Evergreen bagworm, Thyridopteryx ephemeraeformis (Haworth) (Lepidoptera: Psychidae) is commonly attacked by two species of ichneumonid parasitoids in central Illinois: the endemic Itoplectis conquisitor (Say) and Pimpla disparis (Vierick), a generalist introduced as a biological control agent for gypsy moth in the eastern United States. We have conducted experiments to determine whether the two parasitoid species compete for hosts, and whether oviposition preference was influenced by previous experience with host species. This research allows an assessment of the degree to which natural control of a pest species is compromised by adding exotic species to its guild of natural enemies.