Torymus sinensis against the invasive chestnut gall wasp: Evaluating the physiological host range and hybridization risks of a classical biological control agent

Background: A common strategy to limit the negative impact of biological invasions is biological control through the release of specialized alien natural enemies. However, biological control plans are not without risks, which include parasitism of native hosts and hybridization with related native species, particularly those that are potential natural enemies of the invasive species. Here, we evaluate these potential risks resulting from the introduction of the parasitoid wasp Torymus sinensis (Hymenoptera: Torymidae) in Europe to control the invasive Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae). Results: We found that, under laboratory conditions, the physiological host range of T. sinensis includes several native non-target species of Cynipidae, with oviposition observed on the galls of 8 of the 11 species tested. However, physiological host range of T. sinensis appears to be limited under field conditions, as we observed only one parasitized gall of Andricus curvator in the field. Regarding hybridization, inter-species mating between T. sinensis and its phylogenetically closest native Torymus species was not observed in the laboratory. Moreover, discordance between nuclear (ITS2) and mitochondrial (coxI) data does not support the presence of genetic introgression, suggesting that hybridization between T. sinensis and native Torymus species does not occur. In addition, we cite and discuss the unexpected presence of one individual of the related alien species Torymus beneficus in Spain. Conclusion: Our results suggest that T. sinensis may negatively impact several non-target species, highlighting the need for careful monitoring of the extent of such undesired behaviour and its effects on the native fauna.