The black ant (Lasius niger) make their nest in the soil, on grassland including lawns, at the base of walls, under flat stones and sometimes in hollow trees. Nests are often very numerous near buildings and occasionally may be situated close to or actually in the foundations. The worker ants are dark brown to black in colour and about 5mm in length. The queen ant is the same colour, but about 15mm in length.
The foraging workers follow fairly well trails to their feeding grounds which may be many yards from the nest. They have varied feeding habits. They may enter buildings, often through very narrow crevices, and if food, especially sweet food, is found by one ant there will soon be many others to share the feast. Houses, restaurants and food shops, office buildings and hospitals etc. may be entered this way and the ants may cause a considerable nuisance and some damage but their nests are usually outside the buildings. Ants are a nuisance, but not known to cause disease.
During the summer great numbers of winged females (which are potential queens) and males are reared in the nest and on one or two warm summer afternoons between mid-July and mid-September they swarm out and take to flight often in quite spectacular numbers. This usually happens simultaneously over a wide area of country. Sometimes, if a nest is situated in the foundations, these winged ants may swarm inside buildings. During the flight, the ants mate. Many thousands are eaten by birds and in about two or three hours it is all over; the males soon die, the queens shed their wings and make themselves a cell, generally in the soil where they pass the winter before attempting to start a new nest the following spring. A few – but enough – succeed. Some may find shelter in existing nests but these will generally only tolerate one queen. Under favourable conditions the queen and therefore her nest, may survive for several years.