Those tiny black ants that swarm your yard and your kitchen have laid claim to a far bigger piece of real estate. Studies by researchers from the University of California, Davis, and UC San Diego show that from Ukiah to beyond the Mexican border, California is one huge supercolony of Argentine ants.

Back home in Argentina, competition between rival colonies keeps their numbers in check, but most of the California imports recognize each other as family, said Neil Tsutsui, a postdoctoral fellow in the Center for Population Biology at UC Davis.

"In ants, usually their biggest competition is within the same species. But here, colonies are so closely related they even exchange workers," says collaborator Andy Suarez, a former UC Davis entomology postdoctoral fellow now at UC Berkeley.

The ants' lack of genetic diversity is the result of a population bottleneck, Suarez and Tsutsui found. Their close-knit sisterhood and the fact that their colonies have multiple queens have allowed the Argentine ants to displace native ants and become one of California's leading household and agricultural pests.

Tsutsui and Suarez, working with David Holway and Ted Case at UC San Diego, used DNA comparisons to show that Argentine ants in California are genetically similar to ants along the southern Parana River in Argentina. They are particularly close to ants from the port city of Rosario.

Efforts to identify natural enemies of the Argentine ant for biological control should focus on the southern Parana River watershed, Tsutsui said.

Argentine ants first arrived in the United States in 1891 in the New Orleans area. They are a major household pest and cause problems for farmers by shepherding aphids for their "honeydew" excretion. They kill native ants that are a major food source for native horned lizards and other animals. They also cause problems in other regions of the world such as the Pacific Islands, Hawaii, Southern Australia and South Africa.

The study is published in the September issue of Molecular Ecology.

Editor's note: An image of an Argentine ant is available. Contact Patricia Bailey for details.