Potential interactions of larval and pupal drosophila parasitoids and their implications for biological control of Drosophila suzukii

The parasitoid guilds attacking Drosophila spp. consist primarily of larval and pupal parasitoids. Most larval parasitoids are koinobiont and do not kill their hosts until they form pupae within puparia. Thus, a host parasitized by a larval parasitoid could be attacked subsequently by a pupal parasitoid via multi-parasitism or hyperparasitism. Ganaspis kimorum is a key larval parasitoid that has recently been introduced into Europe and the US as a classical biological control agent against Drosophila suzukii. Pachycrepoideus vindemiae and Trichopria drosophilae are two cosmopolitan pupal drosophila parasitoids that may affect the performance of G. kimorum. This study investigated potential interactions of P. vindemiae and T. drosophilae with G. kimorum in D. suzukii puparia and with Leptopilina heterotoma (a common larval parasitoid of Drosophila melanogaster) in D. melanogaster puparia. Both no-choice and choice tests were conducted to determine the pupal parasitoids’ preference to attack unparasitized or parasitized puparia containing a third instar larva of the larval parasitoid, which becomes ectoparasitic and can be easily recognized via puparium. No-choice tests demonstrated that both pupal parasitoids could parasitize puparia previously parasitized by either larval parasitoid, but only P. vindemiae successfully developed from multiparasitized puparia, with a significant reduction in female offspring. In choice tests, both pupal parasitoids preferred unparasitized over parasitized puparia, although no differences in the progeny sex-ratio were observed. Stereomicroscopic observations of multiparasitized D. suzukii puparia suggest that P. vindemiae would outcompete the ectoparasitic G. kimorum through physiological host suppression. These findings provide new insights into the potential interactions between larval and pupal drosophila parasitoids and their implications for biological control of D. suzukii.