Semi-natural habitats promote biological control of Halyomorpha halys (Stål) by the egg parasitoid Trissolcus mitsukurii (Ashmead)

The invasive alien species Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) is a major pest of kiwifruit and other fruit crops. Adventive populations of the Asian egg parasitoid Trissolcus mitsukurii (Ashmead) (Hymenoptera: Scelionidae) were found in the north of Italy in 2016. Halyomorpha halys abundance was monitored in 14 kiwifruit orchards located in north-eastern Italy, and H. halys egg masses were sampled within kiwifruit orchards to assess the impact of egg parasitoids. Both H. halys populations and parasitoid impacts were evaluated considering the influence of ecological structures, and in particular the distance from semi-natural habitats constituted by riparian vegetation buffers and the presence of hedgerows surrounding the orchard. Halyomorpha halys populations fluctuated during the two seasons surveyed, and adults were more abundant during the first year of the study. In addition, more adults were detected in kiwifruit orchards located close to the riparian vegetation and decreased along with the distance from this habitat. Trissolcus mitsukurii had higher parasitism rates on H. halys eggs than the native Anastatus bifasciatus (Geoffroy) (Hymenoptera: Eupelmidae). No effect of hedgerows was observed on parasitoid impacts in orchards, while a relationship was detected with distance from riparian vegetation: a high T. mitsukurii parasitism rate was observed in kiwifruit orchards close to riparian vegetation buffers. Our findings demonstrate that semi-natural habitats constituted by large patches of unmanaged vegetation promote biological control of H. halys by the introduced egg parasitoids.