DDT Deposits on Grapes as Affected by Growth and Weathering1

Abstract

Disappearance of surface deposits of DDT on grapes was determined for sprays applied for control of the grape berry moth, Paralobesia viteana (Clem.) . Methods of sampling grapes for these determinations are described. The results of analyses from 7 years are reported for samples collected in the period beginning immediately after application of final spray and extending into the harvest period. A deposit of less than 7 p.p.m. was found at harvest when only two first-brood sprays are applied. While residues at harvest were below the tolerance for three sprays when no spreader-sticker was used, such a schedule gives poor insect control. With a spreader-sticker in 6 out of 7 years, thc deposit of DDT was 17.0 to 22.8 p.p.m. immediately after the final spray application and 6.1 to 12 p.p.m. at the beginning of harvest. The contribution of each application to the deposit from a three-spray schedule was determined. Losses in amounts of deposits are given separately for growth and weathering. The former caused decreases averaging 27.7% in 4 years and weathering 37.2%. The influence of adhesive and of fungicides on the initial deposits and subsequent losses arc reported.