Sniff . . . Pow! Wasps use chemicals to start ant brawls

Wasps sneak around in ant colonies thanks to chemicals that send the ants into a distracting frenzy of fighting among themselves, says an international research team.

HIDE AND SNEAK. Top: A wasp will inject its eggs into a rare butterfly’s caterpillars hiding deep in nests of ants (highlighted). By making the ants fight each other (below), the wasps avoid resistance as they slip into the nests. J. Thomas/Winfrith

The wasps infiltrate the colonies to hunt caterpillars hiding therein, explains Graham Elmes of the Winfrith Technology Centre in Dorchester, England. In the May 30 Nature, he and his colleagues identify components of the riot-inciting cocktail and suggest that it could inspire new ways to control pests.

The rare European Maculinea rebeli butterfly tricks little, red, stinging ants called Myrmica schencki into protecting and feeding its caterpillars. At first, caterpillars find shelter within tough seeds but eventually outgrow them and tumble to the ground. Their body chemicals mimic those of Myrmica ant larvae.

In response, adult ants bring the caterpillars home. The foundling “is just like a cuckoo,” says Elmes, and the ants feed it and let it grow in the well-defended nest.

A wasp of the species Ichneumon eumerus, known only from four European meadows, needs to find one of these caterpillars to serve as a depository for a wasp egg and to nourish the wasp larva that hatches. In 1993, Elmes and his Winfrith colleague Jeremy Thomas hypothesized that the wasp incites ant fights as a distraction on two occasions: when the youngster departs from the ant nest and again when a mother enters a nest to lay an egg.

Now, the researchers present behavioral and biochemical evidence of wasps’ deploying chemical disruptors. The researchers washed wasps in solvent and painted small glass pellets with the rinsings. Both the pellets and empty wasp pupal cases provoked aggression in ant colonies in the lab.

Collaborator Toshiharu Akino of the National Institute of Agrobiological Sciences in Tsukuba, Japan, and his colleagues identified six hydrocarbons in the washings and synthesized the chemicals.

Tests with these compounds applied to pellets showed that each does one of three different jobs, explains Elmes. Some attract the ants to a wasp. On contact, the ants pick up–and spread to other ants–other substances that make them aggressive. They also encounter still other chemicals that drive them away from the wasp. The overall effect of the chemical mix is that the ants fight each other and lose interest in the wasp.

“The wasp starts a saloon brawl,” says Elmes. The researchers estimate that such a fracas can distract up to 80 percent of the ants in a wild colony.

Philip Starks of the University of California, Berkeley, who has studied other wasps, points out that the fight-starting mixture may be even more important to the offspring than to the mother. “I know that if I was a freshly pupated wasp, one whose cuticle hadn’t yet hardened, I would be very worried about getting out of the colony before becoming ant chow,” he says.

“The wasps’ ability to elicit aggression among the ants while also repelling them is particularly interesting,” comments Andrew Suarez of the University of California, Berkeley. A close observer of the invasive Argentine ants, Suarez says that he sees promise in using discord as a way for people to control ants.

Joan Herbers of Colorado State University in Fort Collins, who studies slave-making ants, calls the wasp report “most interesting.” Some slave makers release a chemical that causes the ants that they’re raiding to run around in alarm. Another ant species takes the opposite approach, releasing an “appeasement” chemical that renders its victims passive, Herbers says. However, she says, the wasps “have a new twist.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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