If it's a ground floor kitchen, I'm guessing an ants nest under the floorboards perhaps near an airbrick. Have a look around on the outside and see if there are any clues. If the kitchen is fitted, the little blighters will have a couple of routes up the walls behind the cupboards. But it's the source of the infestation that needs getting rid of, ie the nest. If you can see the hole they are coming up from, a kettle of boiling water poured down it is quick and eco-friendly. But it's getting to see where the nest is, that might be the real probelm here.