Flora and Fauna Non Grata: Argentine ants

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I stopped at a cafe recently and bought a muffin on my way to work. Pulling into the parking lot at work, I left my car and spent the day with my usual chores. When I opened my car at the end of the day, the interior was covered with thousands of tiny Argentine ants. On the seat, in the doorway, on the door, everywhere. I quickly found the reason for the insect hullabaloo: three or four crumbs from my morning muffin lying on the driver’s seat.

Somehow an ant or ants had climbed from the parking lot pavement up a tire or two, snooped about in the vehicle until finding the crumbs, then communicating to other ants that tasty morsels were inside the Prius.

If you live in one of the places of Europe, Africa, Asia, Australia, or the Americas where Argentine ants have spread, you know this familiar story. These little insects are food-finding experts, invading campgrounds, office buildings, schools, and homes to feast on just about any type of food that isn’t completely sealed away.

Argentine ants first came to California in 1907 and have established themselves in about one third of the state, mostly in counties along the coast. Their earliest landing in the U.S. may have been in New Orleans in the late 1800s. They have been around so long and are so common most folks don’t think of them as nonnative or even invasive.

As its name implies, the Argentine ant is native to Argentina. More specifically, its native range is the basins of the Parana and Uruguay rivers that run through Brazil, Paraguay, and Argentina. This insect has been nicknamed the “Gheghis Khan of the ant world,” as it has spread so widely, with little getting in its way. Animals that prey on insects and ants tend not to eat them. Unlike other ants that form colonies that may compete with each other, Argentine ants have formed super colonies of billions, spanning thousands and thousands of square miles. They form these massive colonies wherever they have invaded; the largest one we know of reaches across Italy, France, and Spain.

Having multiple queens per nest and multiple nests to a colony furthers their resilience. Their scientific name is Linepithema humile. Linepithema refers to the tendency of the species in this genus to make long trails, or lines, linking nests or where they find or “farm” food, and humile simply means small.1

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If Argentine ants have invaded where you live, one thing you may notice is there are fewer, sometimes zero, other ant species. Argentine ants are very aggressive at pushing out other ants. I’ve worked on the little island at the nature center for about ten years, and I’ve not seen any other types of ants besides the invasive Argentine.

The success of this invasive is due in part to its adaptability. When Argentine ants first invade, they become relentless predators of native insects, mostly native ants. Once they have eliminated the competition, they switch from being protein-rich carnivores to subsisting mostly on carbohydrates, feeding on honeydew nectar from aphids and scales. The parasites’ nectar is basically digested plant sap.2

As the ants feed on the nectar from the aphids and scales, they promote the spread of the parasites to other leaves and branches, and even to other plants. It’s a curious phenomenon. I’ve seen the ants spread parasites on the plants I garden. Sometimes they only affect a few branches and won’t spread to other portions of the tree or shrub; this happens often with lemonade berry trees. I’ve also seen ants spreading both scales and aphids on native Baccharis plants—coyote bush and the broom plant—and they sometimes take over the entire plant.

In the video below you can see Argentine ants racing up and down a Baccharis. The dark spots on the branch are scales. Note the dead and dying foliage.

Scientists have documented this ant/parasite relationship that saps plants of their strength and possibly promotes the spread of plant disease. From my personal experience, Baccharis invaded by Argentine ants will lose color, their leaves going from green to yellowish. I’ve seen a few Baccharis plants die from ant invasions.

Native ants forage blossoms and will also feed on seeds; as such they perform ecological functions of pollination and seed dispersal. Unlike native ants, Argentine ants do not serve as seed dispersers. This means the plants lose one method of reproduction wherever the ants invade. When I watch bees and butterflies pollinate plants at the nature center, there are almost always Argentine ants crawling on and around the blooms, not native ants. Studies indicate bees are less likely to visit flowers that have Argentine ants crawling on them. At least one study in Southern California found the same thing happens with hummingbirds; they avoid blossoms that have these ants.

It is not only other ant species that suffer when Argentine ants invade, they also affect populations of spiders and beetles. Moreover, there is a cascading effect on other animals where these ants invade. For example, here in Southern California, the coastal horned lizard feeds on large ants. Where the smaller Argentine ant has invaded and wiped out the larger insects, the lizards lose their food source. This is a major factor in the decline of this California species.

Just on personal observation, the ants may be worsening. They now occupy every square inch of my greenhouse, with lots of them wandering about where they hadn’t before, such as on bookcases and places where I keep my tools.

I also see more plants afflicted with the ants and the parasites they spread. I would say that about a third of the Baccharis on our grounds show signs of being invaded by the ants and their parasitic friends. And in years past, I don't remember seeing any plants die from infestations as they do now. Our weather is warmer and more humid than it used to be. It’s possible this has something to do with it.

Their aggressiveness and ability to form super colonies make the Argentine ant almost impossible to eradicate on large landforms. But there is some hope for islands, where humans can perform comprehensive eradication projects. One of those is ongoing on Santa Cruz Island, off the coast of California. The ants have spread on the island since first being introduced there in 1980. The Nature Conservancy and National Park Service are using chemical methods to kill the ants.

How about you? Do you live where Argentine ants have invaded? How much of a pest are they? Do they affect the ecology where you live? Please share by clicking the “Leave a comment” button below.

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