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July 19, 2016

Why do mosquitoes bite me and not you?

Timothy Gibb, Extension Entomologist, Purdue University

Are mosquitos selective in who they bite?  How do they discriminate?  Is there something I can wear or spray or eat or pray for, that will decrease my ranking on the 10 most wanted by mosquitos list?

The question is a good one and if it gives you any comfort, you are not the only one asking it.  There are many who are convinced that they are on the list.  The complete answer as to whether mosquitos discriminate or not, however, is not simple. It includes a few things we know, and many things that we wish we knew.

It is always best to start with what we know.  There are about 75 different species of mosquitos in the U.S.  Each has its own biology, life history and behavior, so making generalizations covering all mosquitos is risky.  Given this caveat, we do know that bites from mosquitos are the end result of the female mosquito (not the male) searching for a blood meal to help her develop eggs. 

Her search for a potential blood meal begins long before landing on your skin, however.  Mosquitos cue into traces of carbon dioxide exhaled from people and other animals (yes mosquitos bite other animals quite frequently, and I have yet to hear a dog complain that it gets bitten more often than its master).  As a hungry mosquito flies toward ever higher concentrations of CO2, which means it is getting closer to the actual source, it also begins searching for heat in the same way.  I imagine that the closer it gets to a warm, breathing body, the more excited it becomes.  By the time that it has located the party, the mosquito is nearly giddy with excitement, and it then begins selecting which of the victims to bite.  Some suggest that it is more than a random choice.  At this point other sensory receptors (sight and smell) begin working.  Some studies have shown that hungry mosquitos are attracted to dark shapes and movement.  It is true that mosquitos see contrasts of light and dark and the greater the light/dark contrast, the easier it is picked up by their little beady eyes.  So if you dress with highly contrasting colors you are more likely to be bitten than someone who dressed in monotones.  What the study fails to point out is that if both people are dressed alike, both get bitten. 

Movement is also attractive to mosquitos.  I suspect that they associate movement with a live animal – and mosquitos definitely are most attracted to live bodies, because that is where the blood is.

Once they zero in on a live body, mosquitos begin using yet another set of sensory receptors – their formidable sense of smell.  Human are replete with a very elaborate array of smells, detectable to mosquitos.  Lactic acid, estrogen, steroids, among many other substances, occur naturally on a person’s skin in body secretions, oils, sweat, etc.  Other smells are released during respiration.

While there is no doubt that mosquitos have evolved to recognize these chemicals as indicators of potential hosts, it is also true that odors vary from one person to another. What is unclear is exactly which and at what concentrations the various substances on a person’s skin or in their breath that mosquitos prefer, and if this preference is strong enough to cause a mosquito to leave one person in favor for another. 

So, the answer to the question,  ‘Do mosquitos prefer me over my neighbor?’ depends on what exactly mosquitos prefer, how human respiration and skin chemistry differs and how strong these attractions to the mosquito are.  These are the things we do not know. 

So, my personal theory is that, if there is a preference, I suspect it is at this level (respiration or skin chemistry) that the mosquitos show a preference for one person above another.

If body odors are detectable by mosquitos and we know that they are, we can’t really do much about it.  It is part of our physiology.  It is also true that human odors differ from one person to the next.  Some of this is genetic.  However, bacteria on a person’s skin are responsible for a lot of the smells (I know this from living with a teen-ager).  These change from person to person and from day to day.  Lactic acid on the skin, for example, increases when a body is perspiring.  Some studies show that certain mosquitos key in on lactic acid. 

Some people have concluded that altering how people smell may confuse the mosquito and to that end have tried applying perfumes, lotions, or even changed diet by consuming things such as garlic or lemons, vitamin B-12 tablets, bananas or beer.  None of these, even in large quantities have proven to be effective. 

One small, recent study suggests that people with a particular blood type, (type O) makes them more attractive to mosquitoes. Bear in mind that this is the conclusion of only one small study and uses only one species of mosquito. It was an interesting study but results are likely to be completely different if another species is used.
Why don’t we know more?  Where are the so-called, mosquito scientists when we could really use them?  Well, in defense of the entomologists (I am one), I offer the following note.

For those trained in statistics, it is easy to see why mosquito attraction studies are difficult to produce and scarce.  For each of the 75 species of mosquito, scientists would have to recruit a number of people who look, smell and respire exactly the same and convince them to join the mosquito biting study (who is going to volunteer as bait for a study like this?)  Then the scientist must be able to manipulate one single factor such as a specific smell or chemical compound and change half of the volunteers smell this way in order to test them against the original (control) group.  Then a number of identical mosquitos - all in exactly the same developmental state, age and in the same mood (this alone would be difficult to survey) would have to be released and the number of resulting bites would have to be measured, tallied and then the whole experiment would need to be replicated enough times to provide confidence in any differences. Yup, it would be difficult test.

Compare that with simply applying an insect repellent.  We know that that works.

The bottom line is that if people simply concentrated on avoiding the places and times that mosquitos are most active and applied proven insect repellents, rather than worrying about why mosquitoes don’t bite our friends more, there might be less ‘itching’ all around.

As for me, I like to hang out near mosquito bait.  Remember the story about not worrying about outrunning a bear, just worry about outrunning your friend?  The same general strategy can be used for mosquitos.  Just make yourself less attractive than the next guy. Wear long sleeved clothing, avoid excessive working, or moving or breathing if possible.  Look for smelly teenagers who are wearing dark colors. Hang particularly close around a big person (= greater exposed skin and higher respiration), maybe ask them to run over and get a beer (= more panting and possibly some more sweating).   That way, if there are only one or two mosquitos at the party, you should be ok.

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