Stink bugs have propogated their way across the USA in the last 5-7 years, and there's a variety of other bugs, fish, crustaceans, and even rodents that are prevalent that simply didn't exist here a decade or two or three ago.
Obviously some are accidental stow aways in cargo ships etc, but all of them? Why not force your enemy to deal with a common plague that your country already has?
Remember they were sending seeds randomly not that long ago?
Oh damn that's right.