Friday, June 18, 2010

Argentina's Six-Legged Invaders

There are countless unwanted Argentines in California, but they keep a far lower public profile than the barras bravas (soccer hooligans) expelled from South Africa during this month’s World Cup. Rather, they’re six-legged invaders who arrived surreptitiously in fruit shipments at the port of New Orleans about a century ago, and spread by rail around the country. Today, Argentine ants (photograph courtesy of Penarc) form mega-colonies that extend from the San Francisco Bay Area to San Diego. Coastal California’s mild climate, apparently, is particularly conducive to them, though they are also troublesome in Mediterranean Europe, Japan, and New Zealand.

According to the Smithsonian Institution’s Mark Moffett, the “Indiana Jones of Entomology,” Linepithema humile is an aggressive species that has displaced or even wiped out native ants to the point that California now has an “ant crisis.” Unlike the state’s native species, Argentine ants do not disperse seeds of indigenous plants, for instance, and are unpalatable to native lizards. They also cause crop damage by encouraging aphids.

In their home range in the Río Paraná lowlands, Argentine ants co-exist with other ant species, but in California they don't always co-exist among themselves - in reality, there are competing colonies at war with each other. They distinguish themselves by scent and, along the borders between competing colonies, their mortality rates can be astronomical.

Those mortality rates, though, do not keep them from being a garden and household nuisance - according to Moffett, the average back yard garden in San Diego may be home to more than a million of these tiny creatures. During the rainy season, especially, they stream through cracks in the foundations or gaps in windows, to the point that it’s impossible to leave a dirty dish in the kitchen sink without attracting hordes of them.

Moffett concludes his new book, Adventures Among Ants, with a chapter about the Argentine ant. He also spoke about the book, and Argentine ants, in a recent interview on the PBS radio program Fresh Air, with Terry Gross.

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