Insecticide Washoff from Cotton Plants as a Function of Time Between Application and Rainfall
ABSTRACT METHYL parathion (MP), toxaphene (TOX), and fenvalerate (FEN), as emulsifiable concentrates, were tank mixed with water and applied at 1.15 + 2.30 + 0.11 kg active ingredient/ha to cotton plants (triplicate plots) by ground equipment. Plant sampling showed that insecticide loads on the plants decreased hyperbolically with time. Simulated rainfall (51 mm) was applied at 53 mm/h to a new test plot at 2, 6, 29, 50, 98, and 146 h after insecticide application to determine the fractions of the insecticides washed from the plants. The fraction of MP washed from the plants decreased exponentially with time after application and linearly with load on the plants. In contrast to MP, the fractions of TOX and FEN washed from the plants were relatively constant at 10 and 7%, respectively, regardless of time after application and loads on the plants. About 50% of the MP, TOX, and FEN washed from the plants by any one storm was washed off by only 7 to 8 mm of rainfall, regardless of when rain occurred. This information will improve the accuracy of predictions made in mathematical simulations of foliar washoff and routing and may aid in developing guidelines for respraying for pest control following natural rainfall or overhead irrigation..