Banker-plant systems with Aphidius species have been used to control pest aphids in greenhouses in Japan. However, the occurrence of secondary parasitoids, which lead to aphid-control failures, and of minor aphid species, are problematic. In this work the ecological characteristics of Ephedrus nacheri Quilis (Hymenoptera: Braconidae), an alternative biological control agent to Aphidius species against the pest aphids Aphis gossypii and Myzus persicae, were examined under laboratory conditions at 25 A degrees C. Attack rate, success of parasitization, and offspring sex ratio were highly dependent on aphid species and instar stage. Development time and offspring body size depended on aphid species. Because of insufficient honeydew production, survival of the female wasp decreased when the host density was low; as a result, a type III functional response curve was observed for the two aphid species in most stage combinations. The intrinsic rate of natural increase estimated by use of the bootstrap method ranged from 0.128 to 0.258, depending on the larval mortality, host species, and stage. These values were insufficient to support inoculative use of E. nacheri for aphid control. In future work we will conduct a survey to discover a better combination of banker plant and host aphid which maintains a sufficient population and offspring sex-ratio, and decide the scale of the banker-plant system in the greenhouse that will supply a sufficient wasp population.