I agree with the poisoning.
Bees also at this time of the year, will collect about anything. Sawdust, seed/feed dust, etc. So maybe they are getting into something they normally would not.
If the bees can get out regularly, and are still pooping all over the hive, I would also look at feeding. Is he feeding, or was he feeding all winter? And at this time, the bees could be using up bad or poor pollen stored months go, yet only feeling the full effects now.
You really need to analyze many things that may be playing off of each other.