Ants in the house: how to get rid of them and keep them out

First find out where the ants come in and where they are heading. Then remove accessible food and moisture, clean the surfaces, and seal the gaps you can reach. If the ants still come back, the next step is a targeted approach, such as bait, or advice from a professional.

The ants you see in a room are usually a small part of the colony (the family of ants living in one nest): workers look for food, while the nest and the egg-laying queen, or several queens, stay elsewhere. So removing the visible ants does not yet mean the problem is solved.

1. Watch before you wipe. Follow the ant trail for a few minutes and see where the ants come in and where they are heading. Once the trail is cleaned, this is much harder to work out.
2. Check the places you can reach: the food cupboard, around the sink, skirting boards, door and window frames, and where pipes pass through the wall. Do not open electrical equipment or take walls apart: if the trail disappears somewhere out of reach, that is useful information for a professional.
3. Remove food. Pick up crumbs and spills, especially sweet ones. Store food in tightly closed glass or hard plastic containers rather than cardboard boxes or paper wrapping, and use a bin with a tight lid. Keep pet food stored closed too and keep the area around the bowl clean; your pet should always have drinking water.
4. Deal with moisture. Fix dripping taps and leaking pipes, dry areas under the sink or in the bathroom that stay wet, and do not leave standing water.
5. Clean the trail. Wipe surfaces where the ants were walking, especially near the entry point, with soapy water or an ordinary mild detergent; you can also pick the ants up with a vacuum cleaner. This removes visible ants and their scent trail for a while, but it does not deal with the nest. If you are already using bait, do not wipe the trail around it (see below).
6. Choose the next step. If the entry point is a gap you can reach, seal it with caulk. If the ants keep coming back after food is removed and surfaces are cleaned, or you cannot find where they get in, consider bait or professional help. If you use bait, decide in advance when to seal the gap the ants use to reach it: that is the route along which they carry the bait to the nest. Follow the label or the professional's advice.

Ants mostly come indoors looking for food and water, and sometimes for shelter. According to the University of California Statewide Integrated Pest Management Program (UC IPM), a sudden appearance often follows a change in the weather, such as heat, drought or heavy rain, or a drop in food outdoors.

When a worker ant finds food, it leaves a scent trail (a pheromone) back to the nest, and others follow it. That is why ants often move in a single line along the same route.

Seeing ants does not mean the home is dirty, and it does not mean the nest is necessarily inside. Often they come in from outside through gaps, around doors and windows, or through pipe openings. Some species do nest in buildings: behind skirting boards and trim, under kitchen countertops, in wall voids, or in moisture-damaged wood.

If ants keep appearing in the same place every day even though food has been removed and the known entry point is sealed, it is worth finding out whether the nest is inside the house or in a wall.

A spray kills only the ants it hits directly. According to the University of Kentucky Department of Entomology, the ants walking over open surfaces are a small part of the colony, so spraying only the ones you see usually fails. UC IPM notes that sprays are usually not needed indoors and give only temporary control.

Bait works differently: it combines food that attracts ants with a slow-acting insecticide. Workers carry the food to the nest and share it with others, including the queen. The slow action is essential: the ant has to get the food back to the nest.

An enclosed bait station does not trap ants: the ant goes in, feeds, and carries the food back to the nest. The station reduces direct contact with the bait, but it does not make it completely safe.

Bait does not work the same way on every species. Ants prefer different foods, some sweet, some protein or fat, and according to UC IPM, a bait that does not attract the species present gives no control. UC IPM also advises placing bait outdoors, by the trail or the entry point, where possible, and indoors only when the problem is serious and you cannot find where the ants get in, because bait placed indoors can draw ants inside.

If you use bait:

• Choose a product whose label lists ants and the place where you want to use it, for example indoor living areas. The label sets the rules for placement, replacement and safety.
• Place it beside the ant trail, where the ants actually walk, not somewhere random.
• Do not use sprays or cleaning products around the bait or on the trail leading to it: they put ants off the bait. Normal kitchen cleaning carries on everywhere else.
• Place it out of reach of children and pets, and not next to food or on food-preparation surfaces. Bait contains food ingredients and attracts many animals.
• Check within the first day whether the ants are feeding. If they ignore the bait, remove it and try another product whose label fits, or contact a professional. Do not use more than the label says.
• Results take time. In the first days, more ants at the bait may mean they are feeding and carrying it to the nest; on its own, that is neither a sign of failure nor a guarantee of success. According to the University of Kentucky, activity often drops within a few days, while according to UC IPM and the University of Minnesota, full control can take several weeks or longer. The time depends on the species, the product, the size of the colony and how well the bait is accepted.

Ants often come back because food or moisture is still available (a bin, a pet bowl, a leak under the sink), because the entry point is somewhere else or there are several, or because the nest is out of reach, for example in a wall void. With bait, common reasons are that it does not attract this species or that a spray or cleaner was used around it. These are observations, not a diagnosis: often only identifying the species and an on-site inspection show the exact cause.

One exception matters in particular. According to the University of Minnesota, using an insecticide on Pharaoh ants (Monomorium pharaonis) can make the colony split and form new colonies; they are hard to eliminate, and an experienced professional is recommended. This does not apply to every ant or every product, and you cannot recognise a Pharaoh ant by small size and colour alone. So if small ants keep appearing in large numbers in different parts of the home, have a professional identify the species before you reach for a spray.

In an apartment building, if ants appear in several apartments or in shared areas, raise it with your neighbours or the building manager, such as the homeowners' association: an ant trail can cross the boundaries of a single apartment.

According to the University of Kentucky, professionals have more experience with ants and a wider range of tools. Contact an experienced pest management professional and ask them to explain which species they are dealing with, what they will use, and where. This is especially sensible if:

• Activity continues even after food, moisture and entry points have been dealt with and the product label has been followed.
• You suspect the nest is somewhere out of reach, for example in a wall or under the floor.
• The problem involves several rooms or neighbouring apartments.
• Winged ants repeatedly emerge indoors.
• You see activity around damp or damaged wooden structures.

Colour and size are not enough to identify an ant species. The University of Minnesota Extension notes that ants are often hard to identify even from a photo, and different species may need different treatment. So “black”, “red” or “small” is a description, not a species name: a black ant is not automatically one particular species, a small reddish ant is not automatically a Pharaoh ant, and a red ant is not automatically dangerous.

Behaviour often tells you more than appearance: where they show up, where they are heading, and whether they keep returning to the same spot. If you contact a professional, these observations will help.

Winged ants are the colony's reproductive females and males, so not every winged ant is a queen. One or two winged ants may have flown in from outside through an open window or door. If they appear in a room repeatedly and in numbers, according to the University of Minnesota, that is a sign of a nest inside the building.

A winged ant is sometimes mistaken for a winged termite. An ant has a narrow, pinched “waist” and elbowed antennae, and in winged ants the hind wings are shorter than the front wings. If the insect does not match these features, leave the identification to a specialist.

Some large ants, for example species of the genus Camponotus (carpenter ants), nest in damp or decayed wood and dig galleries in it, but they do not eat the wood. If you repeatedly see ants by a wet wooden structure, fix the source of moisture and ask a professional to assess it.

Home tips often mix up two different things: ordinary surface cleaning and homemade poison. The first is a useful step; the second is in effect a homemade pesticide.

“Natural” or “homemade” does not automatically mean safe. According to the US National Pesticide Information Center (NPIC), home recipes are often untested, do not say how much to use, where or how often, and carry no safety warnings; even a low-toxicity ingredient can be harmful in excess. For each remedy below: what it may be useful for and its main limitation.

• Soapy water or a mild detergent. Useful for wiping away the trail and its scent and removing visible ants for a while. Limitation: it does not deal with the nest and is not used around bait.
• Vinegar diluted with water. According to the University of Minnesota, wiping the trail with it temporarily disrupts ant activity. Limitation: the effect is temporary, and it does not destroy the colony.
• Baking soda, salt, cinnamon, essential oils and similar. They are often presented as a “barrier” or a “poison”. Limitation: the sources this guide relies on give no basis for recommending them as reliable colony control. That does not mean any effect has been proven impossible.
• Homemade boric acid or borax mixed with food. Limitation: this is a homemade pesticide with no label, no dosing and no safety warnings, and a sweet mixture also attracts pets. A manufactured bait that may contain boron compounds is a different thing: it has a label and rules of use, and a homemade mixture is not its equivalent.

Ants in the house are usually not an emergency, although some species can bite or sting, and misusing chemical products adds risk of its own. The US Environmental Protection Agency (EPA) recommends prevention before chemicals: removing sources of food, water and shelter.

If an ant bites or stings you, according to the NHS, it is usually not serious: there may be pain, itching or swelling that passes within a few hours or days. Call 112 immediately if the lips, mouth, throat or tongue suddenly swell, breathing or swallowing becomes difficult, the skin or lips turn blue, grey or pale, or the person suddenly becomes very confused, drowsy or dizzy. Seeing ants is not in itself a reason to call 112: call 112 when the immediate arrival of emergency services is vital.

If you use a chemical product:

• Use only a product whose label lists ants and the place of use. Do not use a product meant for outdoors inside the home: according to the EPA, it stays toxic longer indoors.
• Do not use more than the label says. According to the EPA, using too much pesticide can endanger your family's health.
• Prefer a ready-to-use product that needs no mixing, and do not mix household chemicals together.
• Do not spray a whole room. Apart from baits and traps, a pesticide is applied only to targeted spots.
• Place bait and other products out of reach of children and pets. An enclosed station reduces the risk but does not make the bait completely harmless.
• Protect food, dishes and food-contact surfaces as the label instructs.
• Keep pesticides in their original packaging, out of children's reach. Never reuse an empty container for anything else, least of all food or drink.

According to UC IPM, keeping ants out of the building and removing their food and water sources is the main approach. Once the ants are gone, a few routine habits help keep them away:

• Wipe up sweet spills and crumbs straight away, and keep open food in closed containers.
• Use a bin with a tight lid, and keep the area around the pet bowl clean.
• Fix leaks promptly and replace moisture-damaged wood.
• Seal accessible cracks by skirting boards, frames and pipe openings with caulk; this also keeps out other insects, such as stink bugs. Do not block vents or drains, and do not work on wiring or sockets yourself.
• Outside, trim plants and branches so they do not touch the walls or roof: ants use them as bridges.