Omnivorous insects make foraging decisions between plant and prey resources depending on their accessibility, availability, and nutritional value. This shapes the stability and complexity of food webs, but also pest control services in agroecosystems. The mirid bug Lygus pratensis is a common cotton pest in China, but it also feeds on a variety of prey species. However, little is known about how different types of available resources affect its fitness and foraging behaviour. In laboratory experiments, we measured the fitness (survival, longevity and fecundity) of L. pratensis fed with bean pod only, bean and Aphis gossypii nymphs, or bean and Helicoverpa armigera eggs, and we also conducted focal observations of its foraging behaviour when provided the latter two. Adding H. armigera to its diet increased its fitness (both survival and fecundity), while adding A. gossypii was marginally detrimental. The different diets did not affect the time spent walking (searching for food resources) or preying, but significantly affected the time spent sapping bean tissue depending on L. pratensis life stage. Nymphs spent more time sapping plant when provided with H. armigera than A. gossypii (possibly through higher efficiency of handling prey). In addition, adults spent less time sapping plant than did nymphs (possibly through good efficiency of handling A. gossypii mobile prey and due to higher protein requirements). This special case of life-history omnivory highlights the complexity of natural food webs, where a major crop pest at the juvenile stage may become a biological control agent at the adult stage.