Brown ants in the house: the invasive garden ant and how to spot it

Most ants in British homes are black garden ants. When people describe brown ants that keep coming back whatever they do, turn up in enormous numbers, or gather inside sockets and light switches, it is worth considering a less familiar species: the invasive garden ant.

01What brown ants are

"Brown ant" is not a single species. People use it for any ant that looks lighter than the usual black garden ant. It can mean a black garden ant seen in bright light, a native species such as the brown tree ant, which nests in the dead wood of old trees, or the invasive garden ant. The last is the one that matters most to homeowners.

02The invasive garden ant

The invasive garden ant (Lasius neglectus) is thought to have originated in Asia Minor and has spread across Europe, mostly in soil and potted plants moved by people. It was first confirmed in the UK at a National Trust garden in Gloucestershire in 2009, where it was found in very large numbers and getting into buildings and electrics. It has since been recorded at other sites.

What sets it apart is the way it lives:

• Supercolonies. Instead of one queen per nest, a colony can have many queens spread across many connected nests. Numbers can run far beyond what a black garden ant colony reaches.
• No mass flying ant days. Queens usually mate inside the nest and the colony spreads by budding, with queens and workers walking off to start new nests nearby. You will not see the summer swarms of flying ants that black garden ants produce.
• Electrics. Workers are attracted to electrical fittings and equipment, and large numbers can gather in sockets, switches, junction boxes and outdoor lights. That can cause faults.

03How to tell it from a black garden ant

By eye, it is difficult. Workers are about 2.5 to 3.5mm long and a slightly paler, browner shade than black garden ants, but the reliable differences need a microscope. These are the signs that point towards it:

• Very large numbers, far more than you would expect from a garden ant colony.
• Ants active over a wide area of the garden, or along a whole street, rather than one trail to one nest.
• Ants inside electrical fittings.
• Ants active early in spring and late into autumn.
• No winged ants at swarming time, despite huge numbers of workers.

04Why DIY treatment struggles

Sprays and powders kill the ants they touch, but with many queens across many nests, the colony barely notices. Boiling water poured into one nest leaves the others untouched. Even baits, which work well on black garden ants, need to be the right type, in the right quantity, in the right places, and usually more than once. Disturbing a multi-queen colony can also spread it.

The useful things you can do yourself are the same as for any ant: keep food sealed and surfaces clean, fill gaps around doors, windows and pipes, and wash ant trails with a detergent or vinegar solution to remove the scent the ants follow. These make the house less attractive but will not remove the colony.

05Electrical safety

If ants are gathering in a socket, switch or consumer unit, do not spray insecticide into it and do not take the fitting apart yourself. Switch off the circuit if it is safe to, and have a qualified electrician check it. Treatment of the ants can then be planned around the electrics.