I mean, maybe it is the thousands (millions?) of gallons of general pesticides sprayed from aircraft over many US counties?
There are mosquito control programs everywhere in the US that use pretty non-selective pesticides and blanket large areas from aircraft. There are better more eco friendly options but the general stuff from fixed wing planes is the cheapest option.
I'd read, years ago, in the science news-headlines sites, that someone had cracked Colony Collapse Disorder:
They'd said that in ALL the hives which didn't have CCD, there was never the coincidence of both a specific virus and a specific fungus, infecting them.
They'd said that in all the hives which did have CCD, both that specific virus & that specific fungus were in-play.
Apparently, to them, the infection of both of those specific-pathogens, meant that their immune-systems were defeated, & everything else got them, then.
Since I can't find that article, here's a different article which identifies some specific pathogen, which I hope includes both the specific-virus & specific-fungus that the other researchers had found to be the culprit: