Bees need some diversity in their diets. They consume nectar and pollen true, but that doesn't go far enough. If I offered you a diet of only Sugar cubes, Soya protein extract and Olive Oil you'd quickly die. Why? That diet has Carbs, Protein and Fat. That's everything a person needs!
It's a known issue that colonies cannot survive long term on substitutes. Why this is isn't clearly known yet but the assumption is that they are missing some dietary analogue to vitamin C.
Once again: I am not discussing bee diets. I brought up canola because (a) it's the crop discussed in the study we're commenting on, and (b) it's the worst-case scenario for bee exposure to neonicotinoids (virtually all canola crops are treated).
In fact: the idea that bee colonies would be stressed by a diet solely of oil rape nectar (note: no commercial bee colonies have such a diet, because they're moved around to take advantage of growing seasons elsewhere) favors my argument: it's another way in which bees working canola crops should do far worse, given neonic exposure, than they actually do.
It's a known issue that colonies cannot survive long term on substitutes. Why this is isn't clearly known yet but the assumption is that they are missing some dietary analogue to vitamin C.