BACKGROUND The application of plant protection products (PPPs) leads to the formation of residues in treated crops. Even though PPPs contain considerable amounts of co‐formulants, regulation and monitoring of residues normally focus on the active substances (a.s.) only. For our study we selected four commonly used co‐formulants (three anionic surfactants and one organic solvent) and investigated the formation and decline of residues in vegetables and apples under field conditions. The aims were to characterize the behavior of co‐formulant residues on crops and to provide a basis for future investigations on consumer exposure. RESULTS The development of robust and sensitive analytical methods allowed the quantification of residues in the low μg/kg‐level. After treatment with PPPs, co‐formulants were detected up to approximately 10 mg kg–1 in vegetables. In general, these residues declined fast with half‐lives of a few days. Wash‐off and volatilization were identified as important removal processes for anionic surfactants and the organic solvent, respectively. However, in specific crops (parsley and celery), organic solvent residues were still considerable (≈2 mg kg–1) 2 weeks after treatment. We further demonstrate that it is feasible to estimate co‐formulant residues using publicly available data on pesticide a.s. CONCLUSION To date no information on co‐formulant residues in food is available. The findings from our field trials, as well as the presented approach for the prediction of residues, provide key elements for future consideration of consumer exposure to PPP co‐formulants. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

BACKGROUND Although co‐formulants constitute a substantial portion of the total plant protection product (PPP) mass applied to crops, data on residue formation and the behaviour of these substances on plants are scarce. In an earlier study we demonstrated that co‐formulants commonly used in PPPs can form considerable residues, i.e., in the low to medium mg/kg range, but normally decline rapidly within few days. In the field trial reported here, we aimed to identify the major decline processes of co‐formulants. Residues of co‐formulants were therefore monitored in parsley and lettuce grown in an open field as well as under foil tunnels equipped with either an overhead or a drip irrigation system. RESULTS Dissipation of three anionic surfactants was markedly faster when crops (parsley and lettuce) were exposed to natural rainfall or irrigation from above compared to drip irrigation. In contrast, the decline of three volatile organic solvents was not affected by rain or irrigation, but was dependent on the crop, with much shorter half‐lives in lettuce than in parsley. Furthermore, dilution through plant growth contributed significantly to the reduction of residues over time. CONCLUSION In this work we substantiate earlier findings on the magnitude and dissipation of residues of anionic surfactants and solvents representing the most important co‐formulant classes. The chosen experimental setup allowed differentiation between decline processes and we confirm that foliar wash‐off is a major dissipation process for anionic surfactants. For volatile organic solvents, dissipation appears to depend on the properties not only of the substance but also of the plant (surface). © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.