Do Torymus sinensis (Hymenoptera: Torymidae) and agroforestry system affect native parasitoids associated with the Asian chestnut gall wasp?

The Asian chestnut gall wasp, Dryocosmus kuriphilus Yasumatsu, is one of the most serious pests of Castanea spp. worldwide. Since local parasitoids did not effectively control this exotic pest, the Chinese parasitoid Torymus sinensis Kamijo was released into Japan and Europe resulting in a successful control of the invader. A survey of native parasitoids usually infesting oak gall wasps but associated with D. kuriphilus was carried out, sampling chestnut galls in Italy in 2013. The effect of predictive variables (years since T. sinensis release, oak occurrence, type of chestnut formation, latitude) on composition and density of native parasitoid populations was modelled with Generalized Linear Models. In total, 4410 chalcid parasitoids were recorded, and the most abundant species were T. flavipes Walker, Eupelmus spp., and Bootanomyia (= Megastigmus) dorsalis (F.). Introduction of T. sinensis and oak occurrence influenced native parasitoid richness, conversely to the latitude of the study area. In particular, every additional year of presence of T. sinensis was proved to determine the loss of about 14% of native species, and 32% of their population density. A positive correlation between the oak density occurring in the infested chestnut forests and the richness and abundance of native parasitoid species was recorded as well, highlighting how generalist parasitoids may switch hosts easily. A positive effect of increasing densities of oak trees was recorded on most of the taxa, except for T. geranii (Walker), Eupelmus spp., and Eurytoma pistaciae Rondani. Our results provide insight that both T. sinensis and oak trees affect native parasitoids, shedding light on the possible implications related to the release of exotic biological control agents on the recruitment and accumulation of native natural enemies.