Decline of pesticide residue on treated crop seeds: An analysis of comprehensive industry data and implications of the current risk assessment scheme for plant protection products.

For plant protection products applied as seed treatments, the risk to birds and mammals possibly feeding on treated seeds must be addressed in the EU to register products for commercial use. One assumption of the Tier 1 long-term risk assessment of the European Food Safety Authority (EFSA) is that residues of pesticides on treated seeds do not decline over time after seeding. Consequently, a time-weighted average factor (f ) of 1 (i.e., no dissipation) is used to calculate residue concentrations on seeds. In contrast, for spray applications, a default dissipation half-life (DT ) of 10 days is considered corresponding to an f of 0.53. The aim of this study was to establish a default f for treated seeds based on 29 industry-conducted seed dissipation studies, providing 240 datasets covering different active substances, crops, and regions. For f calculation, two approaches were used: (i) kinetic fitting and (ii) using measured data without kinetic fitting. From kinetic fitting, 145 reliable DT values were obtained. Because there were no significant differences in DT values between crops and between the central and southern EU, the DT data from all studies were pooled. The geometric mean DT was 3.8 days and the 90th percentile was 13.0 days, corresponding to 21-day f values of 0.27 and 0.59, respectively. Twenty-one-day f values could be calculated directly from measured residues for 204 datasets. The resulting 21-day f values were comparable with those from kinetic fitting (geometric mean: 0.29, 90th percentile: 0.59). The results demonstrate that residue decline on seeds is comparable with foliar dissipation after spray applications. Therefore, the risk assessment scheme by EFSA should implement a default f  < 1.0 in the Tier 1 risk assessment for treated seeds (e.g., either 0.53 as for foliage or 0.59, the 90th percentile f in seeds reported in this study). Integr Environ Assess Manag 2024;20:239-247. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).