Now that the oak moths have abated in my yard, having picked clean my sad little coast live oak for the second time this year, I have a few weeks’ breathing space before the next cyclical pest makes its move. Once we get another spell of heavy rain, I can be expecting their visit, en masse.

You know what I’m talking about. Ants. Most of the time, we go about our business, they go about theirs, and everything’s fine. In a cold, wet winter, though, they like to be indoors as much as we do. And our houses suit them just fine, thank you very much. That’s when we spring into action with ant sprays, baits, and traps.

The ants that come to call aren’t just any ants--and they’re not native Californians, either. These are Argentine ants (Linepithema humile)--light to dark brown, smooth-bodied, without a sting. They arrived in this country in the 1890s, in shipments of coffee or sugar that were offloaded in New Orleans. From there they spread across the southern states, most likely by train, arriving in Baja California around 1907. Ever since, they’ve been on the move, conquering territory as far north as Chico and eastward into the foothills of the Sierra Nevada--with sporadic forays into our homes as they go.

The climate of coastal California is pretty near perfect for the Argentine ant, which thrives in regions with moderate temperatures and moderate moisture levels. It is now found in 21 U.S. states and on six continents. It does especially well in areas characterized by a Mediterranean climate of dry summers and wet winters, such as South Africa, southern Australia, Spain, and Portugal. Drier parts of Hawaii, too, such as Maui’s Haleakala Crater. It also does very well in human-modified environments, where moisture and temperature balances are somewhat regulated. Although its natural habitat is the ground, where it establishes nests just beneath the soil surface, it also happily inhabits cracks in concrete walls, spaces between boards and timbers, and even personal belongings tucked safely away inside our houses. This ant is a bold survivor.

Intruders Overcome Natives
California has about 200 species of ants. Over a dozen of those live in Stanford University’s Jasper Ridge Biological Preserve, a hilly expanse of grassland and woodland just upslope from the main campus. There, for the past 15 years, biology professor Deborah Gordon has been surveying the distribution of Argentine ants and native ants (and other arthropod species), trying to understand population dynamics and patterns of aggression and competition.

“We find that the Argentine ants are much more abundant around the edges [of the preserve] where there are residential areas, partly because people provide refuge where there are houses, but also because people carry Argentine ants in landscaping plants, including natives,” she said. In those areas where the Argentine ant has become established, it has completely changed local ant distributions. “In places where the Argentine ant hasn’t reached there are certain configurations of ants that tend to go together,” Gordon said. The harvester ant (Messor andrei)--the big black ant that collects seeds and adorns its nest with piles of chaff--and the carpenter ant (Camponotus semitestaceus), which feeds on dead insects, sugar-producing vegetation, or decaying substances, for example, often share higher-elevation territory. Each species moves out from a central nest to forage, remaining separate from other ant species in a manner consistent with competitive dynamics. Wherever the Argentine ant has been introduced, however, the previously distinct populations begin to mill around together, forming a random aggregation that weakens each group’s competitive advantage.

This disruption is not only bad for the ants; it affects the entire ecosystem. Native ants carry seeds around and, “when the Argentine ant comes along, it eliminates the other ants that are dispersing the native-plant seeds,” Gordon said. This can benefit invasive plant species such as star thistle, which, she pointed out, does better in areas where there are Argentine ants, possibly because there is less distribution of the native grass seeds that compete with the thistle. And although we think of bees and butterflies first when it comes to pollination, ants participate in that crucial activity too. So when their patterns of movement are disrupted, vegetation patterns follow suit.

The Argentines tend to establish long highways off which they forage. They work together so well that they are formidable opponents to any native ants in their path. Although they are relatively small--the wingless workers are 2-3 millimeters long, compared to the native harvester ant at 4.5-6 millimeters--size doesn’t impede them. They’ve got numbers on their side, as well as efficiency.

“It seems as though the Argentine ant isn’t succeeding because it’s more aggressive,” said Gordon, “but only because it’s such an effective searcher. So it finds the food first,” before the native species. “And once it gets there, the native ants don’t try to take it away from them.”

The invader ants will eat anything, and they’re relatively tolerant of both higher and lower temperatures; as a result, they can cover a wider range of seasonal conditions and habitats than native species. “The native species have evolved to deal with each other, but they haven’t evolved to deal with these intruders,” said Gordon.

Native Winter Ant Resists
There’s a lot we don’t know about Argentine ants--including their natural history in their lands of origin. In Argentina and Brazil, they are hard to find and little studied. Yet they do seem to be very territorial, forming discrete colonies with limited range. Intraspecific aggression helps them keep these lines drawn.

In California, in contrast, “Argentine ants from different nests don’t fight with each other much,” Gordon said. “It was thought that that meant they were all genetically related and the whole coast of California was essentially one big supercolony.” Through genetic and behavioral study, Gordon’s team has refined that understanding, observing that they maintain dispersed nests seasonally: during the wet months they live in one large colony, all together, and in the dry season they fan out and establish multiple nests, often with multiple queens. “We don’t know how they decide who ends up in which nest,” said Gordon. It does appear that even when they’ve established satellite nests, they are not strongly territorial. Ants still move around, contributing to colony maintenance. And they still get along just fine.

Gordon has a theory for the Californian population’s easygoing ways. When ants eat, they incorporate hydrocarbons into their bodies, chemicals that are then used to signal colony odor. In their South American homeland, each colony may have a discrete diet, and so each colony will signal “difference” through its smell. In California, Gordon said, this sensitivity to smell seems to be diminished. “That may have something to do with diet,” Gordon said--specifically, “the extent to which they are all eating McDonalds out of garbage cans. Maybe their diet is so homogeneous in the urban places where they are living because our diet is so homogeneous.”

One native species on the Jasper Ridge preserve, however, does seem to be withstanding the foreign onslaught. The winter ant (Prenolepis imparis) looks a lot like the Argentine ant, but it’s a little bigger. Like the Argentine ant, it is a dietary generalist, and it also tends aphids. (The aphids suck sugars out of the plant and then secrete “honeydew,” which the ants eat; in return, the ants defend the aphids from potential predators.) “It may be,” said Gordon, “that the winter ant can resist the Argentine ant. What we see is places at the edge that blink back and forth from year to year, where the Argentine ants move in and then get pushed back. By looking over the climate data for the last 14 years, it looks like wet years are better for Argentine ants.” Last year was dry, and sure enough, the winter ant’s range has expanded. Gordon’s team is watching this rivalry with interest, hoping to learn more about interspecific competition and the invasive-native balance of species.

They’re On Their Own Time
What about us humans? Can we push the intruders back when they come streaming into our territory--namely, our houses?

First, said Gordon, we have to understand what’s going on. They don’t come looking for food--though if we leave crumbs and food scraps on our kitchen counters, they won’t ignore them. Instead, in the winter (December through March) they come in looking for a warm, dry place--shelter from the rain and cold; and in summer (July and August) they come in looking for water.

In 2001, Gordon conducted an 18-month survey of 69 households around the Bay Area asking about ant infestations, and she learned something surprising: they came into people’s houses at precisely the same time, and they left at the same time, regardless of what measures people took to keep them out. “That means that putting out pesticides--well, you might as well pour them directly into the Bay. It really does no good against the ants. They come in when they’re ready, and they leave when they’re ready.”

Nevertheless, we do want to think we have control, don’t we--even if it’s an illusion? And so we take action. Some of the measures that people resort to, however, are downright toxic, such as malathion. Gordon mentioned a so-called chalk powder that is sold in Asian flea markets, “and it’s sold there only because it’s illegal--it’s such a potent carcinogen that it’s actually been banned by the EPA. It not only kills ants, but it’s not good for people or pets or anything else.”

There are more environmentally friendly, healthful actions that can be called into play. Plugging up holes in walls with petroleum jelly or poster putty is an option. “People use clove oil or lemon oil,” Gordon said; “or I use Windex: anything that’s strong-smelling. If you wipe down the place where they’re putting down a chemical trail, you can stop them for a while.” Until they find another way in.

Or until they decide it’s time to leave on their own. Because that’s the good news: “They would really rather be outside,” in their own nests, doing what Argentine ants do so well: foraging food out from under the noses of the native ants, trying to survive.