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Why are ants invading my home?

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Megan Burke
Ants scouting in a San Diego kitchen on Aug. 31, 2022.

It happens to the best of us. You wake up one summer morning, casually stroll through your home, and are confronted with ants. Ants swarming on the cat food, ants marching one by one into your garbage, ants, claiming a beach head in your bathroom. Where do they come from? Why are they here? And what can make them go away?

Professor David Holway is in the department of ecology, behavior and evolution at UC San Diego. He studies the intricate structure of ant colonies, the different species of ants in California and why those tiny ants in your sink may be a problem for our ecology. He spoke to Midday Edition on Tuesday. The interview below has been lightly edited for clarity.

Q: The kind of ants I usually see these days are extremely small. Not the bigger ants that I remember at picnics on the East Coast. What kind of ants are these tiny ones?

A: Well, we have between 100 and 200 different ant species here in San Diego County. But the most common ant that comes into people's homes, especially at this time of year, is the Argentine ant, which is a species that's been in California for over 100 years.

Q: So they're not native to California?

A: That's correct, as their name would imply. They're native to southern South America, northern Argentina and surrounding regions, but have been introduced into new environments all over the world.

Q: How big is the colony of Argentine ants in California?

A: Well, that's a good question. Argentine ants in California form what are often called super colonies. And what biologists mean by this is that the workers from different locations tend to not act aggressively with one another. The super colonies are made up of ants that live in individual nests. And what makes the Argentine ant interesting is that they will relocate their nests often, throughout the year, in response to changing environmental conditions and opportunities such as food in people's homes.

Q: Do they pose a threat to our ecology since they are not from around here originally?

A: Well, they do. The Argentine ant is an interesting species because in addition to being an urban pest where it comes into people's homes, it's also a conservation problem. In the displacement of native ants is a well-documented phenomenon associated with Argentine ant invasions. But Argentine ants disrupt ecosystems in a variety of other ways as well. They interfere with plant pollinator interactions. They also interfere with seed dispersal mutualisms. And another way in which they affect our local ecosystems is that there are organisms that prey upon native ants that don't prey upon Argentine ants. And one that seems to have declined locally is the coastal horn lizard. Coastal horn lizards consume arthropods, but seem to like large bodied ants: harvester ants, carpenter ants, things like that. But they don't feed on Argentine ants.

Q: Why do the ants make incursions into houses? What are they looking for that they can't find outside?

A: Well, at this time of year, the Argentine ants is mostly looking for water and this will bring them into people's homes. And when they come into people's homes and they find food, they'll also take advantage of the food as well. The Argentine ant will also come into people's homes at the start of winter because they'll get flooded out of their nest sites. So the period of the year when Argentina seemed most conspicuous tend to be July through the first part of winter, November, December.

Q: But sometimes you find lines of ants in a closet or some other place where there's no water, there's no food. So how does that happen?

A: Well, they do explore their landscape, so they try to find nest sites that are favorable to them that have the right temperature and humidity. And in people's homes, this can sometimes result in them coming into portions of the house where there aren't obvious sources of water or food. But they tend to be temporary visitors in those areas and they will go away if there's not a suitable nest site.

Q: Now, occasionally you'll see one or two ants on their own. Does the colony send out scouts to see if they're entering a good habitat?

A: They do. So that's very common among ants. Scouts will look for food. In some cases, they'll look for new nest sites. And the Argentine ant has a very well-developed chemical recruitment system. So if a worker is looking for food, say, and finds what seems to be a good find, it will lay down trail pheromone from that food source back to the nest and workers will smell that trail pheromone and go out to the food. And if they like that food, they'll lay down more trail pheromone. And within minutes, depending on how far the food is away from the nest, within a short period of time in any case, you can get a large number of ants traveling from the nest to take advantage of food.

Q: Now, if you see an ant, the first reaction for many people maybe to get out the can of Raid. Is that an effective option?

A: Well, pesticides are often very effective at killing insects and there are a variety of products available that one can use to kill ants. Doing so is often unnecessary because, as we've discussed, ants are typically temporary visitors inside homes and pesticides have well known environmental impacts. Many of the pesticides that are commercially available can be environmentally persistent and be carried in stormwater into freshwater ecosystems where they have effects that are far reaching and extend well beyond use outside of a house, for example. The other thing I can say is that despite the fact that Argentine ants are usually perceived as a nuisance inside the house, they are completely harmless. They don't sting, and they also don't carry any diseases.

Q: Professor, what do you do when you see ants in your house?

A: Well, I try to figure out why they're in the house. And oftentimes they're coming in to seek out sources of water like we've discussed. But I have to say that we stopped irrigating very much in our yard several years ago. And the numbers of Argentine ants in our yard have dropped greatly to the point where we haven't had Argentine ants in our house for some time, well over a year. And we have done experiments in the field where we've used irrigation to encourage our Argentine ants to spread. And those experiments were very clear. Argentine ants really seek out water, and if the water is eliminated, Argentine ants will retreat.

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