Black garden ant Lasius niger

Black garden ants nest under paving, lawns and foundations and send foraging trails indoors in search of food, usually through the smallest gap in a skirting board or door frame. Follow the trail back and the nest is rarely far from the door — a more useful clue than treating wherever the ants happen to be seen.

Treating the visible trail alone rarely lasts, since the colony behind it is untouched — we work back to the nest itself where we can reach it, and use bait where we can’t, so the colony is dealt with rather than just the ants currently indoors.