Aphids on crucifers: multitrophic and selective insecticide interactions for enhanced control

Most studies on chemical and biological control of arthropod pests of agriculture do not consider the effects of partial host plant resistance. We present research using two partially resistant and a susceptible cultivar of common cabbage (Brassica oleracea var. capitata L.) and determining specific effects of interactions between the host plants, one or two common crucifer aphids (Myzus persicae Sulzer, a generalist species and Brevicoryne brassicae L., a crucifer specialist), selective insecticides (pirimicarb and a neem seed kernel extract) and a predator (the predatory gall midge, Aphidoletes aphidimyza Rondani). A pirimicarb dose equating to c. 15% of the recommended field concentration caused equivalent toxicity of M. persicae on a partially resistant cultivar compared with aphids treated with a three-fold greater dose and reared on the susceptible cultivar. Cultivar-mediated differences in mortality caused by a neem extract tested for systemic and translaminar activity (at the recommended concentration) were not apparent. In a laboratory-based tritrophic system including both species of aphid, three common cabbage cultivars and A. aphidimyza, cultivar effects were particularly pronounced in relation to predator survival from egg to adult, the survival rates being at least 2.5-fold greater on the susceptible cultivar (compared with the two partially resistant cultivars) with both aphid host species. Predator larval body length were also less on the partially resistant compared with susceptible cultivar with both aphid species as hosts. Considerable variation (between host plants/cultivars and sites) in M. persicae infestation intensity were noted in field studies in two different zones of the Cameron Highlands, Malaysia. The results are discussed in relation to ways in which compatibility between partial host plant resistance, selective insecticides and biological control may be manipulated and optimised in crucifer aphid management systems.