Those Awesome Argentine Ants

If you have lived in Riverside for any period of time, you have very likely encountered California’s most ubiquitous wildlife in your home. We are talking about Argentine ants, Linepithema humile (= “the humble Linepithema). These are the small (about 1/8 inch) brown ants that magically appear in your home out of nowhere. First, you might see two or three on a counter; come back in fifteen minutes, and there can be dozens (or more!)

Such ant attacks are often predictable. Beware of any major change in the weather because Argentine ants like to be comfy. If there’s a cold snap, then a warm house looks good to these critters. A few days of intense rain and their shallow nests will be flooded; a dry home becomes an attraction. And, of course, when the hot, dry days of summer are finally upon us, the mere humidity of a kitchen or bathroom can generate a full-scale invasion of thirsty ants. To add injury to insult, even though this species of ant doesn’t sting, the occasional individual will bite humans.

Indeed, these visitations are true not only for residents of Riverside but for Californians from the Mexican border to the Bay Area. As you have probably guessed, Argentine ants are not native to California. They are native to subtropical South America, specifically northern Argentina, southern Brazil, Paraguay, and Bolivia. But they are extraordinarily successful invasive species at the global level. In fact, they have successfully invaded subtropical areas in every continent except Antarctica. Islands are not immune either; Argentine ants are pests in various oceanic outposts from New Zealand and Hawai’i to Tristan da Cunha.

What makes L. humile so awesome? In this case, I am using “awesome” with its definition of “overwhelming.” Argentine ants are capable of forming “supercolonies.” To understand this Argentine ant superpower, we need a bit of ant social biology.

Of the 15000 or so known ant species, the typical social structure is that the ants live in individual colonies, each colonywith a single nest. A colony is composed of an egg-producing queen, a small number of drones (males), and a massive number of sterile female workers who are the queen’s daughters and are sisters to each other. With the exception ofreproductive tasks, the workers maintain the day-to-day functions of the nest, tending to the queen, drones, and larvae (baby ants). The social structure of an ant nest is similar to that of a beehive. Indeed, ants and bees are in the same order of insects called the Hymenoptera.

Interactions between unrelated ant colonies are antagonistic and competitive. Ants will also have aggressive behavior towards other ant species. And the same is true for Argentine ants in their home range. Inter-colony aggression limits how far any individual colony can spread.

However, suppose an Argentine ant colony stows aboard a transport and eventually has an opportunity to establish itself in a new land that is yet unoccupied by Argentine ants. If the newly founded colony is successful because the habitat is suitable and free of competition, it can become large enough so that the queen will lay eggs that will grow into queens. Each of these queens will establish their own adjacent colonies. In the case of Argentine ants, a single colony provides a genetic bottleneck such that a new colony that buds off from an old one is very closely related. They are so closely related the members of the two colonies do not recognize a difference. That is, the workers of the sister colonies recognize each other cooperatively. The two colonies with two queens behave as one.

Now, imagine what would happen if colonies with a shared ancestry continue to propagate. Things can get pretty extreme. Indeed, scientists have done experiments to find out just how big a giant multi-queen complex of colonies has become in California. They took individual workers from different locations and paired them to see if they acted mutually aggressively (that is, the ants recognized each other as a member of a different colony) or acted mutually cooperatively (that is, the ants recognized each other as a member of the same colony).

The results were striking. The scientists found that multi-queen colonies can be remarkably large. In all of California, there are only TWO multi-queen Argentine ant supercolonies, each spanning hundreds of square miles. One supercolony ranges from the Mexican border (at least) north to San Luis Obispo; the other supercolony ranges from San Luis Obispo north to and through the San Francisco Bay Area. Each supercolony is made up of many thousands (millions?) of nests and zillions of individuals. The numbers are mind-boggling and, frankly, awesome.

That’s what makes these guys so difficult to control in the broad sense. In supercolonies, the individual colonies are not autonomous; nests are clumped and connected. Distinctive trails indicate some of the inter-nest highways. Suppressing one nest has little consequence for the supercolony.

Nonetheless, the homeowner has options to limit ant attacks. UC IPM (see below) has some sound and straightforward management tips. Prevention methods include filling cracks and outside baiting before the weather changes. Workers will take the bait back to the nest and feed each other and the queen. Baiting crashes the local nests around your home. Boric acid-based sweet baits are natural and organic. If an invasion occurs in your home, sponging with soapy water will eliminate the invaders, and indoor baiting will crash the local nest that is the source of the colonists.

Given their remarkable success and supercolony social structure, California’s Argentine ants are likely to be with us for a while. The best we can do is to find a level of coexistence – ants on the outside, but not on the inside of our homes. Perhaps there’s a nugget of advice in that view for peace on Earth in 2025.

Happy New Year!

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