noneEnter The behavioral response of the parasitoid Cotesia kariyai Watanabe (Hymenoptera: Braconidae) to frass of its host Mythimna separata (Walker) (Lepidoptera: Noctuidae) was investigated in a wind tunnel or in a Petri dish arena. The presence of M separata frass either on intact plants or on host-infested plants did not increase the attractiveness of C. kariyai females in wind tunnel bioassay over that of control. Indeed, flight response of C. kariyai-frass experienced females did not differ from those of naive ones. In Petri dish bioassay, wasps were arrested in patches treated with host frass extract and the parasitoid intensified the search in the treated patches, particularly near the edges and exhibited both orthokinetic and klinotaxic responses. Parasitoid searching time and its arrestment response to host frass decreased with increasing time after application indicating an associative learning for this wasp. The parasitoid responded qualitatively to frass from different larval instars of M. separate, with no significant differences among the searching times in patches treated with frass extracts of the suitable larval instars for parasitization (third to sixth instars), but differed with frass from the unsuitable larval instar (second instar). Moreover, the presence of host frass increased rates of parasitism either in cage or in Petri dish experiment, with subsequent significant decrease in times needed for wasp females to locate host larvae. The possible differences in the responses of C. kariyai to M. separata frass were discussed.