Landscape Scale Study of the Net Effect of Proximity to a Neonicotinoid-Treated Crop on Bee Colony Health

Since 2013, the European Commission has restricted the use of three neonicotinoid insecticides as seed dressings on bee-attractive crops. Such crops represent an important source of forage for bees, which is often scarce in agro-ecosystems. However, this benefit has often been overlooked in the design of previous field studies, leaving the <i>net impact</i> of neonicotinoid treated crops on bees relatively unknown. Here, we determine the combined benefit (forage) and cost (insecticide) of oilseed rape grown from thiamethoxam-treated seeds on <i>Bombus terrestris</i> and <i>Apis mellifera</i> colonies. In April 2014, 36 colonies per species were located adjacent to three large oilseed rape fields (12 colonies per field). Another 36 were in three nearby locations in the same agro-ecosystem, but several kilometers distant from any oilseed rape fields. We found that <i>Bombus</i> colony growth and reproduction were unaffected by location (distant versus adjacent) following the two month flowering period. <i>Apis</i> colony and queen survival were unaffected. However, there was a small, but significant, negative relationship between honey and pollen neonicotinoid contamination and <i>Apis</i> colony weight gain. We hypothesize that any sublethal effects of neonicotinoid seed dressings on <i>Bombus</i> colonies are potentially offset by the additional foraging resources provided. A better understanding of the ecological and agronomic factors underlying neonicotinoid residues is needed to inform evidence-based policy