What is the geographic range of leaf cutter ants (Atta spp) in the United States? Are there any known colonies of these (or other) fungus growing ants north of the Gulf States (in the Eastern US)?
You may be surprised to know that leaf-cutter ants can be found in the United States! Although mostly a Neotropical group, both true leaf-cutters and the other fungus-growing attine ants can be found from South American through North America. There are ~230 described species of attine (fungus-growing) ants. In fact, attine ants are the world's first farmers and have been growing their fungal food crops for around 50 million years.
Here are the geographic distributions of the two true leaf-cutter ant genera (Atta and Acromyrmex) from the Ant Genera of the World website:
Recent work by Ted Schultz and Sean Brady has greatly contributed to our understanding the evolutionary history and timing of fungus-growing and leaf-cutting in the Attini ants [Ted R. Schultz & Sean G. Brady (2008) "Major evolutionary transitions in ant agriculture" PNAS 105 (14): 5435-5440.] You can read a nice review of the findings of this study on Myrmecos Blog here.
If you are interested in the geographic distributions of ants, I highly recommend checking out the Ant Genera of the World website.
Corrie Moreau & the AntAsk Team