Abstract
To predict the possible effect of native parasitoids on the effectiveness of an introduced parasitoid, native parasitoid complexes of the chestnut gall wasp, Dryocosmus kuriphilus were investigated in Miyagi (38°10′N) and Okayama (34°50′N) Prefectures where T. (S.) sinensis has not yet occurred. The results were compared to those from a chestnut orchard in Kumamoto Prefecture (31°50′N) where the parasitoid has already been released and become established. The parasitoid complex structure and size differed with locality. The number of native primary parasitoid species tended to increase toward lower latitudes. The dominant parasitoid was T. (S.) beneficus in both localities, but the degree of dominancy differed with locality. The parasitism rate was not different between localities, nor did it correlate with the number of parasitoid species at each sampling site. T. (S.) beneficus suffered high mortality during its mature larval stage. However, the mortality showed no significant relationship with initial density of mature larvae in Miyagi, whereas it was negatively correlated in Okayama. Hyperparasitism by facultative hyperparasitoids suffered an inverse densitydependent mortality to T. (S.) beneficus populations in Okayama, whereas in Miyagi the level was low and had little effect. Thus, it was predicted that populations of T. (S.) sinensis, which is closely related to beneficus, should take a long time to increase when introduced into Okayama because of the inverse density-dependent effect of facultative hyperparasitoids, but not in Miyagi.
