Direct Effects of Wide-Area Malathion Application on Fish

The primary objective of this investigation was to determine if fish mortality would result directly from a wide-area application of 561 fluid grams, technical grade malathion per hectare. This cooperative study with the Department of Entomology, University of Nebraska, was conducted in Dawson County, near Lexington, Nebraska. Field tests were conducted on two adjoining 9.94-square-kilometer areas. The area receiving spray served as the experimental area; the other area served as a control. Mortality rates were determined both for captive bluegill (Lepomis macrochirus) and for fish populations native to Buffalo Creek. Data obtained throughout the course of this study indicated that malathion, when aerially applied at an ultra low volume rate over a wide area, did not result in direct mortality of fishes. INTRODUCTION The objective of this investigation was to determine if fish mortality would result directly from an application of 561 fluid grams, technical grade malathion, per hectare. The area treated measured 9.94 km2. In recent years a trend has developed towards greater use of nonpersistent insecticides with relatively low mammalian toxicity and a narrower spectrum of activity. These chemicals, the primary group of which are the organo-phosphates, have mitigated the problems of persistent pesticide residues being incorporated into food chains. However, the control of injurious insects has become more complex. With individual farmers applying insecticides at different times during the growing season, repeated applications may often be necessary to combat reinvasion by insects from adjacent untreated fields. To eliminate the necessity of repeated applications, research entomologists are studying the feasibility of treating large areas with the relatively nonpersistent organo-phosphates. However, the effect of such a wide-area treatment on the total environment is unknown. Malathion, O, O-dimethyl dithiophosphate of diethyl mercato-succinate, has been one of the most widely used insecticides (6 to 12 million pounds a year) in the United States (Kennedy and Walsh, 1970, p. 3)