Field releases of the exotic parasitoid Trissolcus japonicus (Hymenoptera: Scelionidae) and survey of native parasitoids attacking Halyomorpha halys (Hemiptera: Pentatomidae) in Michigan.

An adventive population of the exotic parasitoid wasp, Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae), discovered in Michigan in 2018, is a promising biological control agent of the invasive Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Following its discovery, field releases of Tr. japonicus were conducted over 2 yr in southern Michigan, to test how release size or release frequency impacts establishment. Sentinel eggs of H. halys and of the native Podisus maculiventris (Say) (Hemiptera: Pentatomidae) were used alongside yellow sticky cards to monitor parasitoids. In 2019 and 2020, 7,200 Tr. japonicus were released at 16 sites. Monitoring between 2019 and 2021 yielded only 49 individuals. The captures suggest reproductive activity and overwintering success in the field but do not allow for evaluation of best release methods. Parasitism by native parasitoids was below 7%, which is similar to other states and unlikely to provide sufficient control of H. halys. The placement of sentinel eggs or sticky traps either in the lower or middle canopy of trees did not influence parasitoid capture rates. Frozen and fresh H. halys sentinel eggs were attacked at the same rate, but more native parasitoids emerged from frozen eggs. We did not find signs of nontarget effects on P. maculiventris thus parasitism rates overall were very low. These results could indicate dispersal of Tr. japonicus from the release sites or slow population growth. The latter may be due to the relatively low densities of H. halys in Michigan or may stem from the small founding size of our laboratory colony.