Can't be ruled out, but I would first look at (improper) pesticide use and monoculture. At least this is the general observation in my beekeeping community. We

Can't be ruled out, but I would first look at (improper) pesticide use and monoculture. At least this is the general observation in my beekeeping community. We noticed a gradual decline in colonies since the local farmers started growing rapeseed for biodiesel about 7-8 years ago. It has only gotten worse recently. Apart from the intensive use of pesticides, compared to other plants, one other problem that I face with rapeseed, is that the honey crystallizes really fast. If such honey is left in the hive during winter, its a sure recipe for failure. Even in the summer, it creates some irregularities in the food supply (I've seen bees taking it out of the combs after crystallization).