Usually this happens when they get flooded out and then head for the surface.
Ants, I have found out, especially the fire ants will not die from mound washes or chemicals, they just move to the neighbors yard.
I use Andro and apply a sprinkling to the mound and in a day or two they have moved. This has to be treated as you see the mounds and you have to actually herd them off your land.
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