Southern California residents, having weathered fire, floods, riots, and a 6.7 earthquake in recent years, are now enduring—what else?—pestilence. The culprit is Linepithema humile, a tiny ant that hitchhiked to New Orleans aboard ships from Argentina (or perhaps Brazil) sometime around the turn of the century and has since spread over much of the United States. Last August, University of California at San Diego researchers reported that Linepithema, besides being a tenacious household pest—They don’t sting or bite humans, but once inside your house they’re a pain to get rid of, says graduate student Andrew Suarez—is also wreaking agricultural and ecological havoc in California.
The Argentine ants, as they’re called, are prolific and mobile. Each colony contains multiple queens, each queen produces thousands of eggs a day, and the same ant family, ucsd ecologist Ted Case has discovered, may found colonies that are miles apart. Too much rain, or too much heat, and the ants flee their shallow nests to colonize the cooler, sheltered habitats of humans—in droves.
The threat they pose to agriculture is more indirect but involves perhaps more serious consequences. The ants love honeydew, the sweet excrement of aphids, says Suarez. They’ll actually cultivate aphids and protect them from other predators. Aphids, of course, are the bane of farmers and backyard gardeners alike, destroying tomatoes, citrus fruits, and other crops by sucking juices from leaves and stems.
While pampering aphids, the Argentine invaders are savaging California’s native harvester ants, ecologically useful insects that consume, and thus help spread, plant seeds. Argentine ants don’t eat seeds, but in battles for territory they mob and dismember the larger harvester ants, as shown in the photo at left. The impact of that battle reverberates up the food chain—to horned lizards, for example, which prey on harvester ants. By strapping dime-size radio transmitters to lizards’ backs, the ucsd researchers found that as soon as Linepithema drives the native ants out of an area, the lizards invariably leave, too, and change their diet from ants to beetles. What effect that has on the lizards’ ability to grow and reproduce or on their own predators—such as birds and snakes—is still unknown.