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,' different species of mosquitoes have different methods of locating their host.

A fact sheet for mozzies is imperative, therefore, to minimize the annoyance these insects can bring.

All mosquitoes use CO2 from human and animal breath as a guide to the location of a potential host.

Ada McVean writing for Canada's McGill Office for Science and Society, explains the surprisingly complicated way in which mosquitoes try and find their target in a recent article. According to McVean, if you've ever seen a mosquito 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 odors 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.

Most of us will have had a conversation or been in a situation at least once, wherein one person expresses their annoyance of mosquitoes which seem to be leaving someone else alone. It could be that the bitten person has a particular blood type which mosquitoes prefer.

Many studies have been conducted looking to see if a human's blood type has some relation to their vulnerability to mosquitoes, and many of them seem to suggest that different species prefer different blood types.

A Sri Lankan study from 2019 found that 30% of starved female Aedes aegypti mosquitoes, the majority preferred O-type blood, compared to 25% for AB, and 20% for B. This species is famed for its spread of diseases like Dengue, Chikungunya, and Yellow Fever.

Another study however found that it was a combination of blood type, and a genetic feature some humans possess that cause them to secret substances related to their blood type onto their skin, which brought the mosquitoes, in this case a malaria spreader called Anopheles gambiae, to the host.

If someone with an O or B blood type were a secretor, their chance was 15-20% higher for an unpleasant visit, whereas A nonsecretors were landed on more than if they were a secretor, showing the complexity of the human-mosquito feeding methods.

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. Craghoppers, the English outdoor brand, even recommends throwing away any treated clothes after a set number of washes.

While the world still struggles to come up with a breakthrough malaria medication, mosquito netting has been saving thousands of lives in the tropics for years, and while the mosquitoes in the U.S. aren't carrying malaria, the net is still a proven technology.

After it was developed by the U.S. Army in 1946, DEET was registered for use by the general public in 1957.

According to the EPA, every year an estimated one-third of the U.S. population use DEET to protect themselves and their families from mosquito-borne illnesses like West Nile Virus, the Zika virus or malaria and tick-borne illnesses like Lyme disease and Rocky Mountain spotted fever.

As the world's most thoroughly studied insect repellent, DEET, has been shown to be remarkably safe. (Here is a 2020 study offering more 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.

While the CDC doesn't make any recommendations for use or concentration of DEET in a given product, Canada's government 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 of the study pointed out that natural insect repellents based on essential oils tend to be entirely untested, something repellent shoppers should take note of.

"There are no published data describing the repellency of oils of canola, goldenseal, patchouli, rhodiumwood, sage, sweet birch, tansy, tea tree, or vetivert to mosquitoes, for example, nor has the repellency of glycerin, lecithin, menthol, extracts of barberry, chamomile, goldenseal, or myrrh to any pest or vector species been shown in scientific studies," they write. "Of the 26 oils incorporated in the products we tested, only geranium, lavender, and peppermint oils, at the 100% concentration, repelled A. aegypti for (more than) 1 hour."

Some products with soybean oil are marketed as insect repellents, but 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.

Sweet lemony-scented citronella candles or incense coils are often sold as insect repellent smokescreens. Citronella candles and diffusers have been shown to work with minimal protection, especially outside, but linalool and geraniol, two plant oils often included within citronella candles, were found to fare much better.

In short, buy a product that has all three for a pleasantly-smelling, and actually functional mosquito repellent, and if one can find an oil diffuser instead of a candle, even better.

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."

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