Diadromus collaris (Gravenhorst) (Hymenoptera: Ichneumonidae), a solitary pupal endoparasitoid of Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a candidate for introduction into Canada as a biological control agent. It is important to assess the parasitoid's host specificity before its release. To maximize the wasp's expressed host range, I tested five variables to determine which experimental conditions would motivate D. collaris to oviposit. Of these variables, wasp diet, exposure length, and the presence or absence of diamondback moth cocoons resulted in statistically significant differences in D. collaris emergence or diamondback moth mortality. To determine the parasitoid's fundamental host range, I exposed pupae from eight species of non-target Lepidoptera to female D. collaris in a series of no-choice tests. Three species, Plutella armoraciae, Plutella porrectella, and Acrolepiopsis assectella, were suitable hosts for D. collaris development. The results from this study provide insight into the suitability of the parasitoid for introduction into Canada.