A DELAYED INCREASE IN THE POPULATION OF AN IMPORTED PARASITOID, TORYMUS- (SYNTOMASPIS)-SINENSIS (HYMENOPTERA, TORYMIDAE) IN KUMAMOTO, SOUTHWESTERN JAPAN

The Torymus (Syntomaspis) sinensis population established in an experi mental chestnut orchard in Kumamoto, southwestern Japan, where the par asitoid was released in 1982, had not increased in six years following its release. However, it has increased since 1989; more than seven ad ults of the parasitoid emerged per 100 galls of the host cynipid, Dryo cosomus kuriphilus, and the parasitism rate reached a level of ca. 17% in 1993. Although total parasitism by all parasitoids including nativ e species tended to increase to ca. 41% in 1993, the tendency might no t be caused by Torymus spp. but by native parasitoids other than Torym us (Syntomaspis) beneficus. The density of the host cynipid has not ye t decreased but fluctuates at a high level. Two factors might play an important role in the delay in the increment of the introduced parasit oid; the low female sex ratio during several years after establishment , and high mortality associated with the activity of native facultativ e hyperparasitoids.