First year, we have two colonies located about 7 feet apart. We are near Boise in hardiness zone 6b.
Picked up as nucs in April and seemed to do well, each filling two deeps as brood chambers, and two 8 frame mediums, the mediums we harvested on Labor Day weekend.
Treated for mites and the counts are pretty low (<10 /24 hrs) monitored with a sticky board under a screened bottom.
I've been offering 2:1 syrup through a mason jar top feeder and also placed Bee Pro patties above the upper deep.

1. Both hives have always taken syrup before, but both stopped consuming the syrup this week. This is odd?

2. Yesterday I noticed 100 - 200 dead bees on the ground mainly in front of one hive (Yellow). I had not encountered that before.
The other hive (Blue) was normal, no bee fallout.. The Yellow hive has always been stronger, but now this.

3. Today I went out to check on them and I saw more sick bees exiting, one at a time one leaving about every minute.
Normal bees going out or back from forage were also observed.

4. The sick bees were kind of staggering in a jerky kind of motion then either just rolling over dead or flying off the deck right
into the ground where they buzzed around frantically in circles on the ground until they collapsed.

5. Although the temperature was about 60F I decided I needed to take a look inside.
There appeared to be the normal population of bees below the cover, and the patty I put there on Saturday was 80% consumed.
I did not rip everything out and inspect due to the low temperature.

6. I think I saw white material (looked something like white paint) on two or three bees yesterday but not on any of the casualties today.

I have no idea what is happening to the Yellow hive.
Temperature change?
Exposure to some toxin?
A common bee disease?

Any ideas or suggested courses of action would be appreciated.

Sorry for the long story.