Aug. 25, 1999 GAINESVILLE -- If you think mosquitoes like you better than they like other people, you're probably right, say University of Florida researchers.
In a study to determine whether the tiny vampires choose their victims or feed indiscriminately, UF entomologist Jerry Butler and research assistant Karen McKenzie found that mosquitoes do, indeed, choose.
"Undoubtedly, mosquitoes have preferences," Butler said. "People do differ, and in any group of 10, one person will be fed on more than others."
Mosquitoes have evolved and survived -- even thrived, Butler points out -- because of their ability to choose the best hosts for their blood meals, which they need to lay eggs. They find their hosts, initially, through a keen sense of smell.
All people have to do to attract mosquitoes from even 40 miles away is breathe. As they exhale, their carbon dioxide and other odors mix to produce a plume that travels through the airstream. The plume acts like a dinner bell to mosquitoes, letting them know a warm-blooded meal is within range.
They fly up the plume in zigzag fashion until they arrive, for example, at a backyard cookout. Then they localize on eddies of other odors in the airstream and then, within yards of a person, they use vision and heat sensing to make a selection.
"Mosquitoes use odor to sort attractive people from the unattractive people to find those that are most tasty," Butler said. "They are looking for the highest rates of human attractants."
What are those attractants? That's the next challenge for Butler and McKenzie. Already they know that natural excretions through the skin and skin care products affect mosquitoes' appetite.
Take perspiration. By itself, it appears to be neutral, but as it ages bacteria begin developing, and that makes perspiration into a very strong attractant, Butler said. Bathing helps, but some after-bath products don't.
"The things you put on your skin to soften it and make you beautiful can be very strong attractants," Butler said. "Many of the ingredients in cosmetics and will attract mosquitoes. And while a repellent may offset that, most times the cosmetics and creams last longer than the repellents."
Medications, too, can change an attractive person into one who is repellent or vice versa. These include heart and blood pressure medicine and drugs to treat high cholesterol.
McKenzie saw this effect firsthand when a research volunteer was diagnosed with a brain tumor in the middle of her experiment. Before his tumor was removed, he was repellent. After surgery, however, he became very attractive to mosquitoes.
Such findings have added to the intrigue for Butler and McKenzie. They theorize that mosquitoes, who need cholesterol and B vitamins but can't make them on their own, can sense which host is the richest source of these ingredients.
In their laboratory, Butler and McKenzie are screening materials to determine whether they attract or repel mosquitoes. They built an olfactometer, a machine that measures mosquitoes' preferences for various odors, and connected it to a computer.
Small discs of cattle blood and odorous gel are covered with a membrane to mimic skin, and then mosquitoes are released into the olfactometer. When a mosquito chooses a "host" and feeds, an electrical charge is transmitted to the computer, which records the feeding. Butler and McKenzie can then analyze the data to determine which substances the mosquitoes found most attractive.
Their research goes beyond relief for barbecue guests. They hope to help people to protect themselves from mosquito-borne diseases.
"If you reduce the feeding rates just a little, you reduce the probability of transmission of diseases greatly," Butler said. "Only one mosquito in a thousand carries disease organisms.
"People who think they attract mosquitoes are the ones at largest risk of mosquito-borne disease," Butler said. "They'll have a hundred mosquitoes feed on them when a normally repellent person might have five. It's that kind of ratio."
After mosquitoes choose a host, they look for a landing strip. It's no accident that they often land in hard-to-reach places.
"They like to feed on humans, and they know what the human response is going to be so they sneak in and bite when it's the least dangerous to them to feed on you," Butler said. "Then they feed quickly, just eight to 10 seconds, and they're gone; the length of time they're on a host regulates whether they get squashed."
If you've ever watched a mosquito land on you and noticed it doesn't bite right away, there's a reason for that. Just any old spot won't do.
They search the skin surface with their mouth parts till they find a site that feels or tastes good, then insert their stylet. The stylet moves through the tissue "like an oil-drilling rig" working around objects and turning at angles. When it detects the mother lode -- a capillary -- it taps right in.
Butler says the study has far-reaching implications for backyard barbecues -- and who is on the guest list.
"If you can figure out who, among your friends, is attractive to mosquitoes and be sure to invite that person to all your outdoor gatherings, you might be able to spare your other guests from mosquito bites," Butler said. "In any group, there should be one person who is highly attractive to mosquitoes."
What a price for popularity.
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The above story is based on materials provided by Institute Of Food And Agricultural Sciences, University Of Florida.
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