In a study published in Molecular Ecology (September 2001), scientists from UC Davis and UC San Diego showed that California harbors a huge supercolony of normative Argentine ants, extending from Ukiah to beyond the Mexican border.

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 UC Davis Center for Population Biology.

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

Because they were initiated by a relatively small number of individuals, introduced populations of Argentine ants have reduced genetic diversity and are genetically similar to one another, the scientists found. This close-knit sisterhood allows Argentine ants to form large supercolonies, which then 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 a type of DNA fingerprinting to show that Argentine ants in California are genetically similar to ants along the southern Parana River in Argentina. Efforts to identify natural enemies of the Argentine ant for biological control should focus on this area, Tsutsui said.

Earlier research by Suarez and the same colleagues traced the rapid decline of coast horned lizards in California to indirect impacts of invading Argentine ants. The invaders displaced indigenous ants, the lizard's favored food source. They are not a palatable substitute.

In related research published in several journals, the four scientists also showed that the loss of genetic diversity in introduced populations led to reduced aggression among the ants, allowing the formation of the supercolony in which queens and workers mix freely among separate nests. The invaders wipe out indigenous ants through sheer numbers.