GAR: I would love to see that local co-op manager's face if an inspector for your state dept of agriculture walked into his store and asked for the person who was recommending diuron for aquatic treatments; or your farmer-friend's face for that matter.
I can assure you, their smug attitudes and winking smirks would disappear in a heartbeat.
I'm betting that the farmer would never contaminate his crop with a non-labeled pesticide: which begs the question, why would he do otherwise with his own pond - or encourage his neighbor to commit the same blunder. Evidently, he considers himself smarter than an impressive number of highly educated research chemists and environmental toxicologists.
To paraphrase an old Will Roger's quote: -That which we don't know rarely gets us into as much trouble as what we know, that simply ain't so.-

Bill: Interesting analogy, and quite accurate.