| Summary: | <i>Trissolcus kozlovi</i> (Hymenoptera: Scelionidae) emerged from field-laid eggs of <i>Halyomorpha halys</i> (Hemiptera: Pentatomidae) in North Italy, and it emerged in significantly higher numbers from fresh <i>H. halys</i> eggs compared to other native scelionids. Since few data on <i>T. kozlovi</i> are available, its host-specificity and some biological traits were investigated in laboratory tests, and its impact after augmentative releases was evaluated in two hazelnut orchards. Among the 12 tested bug species (Hemiptera: Pentatomidae, Scutelleridae), only <i>Nezara viridula</i> was an unsuitable host, while the highest offspring proportions were obtained from <i>Arma custos</i>, <i>Pentatoma rufipes</i>, and <i>Peribalus strictus</i>, followed by <i>Acrosternum heegeri</i> and <i>Palomena prasina</i>. Furthermore, when reared on <i>P. strictus</i>, <i>T. kozlovi</i> showed a high longevity as well as a high adaptation to <i>H. halys</i> eggs. In both hazelnut orchards, <i>T. kozlovi</i> emerged from <i>H. halys</i> eggs after field releases, but it was not found in the next two years. The physiological host range of <i>T. kozlovi</i> was quite similar to that of <i>T. japonicus</i>, and probably <i>T. kozlovi</i> has just begun to attack <i>H. halys</i> as a new host. This aspect needs to be further investigated, as well as its favorable environmental conditions, its distribution and also its possible interaction with <i>T. japonicus</i>, currently present in Italy.
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