Biologists at the University of California, San Diego have discovered that the proliferation in California of the introduced Argentine ant, a major pest that has invaded homes and displaced native species of ants in much of the coastal regions of the state, is due to the lack of genetic diversity among individuals up and down the coast.
In paper published in the May 16 issue of the Proceedings of the National Academy of Sciences, the scientists conclude that this reduced genetic variation has essentially allowed a giant "supercolony" of closely related ants to grow unchecked from San Diego to Ukiah, 100 miles north of San Francisco. In Argentina, by contrast, fighting among the more genetically dissimilar, territorial ants has managed to keep these insects in check and in smaller, much more sharply defined colonies than those in California.
"When we did our field work in Argentina, it was surprisingly difficult to find Argentine ants, compared to our experience in California," says Ted J. Case, a professor of biology at UCSD who headed the research team. "They are a relatively inconspicuous feature, both in the urban and in the natural environment."
Not so in California, particularly in the southern part of the state, where the ant is regarded as the number one pest extermination companies are called upon to remove from people's homes. "If you live in urban San Diego or Los Angeles, this is the ant that's in your house and is invading your garden and office," says Neil D. Tsutsui, a graduate student and the first author of the study.
The tiny dark-brown and black ants, which are about two millimeters in length, are thought to have entered the United States aboard ships carrying coffee or sugar from Argentina during the 1890s, then expanded throughout California and the southern parts of the United States. In the Southeast and much of the South, their proliferation is now limited to some extent by the introduction of fire ants.