Winter: November to February
Outdoor colonies are dormant. The queen and workers cluster deep in the nest, below the level where the ground freezes, and do not feed or forage. You would normally see no garden ants at all in these months, even on a sunny day.
The exception is a heated building. Pharaoh ants live only indoors, in places such as blocks of flats and commercial kitchens, and keep going all year; small pale ants in January point to them rather than to a garden colony.
Spring: March to May
As the soil warms, the colony wakes and the first foragers come out, often on the first properly mild days of March or April. Numbers are low at first, because the colony comes out of winter small and the queen is only just starting to lay again.
This is when the first scouts find the kitchen. A handful of ants on a worktop in April is the beginning of the year's trail, and dealing with the food and the scent then is far easier than in July.
Early to High Summer: June and July
The busiest foraging months. The colony is at full strength and raising its largest brood, which needs protein and sugar, so the trails are long, steady and determined. Warm afternoons bring the most activity; a hot spell can bring a trail indoors almost overnight.
It is also the easiest time to find a nest, because the traffic is heavy enough to follow and fine soil builds up at the entrances between paving and at wall bases.
Flying-Ant Days: July and August
On a few warm, humid days in mid to late summer, mature nests release their winged males and new queens, and many nests across a district fly on the same days. The air and pavements fill with them for an afternoon, then it is over. The biology is in what flying ants are; the day matters for the calendar because each mated queen that survives starts next year's colony.
Flying ants coming out of your garden are normal and pass in a day or two. Flying ants coming out of a skirting board or a floor indoors mean a nest inside or under the building, and that is worth acting on.
Autumn: September and October
Foraging slows as the nights get colder. The colony is no longer raising brood and the workers become fewer and slower, particularly after the first chilly spell. Ants can still come in on a warm September afternoon, but by late October most gardens are quiet.
Autumn is a good time to seal the gaps you noticed during the summer, since a nest that has gone quiet will not simply move to the next one along. How the whole cycle feels from inside one house, from the first ant on the worktop to the last, is covered on the season at home.
What This Means for Timing a Treatment
A treatment works best while the ants are active, because both bait and residual products rely on foragers moving along the route. That makes late spring and summer the natural time. In deep winter there is usually nothing outdoors to treat, and the sensible step is to note where the trail ran and deal with it when activity returns.