For a couple years now I've had colonies collapse in October and I'm struggling to identify the clues left behind. 2 of the 4 colonies from this year produced honey (Northern spring nucs) and looked pretty good going into September following a mid-summer formic treatment. The other 2 colonies were early summer splits and are doing ok, but didn't produce honey this year.
I applied Apivar strips to all hives after pulling honey in September and also supplied supplemental feeding to bolster the stores since we had a slow goldenrod flow this year. At the end of this month these two producer colonies had died, or otherwise left the hive. Little to no bees remain, but there is plenty of honey in and above the brood chambers in a medium. I feel like I did a good job with mite treatment this year and was conservative with honey harvesting, in addition to feeding this fall. Why would colonies disappear at this point in the fall? Is there something or a combination of things that are red flags that I'm not seeing?