We measured the parasitization rates of Mythimna separata by a parasitoid, Cotesia kariyai, in a climate room on one species each of corn plant, kidney bean plant and Japanese radish plant. The trend of parasitization by wasps was corn>kidney bean>Japanese radish. In a cage, C. kariyai had greater olfactory responses to leaves (of corn, kidney bean, and Japanese radish) infested by the host larvae than to uninfested leaves. When we compared infested leaves using pairs of plant species, the wasps showed preference in this order: corn>kidney bean>Japanese radish. From GC-MS analysis, we found qualitative and quantitative differences in the blend of volatiles emitted from infested leaves of the three plant species. Such differences were considered to be one of the factors affecting the parasitization rates.