Pesticides is my thought, lately what they have been finding in the dead colonies is a fungus and a virus the question is does it come after the deaths of the bees (Having a lot of dead matter around can cause disease.) or is it killing them. My crop isn't dying in the summers but rather over the winter. 4 out of 70 of my colonies made it through the winter last year, so most of my crop this year came from new packages. This year I'll be treating with formic acid so I'll see if that works any better than what I did last year. I did pull in a reasonably good crop this year however despite the troubles I came across, other beekeepers in my area didn't do quite so well, but it seems to vary from location to location.