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

The influence of elapsed time between spray treatment and initial rainfall on insecticide washoff from cotton plant foliage was explored. The 50% disappearance time for azinphosmethyl and fenvalerate applied to mature plants as tank-mixed emulsifiable concentrates was 3.9 and 237 h, respectively. Plants were subjected to simulated rainfall of 51 mm in 1 h at 2-146 h after chemical application. Residues of both compounds were increasingly resistant to washoff with increasing time interval between application and initial rainfall. Foliar pesticide application should be postponed when rain is imminent.