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

The brown marmorated stink bug <i>Halyomorpha halys</i> 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 <i>H. halys</i>. Adventive populations of two candidates for releases, <i>Trissolcus japonicus</i> and <i>Trissolcus mitsukurii</i>, 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 <i>Tr. japonicus</i> and <i>Tr. mitsukurii.</i> Within four years after their first detection in Europe, both species have rapidly spread into all types of habitats where <i>H. halys</i> is present, showing a wide distribution and continuous expansion. Both exotic <i>Trissolcus</i> showed high levels of parasitism rate towards <i>H. halys</i>, while parasitization of non-target species was a rare event. The generalist <i>Anastatus bifasciatus</i> was the predominant native parasitoid of <i>H. halys</i>, while the emergence of native scelionids from <i>H. halys</i> eggs was rarely observed. The presence of the hyperparasitoid <i>Acroclisoides sinicus</i> was also recorded. This study provided fundamental data that supported the development of the first inoculative release program of <i>Tr. japonicus</i> in Europe.