FunNature & AnimalI have ants at home! Causes and solutions

I have ants at home! Causes and solutions

Ants are practically omnipresent animals. We know of more than 14,000 different species and they are present on all continents, with the exception of Antarctica; in almost all terrestrial ecosystems, from sea level to above 2,000 meters, and urban ecosystems are no exception.

Colonizing other people’s houses

Cities provide ants with a large number of places to live: from flowerbeds, tree groves or garden areas they can extend their nests under asphalt or cobblestones, which gives them greater security, provides them with necessary heat from the sun when temperatures are low, and prevents them from problems such as floods. And above all, the city generates a large amount of organic waste that the ants use as food.

Some particularly small species can infiltrate cracks in the concrete, and even pierce it, removing the particles that compose it grain by grain. That allows them to make nests inside human constructions. Others enter through cracks, gaps under doors, or fly through the window. And of course, it gives them access to a new and optimal ecosystem: our homes. In them they can achieve a stable temperature —the ants do not like the cold—, and a source of food available throughout the year. In fact, they are particularly common where they can find more food: in the pantry, in the kitchen and in the bathroom.

How do I invite them out of the house?

In the book ‘The Day of the Ants’, the author Bernard Werber exposes a peculiar approach, in the mouth of the character Edmond Wells:

By what right would your kitchen belong more to you than to the ants? You bought it, okay, but who? To other humans who made it with cement and filling it with food from nature. Your property is a convention between you and other people. But it’s just a human convention. Why should the tomato sauce in your cupboard belong to you more than to the ants? Those tomatoes came out of the ground! The cement came from the earth, the metal of your forks, the fruits of your jam, the brick of your walls, come from the planet. All the human being did was put names, labels and prices on them. That’s not what makes you “owner”. The earth and its riches are free for all its inhabitants… However, this message is still too new to be understood.

Bernard Werber, ‘The Day of the Ants’, 1992

Although it is a work of fiction, Edmond Wells is right. The only reason why we consider these things to be ours is because we have assumed it that way. The ants have found a favorable environment and have taken advantage of it, as human beings have done throughout history with almost the entire planet. Only in this scenario, we are the injured species.

However, our discomfort with a colony of ants swarming in the kitchen is understandable, so we have invented methods to get rid of them that, within limits, can work better or worse.

The best is, of course, to maximize hygiene, especially with regard to food or its remains. Ants go where there is food; if they find nothing available to them, they will leave. But that is not always possible, and sometimes more drastic solutions are necessary.

Trying with imidaclopride

Imidacloprid is an insecticidal chemical, available for sale in gel form. It is a compound that must be used with the utmost caution, as it is also toxic to humans and pets, and although manufacturers often use substances with an unpleasant odor or taste as excipients, prevention is always better than cure, and it is preferable to use it in places where people and pets do not have easy access.

The way imidaclopride works takes advantage of a peculiarity of ants: trophallaxis , or transfer of food from mouth to mouth. It is a way of transporting food to the interior of the colony, and the way in which the workers feed the larvae and the queen.

Imidaclopride acts in a delayed manner. It doesn’t kill the insect instantly, but takes several days to take effect, and it’s cumulative, so an ant can continue to consume the toxin until the lethal dose is reached. When a nutritive bait impregnated with this substance is made available to the workers, they take it as if it were food, and take it back to the colony. The toxin is transmitted from ant to ant by trophallaxis, and hopefully reaches the queen. That is when imidaclopride acts, intoxicating all the ants that have had contact with the bait.

Trials with imidaclopride baits show a delay of between 24 and 36 hours in the onset of lethal effects, maximum mortality between 7 and 14 days after application, and reduce ant colonies by between 77% and 100%; reductions that are sustained over time. However, imidacloprid does not work with all species.

Why does it sometimes not work?

There are more than 300 known species of ants in Spain, with very diverse characteristics. The most relevant when it comes to knowing how to avoid an ant infestation at home are related to reproduction, the number of nests per colony or eating habits.

For example, a bait in the form of sugary nectar does not work the same for an ant that feeds on this type of product in nature, than for another that feeds on seeds or insects.

Regarding the number of nests, those that organize their colony around a single nest —monodomics— are easier to eliminate than those that have several nests —polydomics.

Regarding the reproductive aspect, there are some species whose anthill is dominated by a single queen. These species are called monogynous, and when the time comes, the males and sexual females make a nuptial flight, after which the fertilized females found their own anthill. The death of the queen of a monogynous anthill implies the end of the colony. Imidacloprid baits with these species are therefore often very effective.

On the other hand, there are species with many queens; They are called polygynous. When one of the queens dies, another takes her place. This behavior entails a higher energy cost —the queens consume a lot—, but in return it has certain advantages. The constant replacement of breeding females allows them to have larger colonies, and also reduces the genetic relatedness between colony members. These species may be temporarily affected by an imidacloprid bait, but it is rare for all queens in the colony to be affected, so even if the product works temporarily, in the long term the ants are likely to return.

In fact, some of the species associated with human populations are polygynous and polydomic, such as the ants Monomorium pharaonis , Tapinoma nigerrimum , or the invasive Argentine ant ( Linepithema humile ).

The only possible solution in these cases is to maintain, as already indicated, the highest level of hygiene; if at all, continue to use the baits every time the ants proliferate; and if there is no other, perhaps learn to live with them.

This article has been reviewed by José Manuel Vidal Cordero, entomologist at the Doñana Biological Station (CSIC)


Boulay, R. et al. 2014. The ecological benefits of larger colony size may promote polygyny in ants. Journal of Evolutionary Biology, 27(12), 2856-2863. DOI: 10.1111/jeb.12515

Braness, G. A. 2002. Ant bait development: an imidacloprid case study. 4thInternational Conference on Urban Pests

PubChem. 2022. (2E)-1-[(6-chloropyridin-3-yl)methyl]-N-nitroimidazolidin-2-imine[imidacloprid] (N.o 86287518).

Rondeau, G. et al. 2015. Delayed and time-cumulative toxicity of imidacloprid in bees, ants and termites. Scientific Reports, 4(1), 5566. DOI: 10.1038/srep05566

Vidal Cordero, JM 2021. The Ants . CSIC, Cataract.

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