The scientists report in the March 30 issue of the Journal of Animal Ecology that they were able within a matter of weeks to increase the abundance and encourage the spread of Argentine ant ants by irrigating normally dry land. Once irrigation ended and soil moisture declined, the researchers found, the number of ants returned to pre-irrigation levels.

Biologists have long suspected that water is critical to the survival of the tiny ants, which thrive in coastal California and invade new subdivisions that have transformed much of Southern California's arid landscape into a lush oasis. But until now, no one knew for sure.

"This is the first, definitive study that provides unambiguous experimental evidence that soil moisture can control invasions of Argentine ants," says David Holway, an assistant professor of biology who conducted the study with Sean Menke, a graduate student at UCSD.

It also suggests that residents who stringently limit watering their gardens and yards may be able to reduce ant problems without using costly toxic pesticide treatments.

"Our study shows that if you irrigate, you may well have ant problems," says Holway. "And if it's dry, you're not going to get Argentine ants. If it's at all possible to limit your irrigation, you can decrease problems associated with Argentine ants."

The tiny dark-brown ants, which are about three millimeters in length, are thought to have entered the United States aboard ships carrying coffee from Brazil during the 1890s, then expanded throughout California and the southern parts of the United States.

In the Southeast, their proliferation may be now limited to some extent by the introduction of fire ants. But in California, where those competitors are largely absent, the ants thrive, killing and displacing native ants. In Southern California, Argentine ants are second only to termites as household pests.

The UCSD biologists conducted their experiments at five sites San Diego County: two reserves in the University of California Natural Reserve System (the Elliot Chaparral Reserve near the Miramar Naval Air Station and the Dawson-Mono Reserve near Carlsbad), the Los Pe�asquitos Canyon Preserve, the Cabrillo National Monument and Tijuana Slough National Wildlife Refuge. At these ecologically different sites, the biologists manipulated soil moisture with drip irrigation for six months during the dry season, from May to October, and observed the effects on the ant populations.

After three months, the Argentine ants had 54 percent more nests in the irrigated sites. And when the water was shut off, the ants gradually retreated, reaching the levels of control sites after another three months.

Because irrigation increased plant growth around the study sites, the scientists also conducted experiments in which they suppressed plant growth with an herbicide. They discovered that the ant populations were 38 percent higher in the irrigated plots with plants than in those without plants, probably because of the presence of aphids, which provide an additional source of food for the ants.

However, the slight increase in the ant populations provided by plants was far less than that provided by water alone, leading the scientists to conclude that water, not plant cover, was the primary attractant for Argentine ants.

"These results are extremely clear," says Holway. "If you have wet soil outside of your home, if you irrigate your yard frequently, you're providing an environment that is attractive for Argentine ants."

The results also have implications for land-use managers who must deal with streams of irrigated water and other urban runoff from suburban developments year round.

"The extent to which urban runoff can be reduced or contained, problems associated with Argentine ants should diminish," says Holway, adding that he knows first hand how much of a nuisance Argentine ants are for residents in Southern California. "Whenever I'm out in the field and someone asks what I'm doing and I tell them I study Argentine ants, the first thing they want to know is, How do I get rid of the ants from my home?"

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