"In every single physical bout we were watching, it was always the loser that released," he added.
The researchers are uncertain whether the chemical, known as a spiroacetal, is a signal or a weapon.
But the team speculates that the insects use the chemical like a human would use pepper spray on an attacker: a potent distraction that buys an opportunity for escape.
"I think the wasps release it when they are wanting to get away, and it somehow negatively affects the one receiving it," Hardy said.
He and colleagues report their findings in the November 22 issue of the journal Proceedings of the Royal Society B.
What's That Smell?
Juan Barrera is an entomologist at El Colegio de la Frontera Sur in Chiapas, Mexico, who studies bethylid wasps.
He says that it makes sense that G. legneri's chemical release could be a weapon for "tactical withdrawals."
"However, I think more studies are needed to discard other possible behavioral functions," he wrote in an email.
Overall, he says, understanding the basic biology of bethylid wasps will help researchers make more informed decisions on how to use the parasites as pest controls.
"The lack of ecological and behavioral information of these insects can explain many unsuccessful biological control programs in the past," he said.
For example, Nottingham University's Hardy says, the bethylid wasp Cephalonomina stephanoderis has potential as a control for coffee berry borers (get coffee facts, legends, maps, and more).
But when lab-reared adults were released into a coffee plantation during a trial program, only one percent of the population stayed put.
In 2005 Barrera and colleagues reported that C. stephanoderis releases a fecal-smelling chemical called skatole when agitated.
Hardy surmises that the lab-reared wasps were agitated during the bumpy ride from the lab to the field and were releasing skatole.
When the jar was opened, the strong chemical signal drove the wasps away.
In contrast, when wasp pupae were set in the coffee plantation and the insects emerged naturally, four percent of the wasps stayed.
Chemical release, Hardy said, is "probably a very strong candidate explanation for why they get a better ratio of establishment in the field when [the wasps] emerge naturally rather than [after being] shaken about."
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