So I have seen the North Koreans say this multiple times in documentaries.
They say that the US dumped insects on them as a form of biological warfare during the Korean war. I think it was to destroy crops.
So normally, I don't really trust the people who say that Unicorns exist, and that their dear leader invented the hamburger.
But I could actually make a case for this happening, considering the fact that we (US):
-Caused catastrophic damage on the civilian population during that war.
-Weaponized industrial herbicide during the Vietnam War.
What do the historians have to say?
No, they did not. The allegations continue to be leveled in certain political circles, but as far as the historical record goes, we have ample reason, from several different approaches, to say that the allegations were false. I've written more on this here.