Does the fundamental host range of Trissolcus japonicus match its realized host range in Europe?

Unintentional introduction of natural enemies has increased in recent years due to a massive rise in global trade and tourism. One such natural enemy is the Asian egg parasitoid Trissolcus japonicus, a promising agent for classical biological control of Halyomorpha halys. In Europe, adventive T. japonicus populations have been detected in Switzerland, Italy and Germany. Host specificity testing demonstrated that its fundamental host range is fairly broad; however, it is unclear whether spatial or temporal refuges reduce parasitism of non-target species in the field. To address this, the realized host range of T. japonicus was assessed over three years by exposing sentinel egg masses of H. halys and 18 non-target species and collecting naturally laid egg masses in Switzerland and Italy. In total, 15 of 18 non-target species were successfully parasitized by T. japonicus in the field, confirming its broad fundamental host range. However, most non-target species were less parasitized by T. japonicus than H. halys, profiting from either partial temporal or spatial refuges from parasitism. Species with an unusual life cycle and the same ecological niche as H. halys, such as Pentatoma rufipes, which was the most parasitized non-target species in both countries, potentially face an increased risk of parasitism. In contrast, beneficial non-target effects may occur for the invasive pest, Nezara viridula, which suffered high non-reproductive mortality induced by T. japonicus. In both cases, life table studies will be needed to determine the impact of non-target parasitism and the potential consequences at the population level.