Factors influencing short term parasitoid establishment and efficacy for the biological control of Halyomorpha halys with the samurai wasp Trissolcus japonicus

BACKGROUNDClassical biological control has been identified as the most promising approach to limit the impact of theinvasive pest species Halyomorpha halys (Heteroptera: Pentatomidae). This study investigated the parasitismrate at sites where the biocontrol agent Trissolcus japonicus (Hymenoptera: Scelionidae) was released andwhere its unintentional introduction took place, in the Trentino-South Tyrol region. The effect of land usecomposition was studied to understand which factors favor the establishment of hosts and parasitoids,including native and exotic species.RESULTSThe released T. japonicus were detected a year after the start of the program, with a significant parasitoidimpact and discovery, compared to control sites. Trissolcus japonicus was the most abundant H. halysparasitoid, Trissolcus mitsukurii and Anastatus bifasciatus were recorded as well. The efficacy of T. mitsukuriiwas lower in sites where T. japonicus was successfully established, suggesting a possible competitiveinteraction. Parasitism level by T. japonicus at the release sites was 12.5% in 2020 and 16.4% in 2021. Thecombined effect of predation and parasitization increased H. halys mortality up to 50% at the release sites.Landscape composition analysis showed that both H. halys and T. japonicus were more likely found at siteswith lower altitude and with permanent crops, while other hosts and parasitoids preferred differentconditions.CONCLUSIONTrissolcus japonicus showed a promising impact on H. halys, at release and adventive sites, with minor nontargeteffects, mediated by landscape heterogeneity. The prevalence of T. japonicus in landscapes withpermanent crops could support IPM in the future.