Ants are not strictly a garden pest because they do not damage plants. However, their presence in large numbers can be a real nuisance. Ants are social insects, living in nests in dry soil under stones and paving.
They actually farm colonies of greenflies for the honeydew they excrete and they often move greenflies to new plants. Burrowing by ants while building nests sometimes undermines small shrubs, leaving them high and dry and prone to wilting in a warm summer.
There is no effective method of physical control. Pouring hot water into the nests does not work. If the nest is hard to find, leave some sugar in a little heap where the ants have been seen and a couple of days later there will be a trail of ants back to the nest.
Apply ant-killers at the entrance to the nest. These include Anti-ant Powder, Murphy’s Antkiller. They can need to be renewed several times until the ants are no longer seen.