Plants can affect the effectiveness of carnivorous insects in various ways. An important aspect is that herbivory results in the emission of plant volatiles that are used by carnivores to locate their herbivorous victims. Here we show that such plant volatiles may affect parasitoids differently in two different tritrophic systems. The parasitoidCotesia kariyai(Hymenoptera: Braconidae) is attracted to volatiles from uninfested plants, both host-food plants and non-host-food plants. However, females of this parasitoid clearly prefer host (Pseudaletia separatalarvae)-infested corn plants over uninfested corn plants or artificially damaged plants. This response is quite specific: feeding by early larval instars results in attraction of the parasitoids, while feeding by late larval instars does not. Another parasitoid,Cotesia glomerata,is a specialist ofPieris rapaelarvae in mainland Japan where they feed on crucifers. The wasps preferred uninfestedRorippa indicaplants to non-host-food plants and host-infestedR. indicaplants, or artificially damagedR. indicaplants to uninfestedR. indica.This response was not specific: in a two-choice test the wasps preferred volatiles from artificially damaged plants over those from infested plants. The differences in plant cues available to the two parasitoids are discussed in terms of plant effects on biological control agents.