The brown marmorated stink bug, Halyomorpha halys, native to East Asia, is an invasive alien pest that arrived in Europe in the early 2000s and poses an imminent threat to a wide variety of crops. Adventive populations of the Asian egg parasitoid Trissolcus japonicus, the most promising agent for classical biological control of H. halys, have recently been detected in Italy and Switzerland. Its prospective fundamental host range in Europe was evaluated in behavioural no-choice tests, followed by large-arena choice tests presenting host plants with naturally laid egg masses of target and non-target hosts. Developmental suitability of European non-target host species for T. japonicus was demonstrated, via no-choice tests, by offspring emergence (successful parasitism) from eleven out of thirteen non-target species tested (85%). Whereas successful parasitism of most non-target species was significantly lower, acceptance of Arma custos, Palomena prasina, Pentatoma rufipes, and Rhaphigaster nebulosa was not significantly different from H. halys controls. When eggs of H. halys and non-target species were exposed in a semi-natural situation in large-arena choice tests, the degree of non-target parasitism was substantially reduced for three out of four tested species, whereas parasitism of Pa. prasina eggs was not. It remains unclear if there are behavioural barriers to parasitism that may exist and preclude excessive parasitism of potentially threatened species in the field, but field data from the invaded areas in Switzerland and Italy could contribute to a risk–benefit evaluation of releasing or re-locating adventive T. japonicus populations into other parts of Europe.

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.

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.