Leslie L. McDowell

Methyl parathion and EPN washoff from cotton plants by simulated rainfall

Modeling the amount of pesticides that will be transported in runoff and sediment from agricultural lands to aquatic environments requires ability to predict amounts washed from plant canopy to soil by rainfall. Methyl parathion (MP) [ 0,O-dimethyl 0-(p-nitrophenyl) phosphorothioate] and EPN [ 0-ethyl 0-(p-nitrophenyl) phenylphosphonothioate] are used extensively for cotton (Gossypium hirsutum L.) insect control in the South during July, August, and September-months of high intensity thunderstorms. The MP and EPN washed from mature cotton were measured by the use of a multipleintensity rainfall simulator. Concentrations and amounts of MP and EPN washed from plants were independent of rain intensity when 24 mm of simulated rain was applied at 11,26,52, and 111 mm h-l just 2 h after MP and EPN were applied at 0.64 + 0.64 kg hab1. About 88 f 32% of the MP load and 62 f 18% of the EPN load on the plants were washed off. MP and EPN concentrations in storm runoff decreased with runoff volume as hyperbolic functions. Only rainfall amount affected MP and EPN washoff. This information greatly simplifies modeling the movement of MP and EPN from plant canopy to soil during natural storms when intensities vary greatly within storms and from storm to storm.

Nitrogen and phosphorus yields in run-off from silty soils in the Mississippi Delta, U.S.A.

Nutrients and sediment concentration in surface run-off from an 18.7-ha watershed, planted to continuous cotton (Gossypium hirsutum L.), were measured from March 1973 through February 1979 to determine: (a) nutrient and sediment yields; (b) mode of nutrient transport; (c) nutrient concentration frequency distributions; (d) nitrogen (N) and phosphorus (P) enrichment in sediments. The watershed, consisting of silt loam (72%) and silty clay (28%) soils, had been land-formed to a mean slope of 0.2%. The 6-year average annual rainfall, run-off and sediment yields were 1335 mm and 20.8 t ha−1, respectively. Nitrogen fertilizer was applied at the rate of 171 kg ha−1 year−1. P fertilizer was not applied because cotton growing on these soils does not respond to P fertilization. The 6-year average soluble N (ammonium N plus nitrate N) and P yields were 7.7 and 1.6 kg ha−1 year−1, respectively, compared with sediment N and P yields of 34.6 and 19.6 kg ha−1 year−1. Thus, about 18 and 7% of the total (sum of solution and sediment) N and P yields were transported in solution; the remainder was transported by the sediments. Sediment and nutrient yields were greater than expected from these flatlands. Without P fertilizer additions, soluble P concentrations in 77% of the run-off exceeded the 0.1 mg l−1 criterion proposed by the U.S. Environmental Protection Agency. Improved run-off and erosion control practices are needed to significantly reduce sediment and chemical yields.

Carbaryl washoff from soybean plants

Both the efficacy and fate of most foliar-applied pesticides may be affected by weather variables, especially rain. A multiple-intensity rainfall simulator was used to determine the effects of rainfall intensity and amount on concentrations of carbaryl (Sevin® XLS Plus) washed from soybean plants. Two hours after carbaryl was applied at 1.12 kg/ha, 25 mm of rain was applied at intensities of 13.0, 27.4, 53.8, or 105.1 mm/h. About 67% of the carbaryl on the plants was washed off by 25 mm of rain. Rainfall intensity affected carbaryl concentrations in washoff; higher concentrations occurred at lower intensities. Even though the experimental conditions were designed for “worst-case” conditions, washoff patterns suggested improved carbaryl rainfastness when compared to carbaryl (formulated as a wettable powder) washoff from cotton plants in earlier studies. Rainfall amount had a greater effect on carbaryl concentrations in washoff than rainfall intensity.

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.