If you need to dig where there is a mound, put a lot of ice on the mound and wait until the ice melts before digging. I did this with a rose bush that was in the center of a mound, and watched as soldier ants would try to bite the ice and die of hypothermia. Within half an hour there was no evidence of live ants at all, including in the root ball of the rose. Another thing that I noticed is that they interbreed with local ants - one year we had stinging sugar ants falling on us from our sugar maple. After that season the sugar fire ants disappeared and we could sit under our maple without brushing off falling ants of any kind. And another observation is that they interbred with tiny black ants, and instead of forming mounds, formed granular 'tunnels' on the surface. These ants had the most persistent aftereffects of stings, and definitely took the pleasure out of gardening barefooted.