The concept of ecological generalism (or polyphagy) is widely used in ecology to describe and predict aspects of the behaviour of predatory insects, notably with regard to the management of insect pests. The assumptions of ecological generalism have their basis in competition and optimization. We review the historical context and perspectives that led to the development of the concept of ecological generalism as it is applied to diet breadth in predatory insects. We identify how flaws in this concept limit its utility in describing and predicting the behaviour of organisms. We re-analyse the behavioural expectations of putatively generalist organisms with a focus on the mechanistic aspects of predation; that is, we develop an interpretation based on the adaptations that mediate their responses to particular cues in their environment. This mechanistic analysis emphasizes the need to determine the primary environmental associations of putatively generalist species, and the adaptations that underpin those associations. This approach thus highlights the importance of appropriate matching of predator species to prey species in biological control, based on an understanding of these adaptations and associations. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.