We bought a nuc about 3 weeks ago, last looked in on it just over a week ago, all was well. Lots of brood, bees looked happy and healthy. We've observed them from outside the hive every few days since then, the ladies were buzzing around like normal a few days ago, but this evening we went out to make sure all looked normal and found a huge pile of dead bees instead. Piles of dead near the entrance, and on the ground below being slowly taken away by ants. Brown blotches, which we assume to be diarrhea, on the hive itself and in the grass, and some bees which are in the "dead" piles still alive, but barely moving- clinging to blades of grass, trying to climb into the hive. In the 15 minutes we observed, maybe three "normal" seeming bees flew by. It was a cooler evening, but not cool enough to slow them down so much.
We opened the top to peek in, but didn't go in the hive as we didn't want to subject them to a chill without having a solution to offer. There looked like maybe 1/3-1/4 of the hive inside at time of opening.
The person we bought the nuc from said it might be poisoning- for it to have happened so suddenly. He's coming out Sunday, but we don't really expect to still have any left by then- not the way things look now. We're at a complete loss. We were so worried about keeping our eyes open for mites, we've never even heard of anything like this. Any ideas what's happened here? Is there anything we can do? We're at a loss and are desperate to do anything that might save the survivors.