Ants in the House: Identification and Signs

The black garden ant (Lasius niger) is the species most people find in the kitchen. Workers are 3 to 5 mm long and dark brown to black. They nest outdoors in soil, under paving slabs and at the base of walls, and follow scent trails into buildings to reach food.

Ants have a narrow waist and elbowed antennae, which makes them easy to tell apart from other small insects. Once one worker finds food, it lays a trail for the rest of the colony, which is why a few ants can become a steady line within hours.

Flying ants are not a separate species. They are the winged males and young queens of the garden ant, which leave the nest to mate on warm, humid days in July and August. Swarms often appear in many gardens on the same day.

A swarm usually lasts only a few hours and does no harm. If flying ants appear inside your home in large numbers, however, it can mean there is a nest under the floor or within the walls.

The pharaoh ant (Monomorium pharaonis) is a tiny, yellow-brown tropical ant only about 2 mm long. In the UK it survives only in heated buildings, so it is found in blocks of flats, hospitals, hotels and commercial kitchens.

Pharaoh ants have many queens and many linked nests hidden in wall voids and behind fittings. Spraying them makes the problem worse, because the colony splits and spreads. They also carry bacteria from drains and waste onto food, so they need a professional baiting programme.