PERMETHRIN WASHOFF FROM COTTON PLANTS BY SIMULATED RAINFALL

Developing models for predicting amounts of pesticide transported in runoff and suspended sediment from agricultural land of aquatic habitats has been hampered for foliarly-applied pesticides by a lack of knowledge about the processes of pesticide washoff from plants by rainfall. A multiple-intensity rainfall simulator was used to determine the effects of rain intensity and amount on permethrin [(3-phenoxyphenyl) methyl (.+-.)-cis, trans-3-(2,2-dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate] concentrations and amounts washed from mature cotton (G. hirsutum L.) plants. Permethrin concentration in plant washoff was independent of rain intensity when 25 mm of simulated rain was applied at 6.6, 13.0, 25.7, and 51.3 mm h-1 2 h after permethrin was applied at 0.224 kg ha-1. Concentrations were less when the same amount of simulated rain was applied at 106.4 mm h-1. Permethrin concentrations in plant washoff decreased rapidly during the early phases of washoff. About 35% of the permethrin load on the plants 2 h after application was washed off by 25 mm of rain; an additional 76 mm of rain removed only 11% more of the permethrin from the plants. Rainfall amount had greater influence than rainfall intensity on the amount of permethrin washed from the cotton plants. This information greatly simplifies modeling the movement of permethrin from the plant canopy to soil during natural storms when intensities vary greatly within storms and from storm to storm.