Integrating Chemical Ecology with Behavioural Bioassays to Understand Host Preferences in the Stink Bug Egg Parasitoids Trissolcus basalis and Trissolcus oenone (Hymenoptera: Scelionidae)

Current approaches to assessing potential non-target risks associated with biological control agents are conservative, and they often rely on oviposition experiments conducted in quarantine laboratories. By their nature, such tests offer robust evidence of a parasitoid's ability to attack and develop in a host. However, they exclude many important chemical cues present in the natural environment, which play a key role in the ability of a parasitoid to search for and locate hosts. We conducted a series of experiments with Trissolcus basalis and Trissolcus oenone to better understand the chemical basis mediating differences in host-specificity between these parasitoids. First, we compared the searching behaviour of T. basalis and T. oenone in open arena arrestment bioassays contaminated with footprint compounds of Nezara viridula or Cuspicona simplex. Trissolcus basalis spent four times longer searching for N. viridula than C. simplex, while T. oenone spent four times longer searching for C. simplex than N. viridula. We then conducted competition experiments to assess factors important to determining the outcomes of extrinsic and intrinsic contests between these parasitoids when they are simultaneously exposed to C. simplex egg masses. Trissolcus oenone was the superior competitor in extrinsic and intrinsic contests. Finally, we recorded the antennal responses of T. basalis to egg extracts of N. viridula, to tentatively identify potential contact kairomones used by this parasitoid to recognise and accept hosts. We discuss our results in the context of combining behavioural and chemical ecological techniques for pre-release risk assessments of classical biological control agents.