The assessment of Aleochara bipustulata as a potential biological control agent of Delia radicum

Aleochara bilineata (Gyllenhal) and A. bipustulata (L.) first instar larvae are important pupal ectoparasitoids of Delia radicum (L.) in many Brassica crops in Europe. In North America, however, A. bipustulata does not occur and therefore is a potential candidate for biological control against D. radicum in Canadian canola. Prior to its introduction, further ecological data such as how the two species interact with each other, as well as methods to implement the introduction of A. bipustulata, are needed. Field cages studies determined that the two species occurring together would increase parasitism of D. radicum compared to each species occurring individually. Parasitism levels were higher when three pairs of each of the two species were present than when six pairs of either of the species were present alone. Laboratory studies showed that, regardless of whether there was choice, the Aleochara species inside a puparium, or the instar of the parasitoid inside a puparium, first instar larvae of A. bipustulata seldom entered a previously parasitized D. radicum puparium. Frequency of entry of A. bilineata larvae into a previously parasitized D. radicum puparium was higher particularly in the absence of choice of an unparasitized puparium. Frequency of entry of A. bilineata into a parasitized puparium was less if the parasitoid inside was a conspecific and was inversely related to the instar of the parasitoid inside. Mustard seed meal applied to canola was attractive to A. bipustulata but not attractive to A. bilineata. Y-tube olfactometer tests showed that mustard seed meal was attractive to A. bipustulata adults, regardless of whether the meal was wet or dry. In the field, catches of adult A. bipustulata parasitism were enhanced by mustard meal mulch. Mustard meal did not affect A. bilineata. It is concluded that Aleochara bipustulata potentially could coexist with A. bilineata in the Canadian Prairies. Larval-larval and adult-adult interactions have shown that A. bipustulata adults are not aggressive toward A. bilineata and since the first larval instar prefers to parasitize unparasitized puparium they do not pose a major threat against A. bilineata larvae. The application of mustard meal to canola could enhance the release and establishment of A. bipustulata and allow them to locate D. radicum hosts as well as finding other A. bipustulata with which to mate.