Mosquitoes target favorites

August 20, 1991|By Linell Smith | Linell Smith,Evening Sun Staff

AFTER A PLEASANT evening socializing in their neighbor's backyard, the family has barely reached the front door before Jane Doe rolls up her pants to display her battle-scarred ankles.

"See?" she says, launching into the summer refrain of recreational martyrdom: "See? I got all the bites again."

She did.

Scientists say it's true that mosquitoes prefer to feed on some more than others. So do black flies, fleas and no-seeums. The attraction doesn't seem to have much to do with whether you're young or old, male or female, perfumed or naturally scented, mean-spirited or charitable.

In fact, mosquito-to-human attraction is so complex that scientists all over the world are unable to say why the most effective repellents work.

But it's not for lack of trying.

"Since the 1920s, people have been busting their butts trying to understand what attracts mosquitoes," says entomologist Dennis La Pointe of the University of Massachusetts at Amherst. "There's not a simple chemical cue here. It depends on a combination of chemical cues and visual cues and parameters around the host."

"There are countless odors that everyone emits . . . What folks don't realize is how many different chemicals, odor-wise, come off an animal," says Ken Holscher, associate professor and extension entomologist at Iowa State University.

The quest to understand why some of us are so tasty to mosquitoes continues to generate plenty of conjecture. Although the insect has been one of the best studied -- primarily because of its role in the spread of such deadly diseases as equine encephalitis and malaria in some parts of the world (though not in Maryland) -- its secrets continue to elude scientists.

Here are some commonly asked questions and some scientific response.

* Why do mosquitoes need blood?

Female mosquitoes seek blood to provide proteins necessary for the development of their eggs.

Scientists say mosquitoes prefer the blood of horses, cows and other large animals to human blood because it is actually more enriching. As long as cities and suburbs remain short on livestock, though, mosquitoes will continue to become better adapted to human blood.

* How do mosquitoes find you?

Studies show mosquitoes are attracted to carbon dioxide, the natural by-product of respiration. The insect appears to fly upwind to her target after CO2 triggers her host-seeking behavior. According to entomologist Holscher, a mosquito will turn into the wind and fly a zig-zag pattern that allows her to stay in the funnel-shaped stream of air carrying the molecules that attract her.

"There is this incredible range of behaviors that gets the mosquito from 100 yards under a fern leaf to you in a field somewhere," La Pointe says. "And we can't expect that all of the cues are the same at 100 yards and at 2 yards . . . If the wind is right, and the chemical is right, then the mosquito is going to detect you and make that big journey."

* Do mosquitoes prefer women to men?

Only anecdotal evidence supports this claim, which is based on the attractiveness of female hormonal chemicals.

Some scientific studies have shown that there is no statistical difference, says Dr. Robin Todd, entomologist and director of Insect Control & Research Laboratory in West Baltimore.

* Do cologne and hair spray attract mosquitoes?

If Skin So Soft, a sweet-smelling Avon product, has demonstrated an ability to repel insects, then all perfumes can't be said to attract biters, Todd points out.

* How effective is Skin So Soft?

Popular belief in its powers swelled in the 1980s, prompting even the U.S. Army -- perhaps the world's most long-standing mosquito researcher -- to test the efficacy of the aromatic bath oil. Researchers found that it offers some protection but that insect repellents on the market work much more successfully.

Similarly, such traditional anti-mosquito plants and herbs as pennyroyal and citronella repel mosquitoes -- but not for very long. Scientists say it is fairly easy to find substances that repel insects, but difficult to find ones that are long-lasting, safe to use, effective against many different kinds of biters . . . and acceptable to the human nose.

* Will eating garlic and/or lots of vitamin B keep those pests away?

No conclusive scientific evidence. Keeps people away, though.

* Do people who exercise attract mosquitoes?

Maybe. Physical activity does increase your levels of carbon dioxide, an element that triggers mosquitoes. Scientists also suspect that mosquitoes are attracted to heat, moisture and lactic acid -- all of which are present after someone has exercised.

* Will bug zappers kill mosquitoes?

Absolutely not, says Gaye Williams, an entomologist with the Maryland State Department of Agriculture. Bug zappers tend to only kill moths and beetles, which do not bite humans and sometimes eat mosquitoes. (They don't kill flies or wasps, either, because these insects sleep at night.)

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