We had a problem with Ants last year, we couldn't get rid of them so we called in pest control. The guy had a "special" bait that he placed amongst the trail of Ants, they think it is food and take it back to the nest and it takes them all out. Might be worth a try if they keep coming back.
we recently had a huge ant problem, the only thing that worked was two mixtures, one was borax and very finely grated parm cheese the other was borax, icing sugar and cornflour, it took weeks, yes weeks to kill the entire nest but it did work, it was sooooo frustrating, we had them everywhere, throughout my pantry (nothing kept the away) bathroom, shower etc...
we were told not to spray the nest as doing that just kills those in it which therefore means the remaining ants start new nests
we also did capfuls of polenta once, with the theory behind it that they couldn't digest it. The ants did disappear, but a colleague (it was at work) was secretly spraying when the boss wasn't looking, so we don't know which one it actually was.
Bookmarks