Determining the effects of reduced leaf area caused by the grape leaf folder on Thompson Seedless grapevines

With the increased interest in reducing the amount of pesticides applied for insect control, this study was initiated to determine an economic threshold for the grape leaf folder, Desmia funeralis, on Thompson Seedless grapevines in the San Joaquin Valley. Various levels of the insect at 100, 250, 350 and 500 larvae per vine were placed on vines and their damage was compared to artificial defoliation at rates of 16%, 20%, 25%, and 50% used as a reference, measuring the crop yield and quality. The results showed that reduced leaf area by Desmia had no significant effect on total soluble solids, total acid, pH or crop weight attributable to the insect. The data indicate that even at 50% defoliation, control of this insect may not be necessary. Vine vigor and the extensive vegetative growth of the vines accounted for the failure of removal of half the leaves to produce any adverse effects. Time of defoliation in relation to anthesis is the critical factor of leaf removal when considering damage.