Impressionable wasps: How rearing host affects the foraging ecology and field efficacy of an augmentative biological control agent of stored products

A post-harvest biocontrol agent is Theocolax elegans (Hymenoptera: Pteromalidae) whose host range includes the rice weevil, Sitophilus oryzae (Coleoptera: Curculionidae), and lesser grain borer, Rhyzopertha dominica (Coleoptera: Bostrichidae). Both species destructive primary pests of stored products, and either could be used when mass rearing T. elegans. However, the Hopkins-Host Selection principle suggests that important foraging semiochemicals may depend on the natal host environment in which an organism is reared. In order to investigate the impact of natal host environment on the behavioral response of T. elegans to potential hosts, we reared T. elegans on either S. oryzae or R. dominica for multiple generations. In a four-arm, still-air olfactometer, we evaluated the orientation and taxis of T. elegans to six treatments: damaged grain+S. oryzae, damaged grain+R. dominica, S. oryzae alone, R. dominica alone, damaged grain+insects from the natal environment, or a control. Further, we characterized the volatiles from these treatments using headspace collection and gas chromatography coupled with mass spectrometry to analyze these volatile differences with an ordination procedure. Additionally, in a pilot-scale elevator setting, we evaluated the ability of host volatiles to attract T. elegans and oviposit at increasing distances from a release point. We found that the natal host and odor source significantly affected chemotaxis of T. elegans, and our treatments emitted unique blends of semiochemicals, suggesting that careful attention should be paid to the mass rearing procedure for this parasitoid. Mixed host cultures should be used for mass rearing in the future.