Ants Pawns In Battle Of Wasps, Butterflies

Ben Harder
National Geographic News
May 30, 2002
Even when they win, they lose. Hapless ants, like mortals caught in a crossfire between competing gods, are exploited in turn by two other insects—a butterfly and a wasp. When the wasp attacks the larval butterfly, it drives the ants to attack each other, turning them into incidental casualties.

The ants, researchers have discovered, are manipulated by chemicals produced by both sides in the struggle. Now scientists are wondering whether humans can use the wasp's powerful chemical cocktail as a non-toxic form of insect control. The research is reported in the May 30 issue of the journal Nature.

The highly sophisticated chemical warfare that has evolved between the Ichneumon eumerus wasp and Maculinea rebeli butterfly in western Europe has resulted in Myrmica schencki ants being used as pawns.

The ants are duped by chemicals into accepting, nurturing, and protecting the butterfly caterpillar as one of their own. But when the wasp detects a caterpillar hiding inside an ant colony, it uses its own clever chemical trickery to sow confusion in the ranks of the ants, allowing it to gain access to the caterpillar.

The intruding wasp, by secreting a cocktail of compounds called pheromones, throws the ants into such a frenzy that they attack and even kill one another. In the chaos, the wasp slips unnoticed through the ant nest and preys on the unguarded caterpillar.

If the chemicals can be synthesized and mass-produced, they could replace the poisons and toxic repellants currently used in ant traps, researchers say.

Pheromones have long been [considered for] use in controlling ants, said Graham W. Elmes of the Natural Environmental Research Council's Centre for Ecology and Hydrology in Dorset, U.K. New pheromones he and his colleagues in Britain and Japan have recently discovered last longer than most such chemicals, making them particularly promising candidates for insect control.

Time could be running out, however. In recent years, the endangered wasps that produce the chemicals have been found in just four alpine meadows—two in southern France, two in northern Spain. None of the sites are protected from agricultural development.

A Cast of Critters

Elmes describes the Myrmica ant as "a rather splendid beast" with large antennae that make it slightly resemble a minute stag. The ants live in highly complex, underground nests hidden beneath inconspicuous openings.

Before developing into a beautiful dark-blue butterfly, the endangered Maculinea spends an early stage of its life inside an ants' nest. About two to three weeks after the female butterfly lays her eggs on a plant, the larval caterpillars drop to the ground.

"At this stage, they are very small and resemble an ant larva," says Elmes. The caterpillars also smell like ant larvae, which fools the ants into treating them as such.

"If found by a foraging Myrmica ant, they are picked up, taken back to the nest, and placed among the ant brood," Elmes said. Thus deceived, the ants feed and protect the caterpillar, while the caterpillar grows ever larger.

If it's lucky, the caterpillar will eventually have its fill and make its way out of the ant nest, where it eventually becomes a butterfly and takes wing.

But when Ichneumon eumerus wasps are around, the caterpillar often isn't lucky. And when a caterpillar finds itself in the crosshairs of the parasitic wasp, the luck of the host ants runs dry, too. "Ants and such wasps are deadly enemies," says Elmes.

All the same, it's the caterpillar the wasp is really after. Female parasitic wasps lay their eggs on the caterpillars. As the caterpillar matures, the wasp eggs develop into larvae and eat their way through the caterpillar, killing it.

To do that, though, the wasp must get by the ants, which naïvely protect the caterpillar as if it were one of their own.

Chemical Weapons Create Panic

The trick to the wasp's subterfuge is the chemical weapons it brings to bear. In their lab, Elmes and his colleagues tested secretions produced by the wasp. They identified four compounds previously unknown to science and two others of previously mysterious function.

Four of the six compounds affect ant behavior, the researchers determined through experiments. Individually, they either attract or repel ants, or cause them to become agitated and aggressive. Mixed together, the compounds form a cocktail that elicits a dramatic reaction among the ants. The ants are first drawn toward the wasp but, on contact with the intruder or her secretions, they run away—only to be attacked by their kin.

"As soon as an ant encounters another that has been contaminated by the wasp chemical, they seize hold of it and try to immobilize it by pulling an appendage," Elmes said. "This can be quite successful when more than two ants attack a third."

The attacking ants may themselves become contaminated in the process of immobilizing their nestmate, making them vulnerable to attacks by other ants.

"The fighting spreads [like] the bar-room brawl in old Westerns," Elmes said. "The hero pushes someone, who falls on three others, who immediately turn 'round and hit the man nearest them—so the fighting rapidly spreads out to envelop the whole bar."

"Meanwhile," Elmes says, "the hero emerges unscathed." As the usually cooperative ants tear one another to pieces, the wasp penetrates deep into the nest, lays an egg on the caterpillar, and makes its getaway.

The wasp's chemicals can still provoke ants to aggression 50 days after being secreted by the wasp, Elmes and his colleagues reported in Nature. That impressive staying power—longer than most pheromones last—would be an asset if the chemicals become integrated into future pest-control formulas.

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