Synergistic effects of a neonicotinoid and an EBI fungicide in honey bees, bumblebees and red mason bees

Bee diversity has been declining worldwide in the last decades. Although many factors have surely contributed to these declines, neonicotinoid insecticides have often been signaled as one of the main factors. Most studies on neonicotinoids tests single compounds. However, in field conditions, bees are exposed to multiple pesticide residues, some of which may interact synergistically. In addition, different bee species have different sensitivities to pesticides, and therefore may respond differently to these synergistic effects. The aim of this study is to assess the possible synergistic effect between a neonicotinoid (clothianidin) and an ergosterol-biosynthesis-inhibiting EBI fungicide (propiconazole) in three bee species of bees with contrasting life histories (Apis mellifera, Bombus terrestris and Osmia bicornis). We orally exposed bees to non-lethal doses of clothianidin (LD10 of each species) and propiconazole (7 µg/bee), singly and in combination. There was a strong synergistic effect on mortality in all three species, in particular during the first 48 hours after the exposure phase. The level of synergism and survival rates throughout the experiment varied among species. Osmia bicornis was the most sensitive species and A. mellifera the least. These results highlight the need to test combinations of pesticides likely to co-occur in agricultural environments and to include several species in risk assessment schemes.