"This is the key to their success," Tsutsui said. "It allows the population to boom—to grow to an extremely large size and overwhelm other species just by sheer numbers."
Scientists believe the founders of California's Argentine ant gang stowed away on United States-bound cargo ships from South America in the 1890s.
With no need to devote resources to fighting rival gangs—who were left behind in South America—the U.S. invaders quickly multiplied and wreaked havoc on the country's southeast and west coasts.
"In California we now have one huge supercolony [that stretches] from San Diego to north of San Francisco," Tsutsui said (California map).
(Related: "Ant 'Supercolony' in Europe Raises Questions About Getting Along" [April 2002].)
(Some scientists, however, believe the "supercolony" idea is false, arguing instead that the Argentine ants in California are from numerous colonies of genetically distinct ants.)
The Argentine ants have all but ousted the California natives, Tsutsui says. Other critters that eat only native ants, such as the coastal horned lizard, are suffering too.
Homeowners spend gobs of money on sprays and traps in a losing battle to block the invasive arthropods from their food and water.
And farmers routinely see aphids and scale insects—which the Argentine ants protect from parasites in exchange for the insects' sweet secretions—ravage their crops.
Tsutsui and colleagues identified a few small, isolated Argentine ant colonies in California that are distinct from the supercolony. These ants have a different chemical scent.
When the researchers pitted members from these distinct colonies against the supercolony ants, the rivals fought in mortal combat.
Surprised, the researchers compared the chemicals on the bodies of the rival ants and identified 15 compounds that appear to provoke aggression.
The team is now creating these chemical compounds synthetically in the lab and applying them to some colony mates in hopes of starting deadly feuds.
Each artificial chemical has provoked infighting, but the level of aggression differs, suggesting that each scent contributes only a small part to the cues for relatedness.
More closely related ants share more chemicals, and thus behave less aggressively toward each other, Tsutsui says.
"Not only does this research suggest a potential avenue for controlling Argentine ants, it also allows us to gain insight to the evolution of social structure," he said.
Deborah Gordon is a professor of biological sciences at Stanford University in California who was not part of the research team. She says the concept that large differences in hydrocarbons provoke ant feuds is well known.
"The question is, how much will this affect the success of Argentine ants as an invasive species?" she said.
Humans are the greatest help to the invaders, Gordon adds. Houses provide water in the hot, dry summer and warmth in the wet winter.
"It's an open question whether getting [the ants] to fight each other will offset the help we give them," she said.
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