Frog "Juice" May Be Next Big Bug Repellent
for National Geographic News
|July 24, 2006|
Although the chemical known as deet currently dominates the mosquito-repellent market, nature is full of bug-banishing strategies.
Native Americans once smeared goldenseal root on their bodies to ward off bites. And today candles made with oil from citronella grass help keep the bugs away from backyard barbecues.
Now a team of Australian researchers has isolated one of the most effective natural mosquito repellents yet from an unlikely source: smelly frog skin.
Craig Williams is an ecologist at the University of Adelaide in Australia who led the research team.
"Because frogs have delicate skins designed for absorbing water from their environment, they are vulnerable to things like infections, predators, and [parasites that live in skin]," he said (photos: frogs linked to eco-health).
"So frog skin is really a portable pharmacy" full of chemicals for keeping the amphibians healthy.
Mice in Frogs' Skin
Williams, whose latest findings were published last month in the journal Biology Letters, had suspected for some time that certain frogs emitted substances that protected them from bugs.
"A vast array of compounds had previously been isolated from frog skin: hallucinogens, glues, antimicrobials, and lubricants," he said.
"It made sense that frogs could also develop anti-insect chemicals."
To test this hypothesis, Williams and his colleagues applied low levels of electric current to the skin of five different Australian frog species.
The current caused them to expel larger amounts of their natural secretions than they normally would have.
The scientists then applied the frog fluids to the tails of mice and placed the rodents in cages with colonies of biting mosquitoes.
Mice treated with the frog-skin formula received only a few bites during the first hour of the experiment. Those that received a control treatment of water were almost always bitten in less than 15 minutes.
But mice with tails covered in a deet-based solution avoided bites for up to two hours. (Related feature: Bug repellents tested in the Florida Everglades.)
Williams says the frog extract may have potential to be used as part of a deet-free insect repellent for people who have adverse reactions to the chemical or don't want to expose themselves to it.
Stephen Regenold, a Minnesota outdoor enthusiast who often blogs about his product finds, is intrigued by the possibility.
"I have a friend whose wife is seven months pregnant, and I'd be wary of recommending she put chemicals like deet on her skin," he said.
"Researching natural forms of repellent could be helpful for a lot of people."
But human tests for a frog skin-based repellent haven't been carried out, and Williams and his team already foreseeor foresmellsome obstacles to making a commercial product.
"We tried putting some of the chemicals from frog skin in a lotion," he said, "but the odor was too strong."
Some of the frogs' secretions smell of rotting meat, the team reports. Others give off aromas of nuts or thyme leaves.
The researchers say the strong smell is what repels biting insects.
Brian Weekley is president of the Minnesota-based repellent company Bugg Products.
He notes that just because a compound is natural, that doesn't mean it will be practical for people to wear or for companies to manufacture.
"Every natural compound needs to be tested in humans for safety and efficacy, and the government approval process can cost over [U.S.] $300,000," he said.
"We'll have to wait for more scientific results to see how promising this repellent is."
For now, the University of Adelaide's Williams isn't putting much store in commercial ambitionshe's more focused on finding out what other properties of frog secretions might be useful to humans.
"We need to identify more of the chemicals liberated by frogs and relate them to real-life functions, like mosquito repelling, antipredator cues, and sexual cues," he said.
"There's a lot we don't know about frog chemical ecology."
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