More than likely there are millions of them under your patio if it was laid with fine sand, This post will be deleted if not edited immediately I hate patio slabs, I lifted mine years ago and every ant in Ireland was under there I thought I got them all but I know a few flew, yes flew away I had to lift every slab in the place and that was a lot of slabs, god there were billions of them under the slabs all due to been laid on fine sand, That is why pea gravel is supposed to be used with slabs, It stops them from building roadways as it always fall in, with fine sand they will have duel carriageways, motorways,bridges the lot built, I also believe under the slab is nice and warm a great nest area A water can and DDT or sheep dip is the answer.