Authors: Schwarz, Janine; Knauer, Anina C.;Alaux, Cédric;Allan, Matthew J.;Dean, Robin R.;Klein, Alexandra-Maria;Michez, Denis;Tamburini, Giovanni;Wintermantel, Dimitry;Albrecht, Matthias;

Wild bee pollinators in agricultural landscapes are threatened by various – potentially interacting – stressors, including exposure to pesticides and loss of floral food resources. Since the recent partial ban of neonicotinoid insecticides, novel alternatives such as sulfoxaflor and flupyradifurone are increasingly used. Potential negative effects of field-realistic exposure to these new insecticides and their interactive effects with commonly co-applied fungicides or food stress on solitary wild bees remain largely unknown. We investigated the impacts of sulfoxaflor and flupyradifurone, as well as their interactions with the common fungicide azoxystrobin and food stress, on the solitary bee Osmia bicornis under laboratory and semi-field conditions. Pre-flowering application of sulfoxaflor, alone or combined with the fungicide, did not have significant negative effects on fitness under semi-field conditions. Application of flupyradifurone during crop flowering and food stress, however, caused pronounced synergistic negative impacts on survival, reproduction and foraging behaviour. Under laboratory conditions, worst-case field-realistic doses of both insecticides negatively affected developing O. bicornis bees. Our results highlight the need for improving pesticide risk assessment schemes by considering interactions with further stressors in order to better protect bees and their invaluable pollination services.