A low concentration of atrazine does not influence the acute toxicity of the insecticide terbufos or its breakdown products to Chironomus tepperi.

The acute toxicities of the insecticide terbufos and its major breakdown products individually, as binary mixtures, and in combination with the co-applied herbicide atrazine were evaluated using final instar larvae of the midge Chironomus tepperi. Terbufos, terbufos sulfoxide and terbufos sulfone were highly toxic to C. tepperi with mean 96-h EC50 values of 2.13, 3.64 and 2.59 μg/l, respectively. No interaction was observed between atrazine (25 μg/l) and terbufos or its breakdown products while the binary mixture of terbufos sulfoxide and terbufos sulfone exhibited additive toxicity. The high toxicities of terbufos and its environmentally persistent oxidation products suggest that contamination of aquatic systems with this insecticide pose a threat to aquatic organisms whether or not atrazine is also present.