1. USDA−Agricultural Research Service, Cropping Systems and Water Quality Research Unit, 269 Agricultural Engineering Building, University of Missouri, Columbia, Missouri 65211, U.S. Geological Survey, Water Resources Division, 4821 Quail Crest Place, Lawrence, Kansas 66049, and Department of Entomology, Pesticide Toxicology Laboratory, 112 Insectary, Iowa State University, Ames, Iowa 50011