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 parasitoid Cotesia 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 separata larvae)-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 of Pieris rapae larvae in mainland Japan where they feed on crucifers. The wasps preferred uninfested Rorippa indica plants to non-host-food plants and host-infested R. indica plants, or artificially damaged R. indica plants to uninfested R. 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.