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 Linnaeus, a crucifer specialist), and the predatory gall midge, Aphidoletes aphidimyza Rondani, on three cultivars of common cabbage Brassica oleracea var, capitata cv. Derby Day (green-leaved), Minicole (green-leaved) and Ruby Ball (red-leaved). In a laboratory-based tritrophic system including both species of aphid, the three cabbage cultivars and A. aphidimyza, predator larvae grew most rapidly when feeding on M. persicae or B. brassicae on cv. Derby Day, while growth was slowest on cv. Ruby 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 less 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 differences in predator growth or consumption significant between cultivars. Effects of selective insecticides (pirimicarb and a neem seed kernel extract, NeemAzal-T/S(R)) on bitrophic (aphid-host plant) interactions were also investigated in the laboratory. A pirimicarb dose equating to c. 15% of the recommended field concentration caused equivalent toxicity of M. persicae on cv. Minicole compared with aphids treated with a three-fold greater dose and reared on cv. Derby Day. Cultivar-mediated differences in aphid mortality caused by the neem extract when tested for systemic and translaminar activity were not apparent. The results are discussed in relation to ways in which host plant selection, selective insecticides and biological control could potentially be manipulated and optimized in aphid management systems on brassica crops.