Prediction of the joint toxicity of five organophosphorus pesticides to Daphnia magna

The individual toxicities of five organophosphorus pesticides (dichlorvos, parathion, methyl parathion, malathion and dimethoate) to Daphnia magna were investigated in 24-h immobilization experiments. Using these toxicity data, their combined toxicities were measured in pesticide mixtures designed using either 'equivalent-effect concentration ratios' or 'uniform-design concentration ratios'. The toxicities of mixtures of similarly or dissimilarly acting toxicants are often predicted from the individual toxicities of the component compounds, using one of two distinct biometric models: concentration addition (CA) or independent action (IA). The relative accuracies of the CA and IA models were assessed using the model deviation rate (MDR), which represents the difference between the effect predicted from the individual pesticide concentrations and the observed effect. The mean MDR value of CA was 0.93 (range 0.75-1.31) and the mean value obtained by IA was 3.13 (range 2.52-4.37). We conclude that the CA model is better able to predict the joint toxicities of mixtures of organophosphorus pesticides to D. magna.