Tiny Wasps Alter Plant Growth to Attract Mates

John Pickrell
for National Geographic News
December 2, 2002
Females of a minuscule parasitic wasp species are able to induce the plants they live in to produce a chemical perfume that attracts the males of the species, scientists report.

The discovery, detailed online in the current issue of the journal Proceedings of the National Academy of Sciences, is the first example of an animal stimulating a plant to produce a chemical sexual attractant for it.

"This is evidence of a completely different way that insects and plants can interact at the chemical level," said entomology graduate student and lead researcher John F. Tooker of the University of Illinois at Urbana-Champaign.

"No one has really considered before that insects could manipulate plants to produce a kind of sex pheromone," he said.

Plant-Animal Interactions

Pheromones are chemicals emitted by animals—including humans—that serve as a form of communication with other members of the species. Most often associated with reproduction, pheromones can also be used to signal danger, to mark trails, or to identify food sources.

Scientists have known for many years that plant-eating insects can induce chemical changes in plants. When attacked, some plants produce chemicals that poison the animals feeding on them, or signal nearby plants to ramp up their defense mechanisms.

Tobacco and corn are examples of plants that employ an alternative strategy. When attacked by herbivorous moth larvae, the plants produce volatile chemicals that attract moth predators.

"It's almost like the plants are calling for help," said Tooker's supervising professor and co-author Lawrence M. Hanks.

Tooker and Hanks are working on a project to characterize insect communities in the rapidly diminishing Midwestern prairies of the United States.

Gall Wasp Strategies

As a by-product of that study, they discovered that females of a flea-sized prairie gall wasp, Antistrophus rufus, are able to alert males to their presence by altering the smell of the plants they live in.

The tiny wasps are born within the stems of the meadow herb Silphium. They spend the winter as larvae; in the spring, the males emerge first. The females remain completely sealed within the decaying stems until located by a male. Once located, the females chew their way out to mate.

Tooker and Hanks wondered how the tiny and largely flightless A. rufus males, with a roughly nine-day life span, were able to locate the females.

"We started to look for some of the cues that male wasps might use to locate female wasps in sealed stems," said Tooker.

The scientists suspected that changes in complex plant fragrances might be the answer.

"The females can't use a direct pheromone signal to attract a male [as many other insects might], because they are totally concealed within plant stems," said Tooker.

Working with colleague Wilfried A. Koenig at the University of Hamburg in Germany, Tooker and Hanks compared Silphium stems harboring females awaiting a mate with stems free of the parasites. They found that stems containing the female wasps produced a quite different ratio of strong-smelling defensive chemicals known as pinenes than the wasp-free stems.

The findings suggest that the galls—the bulbous plant growths that develop in a plant when it is colonized by parasitic organisms—cause changes in the ratio of pinenes produced by the plant, said Tooker.

This provides a helpful clue for males to locate mates long after the plant has died, he said.

The study demonstrates yet another way that parasites are able to manipulate the physiology of their hosts for their own benefit, said Arthur Weiss, an ecologist at the University of California-Irvine.

"Gall-makers are remarkable because they trick their host plant into providing a nutritious diet and protection from enemies and the elements," he said. "I don't think anyone anticipated...this eye-opening piece of work," he said.

The finding is novel and very important, agreed Warren G. Abrahamson, a plant geneticist at Bucknell University in Pennsylvania.

It offers "a new reason for host-plant manipulation by gall-inducing insects," he said.

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