New Worlds Of Order, Argentine Ants Succeed By Outnumbering The Competition

January 12, 1999

They are quiet, live mostly underground, and they don't sting humans. But even insect lovers dread the arrival of Argentine ants (Linepithema humile). The voracious appetite of this alien species and its tendency to displace native ants has been well documented, but until recently scientists knew very little about the specific mechanisms used by the insect to gain control of a particular area. Now a new study, published in the January issue of Ecology, reveals some interesting findings about these aggressive ants.

The study, conducted by David Holway at the University of Utah (now at UC San Diego), examined the Argentine ant and seven species of native ants it displaces in the riparian woodlands of Northern California. Baits made of tuna and apple jelly were placed in areas where native ants and Argentine ants foraged in close proximity to one another. By studying their reactions to the bait and their interactions at the bait stations, Holway was able to determine several key things that make the ant such a successful invader.

Argentine ants located bait as quickly or more quickly than the native species. And along the invasion lines, Argentine ants controlled a greater proportion of bait than did their competitors. But in one-on-one interactions between individual Argentine ant workers and workers from the other species, the Argentine species experienced only mixed success.

Argentine ants employed both chemical defensive compounds and physical aggression during confrontations with other ants. Although chemical defense was typically more effective than physical aggression, the chemical compounds of the invasive ants did not appear more repellent than those found in the native species. In addition, Argentine ant workers were not able to overcome native ants on a consistent basis.

But the invasive species -- colonies were more successful on the whole than the native ant colonies. The discrepancy between worker-level and colony-level success suggests that numerical advantage was key in the Argentine ant's success.

The native ants in the study were subject to a common competition trade-off; their interference ability and their exploitative ability were found to correlate negatively. The colonies had to either search for food or fight, and could only succeed at one task by sacrificing the potential success of the other.

In contrast, the Argentine ants were not subject to this trade-off.

"These findings imply that Argentine ants secure a majority of available food resources where they come into contact with native ants," says Holway.

This may account for patterns of dominance within ant communities in general. It may also shed light on the success patterns of success of other introduced insect species.

"The strong competitive ability of the Argentine ant probably stems from a combination of its unique colony structure and the lack of natural enemies on this continent," says Holway.

In their introduced range, Argentine ant colonies support worker forces that exhibit little intraspecific aggression and may contain hundreds of queens. The queens disperse locally to form new colonies which often connect with the original colony. This enables a single "supercolony" to grow steadily larger over time. Infestations can grow at a rate of 200 meters (approximately 650 feet) per year.

Argentine ant invasions have proven problematic across the globe. Introduced to the United States by way of coffee shipments sometime around 1891, the species quickly gained a foothold in many moist regions of the country. They have become a particular problem in southern states, where their damage to fruit crops has been costly. They are probably the most common of the honeydew-feeding species, and they will farm insects such as aphids, mealybugs and soft scale bugs in order to feed upon their sweet residue. As a part of this activity, they will regularly protect these species from predators. They have also been known to occasionally invade beehives and interrupt beekeeper's practices and procedures. Their ability to out-compete native species may also be affecting the feeding habits of lizards and small mammals.
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