Right about dusk I noticed that the bees seemed to be acting a little “differently” on the landing board just outside the entrance. A few of them seemed to be acting like their very back legs weren’t working correctly. And, the bees seemed to be “clumped up” just inside the entrance. I also saw what was probably around a hundred dead bees on the ground outside the entrance. Some of the dead bees seemed completely dried out; this may not have happened all today, but, rather, more gradually, and I just now noticed it.
I spoke with John Babb about it on the phone. He theorized that, most likely, a scout bee got into some plants with pesticides on them but didn’t die before making it back to the hive and telling other forager bees where that source of nectar was. So a whole lot of them visited those plants and got poisoned. They later died in the hive and the worker bees moved them out.