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Taking a Bite Out of Summer Fun

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Washington Post Staff Writer
Saturday, July 12, 2008; Page B01

All Robin Levin has to do is step outside her Takoma Park home, and the hordes descend.

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"Basically, if there's a mosquito within a seven-block radius, they find their way to me," she said. "I'm actually a good repellant for everyone else, because mosquitoes are attracted to me like a lightning rod."

She has the ruddy welts to prove it, evidence that mosquitoes have been piercing her fair skin, injecting saliva and anticoagulants and sucking out her blood for nourishment.

Levin, like so many other humans, is unconsciously attracting mosquitoes: Every time she breathes out, she expels carbon dioxide, a powerful siren song to mosquitoes. Her skin exudes hundreds of volatile organic compounds, such as acetone and lactic acid, as do all humans', but her particular combination sends out a come-hither wink to the neighborhood insects. In addition, she's a woman, and some scientific studies show that the biting mosquitoes, which are female, prefer their own gender, although no one is sure why.

What is clear is that to mosquitoes, some of us are smorgasbords, and others are as tasty as cardboard. As the mosquito season begins in earnest -- if it hasn't already in your microclimate -- those on the buffet line might dread its arrival more than the rest of us, but there's no escaping.

Virtually any place on Earth where the sun's long, warm rays coincide with the smallest bit of moisture, you will find some of the 3,700 species. Only 150 of them live in North America, and about 60 of those species call the Washington area home. Every time a new species bites you, your immune system goes into overdrive, often creating a big, angry welt; bites from more familiar mosquito species are smaller.

Just as not everyone attracts mosquitoes, not everyone who is bitten is bothered by the result.

"Some individuals are more sensitive to bites, so they notice them more," said Jonathan F. Day, a professor at the Florida Medical Entomology Laboratory in Vero Beach, Fla. "Some people produce more cues than others. They give off more carbon dioxide. Anyone who is really active or has a naturally high metabolism. It's more than carbon dioxide, though, or mosquitoes would waste a lot of time chasing cars."

In this area, most mosquitoes fall into three main genera: Culex, Aedes and Anopheles. All of them start their lives as eggs deposited in or near water, even in tiny pools that collect in the bottoms of planters or drainage pipes. A mother can lay up to 300 eggs at a time. The eggs hatch into a larvae, and after a few weeks, they turn into pupae. Within days, those pupae become adults, and if you walk into their path, you are their first meal. They usually live only a few weeks, so their interest in mammal blood is intense.

Mosquitoes find you by searching, the same way a bird dog hunts birds, Day said. Only the females bite, because, as egg-bearers, they are in search of the added nutrition that blood provides. As they zero in on their victim, sight comes into play. The best targets are people in motion, those who provide a large dark target against a light background, and those who provide meals close to the ground, where low wind and high humidity favors insects.

When she lands, she probes for a capillary, then inserts her very thin and sharp proboscis through the skin. She injects you with saliva, which numbs your skin and stops your blood from clotting. Then she drinks for one to three minutes. It's the saliva's proteins, not the bite, that irritate human skin and causes the itchy bump we call a mosquito bite.

The most common native mosquitoes, those in the Culex and Anopheles genera, feed mostly after dusk, when it's easier to bite birds and mammals as they're settling down to sleep. The Culex and Anopheles mosquitoes also are most likely the bugs you hear buzzing near your ear as you lay in bed on hot summer nights. They spend their days in the shade of bushes, grasses or leaf litter, near the twin food sources of moisture and microorganisms, and hidden from predators.


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