1995 Volume 41 Pages 110-113
Parasitism of the chestnut gall wasp, Dryocosmus kuriphilus, by Torymus (Syntomaspis) sinensis in an experimental chestnut orchard in Ohzu, Kumamoto Pref., where the parasitoid was released in 1982, had been very low, less than 1%, in the six years following its release. However, the parasitoid population has increased since 1989; 4. 4 females of the parasitoid emerged per 100 galls of the host cynipid, and the parasitism rate reached 17% in 1993. The density of the host cynipid has not yet decreased but it has fluctuated at a high level. Two factors have already been suggested for the delay in the increment of the introduced parasitoid; a high mortality associated with the activity of the native facultative hyperparasitoids, and the low female sex ratio during several years after establishment. The sex ratio, however, has increased since 1991 and has maintained its normal level, and it has been followed by a rapid increase in the rate of parasitism and the number of females emerging per 100 host cynipid galls in the subsequent years.
