Exposure reduction of seed treatments through dehusking behaviour of the wood mouse (Apodemus sylvaticus).

Seed treatments are widely used on cereals and other annual crops throughout Europe. Most of the formulated pesticide is found on the outside of the seed, the husk. Risk assessments of seed treatments are especially needed for granivorous mice living in the agricultural landscape e.g. for registration using the guidance for risk assessment for birds and mammals (EFSA 2009). The dehusking of seeds before consumption is a known behaviour of these mammals, but so far, no quantitative data on the reduction of exposure of seed treatments by dehusking were published. Therefore, we aimed at providing a first quantitative estimate of this behaviour-related exposure reduction for the wood mouse (Apodemus sylvaticus) with different seed types. We evaluated the efficiency of dehusking behaviour of 20 wood mice captured in the wild for four different seeds (wheat, barley, maize and sunflower). One experimental setup used a fungicide seed treatment where the remaining seed husks of consumed seeds were analysed with a HPLC-MS/MS technique. In the second setup, we measured generic pigment present in a blank seed treatment formulation and determined the leftover pigment in the husks with a photometric technique. The exposure reduction was similar for the fungicide and the pigment design where the same seed types were studied. We could demonstrate exposure reductions ranging from around 60% for cereals to almost 100% for sunflower seeds as a result of the dehusking behaviour. Since exposure reduction was similar in both approaches, working with pigments would be a generic way to estimate the impact of dehusking behaviour on seed treatment exposure. This behaviour can result in a substantial exposure reduction and should, therefore, be considered in a seed-type specific way in the risk assessment of pesticide seed treatments. It is proposed to include a seed-specific dehusking factor in the calculations of estimated theoretical exposure of seed treatments for granivorous mice. The approach of accounting for a dehusking-related exposure reduction by field relevant wild mammal species seems a more promising way to advance the risk assessment instead of using generic species and neglecting behavioural traits. The pigment approach could be used to gather data for exposure reduction for other species and seed types. Its advantage is that it is harmless to the test species and comparatively cheap since no chemical analysis is involved. Seed treatments are used for most of the cereal crops grown in Europe today. Their advantages usually include a lower application rate and the reduction of drift compared to a conventional spraying regime. However, there is a potential risk especially for granivorous mice, and its assessment is challenging in case of a high residue concentration on the dressed seeds. The concept of a dehusking factor in the risk assessment scheme for seed treatments for granivorous mice is a valid approach to account for the behavioural exposure reduction, and generic data could be easily generated also for other wild mammal species and other seed types, possibly analysing the pigment in commercial seed treatment formulations.