Fenvalerate wash-off from cotton plants by rainfall

Information on rainfall wash‐off of foliar‐applied pesticides is needed in developing mathematical models to predict the transport of pesticides to soil and in run‐off from croplands. Fenvalerate [(RS)‐α‐cyano‐3‐phen‐oxybenzyl (RS)‐2‐(4‐chlorophenyl)‐3‐methylbutyrate] is used extensively for cotton (Gossypium hirsutum L.) insect control in the southern United States during July, August and September, which are months of high‐intensity thunderstorms. Fenvalerate was washed from mature cotton plants by the use of a multiple‐intensity rainfall simulator. Fenvalerate concentration in wash‐off from plants was independent of rain intensity. When 25 mm of rain was applied at 7, 13, 26 and 51 mm h‐1 just2 h after fenvalerate was applied at 0.224 kg ha‐1, 38 (±10)% of the fenvalerate load on the plants was washed off. Fenvalerate concentrations in storm wash‐off decreased with wash‐off volume as hyperbolic functions. Rainfall amount affected the wash‐off of fenvalerate from cotton to a greater extent than did rainfall intensity. This information greatly simplifies modelling the movement of fenvalerate from plant canopy to soil during natural storms.