Which influence has sunflower on the development of bee colonies (Apis mellifera)?

In Switzerland, sunflower cultivation extend since the nineties and reaches about 5’000 ha nowadays. The sunflower seeds are not treated with insecticides (imidacloprid, fipronil) but despite that, Swiss beekeepers are complaining about colony losses if the hives are placed close enough to fields of blooming sunflower. We carried out a two years trial, in order to find out whether sunflower by itself is detrimental for bees or if it causes any colony damages. The results are that foraging of sunflower has no adverse effect to bee populations during the blooming period and within the next few month after blooming. There was no impact of sunflowers on the bee losses during winter. The bees were foraging intensively on sunflowers but the quantity of nectar collected was small or nought. The bees were collecting sunflower pollen during a few days after the migration to the sunflower fields but soon they used to change to alternative pollen sources such as maize and clover while abandoning the sunflower completely.