One of the patterns reported by the group at Penn State was that all producers in a preliminary survey noted a period of "extraordinary stress"
affecting the colonies in question prior to their die-off, most commonly involving poor nutrition and/or drought. This is the only factor that all of
the cases of CCD had in common in this report; accordingly, there is at least some significant possibility that the phenomenon is correlated to
nutritional stress, and may not manifest in healthy, well-nourished colonies.
en.wikipedia.org...
I bet this is related to some common supplement, given to bees here in the USA, especially when bee keepers find their hives are "under stress".
I further bet that this supplement is made in China, or some other foreign country.
The reason that this has been so hard to find? The contaminant is no longer present in that supplement, due to better quality control by the foreign
vendor (for now.)
The whole problem might just be THAT SIMPLE.