Density-mediated indirect interactions alter host foraging behaviour of parasitoids without altering foraging efficiency

1. Foraging decisions of parasitoids are influenced by host density via density-mediated indirect interactions. However, in the parasitoid's environment, non-suitable herbivores are also present. These non-hosts also occur in different densities, which can affect a parasitoid's foraging behaviour. 2. The influence of non-host densities can be expressed during the first phase of the foraging process, when parasitoids use plant volatiles to locate plants infested by their host. They may also play a role during the second phase, when parasitoids use infochemicals from the host and plant to locate, recognise and accept the host. 3. By using laboratory and field setups, it was studied whether the density of non-host herbivores influences these two phases of the foraging behaviour of the parasitoid Cotesia glomerata as well as the parasitoid's efficiency to find its host, Pieris brassicae caterpillars. 4. The findings show that a high non-host density, regardless of the species used, negatively affected parasitoid preference for host-infested plants, but that the behaviour on the plant and the total host-finding efficiency of the parasitoids were not influenced by non-host density. 5. These results are discussed in the context of density-mediated indirect interactions.