EFFECTS OF RANGELAND AERIAL APPLICATION OF SEVIN-4-OIL(R) ON FISH AND AQUATIC INVERTEBRATE DRIFT IN THE LITTLE MISSOURI RIVER, NORTH-DAKOTA

Carbaryl (1-naphthyl N-methylcarbamate) is a synthetic organic insecticide that is used to control grasshopper infestations on rangeland and cropland throughout the western United States. This research investigated the effects of two rangeland aerial applications of a formulation of carbaryl known as Sevin-4-Oil(R) on fish and aquatic invertebrates in the Little Missouri River, North Dakota. Pesticide drift was deposited in the Little Missouri River despite a 152 m no-spray buffer zone. Maximum mean carbaryl concentrations during a drought year (1991) and a non-drought year (1993) were 85.1 and 12.6 mu g/L, respectively, and occurred within 2 h of application. Aquatic invertebrates were affected by pesticide application during 1991. That year, invertebrate drift at an impact site on day of pesticide application was more variable than at a reference site. Variability was attributed to increased drift of Ephemeroptera within three hours of pesticide application. Simultaneous monitoring of brain acetylcholinesterase activity of flathead chub (Platygobio gracilis) showed that fish were not affected by pesticide application. Similar studies conducted during 1993 gave no indication of adverse effects. Because this was an unreplicated field experiment, arguments for or against causation are presented using rigorous standards of proof based on Koch's postulates, and concordance of laboratory and field data. The results suggest that although pesticide application probably caused an increase in invertebrate drift during 1991, the observed response was not biologically significant because invertebrates were affected to a greater extent by natural events, and a relatively small portion of the Little Missouri River was impacted.