Written by Joe Ballenger

Hello, I have a friend (yes, true!) who was telling me that she gets bitten from lots of insects yet her husband doesn’t have this problem so why do some people get bitten and others don’t?

-Angela

Hi! When my husband and I are outdoors or in the general realm of bugs, they always go after HIM and not me. I don’t really mind this trend, but I have seen him be CHASED (unprovoked) by mosquitos, flesh flies, regular flies, spiders, wasps, and bees. I can be right there with him yet go unbothered. Why might this be?

-Jasmine

Guys…we get this so much. Like, so much. It’s one of the most common questions we get asked.

When I went to Ecuador with Nancy, about a decade ago now (!), I did not wear insect repellent the entire trip because I wanted to catch a botfly. Botflies are vectored by mosquitoes, and I didn’t wear insect repellent because I wanted one so bad. Everyone else was bitten constantly by mosquitoes, so I thought my chances were really good.

I got bitten by exactly two mosquitoes. The entire time, two mosquitoes.

Why do some people get bitten by mosquitoes, and others get passed by?

I’ve written a little bit about mosquito attractants before, in response to why mosquitoes like cheese. Mosquitoes are attracted to cheese because the same bacteria which make cheese taste and smell like cheese are often bacteria which are located on the skin. These bacteria make smells the mosquitoes like. This is why mosquito biologists will use Limburger cheese for mosquito traps.

So there’s basically two parts to why some people get bitten by mosquitoes and why some people don’t.

The first has to do with regional variation in what mosquitoes are attracted to. One chemical we secrete is lactic acid. If you make pickles at home, there’s a good chance you’re doing lactic acid fermentation. This is the one which makes cloudy pickle juice due to Lactobacillus. We exert it during exercise, and in low doses at rest. Yellow fever mosquitoes, a species which feeds preferentially on humans, are attracted to lactic acid. However, some are attracted to it less than others.

Yellow fever mosquitoes (YFM) from Florida are highly sensitive to lactic acid, being attracted to .03 ug/min to an artificial source. Mosquitoes from Brazil aren’t particularly sensitive to it; they’re attracted to 10.27 ug/min. That’s a 342x range in dose!

There’s also the human side of body chemistry to consider, as well. Bernard, 2002 took someone who got bitten really badly by YFM and compared the chemicals their hands gave off to someone who didn’t get bitten as badly. YFM aren’t particularly attracted to hands, but it’s still a good comparison.

If you look closely at the red arrows, these are the different chemicals your body puts off. You make some of these chemicals, while bacteria make some of these chemicals. These are the chemicals that mosquitoes cue in on when they’re looking for a host. The chemicals between these two people are quite different, which means that one person may not be producing the chemicals the mosquitoes use to find their host.

If they’re not making the proper suite of chemicals the mosquitoes need to find their hosts, that person isn’t going to get bitten as often or as badly.

I’m not going to get too deep into the woods when it comes to chemistry in this post, but many of these chemicals were eventually found to have host-attractant or repellent properties. That, however, is another story altogether.

Barnard, D. R. (2002). Chemical analysis of human skin emanations: comparison of volatiles from humans that differ in attraction of Aedes aegypti (Diptera: Culicidae). Journal of the American Mosquito Control Association, 8(3), 186-195.

Williams, C. R., Ritchie, S. A., Russell, R. C., Eiras, A. E., Kline, D. L., & Geier, M. (2006). Geographic variation in attraction to human odor compounds by Aedes aegypti mosquitoes (Diptera: Culicidae): a laboratory study. Journal of chemical ecology, 32(8), 1625-1634.