[Photo] Argentine ants tending scale insects, whose sugary residue the ants eat when they go on their "all carb" diet.

Carnivorous Argentine ants that have invaded coastal California devour other insects. When that food's gone, the ants become vegetarians.

The amazingly adaptive behavior, detailed in what is the first study of this ant's diet, has allowed the invaders to spread successfully and rapidly.

The tiny dark-brown and black critters, an invasive species originally from Argentina, have infested coastal communities and displaced native ant species, even though many of the locals are 10 times larger than the Argentinians.

The new finding, based on an eight-year study of a population of ants in the foothills southeast of San Diego, reveals how the alien ants thrive so well in a foreign land. Their success is linked to their dietary versatility, according to results detailed in this week's online issue of the journal Proceedings of the National Academy of Sciences.

"Despite the fact that these species are known to cause ecological problems in many countries, scientists really didn’t know what they eat," said study leader David Holway of the University of California, San Diego.

Holway and his colleagues discovered that when Argentine ants first move into an area they become fierce predators of native insects. But as the ants eliminate their competitors ??? and thus, their main source of food, they switch from a carnivorous, protein-rich diet to a largely carbohydrate, sugar-water diet of honeydew nectar.

"Honeydew nectar is essentially digested plant sap excreted by aphids and scales," Holway said. "If you’ve ever parked your car under a tree and found your windshield covered with sticky stuff, that’s honeydew from aphids or scales."

This ability to switch from an all-insect protein diet to a carbohydrate-rich one also allows the ants to expand their populations because plant material is much easier to find in irrigated residential communities, the researchers say.

"By virtue of this great dietary flexibility, Argentine ants are able to consume a variety of sources of food, and it’s this ability to consume carbohydrates that contributes to their success," Holway said.

The discovery of the ants preferred sugar snack also offers a way to help California residents control ant infestations in their homes.

"If you cut down on watering to limit plant growth, Argentine ant numbers should decline," Holway said.