We investigated whether corn plants treated with jasmonic acid (JA) increases the ability of the parasitic wasp, Cotesia kariyai, to find and control the common armyworm (Mythimna separata) under laboratory conditions. The rank order of attractiveness increased from intact plants treated with distilled water (DW) (2 d), JA-treated intact plants (2 d), DW-treated infested plants (2 d) to JA-treated infested plants (2 d). Single JA-treatment to either infested or uninfested plants increased attractiveness to C. kariyai over a period lasting at least 10 d. We then showed that the increase in attractiveness of infested corn plants by JA-treatment resulted in increased parasitism by C. kariyai. These results hold a promise for field application of JA-treatment. First, JA-treatment not only promotes the attractiveness of uninfested plants, but also armyworm-infested plants. Thus, parasitoids are not likely to waste time on JA-treated uninfested plants when JA-treated infested plants are available. Second, the effect of JA-treatment is lasting for at least 10 d, a result now obtained in two independent studies.