Apparent competition is an indirect, negative interaction occurring between two or more host species, mediated by a shared parasitoid. Host species' population growth rates, parasitoid attack rates, and parasitoid population size can mediate the outcomes of apparent competition. It has also been suggested that optimal foraging by a natural enemy can influence the outcome of apparent competition, resulting in increased suppression of the optimal host. While this has been shown in theory, to date no studies have empirically tested the link between parasitoid optimal foraging behaviour and the outcome of apparent competition. The present study examined how optimal foraging behaviour influences the outcome of apparent competition in an aphid banker plant system. First, Aphidius colemani Viereck's preference for Myzus persicae Sulzer or Rhopalosiphum padi L., the non‐pest host on the banker plant, was examined, and then the difference in M. persicae suppression in the presence and absence of R. padi was assessed. It was found that optimal foraging behaviour led female A. colemani to prefer M. persicae over R. padi, due to increased offspring survival and female size. Consequently, optimally foraging A. colemani parasitised significantly more M. persicae in the presence of both aphid hosts than in the presence of M. persicae alone. Understanding the interaction between optimal foraging and apparent competition has important implications for biological control of arthropod pests and could help to predict the outcome of biological control programmes.