Effects of Concentration and Application Method on Decay and Residual Activity of Foliar Chlorpyrifos

The decay of chlorpyrifos applied as emulsifiable concentrate to foliage of commercially grown oranges, greenhouse tomato, and outdoor potted cotton was measured under different application conditions. Foliar residue decay was found to be dependent on concentration of active ingredient, amount, and method by which chlorpyrifos was applied to the crop; increase in tank or surface concentrations resulted in a marked decrease in initial rate of loss of the residue. Depending on concentration and application technique, time required for the initial deposit to lose half its quantity ranged from 15 to 139 h on greenhouse tomato leaves, from 1.4 to 96 h on orange leaves, and from 5.5 to 57 h on cotton leaves. The effect of the concentration on decay of foliar chlorpyrifos was observed for both low-volume and high-volume spray conditions; relative changes in decay rate were larger for low-volume residues than for high-volume residues in most cases. Bioassay of a constant dose of chlorpyrifos applied to cotton leaves at different concentrations and at various drop densities revealed a longer residual activity of the more concentrated treatment against both Spodoptera littoralis (Boisduval) and Bemisia tabaci (Gennadius).