Washoff of Ultra‐Low‐Volume‐Oil‐Applied Insecticides from Cotton Plants as a Function of Time between Application and Rainfall

Abstract Model development for predicting the movement of foliar‐applied pesticides depends on being able to characterize the dynamics of pesticide disappearance from plants, including washoff by rainfall. This study was conducted to determine the effect of elapsed time between spray application and initial rainfall on pesticide washoff from foliage. Methyl parathion [ 0, 0 ‐dimethyl, 0 ‐( p ‐nitrophenyl) phosphorothioate] and fenvalerate [(RS)‐α‐cyano‐3‐phenoxybenzyl (RS)‐2‐(4‐chlorophenyl)‐3‐methylbutyrate] were applied in oil by rotary‐atomizer controlled‐droplet applicators to mature cotton ( Gossypium hirsutum L.) plants. Simulated rainfall (51 mm in 1 h) was applied to the plants at different times after insecticide application to determine washoff characteristics for both compounds. Residues of both insecticides became increasingly resistant to washoff with increasing time interval between pesticide application and initial rainfall. The mean amounts of both insecticides washed from the plants (i) were related to the square of the mean insecticide loads on the plants and (ii) decreased exponentially with time.