed le ss aphid fresh weight when maintained outdoors (mean temperature = 13. 5 degrees C) compared with a constant environment room (20 degrees C). However, in this latter experiment under neither regime were differen ces in predator growth or consumption significant between cultivars. E ffects of selective insecticides (pirimicarb and a neem seed kernel ex tract, NeemAzal-T/S(R)) on bitrophic (aphid-host plant) interactions w ere also investigated in the laboratory. A pirimicarb dose equating to c. 15% of the recommended field concentration caused equivalent toxic ity of M. persicae on cv. Minicole compared with aphids treated with a three-fold greater dose and reared on cv. Derby Day. Cultivar-mediate d differences in aphid mortality caused by the neem extract when teste d for systemic and translaminar activity were not apparent. The result s are discussed in relation to ways in which host plant selection, sel ective insecticides and biological control could potentially be manipu lated and optimized in aphid management systems on brassica crops.