Potential impacts of synergism in honeybees (Apis mellifera) of exposure to neonicotinoids and sprayed fungicides in crops

Assessment of the toxicity of individual pesticides to honeybees is routinely assessed. However, few data have been generated for realistic mixtures of neonicotinoid insecticides and fungicides particularly with regard to exposure levels used. Assessment of the effects of exposure of bees to predicted residues following sprayed applications of ergosterol biosynthesis inhibitor fungicides on the contact and oral toxicity of a range of neonicotinoid insecticides (thiamethoxam, clothianidin, imidacloprid and thiacloprid) showed only low levels of synergism (< 3-fold maximum). Further studies showed that the scale of increase in toxicity was fungicide dose dependent with greater synergy of oral toxicity of thiamethoxam following contact dosing with propiconazole. This underlines the need for the use of realistic exposure levels and routes in studies.