The Argentine ants that infested my kitchen for the last couple of weeks are gone—for now. But when you’re dealing with Linepithema humile, all victories are temporary.
Since they first turned up in California around 1907, these tiny pests have become the state’s most abundant ant species. If you’ve seen an ant lately, chances are it was an Argentine. Like many exotic plants and animals, they’ve left their co-evolved predators and parasites behind—South American parasitic flies, in the ants’ case. Descendants of a handful of founders, Argentine ants in California show little genetic variation. This enables them to form “supercolonies,” sometimes with multiple queens. Ants’ lives are governed by their sense of smell, which allows them to distinguish friend (or kin, at least) from foe. To an Argentine ant from Berkeley, an Argentine from San Diego would smell like family.
The lack of conflict between neighboring colonies frees Argentines to invest more time and energy in foraging. They’re very good at it. Stanford’s Deborah Gordon, author of “Ants at Work,” says they devote more time to the search for food than native ants, are better at locating baits, and recruit more workers to a food source. They’ve also been known to drive other ant species away from baits and kill their queens. When Argentines colonize a new area, most native ants move out.
Argentine ants aren’t just a California annoyance: They’ve spread to every continent except Antarctica, and to oceanic islands like Hawaii. Swiss scientists have discovered a megasupercolony that stretches across northern Spain along the Mediterranean to Italy, comprising millions of nests and billions of individuals—all with the right colony smell. This may be the largest cooperative unit in nature.
The ants in our houses are only part of the problem. Wherever they’ve gone, Argentines have thrown natural communities into disarray by displacing native ants. This has not caused much concern so far (there is as of yet no California Native Ant Society) but it should. Ant species are not interchangeable. Native ants perform valuable ecosystem services; the invaders may push them out, but they can’t take over their roles.
Native ants are part of complex food webs. Horned lizards, those eccentric reptiles perhaps better known as “horny toads,” are specialist ant-predators. (We think of them as desert creatures, but I’ve seen one species, the coast horned lizard, in chaparral on the slopes of Mount Diablo). The lizards either refuse to eat Argentine ants or waste away on all-Argentine diets. Along with habitat destruction, the loss of their prey base has reduced horned lizard populations in California to dangerously low levels.
A wide variety of plants, from bush poppies to trilliums, rely on native ants to disperse their seeds. The seeds are packaged with nutrient-rich food bodies called elaiosomes that attract the ants. They haul the seeds away from the parent plant, where rodents and other seed predators might find them, and back to their nests. Discarded after the elaiosome has been consumed, the seeds often germinate in the ant-tilled soil. Unlike California native species, Argentine ants are not effective seed dispersers.
In South Africa, where many plants are ant-dependent, the Argentine takeover has threatened whole plant communities with extinction.
Some of California’s rarest butterflies, like the Bay Area’s Mission blue, have evolved an intimate relationship called trophobiosis with the local ants. The butterflies’ larvae secrete a sugar-rich substance when the ants stroke their bodies. The ants defend the caterpillars fiercely, carry them to food plants, and shelter them in their nests. To my knowledge, no one has documented Argentine ants as caterpillar-tenders.
So there are consequences when Argentines push out natives. Can they be stopped before ecosystems from California to New Zealand are destroyed by this miniature wave of globalization? Maybe. And, ironically, the Argentine ant’s vulnerability may lie in the pheromones that have allowed the formation of supercolonies.
The story involves a case of ant-butterfly mutualism and its exploitation by a third party, a rare wasp called Ichneumon eumerus, known from only two sites in the French Alps and two in the Spanish Pyrenees. The wasp, like many of its relatives, lays its eggs on the bodies of living caterpillars. Eumerus’s preferred victim is the larva of a blue butterfly that inhabits the nests of ants, where it repays their hospitality by feeding on ant larvae.
It’s impressive enough that the wasp is able to sniff out caterpillars concealed underground. But its secret weapon comes into play once it enters the nest. Eumerus has evolved a set of chemicals that mimic the smell the ants use to signal alarm. Instead of attacking the intruder, the colony defenders turn on each other. Chaos ensues, during which the wasp finds the caterpillar, lays its eggs, and leaves unharmed. When the young wasps reach adulthood, they use their chemical arsenal to make their own exits.
Although the ants in question are native Europeans, entomologists have found that the wasp’s weapon seems to affect several other species in the same way. If the scent can be replicated, we might just have an environmentally benign way of disrupting the colonies of Argentine ants, fire ants, and other pests. One more reminder— if you want to make a pragmatic case for conserving biodiversity—that you never know what small obscure creature may turn out to be really useful.