Novel insight in the life cycle of Torymus sinensis, biocontrol agent of the chestnut gall wasp

Torymus sinensis Kamijo (Hymenoptera: Torymidae) is a biological control agent of the chestnut gall wasp Dryocosmus kuriphilus Yasumatsu (Hymenoptera: Cynipidae). It is reported in the literature as univoltine, but in NW Italy it exhibits a prolonged diapause mainly as late instar larva. Diapause is extended for 12 months, and adults emerge in April as usual, showing a two-year life cycle. Second year emergence individuals are able to mate, and the presence of mature eggs was confirmed in females which parasitised fresh chestnut galls, showing the same parasitism behaviour as first year emergence individuals. Both sexes of second year emergence individuals proved to be smaller than the univoltine ones according to ovipositor sheath length, pronotum width, and hind tibia length. Proving evidence of the extended diapause plays an important role for the establishment of T. sinensis especially in the first years after its release. Future studies are needed to clarify the factors which trigger off this response.