We previously reported that Cotesia vestalis (Hymenoptera, Braconidae), a parasitoid of diamondback moth (DBM) (Plutella xylostella; Lepidoptera, Plutellidae) larvae, was attracted to volatiles from crucifer plants infested by moth larvae kept in a desktop acrylic box, and that a blend of four DBM-induced plant volatiles was responsible for this attraction. In this study, using a specially designed dispenser to release the four compounds, we demonstrated that the wasp was attracted to intact komatsuna plants (Brassica rapa var. perviridis). The experiments were performed in a climate-controlled room, which was approximately 1000 times larger than the acrylic box used previously. Similarly, using the dispenser in the field, C. vestalis females were attracted to intact komatsuna plants with the dispenser from a distance of three metres. We also examined the effect of the volatile blend on the incidence of parasitism of DBM larvae in the field. Three small containers containing DBM-infested komatsuna plants with dispensers, and three control containers containing only infested plants (control) were arranged in two lines running perpendicular to a komatsuna field in which both DBM larvae and C. vestalis populations were maintained, at distances of 12, 30 and 70 m. The results showed that the incidence of DBM parasitism was significantly higher in containers containing dispensers than in the control containers, suggesting that the blend could potentially be applied to DBM control in agroecosystems.