Abstract
Experiments were conducted in 1984 and 1985 in a chestnut orchard in Kumamoto Prefecture where a parasitoid Torymus (Svntomaspis) sinensis introduced from China had been released in 1982, in order to test the effect of two different treatments of pruned twigs on the biological control of the chestnut gall wasp by the imported and native parasitoids. The experimental orchard was divided into two plots, "un-removed" and "removed". All the twigs pruned in the orchard in winter were gathered at the base of each tree in the "un-removed" plot and left there until the parasitoids emerged from withered galls on twigs. The effect of the treatment on the parasitoid effectiveness was not conclusive in the experiment in the first year. In the second year, however, a significantly higher rate of parasitization by a native parasitoid Torvmus (S.) benefcus and the imported T. (S.) sinensis in the "un-removed" plot was achieved than in the "removed" plot. The mortality of host larvae by host-feeding was also significantly higher in the former plot than in the latter. As a result, the mean number of host larvae surviving in a gall was significantly lower in the "un-removed" plot than in the "removed" one. For utilizing native and imported parasitoids more effectively as a biological control method it is suggested that twigs with withered galls pruned in winter should be gathered near the trees and left there until after the overwintering parasitoids emerge from the galls.
