Why mosquitos love us and hate DEET

It's hard to be thinking about mosquitos during the first lake effect event of the season, but maybe it will get you into the mood for summer!

For a long time, I have endorsed the usual understanding of the mechanism by which mosquitos target warm-blooded animals. For quite a while, it has been recognized that there is a complex neurochemical mechanism by which the bugs are attracted to carbon dioxide. From an evolutionary standpoint, this makes a lot of sense: if you are looking for a blood meal, what better way to find it than by going after something which breathes out carbon dioxide? It is also consistent with our backcountry observations. How many times, for example, have you noticed the propensity of these creatures to congregate under the tent fly?

This understanding has also permitted me to pooh-pooh students in my classes who claim that something about their own sweat, soap, or BO is particularly attractive to mosquitos. I have generally dismissed them as whiners, and pontificated that there was no biologic mechanism for such an observation. How could a bug distinguish between Dr. Bronner's and Mountain Suds?

Mea culpa.

Complex modern science has now shown us that the smell detection system of mosquitos is vastly more complicated than we ever could have imagined. In the process, it has also uncovered the biologic explanation for the effectiveness of DEET. Pretty impressive.

The study (by Liu and associates at Vanderbilt) is available online from the journal PLoS Biology (http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000467). The molecular biology here is daunting, and not for the faint of heart. Also keep in mind that the work was done with an African mosquito; it is certainly plausible, however, that similar mechanisms are in place for other species.

These scientists have unraveled the existence of two distinct olfactory signaling pathways in these insects. Each of these pathways (AgOR and AgIR) consists of a family of specific receptors, each of which, presumably, can respond to specific odors. Such responses can be either attractive or repulsive. A specific receptor (AgOR7) appears to be the actual target of DEET; animals in whom this receptor was inactivated by microinjection of specific RNA fragments were no longer affected by DEET.

What can we take away from this elegant science? Carbon dioxide is still a potent attractant for mosquitos, but short of stopping breathing there is nothing we can do about it. DEET works--we know this from a lot of previous behavioral studies, and now we understand it at the molecular level. From a wealth of other studies, we also know that the health concerns of DEET are vastly overblown and can generally be ignored. Mosquitos also seem to be capable of responding to a lot of other olfactory stimuli, but we do not as yet understand what makes something attractive or repulsive.

Bottom line--it is now plausible that certain individuals are more attractive to mosquitos than others. Other than DEET and protective garmets, however, there is little to do other than sucking it up!