Biocontrol implications of multiparasitism by Trissolcus mitsukurii and Trissolcus japonicus on the invasive brown marmorated stink bug

The egg parasitoids Trissolcus japonicus (Ashmead) and Trissolcus mitsukurii (Ashmead) (Hymenoptera: Scelionidae) are the most effective biocontrol agents of the invasive agricultural pest Halyomorpha halys (Stal) (Heteroptera: Pentatomidae) in its native range (east Asia). Trissolcus japonicus and T. mitsukurii are sympatric in the native areas. In northern Italy, where H. halys is a major pest of fruit orchards, adventive populations of both species are spreading, and T. japonicus is artificially released under a classical biocontrol program against H. halys. This laboratory study aimed to assess the outcome of competition when both species share the same resource and possible implications for the biological control of the invasive stink bug. Egg masses of H. halys were offered to each parasitoid after previous parasitization by the other species. Parasitoid behaviour, number of ovipositions, and successfully developed parasitoids were recorded. Additionally, contest behaviour was assessed when both species were released simultaneously on the same egg mass. Results showed that both T. japonicus and T. mitsukurii were able to parasitize an egg mass already parasitized by the other species. Competition occurred within the host eggs and each species outperformed the other when it was the first to oviposit. Importantly, the overall contribution to H. halys mortality was not affected by the interaction between parasitoids, as non-parasitized eggs were 4-6% in the absence of competition and <8% in its presence, respectively. When simultaneously released on the egg mass, T. mitsukurii was more aggressive, engaging in chase-off events in 71% of cases compared to 50% of T. japonicus.