Residue Dissipation, Transformation Products, and Dietary Risk Assessment of Flonicamid in a Rice Paddy Ecosystem

Flonicamid is a systemic pyridinecarboxamide insecticide used to control sap-sucking pests in agricultural crops. An ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method was developed for the simultaneous determination of flonicamid and its metabolites, N-(4-trifluoromethylnicotinoyl)glycine (TFNG), 4-(trifluoromethyl)nicotinamide (TFNA-AM), and 4-(trifluoromethyl)nicotinic acid (TFNA) in rice plants, brown rice, rice husks, paddy soil, and paddy water. Method validation showed average recoveries of 73–115% with relative standard deviations of 1.1–9.9% across different matrices at three fortification levels. Field and laboratory experiments were conducted to characterize the dissipation, degradation, and transformation of flonicamid in the paddy ecosystem, and the dietary risk was assessed. Field dissipation followed first-order kinetics, with half-lives of 5.8, 2.9, and 7.1 days in rice plants, paddy soil, and paddy water, respectively. Under laboratory conditions, degradation proceeded markedly faster under aerobic than anaerobic conditions, with half-lives of 0.8–7.1 and 4.7–17.8 days in five typical Chinese soils, respectively. Six transformation products were identified by UHPLC–Q-TOF/MS coupled with UNIFI software (v1.9.4.0), including a novel product (M210) reported for the first time. At pre-harvest intervals of 7–21 days, terminal residues of flonicamid in brown rice were all below the Chinese maximum residue limit (MRL) of 0.1 mg/kg. Dietary risk assessment revealed that the acceptable daily intake percentages (%ADI) for 12 population subgroups ranged from 0.128% to 1.031% based on the parent compound, and from 0.404% to 3.808% under the EU residue definition. All values were far below 100%. These results indicate that flonicamid, when applied according to Good Agricultural Practice (GAP), poses a negligible dietary risk to consumers.