Ecological risk assessment of imidacloprid applied to experimental rice fields: Accurateness of the RICEWQ model and effects on ecosystem structure

•Current first-tier PEC methods underestimated actual pesticide concentrations.•The higher-tier RICEWQ model adequately predicted imidacloprid exposure profiles.•Imidacloprid leads to effects on ecosystem structure in paddies.•Mitigation measures are needed to avoid spread over waterways and risks to birds.

The fate of imidacloprid and its potential side-effects on biological communities and ecosystem functioning was studied in experimental rice plots. In addition, the influence of applying a withholding period of zero days (actual practices) and seven days (recommended in EU) on this was evaluated. Predicted environmental concentrations (PEC) of imidacloprid calculated with the higher-tier model RICEWQ agreed well with concentrations measured in the field. Methodologies generally used in the EU and USA for lower-tier PEC calculation, however, severely underestimated actual field concentrations and hence appear to need further evaluation and eventually amendments. Effects on several biological communities (especially ostracods, dipterans and coleopterans) were noted following imidacloprid application, with greatest effects in the paddy were as applied a withholding period of 7 days. An increase in the density of snails (Physa acuta), however, may have prevented effects on ecosystem functioning through functional redundancy. Implications of study findings for the ecological risk assessment of imidacloprid and potential mitigation measures are discussed.