Wasps Squirt "Pepper Spray" From Heads in Fights, Study Says

John Roach
for National Geographic News
November 28, 2006
When female bethylid wasps are losing a vicious fight, they squirt an
insect version of pepper spray from their heads before beating a
retreat, new research suggests.

The chemical release is undetectable to humans, but it could represent a crucial behavior that may help biologists use the parasitic wasps as natural pest controls.

(Related news: "Drug-Sniffing Wasps May Sting Crooks" [October 27, 2005].)

Bethylid wasps kill the larvae of several species of crop pests by using them as hosts for their eggs.

For example, the bethylid species Goniozus legneri targets the larvae of pests that plague almond plants.

Female wasps often fight over prime hosts, says Ian Hardy, a biologist at Nottingham University in the United Kingdom.

"Hosts are rare, so [the wasps] don't get too many chances to reproduce," Hardy said.

"So when they do get an opportunity, they make the most of it. They guard the host. When they find one that's being guarded by another female, they try to get it off her" by starting a fight.

Wasp Fights

To better understand the nature of these wasp fights, Hardy and colleagues staged 47 bouts between pairs of G. legneri wasps inside a transparent chamber built to resemble the inside of an almond nut.

Using a combination of video cameras and real-time chemical analysis, the team found that after particularly violent encounters, the loser wasp released a chemical from her head as she ran away.

"The more nasty the fights are in terms of aggression … the much higher the chance of there being a chemical release," Hardy said.

"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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