Receptor characterization (i.e., identifying what will be affected by an activity) is the first step in a risk assessment of biocontrol agents for insects. Development of a representative list of species at risk, based on ecological vulnerability, enables host-range screening of potential biocontrol agents on a manageable group of nontarget insects. A database of 153 species was used to characterize the butterflies potentially at risk from an inundative release of the egg parasitoid Trichogramma minutum Riley. Risk criteria for butterfly species included North American, Canadian, and Ontario geographic distributions; oviposition phenology; number of generations per year; overwintering stage; host-plant preferences; and egg mass type and location. Ecological vulnerability lists of butterfly species were generated for northern and southern Ontario; areas where there have been recent experimental inundative releases of T. minutum for the suppression of forest pests. Based on the above criteria, 2 species and a maximum of 27 species would be potentially at risk, and thus requiring host-range testing if an inundative release were considered for northern and southern Ontario, respectively. The number of species on the ecological vulnerability list for southern Ontario could be reduced to 12 species depending on the specific geographic location in southern Ontario of the inundative release. The six criteria used for receptor characterization for T. minutum, associated primarily with host-habitat location and host-location, can also be used for other parasitoids. They are components of any target host's biology, and thus will affect the scale and impact of any parasitoid attacking eggs, larvae, or pupae. Additional criteria for receptor characterization may also be added that will relate to the specifics of a parasitoid's biology and are associated with host acceptance and host suitability. Development of ecologically based vulnerability lists should become standard practice in determining which nontarget species require host-range testing, for both inundative and classical biocontrol agents targeting insects, and for the potential impact of invasive species.