EU-wide restrictions on neonicotinoids have not entirely eliminated the risk for honeybees foraging on oilseed rape nectar | Les restrictions européennes sur l’usage des néonicotinoïdes n'ont pas vraiment éliminé le risque pour les abeilles qui butinent le nectar de colza

Chronic exposure of bees to sublethal dosages of neonicotinoids in pollen and nectar has been implicated in recent declines of bees, which led the European commission to ban the use of three neonicotinoids in bee-attractive crops. Nonetheless, several studies detected substantial levels of the neonicotinoid imidacloprid in oilseed rape, despite being only permitted for use on winter cereals and sugar beets. Neonicotinoids are highly persistent and relatively water-soluble. These properties ensure systemic protection of treated crops, but also facilitate transport by water and wind as well as the uptake by succeeding crops. It remains, however, elusive what conditions favor carryover from treated crops to insect-pollinated plants. Further studies assessing neonicotinoid levels in mass-flowering crops and their potential effects on bees are needed to inform the debate on a ban of all neonicotinoids and the pending decision on the approval of the currently restricted neonicotinoids, imidacloprid, clothianidin and thiamethoxam. Therefore, we repeatedly quantified neonicotinoid residues in nectar of oilseed rape flowers from 264 fields within a Long-Term Socio-Ecological Research (LTSER) site with documented land use. We detected four out of the five neonicotinoids that are approved for plant protection in the EU. Imidacloprid and thiacloprid were present in all four years of the study. Particularly imidacloprid varied widely in concentrations within and among years and showed large inter-annual differences in prevalence with about 5% of fields being positive in 2015 and over 90% in 2016. We found, however, little relation between neonicotinoid contamination and environmental factors, such as weather conditions, soil type or the cultivation of winter cereals in previous years, suggesting a diffuse contamination of the environment. Based on literature values on the acute and chronic toxicity of neonicotinoids as well as the foraging behavior of nectar foragers, we estimated the mortality risk of honeybees foraging predominantly on oilseed rape nectar and found a considerable threat for foragers at several of the sampled fields. We conclude that the EU moratorium has not entirely eliminated the risk for bees foraging on oilseed rape and that there is an urgent need to better understand the pathways of neonicotinoid spread in the environment.