SYNERGISTIC TOXICITY OF ATRAZINE AND ORGANOPHOSPHATE INSECTICIDES CONTRAVENES THE RESPONSE ADDITION MIXTURE MODEL

A toxic unit (TU) approach was used to test the response addition mode l for mixtures of pesticides with differing modes of action. Atrazine was tested in binary and ternary combinations with the organochlorine insecticide methoxychlor and organophosphate insecticide methyl-parath ion. Atrazine was also tested in binary combinations with additional o rganophosphates. The TU for atrazine was set slightly below its water solubility limits (TU = 20 ppm), which is well below its 96-h 50% effe ctive concentration for the fourth instar of the midge, Chironomus ten tans. In 96-h acute toxicity tests using C. tentans, atrazine was foun d to produce synergistic (greater than additive) toxicity in a binary mixture with methyl-parathion. Less than additive toxicity was found f or the combination of atrazine with methoxychlor (likely due to the lo w TU assigned to atrazine). The ternary combination of atrazine + meth yl-parathion + methoxychlor was found to be marginally synergistic. Re sults of toxicity tests with atrazine in binary combinations with othe r organophosphates indicate more than additive toxicity for all compou nds except mevinophos. Possible hypotheses are presented in an attempt to explain the noted synergistic relationship between atrazine and va rious organophosphorous insecticides. These results suggest that the r esponse addition model does not always accurately predict mixture toxi city for chemicals with differing modes of action.