Azinphosmethyl and fenvalerate washoff from cotton plants as a function of time between application and initial rainfall

Understanding pesticide foliar washoff is important in pest management, in development of integrated management systems, and in environmental modeling. This study was conducted to determine the effect of elapsed time between spray application and initial rainfall on insecticide washoff from foliage. Azinphosmethyl and fenvalerate were applied to mature cotton plants as tank-mixed emulsifiable concentrates with a water carrier. DT₅₀s (50% disappearance time) for azinphosmethyl and fenvalerate were 3.9 and 237 h, respectively. Simulated rain (51 mm in 1 h) was applied to the cotton plants at times ranging from 2 to 146 h after insecticide application to determine washoff characteristics for both compounds. Residues of both insecticides became increasingly resistant to wash-off with increasing time interval between insecticide application and initial rainfall, e.g., about 95% of the plant Xload washed off 2 h after application, whereas at 146 h <50% washed off. The mean amounts of both insecticides washed from the plants decreased with time and were related to the mean plant loads. The results of this study show that foliar pesticide application should be delayed when rain is imminent.