This could be coincidental but several months ago I covered my well, tank and switch with one of those large fake boulders. The very next week my pump was not working and I found a big hill of fire ants had taken up residence under the cover and had nested in the PS.
Because I am concerned about well contamination I did not want to use a pesticide and I happened to have a homemade batch of borax and sugar I had made to kill ants that had gotten into our RV. I sprinkled this liberally on the ground and have no more problems with ants under the cover. A box of Mule team borax is only a couple bucks and it is considered biodegradable.
As a side note it does not seem to work when not protected from the wx as I have a new problem with black ants in my yard and the borax does not seem to have any effect on them, with all the rain we have had this year it could be simply being wash away.
Good Luck!
p.s. Borax kills ants much the same way as diatomaceous earth as JAMES HW suggested but I found it to be much cheaper.