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Rules of attraction

The Baltimore Sun

Many of us learn it by bitter experience. Now scientists say it's true.

Some people, thanks to their genetics, behavior, diet or some poorly understood combination of factors, have body chemistry that draws mosquitoes like linebackers to a loose football. Others just seem invisible to the bugs.

"I am irresistible to mosquitoes," said Michele Karanzalis, 33, a research project manager from Overlea. "I just try to stay inside a lot ... I start to get panic atacks after a while when I feel like I'm getting bit too much."

She's worn jeans and carried citronella torches in a futile bid to ward them off. But "mosquitoes bite through the jeans and fly through the flames. Yes, I am that irresistible."

Such bite victims are further tormented when people such as Randi Brown, a 33-year-old technical writer from Cockeysville, just shrug and say, "I don't know what it is, but I never get bitten."

She doesn't do anything special - no repellents, no particular foods. She just can't remember ever being bitten. "It seems to be only me," she said. "Every other member of my family is a mosquito magnet. It does drive them crazy. They say it's not fair."

Researchers have identified more than 300 chemical compounds that a human emits, chiefly from the skin, and they're trying to figure out which ones most influence mosquito behavior.

Some claim they've isolated a few ingredients that appear to make us undetectable to mosquitoes. They're working to formulate them into a new way of protecting us from biting insects.

What are they?

"I can't tell you those yet. We're still working on patenting," said Ulrich R. Bernier, a research chemist at the U.S. Department of Agriculture's Mosquito and Fly Research Unit in Gainesville, Fla.

They're not repellents, which are analogous to something that smells bad to the mosquito. Rather, they're more like a Klingon warship's cloaking device. making us undetectable, he said.

The hope is to publish findings next year, then seek a manufacturer to license the discovery, develop and market a product. He envisions an aerosol device that would release cloaking compounds into the air.

"It's looking pretty promising so far," Bernier said.

Mosquitoes can be dangerous. In the United States, they can carry potentially deadly West Nile virus, which killed 177 Americans last year. They can also transmit various encephalitis viruses.

Public health officials say the best way to stop the biting is to take precautions. Curtail the breeding by eliminating standing water around your home, from gutters to dog dishes. Avoid going out when mosquitoes are biting. When you must, use repellents with DEET and wear long sleeves, long pants and socks.

But persistent mosquitoes will find the most irresistible of us.

As dry as this spring was, "those little biters are rockin' out there. They're ready to go," said University of Maryland entomologist Mike Raupp. Besides the familiar dawn and dusk biters, Asian tiger mosquitoes arrived in the 1980s to add daytime bites to our worries.

It's the females that torment us. They need blood protein from birds or mammals to produce eggs. And those that feed on people have evolved elaborate mechanisms to find us.

"As human beings move through the world they leave behind a vapor trail," Raupp said. A major component is carbon dioxide [CO2], a byproduct of our respiration. Another is lactic acid, a breakdown product of muscular activity. It's exuded from our skin as a component of sweat.

Once they're downwind of us, mosquitoes' antennae, and mouth parts called palps, can detect our CO2 plume and other compounds. The American Mosquito Control Association says our body chemistry attracts them from nearly 40 yards away.

"And as they move out of the vapor concentration, they turn back in. They turn and correct and keep working their way upwind until they find you," Raupp said.

"Once they're upfront and personal, they're sensing moisture and a whole complex of chemicals," he said. They also can home in on our body heat, our movement and color.

Compound attraction

Your individual chemical complex is the key to how attractive you are to mosquitoes. Bernier's research captured between 300 and 400 compounds from the oily, waxy residue from human sweat. One, dimethyl disulfide, is a byproduct of bacterial activity. It appeals strongly to at least one species - Aedes aegypti - best known for transmitting yellow fever and dengue fever.

The sorts of bacteria and fungi common on human feet have also been shown to attract some mosquitoes, including Anopheles gambiae, the type that spreads malaria. So the Limberger-cheese odor of smelly feet, socks and shoes may make you a target. And it makes evolutionary sense.

"A foot is a great place to bite a human," Raupp said. "It's as far as you can get from your arms - arms and hands are lethal to mosquitoes."

Bernier's work with the lighter, more volatile components of sweat also zeroed in on acetone, a byproduct of fat metabolism. Adding acetone to the mix produced an "extremely potent" lure for yellow-fever mosquitoes.

But that's just one mosquito species among 176 in the United States. "The rest of the species we certainly don't understand at all," Bernier said.

Bernier and his team have also looked for the compounds humans give off that mask at least some of us from the pests.

It's not that different people produce different compounds, he said. "It's just that we produce them in different ratios," some attracting, some masking. In fact, the blend of chemicals we exude probably varies, not just from one person to the next, but for each of us from day to day, depending on our activity.

It's tempting to reason that we could reduce our CO2 and lactic acid output, and keep our body chemistry to ourselves, by remaining still, slowing our breathing and avoiding activity that would produce sweat.

But "that's unknown at this time," Bernier said. It's also unclear whether we can ever reduce our emissions of these compounds - say, by washing our feet - enough to matter.

Ejecting the worst mosquito magnets from our midst might help if cattle and their pests are any guide. Dutch scientists, investigating how insects use smell to identify their targets, discovered that some individual heifers were 64 times more attractive to biting flies than others. By swapping them for less attractive heifers, scientists found they reduced the whole herd's "fly load."

Altering allure

Bernier does believe some things we do can alter our individual mix of attractants and masking compounds, if not always to our benefit. "I have a feeling that diet and exercise can play a huge role," he said, recalling how, when he was "sedentary and overweight," he was invisible.

Then he began to eat healthier food and engaged in "massive amounts of exercise ... I am now at the high end of attraction," Bernier said. The bugs love him, suggesting that being fat and sedentary might make us unappealing to mosquitoes.

Lots of people have changed diets to ward off bites. Garlic, a peculiar British concoction called Marmite, vitamins B6 and B12, yeast - the list goes on - have all been advanced as mosquito shields. Bananas are said to attract them. But it's largely folklore, unsupported - even debunked - by science.

Other products, too, claim protective properties. Skin-So-Soft formulations, made by Avon, have been promoted as mosquito repellents. But its Bug Guard and Bath Oil formulations worked for an average of just 10 minutes in University of Florida tests.

Here's what is known, according to the American Mosquito Control Association:

Repellents containing DEET work best, the Florida tests found. DEET doesn't kill mosquitoes but jams the sensors that guide them. We're cloaked. Follow the directions closely, don't apply it under clothing and don't let kids apply their own.

Clothing thick enough to prevent a mosquito's mouth parts from penetrating works, as do clothes treated with permethrin or sprayed with picaridin or oil of lemon eucalyptus.

Wind works. Mosquitoes are weak flyers. Put a big fan on the patio and they'll have more trouble getting to you.

Smoke works. Any sort will do. Citronella candles have only a slight additional repellent effect.

Bug zappers don't work. Mosquitoes aren't drawn to "black" light. One study found female mosquitoes made up just 0.13 percent of the bugs fried while billions of mostly harmless or beneficial insects died.

Floral perfumes or scents do attract mosquitoes, as does drinking beer. Mosquitoes also track dark colors and movement, so wear light-colored clothing when the bugs are biting, sit still and pray for a frost.

Bug lures

Five things we do that attract mosquitoes:

1. Exhaling. Carbon dioxide is a primary lure for any mosquito downwind within about 40 feet.

2. Sweating. Chemical compounds in human sweat, such as lactic acid and acetone, are beacons.

3. Stinking. Smelly feet, socks and shoes offer an appealing waft of the bacterial and fungal metabolism that takes place on our skin.

4. Moving. This is a cue, along with body heat, that signals the presence of birds and mammals whose blood proteins mosquitoes need to reproduce.

5. Drinking beer. Sorry about that.

[Frank D. Roylance]

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