The exotic cabbage maggot (CM) infests canola on the prairies, feeding on roots in its larval stage, which disrupts the uptake of nutrients and water and provides an entry point for fungal plant pathogens. The European staphylinid, Aleochara bipustulata L., may be introduced for control of CM, but only if the risk to other species is low and if A. bipustulata has demonstrable potential to increase mortality already caused by natural enemies in Canada. Aleochara bipustulata could contribute to pest management as a predator of CM eggs and larvae, and as a parasitoid of CM puparia; however, it could affect non-pest species in the same two ways. A variety of invertebrates that share the soil of Brassica fields with immature CM were screened in laboratory no-choice assays to determine what adult A. bipustulata eat. In these assays, immobile or barely mobile invertebrates were accepted regularly and could be at risk. The majority of groups were seldom or never consumed. Also, a molecular assay developed to test for CM DNA in the guts of field-collected A. bipustulata revealed its high potential as a predator, and a similar assay developed for two carabid beetle species showed these to be seldom if ever consumed. Laboratory and field cage assays with other CM egg predators showed A. bipustulata has potential to disrupt other species, particularly the closely related A. bilineata Gyllenhal, as they seem to forage in similar microhabitats. Measurements of field-collected beetles indicate CM is unlikely to be the primary host in Europe, so introducing A. bipustulata to Canada may bring risks to non-target Diptera species. This was observed even though a series of laboratory experiments demonstrated CM is a superior and preferred host relative to the smaller, acalyptrate cheese skipper.