1. Lay sand traps (lot of crater with no way of getting back up for the ants, similar to what dragonfly larvae make) and put them all over near and around the Harvester ant or other species nests that is being attacked by Argentine ants or Imported fire ants.
2. Make sand barriers (like walls) all around the Harvester ant nest. These walls are sandy and hard for the Argentine ant to climb up it.
3. Make small canyons all around the harvester ants in front of the sandy walls, these are good because they are somewhat like sand traps but on a bigger scale and help the sandy walls a lot.
Salt might work to, sea salt works to keep Argentine ants away but that might have a bad affect on the Harvester ants.
Maybe I play too many video games or strategy games :P But I've done all these before and it worked alright, but since I was younger I didn't really build appropriate defenses and the Argentine ants got through
So what it would be like is this. Sand traps in front of everything, sand barriers behind the sand traps, and the canyon type in front of the barriers.
Maybe this thread is crazy, but I thought these ideas were interesting.

