Since the Argentine ant arrived in California in 1907, the species has spread from the southern tip of the Mexico border to San Francisco. The L. humile — a foreign invasive species that kills off native ants in the regions it invades — has become a common pest control problem, disrupted populations of lizards and shifted the ecosystem of coastal California.

“One thing that we’ve done is to try to understand why the Argentine ant invades some natural areas in San Diego and not others,” associate professor in the ecology and neuroscience department David Holway said. “One thing that we’ve noticed is that the abundance of the Argentine ants is positively correlated with soil moisture. They tend to be very common in irrigated areas, urban environments [and] some agricultural areas.”

Experimentation, physiological work and environmental mesh modeling, which uses geographical information systems, show that the Argentine ant’s ability to invade natural environments in San Diego depends on the physical environment’s suitability.

Holway has been studying Argentine ants to find what factors cause invasion in certain areas, how they compete with and replace native species, how they affect the ecosystem after displacing native species. The search has an emphasis on how the ants contribute to the ecosystem.

Citrus farmers in California are using increasingly large quantities of pesticides against the Argentine ants, which mutually exchanges food with ants that produce honeydew. The increase in pesticides affects the produce and also could causes evolutionary changes in the targeted species of insects and aphids.

“One example of an indirect effect of that invasion is what happened to the horned lizard,” Holway said. “Namely, the horned lizards eat large-bodied native ant species — those are killed off by the Argentine ant, and when the Argentine ant invades and kills off harvester ants, the horned lizards don’t have anything to eat.”

The barrel cactus, local to the San Diego coast, has been similarly affected. The cactus feeds by trapping ants, luring them with nectar. The trapped ants also ward off herbivores from eating the cactus. Argentine ants have killed off the local ant species, making the cactus vulnerable to predation.

UC Berkeley ecology and evolutionary professor Neil Tsutsui sequenced the genome of L. humile, and discovered that the ant species can recognize and process various chemical signals due to their large number of chemical receptor genes. By understanding the chemical messages they send and receive, researchers may be able find other ways to thwart the destructive patterns of the ants.

Holway said an eradication of the Argentine ant is impossible. Because Southern California is irrigating in many areas, regions that have naturally dry soil become perfect habitats for the moist soil-loving Argentine ant. Efforts to decrease water usage will not only decrease the density of the ants, but will also help in conserving water.

Areas that practice water conservation seem to have fewer problems involving the Argentine ant compared to places that have not cut down on water usage.

Holway said places like the Torrey Pines State Park and at Cabrillo National Monument, have taken measures in their gardens and in their visitor centers by using less water than they did before.