none Lymantria dispar; HYMENOPTERAN-PARASITES; Cotesia melanoscela; Hymenopteran hyperparasite prevalence, influence of strain biology; HYPERPARASITISM-; Hyperparasite prevalence in hymenopteran parasite, effect of strain biology Cotesia melanoscela; DIAPAUSE-; Strain differences in occurrence; VARIATION-; Diapause & cocoon morphology; HYMENOPTERAN-PARASITES; Hyperparasitism incidence; LEPIDOPTERAN-HOSTS; Lymantria dispar; Hymenopteran hyperparasite incidence; HYPERPARASITISM-; Incidence, influence of strain biology, lepidopteran host; COCOONS-; Morphology, strain differences, effect on hyperparasite prevalence HYMENOPTERA-; HYMENOPTERAN-HOSTS; Cotesia melanoscela; Hyperparasitism incidence, influence of strain biology, lepidopteran host The vulnerability to predation and hyperparasitism of an introduced European strain of Cotesia melanoscela (Ratzeburg) (Hymenoptera: Braconidae) and an Asian strain of the parasitoid with a different cocoon morphology and no diapause stage was compared in southern Ontario, Canada. The rates of predation and hyperparasitism were not related to cocoon morphology but were dependent on the length of time that cocoons were exposed to hyperparasitism in the field. The nondiapause characteristic of the Asian strain decreases its exposure time and hence reduces its vulnerability to hyperparasitism. Inundative release programs involving Cotesia spp. will be more effective using nondiapause strains of this parasitoid and by making releases as early as possible in the season to minimize their exposure to hyperparasitoids.