DDT residues in crop and soil resulting from application to cowpea Vigna unguiculata (L.) walp. in the sub-humid tropics

Abstract

DDT emulsion was applied by knapsack sprayer to cowpea Vigna unguiculata (L.) at a rate of 10kg a.i. ha −1 in each of two growing seasons per year for a four-year period in Ibadan, Nigeria. Partition of spray deposits between plant and soil was assessed at 1:3 respectively by methods involving crop cover estimation and dye recovery. DDT residues in soil and crop were determined by GLC analysis at the end of each growing season and showed disappearance of at least 60% of the applied pesticide. Field measurements were supplemented by studies on air entrainment and volatilisation rates from crop and soil. Accumulation of DDT in soil and its redistribution by ploughing were monitored. Residue levels reached a maximum of 1·7 μg g −1 total DDT after three spraying seasons and declined thereafter, despite repeated applications. In plots treated with a total of 70 kg a.i. ha −1 over the four years, the final residue in soil represented only 2–3% of the amount applied.