The brown marmorated stink bug, Halyomorpha halys (Stål), native to East Asia, emerged as an invasive pest in Europe in the 2000s. In its native range, Trissolcus japonicus (Ashmead) is the dominant egg parasitoid of H. halys, and thus it has been considered for classical biological control in countries invaded by the pest. A survey of native egg parasitoids conducted in 2017 and 2018 with frozen, sentinel egg masses of H. halys revealed that T. japonicus was already present in apple orchards in the Canton Ticino, Switzerland. Trissolcus japonicus was recovered in both years and from three different sites. In total, 17 egg masses were recovered from which 42 adult parasitoids emerged. A genetic analysis using the barcode mitochondrial DNA confirmed the morphological identification of T. japonicus and evidenced a best match of the “Ticino populations” to Japanese populations, but the pathways of entry remain unknown.

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.

Accurate identification of parasitoids is crucial for biological control of the invasive brown marmorated stink bug,Halyomrpha halys(Stål). A recent work by Talamas et al. (2017) revised the Palearctic fauna ofTrissolcusAshmead, egg-parasitoids of stink bugs, and treated numerous species as junior synonyms ofT. semistriatus(Nees von Esenbeck). In the present paper, we provide a detailed taxonomic history and treatment ofT. semistriatusand the species treated as its synonyms by Talamas et al. (2017) based on examination of primary types, molecular analyses and mating experiments.Trissolcus semistriatus,T. belenus(Walker),T. colemani(Crawford), andT. manteroi(Kieffer) are here recognized as valid and a key to species is provided. The identification tools provided here will facilitate the use ofTrissolcuswasps as biological control agents and as the subject of ecological studies.

Sustainable strategies such as classical or augmentative biological control are currently being evaluated for the long-term management of the alien invasive pest Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). A three-year study carried out in northeastern Italy was performed to investigate the distribution and field performance of the H. halys egg parasitoid Trissolcus mitsukurii (Ashmead) (Hymenoptera: Scelionidae), in comparison with other parasitoid species. In the study area, adventive populations of T. mitsukurii were present since 2016, representing the earliest detection of this species in Europe. Trissolcus mitsukurii was the most abundant parasitoid and showed a higher “parasitoid impact” (i.e., number of parasitized eggs over the total number of field-collected eggs) compared to the other species, i.e., Anastatus bifasciatus (Geoffroy) (Hymenoptera: Eupelmidae), Trissolcus basalis (Wollaston) and Trissolcus kozlovi Rjachovskij (Hymenoptera: Scelionidae). The hyperparasitoid Acroclisoides sinicus (Huang and Liao) (Hymenoptera: Pteromalidae) was also recorded. Phylogenetic analysis of T. mitsukurii population distinguished two clades, one covering samples from Italy, Japan and China, the other from South Korea. The present study provides promising results for the biological control of a pest that is having a dramatic impact on a wide range of crops worldwide.

Halyomorpha halys(Stål) (Hemiptera, Pentatomidae) was accidentally introduced to North America and Europe, becoming a key pest of many important crops. In its native range, it is attacked by egg parasitoids, includingTrissolcus japonicus(Ashmead), considered to be the main species, andT. mitsukurii(Ashmead) (Hymenoptera, Scelionidae).Trissolcus japonicuswas detected in North America in 2014 and Europe in 2017, whileT. mitsukuriiwas detected in Australia in the early 20^thcentury and in Europe in 2016. Both species now appear established in the new areas. The present study used the MaxEnt algorithm to clarify the geographic dimension of the potential interaction betweenH. halysand these two parasitoid species, and to indicate where the release of one or the other parasitoid species is more likely to be effective. We found that the suitability for the two parasitoids overlaps theH. halysnative range completely. In invaded areas,T. japonicusshowed higher potential habitat suitability at the global scale, compared toT. mitsukurii, and also higher potential suitability at lower latitudes at continental and national scales. These results can substantially improve the effectiveness of biological control againstH. halys, by targeting the releases of parasitoids to the areas where each species is most likely to succeed.