Population-level impact of egg parasitism on Halyomorpha halys despite a rapid shift in parasitoid species composition

Intentional introductions of the egg parasitoid Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae) have been carried out in Italy since 2020 for the control of the invasive Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), with releases conducted along ecological corridors of untreated vegetation. These introductions took place in an area where unintentionally introduced populations of Trissolcus mitsukurii (Ashmead) (Hymenoptera: Scelionidae) were already present and adventive populations of T. japonicus were just beginning to establish. In this study, we investigated whether T. japonicus releases contributed to the impact of egg parasitism on H. halys populations, and what the total impact of egg parasitism on pest population growth was over four growing seasons (2020–2023) in ten kiwifruit orchards in Italy. Although higher parasitism by T. japonicus was observed in orchards adjacent to release sites, the total impact of egg parasitism on H. halys remained similar over the four years because an increasing prevalence of T. japonicus over the study period was strongly associated with a corresponding decrease in egg parasitism by T. mitsukurii. Using a parameterized stage-structured matrix model, we estimate that the joint action of T. mitsukurii and T. japonicus (average total egg parasitism: 33–39%) prevented an expected 18–29% increase in net reproductive rate (R0) of H. halys over the four years of the study. This analysis suggests that irrespective of year-to-year temperature variation favoring pest reproduction and the displacement of T. mitsukurii by T. japonicus (hastened by releases), egg parasitism has been playing an important and consistent role in the biological control of H. halys.