The Nest Was Never Reached
The most common reason by some way. If the ants were sprayed, wiped or boiled, the workers died and the queen carried on. A black garden ant nest can have a great many workers, and the ones you saw were only the fraction out foraging that day.
The clue: the ants return along the same route, often within days, and in the same numbers. Nothing about the trail has changed because nothing about the nest has changed.
There Is More Than One Nest
A garden can hold several colonies, and a house can be visited by more than one of them. Deal with the nest under the patio and the one at the far wall base still knows where the kitchen is.
The clue: a trail from a new direction, or ants coming in at a different door or a different room from before. It is worth watching where the new ones leave the house on a warm afternoon; the method for that is in the article on how to find an ant nest.
The Nest Is Not on Your Property
In terraces, semis and flats the colony can be under the neighbour's paving or in a shared wall base. Treating your own side stops the ants that cross it, but a busy nest beyond the boundary will keep sending foragers.
The clue: the trail runs along or under the boundary, or appears from a party wall. A quiet word next door often explains a great deal, particularly if they have ants too.
Something Undid the Treatment
- Cleaning too soon. Mopping a threshold or wiping a skirting that has been treated removes it before the ants have picked it up.
- Your own spray over the top. It kills the ants carrying bait and puts the rest off the treated route.
- Heavy rain or jet-washing on outside surfaces soon after treatment.
- Bait left to dry out or cleared away before the colony had taken enough of it.
The clue: activity fell for a few days, then picked up again, and you can point to the clean-up or downpour in between.
The Trail or the Food Is Still There
An old scent line left on a surface can bring in scouts from a nest that was never the original problem. Food left out does the same. If the ants are few and wandering rather than a tight line, the kitchen is probably simply still worth a look.
It Is a New Year, or a New Colony
A nest treated in August can be replaced the following spring by a different colony nearby. On top of that, every summer the flying-ant days send new queens out to start nests, and one that lands in your patio joints is a fresh colony in its own right. Ants in April, a year after a treatment, are rarely the same nest coming back.
The timing is the best clue you have. Back within days points to the nest being missed or the treatment undone. Back the following season points to a new colony, and to prevention rather than a failed treatment.
Once you know the cause, keeping them out is a routine rather than a fight, and the yearly prevention routine sets it out step by step.