We had Pharaoh Ants in my building once. They're very small - I once saw one going around the edge of a USB plug to get into my computer through the socket! I like how they've evolved to be harder to get rid of by lowering the membership requirements in their cafeterias. With other ants you have to be a member of the right species *and* ant-hill to get fed but Pharoah Ants just require you to be a species member. So if there's just one queen left you haven't lowered the number of ants at all. Also most ant poison works on the principle of putting birth control pills in food that the workers bring back to the queen but as the ratio of workers to queens rises, as you sterilise some of the queens, the chances are that your birth control pills will be in surplus food that doesn't make it to the remaining queens.