The acute toxicity of atrazine in binary combination with each of four organophosphate insecticides (OP; dimethoate, omethoate, disulfoton, and demeton-s-methyl) was evaluated with fourth-instar larvae of the aquatic midge, Chironomus tentans (Diptera: Chironomidae) in 48-h bioassays. OP concentrations resulting in an effect in 15% (EC15) of midges were coupled to atrazine concentrations of 0, 1, 10, 100, and 1,000 �g/L to determine the effect of atrazine on the toxicity of each OP. Atrazine alone (up to 1,000 �g/L) did not show noticeable acute toxicity to C. tentans. However, atrazine (³ 100 �g/L) either significantly increased or decreased the toxicity of the OPs (EC15) in the combinations. Synergism or antagonism ratios were calculated to determine the magnitude of the effect of atrazine on the toxicity of the OPs. Acetylcholinesterase (AChE) activity also was examined in the midges treated with individual OPs or the mixtures of atrazine and each OP for 48 h.
Species 1: Diptera Chironomidae Chironomus tentans
Keywords: synergism, antagonism
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