Do sunflowers influence the development of honey bee, Apis mellifera, colonies in areas with diversified crop farming?

In Switzerland, the cultivation of sunflowers has expanded since the 1990s, and today nearly 5000 ha are cultivated. Sunflower seed is not treated with systemic insecticides (neonicotinoids, fipronil) but nevertheless, Swiss beekeepers claim to have observed a weakening of bee colonies after foraging on sunflowers. The purpose of this test was to evaluate whether the sunflower itself causes the weakening and if so, to quantify it. The results show that bee foraging on flowering sunflowers had neither a noxious effect on bee populations during the blooming period nor in the following months. Likewise, winter losses did not increase. The bees visited the sunflowers intensively but harvests of sunflower nectar were non-existent or low. Sunflower pollen was collected in the first few days after moving to the sunflower fields but, in a poly-culture environment like Switzerland, the bees then turned to alternative pollen sources such as corn (Zea mays) or clover (Trifolium spp.).