Toxicity of sediments containing atrazine and carbofuran to larvae of the midgeChironomus tentans

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Abstract
Field and laboratory tests were conducted to determine the 10‐d toxicity of atrazine and carbofuran to larvae of the midgeChironomus tentans.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 μg/kg bound to sediment and 11.8 μg/L dissolved in interstitial water. Slight but statistically significant toxicity was detected at the highest tested concentration of atrazine (9,000 μg/kg, bound). Toxic interaction between the two pesticides was assessed using all possible combinations of four concentrations of both pesticides spiked onto clean sediments. Analysis of variance was used to determine if greater than or less than additive effects occurred when both pesticides were present. No toxic interaction between atrazine and carbofuran was detected. Based on laboratory findings, it was predicted that edge‐of‐field runoff could be toxic to midge larvae if concentrations of carbofuran exceeded 5 μg/kg. To test these predictions, actual edge‐of‐field runoff was collected from a cornfield receiving atrazine and carbofuran at recommended rates and the suspended sediments used in a toxicity test. Edge‐of‐field runoff containing carbofuran at 186 μg/kg was acutely toxic to midge larvae.