Science Shows What Actually Repels and Doesn’t Repel Mosquitoes: DEET, Citronella, Blood Type Myths Busted
With summer barbecue season well underway in the Northern Hemisphere, many of us are wondering if those citronella candles are even working to keep away the biting bugs.
Science tells us that mosquitoes are peculiar insects that can’t be generalized as much as we would wish, which is why, at the end of your BBQ, some of the guests might be more bitten up than others.
While we are happy to sweep all of the wretched bloodsuckers under the same moniker of ‘mosquito,’ there are actually 112 genus containing 3,500 species that fall under that name. Likewise, the bugs’ methods of finding prey, as well as our deterrent and protection strategies, will naturally vary species to species.
A fact sheet for mozzies is imperative, therefore, to minimize the annoyance these insects can bring.
How they find you
All mosquitoes use CO2 from human and animal breath as a guide to the location of a potential host. If you’ve ever seen them zigzag in the air, that’s the sign they’ve just lost the CO2 scent, and are trying to find it again. Once it brings them to the general position, they use visual features to look for signs of heat, and olfactory smells to provide them with the exact position.
Human sweating increases the natural olfactory cues of lactic acid, ammonia, and other chemicals that mosquitoes use to find people, so people who sweat less are less-likely to attract mosquitoes.
They can also find you through sweet-scented deodorant, body wash, shampoo, and other products—but they’ve been sucking our blood for millions of years so don’t need them.
Blood type is another oft-discussed factor of mosquito host-detection. A 1980 study looking at blood type and malaria rates found that people with blood type A, despite representing only 17.6% of the control group, made up 29% of the malaria cases, while O type trialists, of which were double in number to the A types, had only 22% of the malaria cases.
Annoyingly, in complete contrast, a more modern study looked at blood type and the secretions through the skin, and found that type-O secretors were the most vulnerable. So the verdict on blood types, is still murky.
We can’t change our blood type, anyway, so let’s look at what we can control.
Physical measures are the principal ones that should be employed, long sleeves, high socks, etc., but it’s also something most people don’t want to do in the heat of summer.
Some modern outdoor wear has mosquito repellent infused into the threading, which has been scientifically-shown to work. However the treatment fades overtime, and there are question marks in Canada about the safety of placing such chemicals like permethrin next to your skin, especially for children. A common outdoor brand in England even recommends throwing away any treated clothes after a set number of washes.
DEET, a chemical called N,N-Diethyl-meta-toluamide, has been shown to be effective since 1957, and took its place as the common insect repellent ingredient for the U.S. Army, before arriving in civilian stores as well.
As the world’s most thoroughly studied insect repellent, DEET, has been shown to be remarkably safe. (Here is a 2020 study offering more scientific evidence.)
The authors of a 2002 study published in The New England Journal of Medicine wrote, “This repellent has been subjected to more scientific and toxicological scrutiny than any other repellent substance,” continuing, “DEET has a remarkable safety profile after 40 years of use and nearly 8 billion human applications,” concluding, “When applied with common sense, DEET-based repellents can be expected to provide a safe as well as long-lasting repellent effect.”
Most of the concerns can be attributed to people using products with too high a concentration of the chemical, or using it in an improper way. Concerns about rare DEET side-effects, including skin irritation, breathing difficulty, burning eyes, headaches, or
seizures, were reported after, in some cases, the chemical was ingested or applied in very large quantities.
Canada recommends against using a product that has more than 30% concentration, and with a ceiling of 10% for children 12 and under. As a precaution, manufacturers advise that DEET products should not be used on damaged skin, and that preparations be washed off after they are no longer needed or between applications.
Still not convinced? Here are some alternatives.
Icaridin, also known as picaridin, is a good alternative to DEET and provides equivalent protection for up to 7 hours.
It has broad efficacy against various insects such as mosquitos, ticks, gnats, flies and fleas, and is almost colorless and odorless. A study performed in 2010 showed that a spray or cream at the 20% concentration provided 12 hours of protection against ticks.
The compound was developed by the German chemical company Bayer and later evaluated and recommended as a repellent by Consumer Reports. Commercial products containing icaridin include Cutter Advanced, Skin So Soft Bug Guard Plus, and Autan.
Citronella, a formula commonly-found in mosquito repelling candles, was actually pegged as totally ineffective by one study. The authors believe that it’s actually the accompanied plant oils, like linalool and geraniol, that are sometimes added to the flowery-lemon scented citronella, which are actually repelling the mozzies.
Indeed studies looking at citronella candles containing linalool and geraniol found them to be effective at deterring the mosquitoes, by about 85% and 71% respectively.
Some products like bug zappers and others which play high-frequency sounds that supposedly disrupt mosquitoes could be summarized in this snippet from a study which thusly investigated: “We are not aware of any scientific study showing that mosquitoes can be repelled by sound waves and therefore we consider these devices as the modern equivalent of snake oil”.
Some products with soybean oil are marketed as insect repellents, but Ada McVean writing for Canada’s McGill Office for Science and Society, points out the soybean oil likely has nothing to do with the products’ success and is instead attributable to the other chemicals inside these products, such as geranium oil, which is also used in citronella candles.
McVean sums up a good mosquito strategy by saying, “You should stay as scent-free as possible, wear light clothes, avoid bogs, and use an effective repellent (such as those containing DEET or icaridin).
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