The Effects of mixtures of pesticides, in use in Thailand, on the aquatic macrophyte Lemna minor

The aim of the present study was to assess the effects of four herbicides commonly used in Thailand (atrazine, 2,4-D, alachlor, paraquat)on the aquatic plant Lemna minor under differing patterns of exposure (single-, mixture-, and sequential- exposure). The endpoint of interest was the growth rate of plants over time.
In the single-compound toxicity studies, paraquat was found to be the most toxic pesticide followed by alachlor, atrazine and 2,4-D. Mixture studies were then done on the pesticides to understand how they would interact. Comparison of data from toxicity tests on mixtures of the pesticides with modelling predictions indicated that atrazine and2,4-D interact antagonistically whereas alachlor and paraquat interact synergistically. These results are in agreement with other mixture studies with pesticides.
Studies were also done to understand the effects of the different pesticides when applied in sequence. Comparison of the experimental results with predictions from a simple model demonstrated that at low effect concentration herbicides, the model works well but at higher concentrations it falls down. To explore the reasons for this, a further study was done to assess the carry-over toxicity of the study compounds.
This work demonstrates approaches to understand the effects of pesticides under more realistic exposure conditions. It demonstrates that while modelling approaches are available for estimating impacts under more realistic exposures, the accuracy of the predictions is likely to be highly dependent on the mode of action and concentration of the pesticide and the duration of the exposure.