TOXICITY OF SEDIMENTS CONTAINING ATRAZINE AND CARBOFURAN TO LARVAE OFTHE MIDGE CHIRONOMUS-TENTANS

Field and laboratory tests were conducted to determine the 10-d toxici ty of atrazine and carbofuran to larvae of the midge Chironomus tentan s. The pesticides were first spiked onto a clean control sediment, and the midges were exposed to atrazine, carbofuran, or mixtures of both compounds. Laboratory toxicity tests with carbofuran yielded LC50s of 20.5 mug/kg bound to sediment and 11.8 mug/L dissolved in interstitial water. Slight but statistically significant toxicity was detected at the highest tested concentration of atrazine (9,000 mug/kg, bound). To xic interaction between the two pesticides was assessed using all poss ible combinations of four concentrations of both pesticides spiked ont o clean sediments. Analysis of variance was used to determine if great er than or less than additive effects occurred when both pesticides we re present. No toxic interaction between atrazine and carbofuran was d etected. Based on laboratory findings, it was predicted that edge-of-f ield run-off could be toxic to midge larvae if concentrations of carbo furan exceeded 5 mug/kg. To test these predictions, actual edge-of-fie ld runoff was collected from a cornfield receiving atrazine and carbof uran at recommended rates and the suspended sediments used in a toxici ty test. Edge-of-field runoff containing carbofuran at 186 mug/kg was acutely toxic to midge larvae.