I'll toss my thoughts into the hat too. 

As I trap a lot of pollen each season I have seen how often a couple of hives in a yard have completely different color pollen then what the majority of the rest of the yard is collecting. 

With that in mind I might consider taking some dead bees as a sample and have a lab check for an external source of poisoning. 

Could it be possible this one hive got onto a source, maybe a new forage source that had also been recently sprayed? 

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