Abstract: Studies on the seasonal occurrence and control of the spruce spider mite, Oligonychus ununguis(Jacobi), on chestnut trees were carried out during 1984-1986. The winter eggs hatched in the middle May. The mite population occurred from late May to middle August, sometimes to middle September. Generally from early June the population increased. When the number of mites exceeded 50 per leaf most leaves turned brown. Natural enemies including the chrysopids, Chrysopa spp. coccinellids, Stethorus punctilium Weise, and thrips, Scolothrips takahashi priesner have been shown to be effective in controlling the injuries by mites, but the results were different. Excellent control effects by a single application of dimethoate dilution to the basal trunk of the chestnut tree without causing significant injury to the plant were achieved. The best period of treatment was found to be the hatching stage of the winter eggs. For trees treated with this insecticide, the fruit yields increased about 13% in comparing with the untreated trees. No residues of dimethoate were detected in the Chestnut fruit.

Key words: Spruce spider mite, Chestnut tree, Seasonal occurrence