Comparative assessment of the parasitoid community of Delia radicum in the Canadian prairies and Europe: A search for classical biological control agents

As a precursor to a classical biological control programme for Delia radicum in spring-seeded (“summer”) canola (oilseed rape) in Canada, communities of parasitoids of D. radicum in summer canola in Canada were compared with those in brassica vegetables and several types of canola in Europe. Weekly samples of immature D. radicum were collected from sites in Canada and Europe and reared individually to determine identity and importance of parasitoids. On both continents, the most frequently encountered parasitoids were the larval–pupal parasitoid Trybliographa rapae and the pupal parasitoid Aleochara bilineata. In Canada, Aleochara verna was common at some sites, and in Europe, Aleochara bipustulata was common at some sites. Parasitoid communities were separated primarily by the relative importance of T. rapae and A. bilineata in puparial samples: in Canada, the ratio of A. bilineata parasitism to that of T. rapae was higher than in Europe. The negative correlation of these species may be related to the degree of synchrony of A. bilineata parasitism with its host. In Canada, A. bilineata parasitism occurred early relative to host availability and this timing is deleterious to T. rapae already within the host. In Europe, T. rapae was favored during multiparasitism because A. bilineata parasitism was synchronized with, or delayed, relative to its host’s availability. The only important parasitoid in Europe that was not found in Canada was A. bipustulata, which most frequently occurred in summer canola in Europe, suggesting that it could be suited to the same crop in Canada. Consequently, A. bipustulata is being considered as a candidate biological control agent.