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The Empire of the Ants

Posted by novometro on June 29, 2006

The first colonists arrived a century ago, probably as stowaways on a ship carrying coffee. From an invaders point of view, they could hardly have landed on a more hospitable shore than the San Francisco Bay. They disembarked and took control.

Known to scientists as Linepithema humile, the Argentine ant is one of the world’s great colonizers. And this is the time of year when Oakland residents, and other Californians from Ukiah to San Diego, are reminded that their cities are built on top of a shadowy metropolis, whose size and populations we can only guess at.

So much about the Argentine ant is a mystery. The handful of scientists who study it are not exactly sure why the bugs come streaming from their nests into homes when the weather turns hot and dry as it has in recent days, but Dave Holway, an ecology professor at the University of California, San Diego thinks he has an answer. The ants are looking for moisture.

Unlike other ant species, Argentine ants are highly opportunistic, says Mr. Holway. They can set up a nest almost anywhere, they can even camp out in tunnels burrowed by other types of ants. In the dry season they follow moisture. When it rains, they flee flooding. But it is not their adaptability that makes them so interesting, he says. It is their highly aggressive nature, and ability to eradicate competing ant species that fascinates him.

More Worlds to Conquer

Not only do they push out other ants, scientists have fingered the Argentine ant as the culprit behind a decline of the horned lizard population in Southern California. In addition to California, and large swaths of the South, the Argentine ant has colonized parts of the Mediterranean and Australia. Colonies have been discovered in New Zealand, where local officials hold up California as an example of a worst-case scenario if the invasion isn’t stopped.
The native habitat of the Argentine ant in central South America contributed to their success in other lands, says Mr. Holway: “It’s like growing up in a tough neighborhood.” Not only aresurrounded by other fierce ant species on their home turf in Paraguay and northern Argentina, but they are subjected to frequent floods.
While the Argentine ant is merciless when it meets members of a competing species, in California, they are noted for their passivity when encountering one of their own. An Argentine ant from a colony in San Diego could be dropped in the middle of an Oakland nest without a fight.

Supercolony

This behavior has led scientists to speculate that a so-called supercolony extends from Mendocino County down to Baja. Other researchers have raised doubts about the supercolony theory, and the answer ultimately depends on one’s definition of supercolony. And who really cares where the boundaries of a colony lie, so long as they don’t encroach inside the house.

The native habitat of the Argentine ant in central South America contributed to their success in other lands, says Mr. Holway: “It’s like growing up in a tough neighborhood.” Not only aresurrounded by other fierce ant species on their home turf in Paraguay and northern Argentina, but they are subjected to frequent floods.

Mr. Holway has newly published research demonstrating that the ants follow water. His research found that ant populations increased by 38 percent in well-irrigated plots. Mr. Holway says that one way to limit the presence of Argentine ants near one’s home is to water the lawn less frequently.

Old Enemies, New Battlefield

And another remedy of sorts may be on the way. After a century of being the biggest bully on the anthill, the Argentine ant could be facing competition. An old foe is making its way north. The red imported fire ant, from another tough neighborhood in Latin America, has been spotted in San Joaquin County.

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