Dear Straight Dope:
I am again in the midst of the mosquito season, and it is killing me. I've seen some advertisements for garlic-based sprays. My husband is convinced that no spray will work if we are the only ones in the neighborhood that use it. The ads say that the smell is not detectable by humans after a few hours, but the mosquito with its heightened sense of smell can detect it for up to 60 days. Does garlic really scare off mosquitoes? Is their olfactory sense superb? I'm willing to try it, but I can't convince my husband. He is an avid reader of your column. If you say it will work, he'll sign up for it the same day!
This one is quite timely with worries over West Nile virus and such.
Mosquitoes have almost no olfactory powers whatsoever–they have, in fact, a very selective and narrow olfactory "spectrum". But this is actually irrelevant; you see, the way a mosquito "repellent" works is NOT by being detected and driving insects away, but by blocking their ability to smell things that otherwise would attract them, so they ignore you. (In the same way, household "deodorizers" don’t actually eliminate odors, but merely block your ability to smell them.) In the case of mosquitoes, the primary attractants that have been identified are CO2 and lactic acid, though there are other factors. The best substance yet found to block mosquitoes’ powers of detection is DEET, and while a few repellents are equal over a short term, nothing even comes close in terms of residual repellency; DEET’s effects can last for several hours, whereas even the best alternative repellents last at most about 2 hours, maybe 3, if you’re lucky and the conditions are right. Garlic is only one of a list of plant compounds with similar "repellent" effects, including citronella, cedar, verbena, geranium, lavender, pine, cinnamon, rosemary, basil, thyme, allspice, and peppermint. But again, these substances have a very limited efficacy, in terms of duration, and if you are going to be exposed to mosquitoes for more than an hour or two, you should get a good DEET-based spray or lotion and stick with it. Products with 10% to 35% DEET should be fine under most conditions, and anything over 50% is overkill, except in unusual circumstances in which insect biting pressures are truly intense, or under conditions of very high temperature and humidity (e.g., working in a tropical jungle – and I know this from first-hand experience!). Repellents with DEET may damage plastics (such as watch faces, eyeglasses and frames), spandex, leather, and painted surfaces, but don’t damage natural fibers (cotton or wool) or nylon. Most importantly, there is no genuine evidence that DEET is toxic; what evidence exists is largely anecdotal or without adequate experimental control, and about the most one can say is that one should be cautious about applying large doses routinely, especially to very young children.
So, forget the garlic, and go with the DEET. While using a "natural" repellent wouldn’t rank as falling prey to an "old wives’ tale", there’s little reason to use something of such limited efficacy when better repellents are easy to find and cheap.
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