The short answer
Follow the trail to where they are coming in and seal it, wipe the trail away with hot soapy water to destroy the scent path, and use bait rather than spray.
- Watch them. The line will lead to a gap, usually at a skirting, an airbrick, a pipe entry or a window frame.
- Seal the gap with silicone or filler.
- Wash the trail away with hot soapy water or a vinegar solution. This is the important bit, because the trail is a chemical path other ants follow.
- Use bait stations, not spray. Workers carry the bait back to the nest, which is the only way to deal with the colony rather than the visitors.
- Remove the attraction: crumbs, spills, pet food left down, open sugar.
What will ruin it
Spraying the visible ants feels effective and achieves nothing, because the nest is elsewhere and is producing more. It can also scatter a colony into several smaller ones.
Why it works
Ants navigate by pheromone trails laid by scouts. Break the trail and the route is forgotten; leave the trail and they keep arriving even after you have sealed the gap.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.
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