Displacement of Torymus beneficus (Hymenoptera: Torymidae) by T. sinensis, an indigenous and introduced parasitoid of the chestnut gall wasp, Dryocosmus kuriphilus (Hymenoptera: Cynipidae), in Japanese chestnut fields: Possible involvement in hybridizatio

Although there is empirical knowledge to suggest the displacement of Torymus beneficus (Hymenoptera: Torymidae) by Torymus sinensis, an indigenous and introduced parasitoid of the chestnut gall wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae), respectively, in Japanese chestnut fields, the underlying mechanisms are unclear. In this study, the displacement of the early-spring strain of T. beneficus by T. sinensis was surveyed in a chestnut field for nine successive years (n = 418), using two molecular markers, the internal transcribed spacer 2 of nuclear rDNA (ITS2) and the mitochondrial cytochrome oxidase subunit I (COI). We also investigated whether or not hybridization between the parasitoids was involved as an important factor in the displacement. All individuals from 1993 to 1995 were of the early-spring strain of T. beneficus type for both ITS2 and COI. After 1996, individuals with the same types decreased and ones of the T. sinensis type for both ITS2 and COI increased. On the other hand, there was only one individual that had T. sinensis type for ITS2 and the early-spring strain of T. beneficus type for COI, suggesting that individuals are descendants of the F1 hybrid between the early-spring strain of T. beneficus females and T. sinensis males. These results indicate that hybridization between them was not closely related to displacement in the field. We have also found hybridization between T. sinensis and another type of T. beneficus: the late-spring strain; this is the first report to show their hybridization in chestnut fields.