Application of predefined doses of neonicotinoid containing dusts in field trials and acute effects on honey bees

This study explored the effects of insecticidal dusts on honey bee colonies (Apis mellifera L.) after exposure to a priori defined dose under field conditions. For this purpose two different rates of abraded seed dust, containing active substance Clothianidin, were applied on flowering Phacelia tanacetifolia Bentham during bee-flight with a purpose-built dust applicator. We observed dose-related high acute effects on bee mortality at both application rates, 0.25 and 1.0 g a.s. Clothianidin in dust per ha, resulting in up to 4.3 and 17 fold higher mortality compared to pre-application level and an overall increase of mortality during the 7 days exposure period of 2.0 and 9.8 fold. In dead bees, residues detected between both rates applied were up to 2.6 fold higher in the 1.0 g a.s. Clothianidin dust exposure scenario. On day 7, residues up to 28 μg Clothianidin/kg were detected in bee bread of stored Phacelia pollen. The findings of high effects at chosen rates highlight the need to include specific dust drift field trials for seed treatment products with highly toxic insecticides in risk assessment used in crops with potential dust abrasion and emission from seeds. Further work is required to determine appropriate application rates in further semi-field and field testing that reflect field realistic drift exposure levels.