When The Monk and Monkette2B were in Australia, we learned about the hideous and destructive cane toad -- a fat land amphibian that is poisonous, and very deadly poison at that. The fat buggers have poison sacks just behind their heads and animals that try to eat them die within minutes. The cane toads (who were brought over from Hawaii in the '30s to kill bugs that were destroying Aussie sugar cane in Queensland) have thrived and spread throughout the north of Australia. Consider a critter brought in to help farmers in 1800s New England spreading all the way to Montana with no efficacious predator -- that's the parallel.
And this fact of Aussie life makes The Simpsons episode even more sharply satirical than most people know. Remember that Bart brought his pet frog to Oz, at the end of the episode the bugger escaped and frogs spread quickly across the continent? That's the cane toad situation -- the imported pest in a fragile ecosystem destroying the native environment.