Yponomeuta cagnagellus; Yponomeuta malinellus; HYMENOPTERAN-PARASITES; Diadegma armillata; Parasitoid host preferences & egg encapsulation, control implications; USA-; North west; Hymenopteran parasite host preferences & egg encapsulation, control use Diadegma armillata; LEPIDOPTERAN-HOSTS; Yponomeuta cagnagellus & Y. malinellus; Parasitoid host preferences & egg encapsulation; USA-; North west; Lepidopteran host preferences & egg encapsulation, control implications The ichneumonid Diadegma armillata (Cravenhorst) is a candidate for biological control of Yponomeuta malinellus Zeller in the Pacific Northwest of the United States. In Europe, the parasitoid attacks several Yponomeuta spp. that develop asynchronically on various host plants. Investigations on host-parasitoid relationships were carried out to assess the potential value of parasitoids obtained from non-Y. malinellus hosts as biological control agents against Y. malinellus. Host acceptance, host suitability, and parasitoid encapsulation of D. armillata, attacking Y. malinellus and Y. cagnagellus (Hubner), were studied. Four combinations of the 2 target hosts and parasitoids originating from each were studied. Measurement of rates of infestation, host mortality during development, percentages of dissected host containing either living or encapsulated parasitoids, and successful rate of parasitism in reared hosts were compared in the 4 combinations. Results demonstrated that there is no absolute barrier to parasitoids from 1 host using the other host. However, D. armillata showed a higher physiological performance against Y. malinellus regardless of its natal Yponomeuta host species. Y cagnagellus was more resistant than Y malinellus to attacks from D. armillata, and D. armillata from Y. cagnagellus better overcome host defenses. Therefore, D. armillata individuals from Y cagnagellus could be used in biological control introductions against Y. malinellus in North America. In fact, it would be preferred over D. armillata from Y. malinellus. none