Takeover time. Argentine ants show their fighting mettle by attacking a harvester ant. CREDIT: MARC DANTZKER
Evelyn Strauss*
Argentine ants have become common pests worldwide, raiding kitchens from San Francisco to Johannesburg to cart away crumbs of chocolate cake and soetkoekies. These tiny invaders are more than a nuisance: They often upset ecosystems and reduce biodiversity by wiping out indigenous ants. "Argentine ants are one of the two or three ant species that are a huge problem in many parts of the world," says Daniel Simberloff, who studies biological invasions at the University of Tennessee, Knoxville. One secret to their success, a group led by Ted Case of the University of California (UC), San Diego, suggests on page 949, is intraspecies harmony.
Most ants defend their territory against other colonies, particularly those of the same species. But Argentine ants far from their South American home generally don't fight each other, although they still attack other ants. Now Case and his colleagues have obtained the first direct evidence for something that ant ecologists have suspected for some time: The Argentine invaders flourish because they lose some of their aggressiveness against their own kind. In lab studies, the researchers have shown that pairs of Argentine ant colonies that don't fight reach higher population numbers than pairs of warring colonies, possibly because they waste less time and energy. "This helps explain how the Argentine ant can outcompete other insects in its introduced range," says Philip Ward, an ant biologist at UC Davis.
Takeover time. Argentine ants show their fighting mettle by attacking a
harvester ant. CREDIT: MARC DANTZKER
Although most Argentine ants in Southern California don't attack each other, Andrew Suarez, a graduate student in Case's lab, found some that did. Taking advantage of this natural variation, the group reared pairs of colonies, both containing either the aggressive or nonaggressive ants, in a setup that gave the two colonies access via plastic tubing to a shared area that contained food.
The warlike colonies started battling immediately, and many ants died, says postdoc David Holway, a community ecologist in Case's lab. After a few weeks, workers stationed themselves at the openings to the plastic tubes as if to prevent incursions from members of the other nest. In contrast, workers from the nonaggressive pairs "commonly walked [through the shared area] in a file from one nest to the other," says Holway. Furthermore, in the peaceful setting, the ants foraged for food more actively.
After 70 days, the researchers counted worker ants, eggs, larvae, and pupae in the colonies. The nonaggressive colony pairs had significantly larger numbers than the warring colony pairs. The worker populations, for example, differed by almost 100%. Underscoring the costs of aggressive behavior is a separate study by Case's group, to be published in an upcoming issue of Biological Invasions. The researchers found that in the Argentine ant's native range, where the insects don't take over ant communities, they behave more like other ants and fight among themselves. "These results suggest that intraspecific aggression, which is common in their native range, plays a role in limiting colony size, reducing competitive ability, and allowing for coexistence with other species," says Holway.
The researchers don't yet know why the peripatetic Argentine ants lose their aggressiveness, although decreased genetic variability may be involved. In the Biological Invasions study, graduate student Neil Tsutsui showed that Argentine ants in California display less genetic diversity than they do in their native ranges.
The researchers point out that harmony probably isn't the only factor helping the wanderers thrive. The ants may also benefit from the absence of some natural enemies in their foreign homes and, in some locales, from meeker competition by indigenous ants. In Australia, for example, where the Argentine ants seem to fare worse than in California, "the ant competitors are arguably less wimpy than in California," says Ward.
The new findings may spark ideas about how to control Argentine ant invasions. "If Argentine ants could be made to fight more often in the introduced range, their population sizes would probably go down," says Holway. Reintroducing genetic diversity, he suggests, might generate increased aggressiveness, although it could enhance the adaptability of the ants in other ways. Furthermore, Holway cautions, no one has shown that fighting in Argentine ants has a genetic basis. For now, at least, Argentine ants in kitchens around the world will continue to enjoy their peace dividend.