A Delayed Increase in the Population of an Imported Parasitoid, Torymus (Syntomaspis) sinensis (Hymenoptera: Torymidae) in Kumamoto, Southwestern Japan

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The Torymus (Syntomaspis) sinensis population established in an experimental chestnut orchard in Kumamoto, southwestern Japan, where the parasitoid was released in 1982, had not increased in six years following its release. However, it has increased since 1989 ; more than seven adults of the parasitoid emerged per 100 galls of the host cynipid, Dryocosmus kuriphilus, and the parasitism rate reached a level of ca. 17% in 1993. Although total parasitism by all parasitoids including native species tended to increase to ca. 41% in 1993, the tendency might not be caused by Torymus spp. but by native parasitoids other than Torymus (Syntomaspis) beneficus. The density of the host cynipid has not yet decreased but fluctuates at a high level. Two factors might play an important role in the delay in the increment of the introduced parasitoid ; the low female sex ratio during several years after establishment, and high mortality associated with the activity of native facultative hyperparasitoids.
1995-02-25