Tetramorium immigrans and caespitum invaded the U.S. They are considered invasive species. Also, it could be caespitum too, because they look pretty much identical, yet caespitum is bigger. I've seen colonies of both. They are both polymorphic, and I've heard that they are polygynous, yet I've heard that they are not as well. They have mini stingers, are aggressive, and can grow their colonies into the 10s of thousands.
And isn't it such a coincidence that we are always posting on the same topics? (Rhetorik, this includes you too)