Laboratory Investigation of the Toxicity and Interaction of Pesticide Mixtures in Daphnia magna

The probabilistic ecological risk assessment-toxic equivalent (PERA-TE) combination approach is a relatively new risk assessment approach used to assess the toxicity and interaction of chemical mixtures. The validity and effectiveness of the PERA-TE combination approach has been tested previously in field microcosm studies using pesticide mixtures. The related laboratory studies described here, using Daphnia magna, were conducted to verify the conclusions made regarding the toxicity and interaction of the mixtures tested in the microcosms. Two types of pesticide mixture were assessed: the first consisted of pesticides with similar modes of action (chlorpyrifos, diazinon, and azinphos-methyl; OP mixture), and the second consisted of pesticides with different modes of action (chlorpyrifos, endosulfan, and trifluralin; CET mixture). Similar to the field studies, PERA-TE mixtures with a predetermined effect assessment criterion (10th centile of acute toxicity effects distributions) and proportional ratio (89:11 for binary mixture and 80:10:10 for ternary mixtures) were tested. Further assessment of the (PERA-) TE approach was achieved by altering the effect assessment criterion (to EC/LC₅₀ point estimates) and the proportional ratio of the pesticides in the mixture (to 50:25:25). Generally, but with some exceptions, basing mixtures on species-specific effect criteria and/or changing the proportional ratio of pesticides in the mixture redistributed the concentration of pesticides in the mixture to produce an equitoxic response. The ability to produce these equitoxic responses supported the conclusions drawn from the field studies: The pesticide toxicity in the OP and CET PERA-TE mixtures were effectively additive. Furthermore, it is shown that these alternative (PERA-) TE mixtures would be suitable to confirm or reject the interaction of chemicals in a PERA-TE mixture.