Olfactory responses of Trissolcus mitsukurii to plants attacked by target and non-target stink bugs suggest low risk for biological control

Field release of classical biocontrol agents requires prior risk assessment to ensure that non-target effects are averted. No-choice and choice bioassays can provide basic information on the physiological host range of the candidate agent. However, other experimental methods, like olfactometer bioassays of host-plant complexes, can depict more realistically the likelihood that the released biocontrol agent localizes non-targets in the field. Halyomorpha halys Stål (Hemiptera: Pentatomidae) is native to Asia and invasive in North America and Europe, where it represents a major threat for crops. The Asiatic egg parasitoid Trissolcus mitsukurii (Ashmead) is adventive in Italy and exhibits high parasitization ability on H. halys. In the light of evaluating T. mitsukurii for H. halys biocontrol, its physiological host range has been already partially explored. Here, we investigated the ability of the parasitoid to exploit odours associated with H. halys or with the following non-targets: Arma custos F., Dolycoris baccarum L., Eurydema ventralis Kolenati, and Nezara viridula L. (all Hemiptera: Pentatomidae). Specifically, using a Y-tube olfactometer, we tested the response of parasitoid females to plants bearing naturally laid eggs, to physogastric females or eggs of the different species. We observed that T. mitsukurii is only attracted by plants challenged by eggs of H. halys or N. viridula, while it is not attracted by physogastric females or eggs alone. Remarkably, T. mitsukurii was almost repelled by plants bearing eggs of the beneficial A. custos. Our results usefully contribute to a more valuable assessment of the potential non-target risks in case of parasitoid release.