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 gallsthe bulbous plant growths that develop in a plant when it is colonized by parasitic organismscause 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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