Assessing the Distribution of Exotic Egg Parasitoids of Halyomorpha halys in Europe with a Large-Scale Monitoring Program

SIMPLE SUMMARY: The management of invasive alien species is a very challenging task. For the brown marmorated stink bug (Halyomorpha halys), classical biological control has been identified as the most suitable method to sustainably reduce its populations in the long-term. Among its natural enemies, two species were identified as the most promising candidates for biocontrol, Trissolcus japonicus and Trissolcus mitsukurii. Populations of these two species have recently been detected in Europe and to assess their distribution, a large-scale study was performed. Combining several monitoring methods, in four months (May–September 2019), a wide area covering northern Italy and parts of Switzerland was surveyed. The results showed that both species have spread into all types of habitats where H. halys is present and the parasitization of native species was rarely observed. Among native species, Anastatus bifasciatus was the predominant parasitoid of H. halys. This study supported the development of the first release program of Tr. japonicus in Europe. ABSTRACT: The brown marmorated stink bug Halyomorpha halys is an invasive agricultural pest with a worldwide distribution. Classical biological control has been identified as the most promising method to reduce the populations of H. halys. Adventive populations of two candidates for releases, Trissolcus japonicus and Trissolcus mitsukurii, have recently been detected in Europe. To assess their distribution and abundance, a large-scale survey was performed. From May to September 2019, a wide area covering northern Italy and parts of Switzerland was surveyed, highlighting the expanding distribution of both Tr. japonicus and Tr. mitsukurii. Within four years after their first detection in Europe, both species have rapidly spread into all types of habitats where H. halys is present, showing a wide distribution and continuous expansion. Both exotic Trissolcus showed high levels of parasitism rate towards H. halys, while parasitization of non-target species was a rare event. The generalist Anastatus bifasciatus was the predominant native parasitoid of H. halys, while the emergence of native scelionids from H. halys eggs was rarely observed. The presence of the hyperparasitoid Acroclisoides sinicus was also recorded. This study provided fundamental data that supported the development of the first inoculative release program of Tr. japonicus in Europe.