Torymus sinensis is a parasitoid wasp that was introduced from China to Japan to control the invasive chestnut gall wasp Dryocosmus kuriphilus. Interaction between T. sinensis and the indigenous parasitoid T. beneficus has been of interest since T. sinensis was first released in chestnut orchards, as hybridization between them might impede the success of biological control by the introduced parasitoid. Such apprehensions disappeared with the drastic decrease in damage caused by D. kuriphilus. However, the emergence of morphologically intermediate individuals between them after the introduction of the parasitoid has triggered renewed interest in hybridization, specifically on the non-target effects of biological control. In this review, the interaction between both species, specifically hybridization, species composition and species displacement, is surveyed. Molecular markers are used, not only because T. sinensis and T. beneficus are similar morphologically, but also because the latter has two emergence strains (early- and late-spring). Surveys show that 1) T. sinensis has displaced both emergence strains of T. beneficus; 2) hybridization between T. sinensis and early-spring T. beneficus occurs at a low frequency (≤ 1%); but that 3), hybrid F₁s between T. sinensis and late-spring T. beneficus occur at a much higher frequency (≤ 20%). A study of phylogenetic relationships indicates that the late-spring T. beneficus is closer to the different species T. sinensis than the early-spring T. beneficus. This can partly explain why T. sinensis hybridizes more readily with the late-spring T. beneficus than with the early-spring T. beneficus.