OP responding to D Coates: I had previously treated with formic acid (MAQS), wanting to stay with organic material rather than pesticides. It seemed to work well the first uses, but when I lost all of my hives last winter, I wondered if the fall MAQS treatement had anything to do with it, so thought I'd try the oxalic vapor, as it seemed really benign. In retrospect, I probably waited too long to treat this year. These were both new packages in the spring, had done well over the summer (one phenomenally well), and late in the summer suddenly started showing mites. I thought I'd wait to treat until most of the brood had hatched out before winter. That my have been too long to wait - maybe the bees got overwhelmed and were ready to throw in the towel (abscond) by the time I did treat...
I agree - there are so many variables and unknowns with the bees that it's hard to sniff out what the problems are/may be. I had great success my first three years as a beek, and now it seems impossible to keep them going...