Hybridization between introduced Torymus sinensis (Hymenoptera: Torymidae) and indigenous T. beneficus (late-spring strain), parasitoids of the Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae)

Hybridization between introduced biological control agents and native species is a nontarget effect of biological control. However, the genetic impacts related to introduced insects, especially from the viewpoint of post-release evaluation, remain largely unexplored. Torymus sinensis (Hymenoptera: Torymidae) is a parasitoid wasp introduced from China into Japan to control the invasive Asian chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae). After T. sinensis was first released in 1982, the damage caused by D. kuriphilus fell greatly. Previous studies showed that hybridization between T. sinensis and indigenous Torymus beneficus (the early-spring strain) was rare in the field, but individuals morphologically intermediate between T. sinensis and T. beneficus then appeared in the field. Here, we examined the frequency of hybridization between T. sinensis and the late-spring strain of T. beneficus by using the internal transcribed spacer 1 (ITS1) region of nuclear rDNA. Increasing numbers of F1 hybrids were detected every year from 1993 to 1996, reaching 22%. It seems that T. sinensis can hybridize more easily with the late-spring strain of T. beneficus. Morphological indices of the F1 hybrid type ranged between indices of the late-spring strain of T. beneficus and T. sinensis, with considerable overlap. Simultaneous with the occurrence of the F1 hybrids, the indigenous late-spring strain of T. beneficus was rapidly displaced by T. sinensis.