Why Mosquitoes Suck

By Patricia Gadsby|Friday, August 01, 1997
You know a creature is in deep, deep trouble when people don’t think twice about whacking it. Even animal rights activists aren’t so quick to leap to the mosquito’s defense. The only good mosquito, many people would say, is not just dead but well and truly dead, reduced to a bit of insect juice on the wall.

Why do mosquitoes bug us this way? And why some of us so much? (Why me?)

Considering our bad attitude, it’s a wonder mosquitoes even come near us--an insanely risky proposition from their viewpoint. That’s probably why some

Mosquito species figured long ago they’d rather tuck into an unconscious host, an unmovable feast as it were, and voted by and large to nip at night. The nipping’s done with a proboscis, a sort of springy syringe with a hollow needle formed by interlocking mouthparts and an outer sheath that rides up when the needle slides through your skin and probes for blood. But hitting the sweet spot isn’t so easy--ask any intern drawing blood from a patient for the first time. Or ask José Ribeiro, a medical entomologist at the National Institutes of Allergy and Infectious Diseases, who will gladly give you the gory details. Less than 5 percent of skin is blood vessel, so the mosquito has to fish. It casts its proboscis back and forth under your skin, sawing through tissue and probing an area ten seconds at a time. After several such search castings and no luck, the insect withdraws completely and tries another patch of skin. But if it gets a good probe into one of your small blood vessels, it freezes and sucks from the hemorrhage, pumping in little spitballs of vessel dilators and blood thinners to keep its meal running freely. (An allergic reaction to mosquito drool is what produces those itchy red lumps, if you’ve wondered.) A mosquito can suck two to three times its weight in blood, no trouble. That’s tantamount to a 150-pound human vacuuming up 300 to 450 pounds of food.

At this point, stretch receptors in the mosquito’s hugely bloated abdomen, sensing imminent blowout, initiate an urgent message to the brain, saying in effect, Whoa there, skeeter--pull out! It’s a pretty mechanical reflex, apparently. You can get mosquitoes to quit feeding by pumping them up--how can I put this delicately?--from the other end, with saltwater enemas or air. Marc Klowden, the entomologist at the University of Idaho who did these insufflating experiments, also has videos showing what happens when you prevent the signal from the abdominal receptors from reaching the brain: too much is never enough for these mosquitoes, and they eat until they explode.

Even under normal circumstances, once a mosquito has eaten its fill, it’s so heavy it can barely fly, according to Ribeiro. Stuporous and swollen as a blimp, it looks for a place to lie low and do what anyone would do after going on the mother of all drinking binges--it excretes like crazy. After a few hours the mosquito has reduced its blood meal by half into a supernutritious slush.

Thankfully, at any given time, in any population, less than half the mosquitoes are biters. That’s because, first, only females are hematophagous (Greek for blood eaters). Males are sweet nectar-loving types, peacefully sipping at nature’s juice bar. Second, most females feed on blood only when they need the extra protein to finish making their eggs; for routine fuel they’ll use plant sugars, too. In fact, feeding on blood seems a pretty well orchestrated event, dangerous enough that females have built-in controls to switch it on and off--they don’t seek out victims more than they have to. Remind yourself of this the next time you’re being eaten alive. Things could be worse.

Which brings us back to that all-consuming question--or rather, that question that consumes some of us more than others. How do mosquitoes get wind of their prey, and why are certain people more preferable than others? You know who you are. You’re the ones who make backyard barbecues safe--well, tolerable--for the rest of us by acting as live bait, the folks who get fed on right through their cotton T-shirts. You’re the ones whose mothers dabbed lotion on bites and cheerfully consoled: You just have sweet blood.

Entomologists are a blunter lot: they labor in the service of science, not in the warm, fuzzy business of sparing our inner child. Mosquitoes, they tell us, use various cues to find food--color contrast and movement, skin temperature and humidity. But above all, experiments show they’re olfactory creatures. Floral scents help steer them to their nectar meals. And breath and body vapors draw them to their animal hosts, including you and me. For mosquitoes our vapor trail is a no-brainer. It says: Fly this way. Make a right. Eat here. Just do it.

Each time you breathe out and blow off carbon dioxide, you’re telling mosquitoes (and other nasty biting insects like ticks) that there’s a vertebrate, a handy blood container, in the vicinity. Mosquitoes have CO2 receptors on little feelers called palpi and can detect a plume of the gas from about 50 feet away. This is bad news: there’s a world of bloodsuckers waiting for you to exhale and no way you can hold your breath forever. Matters aren’t helped, either, by another vertebrate emanation of ours, a volatile chemical called lactic acid, which mosquitoes pick up on their bristly antennae. Humans exude this compound from their hands, from their faces and shoulders--in fact, from just about every pore of their bodies, in secretions like oil and sweat made by skin glands. Lactic acid escapes from our mouths too, whenever we go in for any heavy-duty exercise. As it happens, some of us are quite a bit more effusive than others.

That may begin to explain why everyone doesn’t get bitten equally. There’s definitely a marked difference in people’s skin exudations and their effect on mosquitoes, says Dan Kline, a research entomologist with the U.S. Department of Agriculture in Gainesville, Florida. He recalled arriving at a lab some years ago when the notion was in its early test stages: People were walking in the hallways with marbles in their hands, and I wondered what sort of institution I’d come into. They were rubbing their marbles and then seeing which ones attracted mosquitoes. Entomologists are pretty sure that personal skin chemistry is the key. But skin exudes hundreds of chemicals, and who’s to say which ones are preferred? Kline thinks diet may play a part. Among other things, he studies octenol, a mosquito-attracting substance first found in ox breath that’s produced by grass fermenting in the ruminant’s stomach. Eating your greens, he mused, may make your skin ooze more octenol.

It’s also becoming clear that mosquitoes are quite fussy--with different species biting different hosts, and even particular bits of their hosts, suggesting they respond to different cues. For example, Anopheles gambiae, one of Africa’s top malaria carriers, is so inordinately fond of humans that it will fly by cattle, ignoring all their bovine odors, to get to the herdsmen. It’s also a foot fetishist, according to a Dutch research group led by Willem Takken at the Wageningen Agricultural University in the Netherlands. In 1993, Bart Knols, then a student with the group, demonstrated this in a series of PG-rated experiments in which he used young men wearing only tight underwear as target practice for hundreds of avid mosquito females. Painstaking bite counts revealed that A. gambiae nips at feet and ankles, while other mosquitoes seek out heads and shoulders. From their flight patterns, it looked as if each species was finding its favorite sites by tracking either breath or foot aromas. Sure enough, piping the subjects’ breath out of the test area threw the headhunters into a tizzy, and they bit other body parts instead. And feet freshly washed with antibacterial soap left A. gambiae completely cold.

Unwashed human feet, maybe you’ve noticed, have a rather distinctive stinkyness. Nothing else springs to mind as smelling quite like feet, except, well, cheese. That’s what sprang to Bart Knols’s mind, too. In Holland they have a word for the odor, tenenkaas, or toe cheese (not to be confused with the snack food Chee-tos). In what Knols admits is a rather anthropomorphic bit of scientific reasoning (hey, whatever works), he decided to check his mosquitoes’ response to Limburger--a famously pungent fromage invented by Belgian monks--by conducting insect flight tests inside a small wind tunnel. A. gambiae mosquitoes were released into the flight chamber of this contraption some five feet downwind from two traps--one wafting Limburger fumes, the other clean air. The mosquitoes, good little lab rats that they were, chose the cheese trap three times as often as the odorless one.

Of course, the Dutch have a theory to explain all this: what cheese and feet have in common are odor-producing bacteria. Bacteria are essential to making cheese--the powers unseen that turn insipid milk into a panoply of Bries, Goudas, Cheddars, Roqueforts, Parmesans, Leerdammers, and Limburgers. And we have loads of bacteria growing on our feet. In fact, midway through his doctoral thesis, Knols notes rather ominously that it has even been suggested that certain bacteria involved in cheese production originate from human skin and therefore that ‘cheese smells of feet rather than the reverse.’ God knows how the good monks inoculated the first Limburger cheese; pray it was a miracle.

As it happens--just a coincidence, perhaps--the bacterium used for Limburger belongs to the same family as a bacterium that often lurks on human feet. Brevibacterium linens is the cheese maker’s friend, and Brevibacterium epidermis is the one that hangs out on the skin between people’s toes.

About 38 percent of people with normal feet harbor B. epidermis, according to James Leyden, a professor of dermatology at the University of Pennsylvania School of Medicine who (only coincidentally?) happens to share his name with a famous Dutch city. But when it’s there, it’s there a lot- -something like a million per toe-web space. A lot of the sulfurous smells we associate with stinky feet are produced by B. epidermis and a diminutive bacterium called Micrococcus sedentarius, which break down proteins in skin debris, Leyden elaborated. But all feet have an odor, he added, mostly due to fatty acids (from the breakdown of fat substances in degenerating skin cells).

Chalet Cheese Co-op, in Monroe, Wisconsin, is the last of the Limburger makers in the United States. An oblong little cheese with a faintly orange rind, Limburger has a yellowish semisoft interior and lots of funky flavor. Amazingly, though, every Limburger starts life with no more character than a brick of cottage cheese. To begin its transformation, it’s placed on curing boards and repeatedly wetted down and rubbed over with something Chalet manager Myron Olson called schmier. This is not the stuff New Yorkers spread on bagels. In Wisconsin, schmier, or smear water, is a cloudy mix of water, salt, and proprietary organisms including B. linens cultured from previous cheese batches. In a matter of days, smeared cheese has gone from white to golden, said Olson, but it’s still crumbly and acidic like feta. Young Limburger is then put in cool cellars where its surface coating of bacteria goes to work on its fats and proteins, softening the cheese from the outside in, mellowing its acidity, and developing its complex taste and aroma. At two months, in Olson’s view, there’s a sweetness to it and even the kids will eat it. At three to four months it’s at its flavor peak, with heady whiffs of fatty acids-- especially butyric acid, the barnyard flavor you can also detect in Cheddar--as well as ammonia and sulfur compounds. (It’s really pretty tasty if you’re game for strong cheese.) By six months, though, you have to be a real Limburger lover to appreciate its whopping potency.

The Dutch researchers, then, could be onto something with their toe-cheese theory. Takken’s group, however, wasn’t content to trust the human nose--an unreliable stand-in, in any case, for mosquito antennae and palpi, and a poor guide to what an insect can smell. So the researchers proceeded to do gas chromatography studies. They took parings of cheese and scrapings from under a volunteer’s toenails, analyzed the chemicals given off by both substances, and found resemblances not only in their more obviously smelly sulfur compounds but in their carboxylic fatty acids. These fatty acids are something of a human trademark (Bloodhounds follow our fatty-acid fingerprint, according to Leyden). They’re produced by bacteria tirelessly breaking down triglycerides in the sebum coating our skin. Thus, the Dutch have concluded, it is tempting to explain the variations in attractiveness of individuals to A. gambiae on the basis of the metabolic activity of their resident skin microflora. You may take that to mean: Skin microbes make stuff that attracts mosquitoes, but my microbes are not your microbes, and mine may make more (or less) of the stuff mosquitoes find so irresistible. Whether this theory will hold for other species of mosquito hasn’t been formally tested yet, but it couldn’t hurt, could it, to pack your Odor Eaters next time you leave home for mosquito-infested parts? And don’t forget the Dial.

There’s a serious reason for all this work, of course. Malaria mosquitoes are a scourge, and researchers like Knols would love to come up with a simple trap, a Mosquito Motel, baited with the olfactory equivalent of a human. But as Knols pointed out, human skin produces hundreds of volatile chemicals. Comparing the volatiles produced by both cheese and feet is just one way to winnow out a few that are of interest to mosquitoes. Knols is now trying out a trap based on fatty acids, including butyric acid. But other chemicals might be blended with them in the future, including those sulfur compounds produced when bacteria break down proteins in skin residues.

In the meantime, if a mosquito does pick on you, there’s always revenge--or at least a really juicy malevolent fantasy. Remember the explosive feeding experiment performed by Klowden? Small boys do something like that, a mosquito researcher let slip one day in an unguarded moment, followed instantly by deep professional misgivings. Well, I’ve heard they do, the researcher said, when pressed. You’re not going to quote me by name, are you? It’s not exactly, uh, scientific.

Here’s the trick: Once a mosquito has landed and begun feeding, you stretch the skin taut on either side of it. Supposedly, if you’re deft, you can trap the proboscis in your skin in midfeed. Stuck in the blood vessel, unable to pull out, its anticoagulants working overtime to keep its blood meal coming, the mosquito sucks until it pops.

Maybe this method of entrapment works. Maybe it works only for small boys. Maybe it’s just a stupid pest trick or one of those urban legends that shouldn’t be put to the test. But, hey, it’s prime time for mosquitoes. You be the judge.
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