Honey, you'd better bee-lieve it!
I'm not a bee expert. But, I don't think one has to be an expert to be convinced by the vital role that bees and similar pollinators play in the ecosystem that we share with them. Based on the persuasive and somewhat inconvenient evidence that the species precipitous decline is due in some part to the overbearing presence of the human species in this shared space I believe it's important that when a member of that species can do something to help, they do.
From what I've read about bees and other pollinators the main areas of their decline are concentrated on some factors that are more closely linked to anthropogenic (human) causes than others. Industrial agriculture's usage of pesticides and an insistence on raising crops in 'mono' are likely two significant factors, but there others that are less obviously attached to our behaviour; such as parasites that feed on bees, pathogens that cause their wings to deform or simply a basic lack of nutrition. Surely, it is difficult to segregate either of the first group of causes entirely from the second. Largely, because this is not the sort of consideration that nature permits.
Evidently, there must be some unavoidable overlap between two things that exist in one space, which makes differentiating between what we think of as naturally occurring events and others we perceive to be human-derived all the more challenging. Conversely, it makes it all the easier for us to mistakenly say that one of these causes is, in fact, the cause of all of the others or to fail to notice when this is actually true. In any case, as I see it, asking who is responsible for the declining population of bees is less important a question as asking who is responsible for their upkeep or repopulation. It is likely clear, given my writing what my attitude towards answering that particular question is. It's not as if they themselves are in a position to do anything about it, is it? Whatever the cause of their decline, their revival must be anthropogenic.