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Views: 0 Author: RIIS WILLIAMS Publish Time: 2024-09-19 Origin: Site
Keep Mosquitoes Away with These Tried-and-True Repellents
Mosquito experts break down how to choose and use the best mosquito repellents
Mosquitoes are masters of speed and subtlety. Rarely do we catch one on our skin mid-bite. And when we do, the damage is often already done: the blood has been sucked, the itch-inducing saliva has been secreted, and, in some cases, a disease has already been transmitted.
This elusiveness means we have to rely on insect repellents for the best chance of evading bites. Not all mosquito-deterring products are created equal, though, says Dina Fonseca, a molecular ecologist and chair of the department of ecology at Rutgers University. Store shelves may be lined with a plethora of sprays and gadgets such as bracelets, candles and bug zappers purported to ward off the insects, but Fonseca and other experts say only those with a few key active ingredients are effective. And as greenhouse gas emissions drive up global temperatures and make already mosquito-prone habitats even buggier, repellents may become increasingly essential.
Scientific American spoke with experts in mosquito behavior and avoidance about which products are worth the purchase and how they protect us from the pests.
What essential active ingredients should people look for in bug sprays?
Despite the vast number of brands and concoctions on the market in the U.S., only those containing Environmental Protection Agency–registered active ingredients such as diethyl toluamide (DEET), p-Menthane-3,8-diol (PMD) or icaridin (also known as picaridin) have been scientifically proved effective. Spray based on DEET, however, have long been considered supreme, Fonseca says. “It is the oldest tried-and-true gold standard among all of the repellents,” she says. “Back when I was a graduate student doing field work in a mosquito-filled bog, DEET was my best friend.”
The U.S. Department of Agriculture developed this synthetic chemical in 1946 for Army personnel deployed to mosquito-infested environments. At that time, it was used in a spray—nicknamed “bug juice”—that was 75 percent DEET and 25 percent ethanol. Bug juice irritated skin with scratches or cuts, and it held mosquitoes off for only a couple of hours. But later, the USDA and Army produced a new, longer-lasting and gentler formula called extended duration topical insect/arthropod repellent (EDTIAR), which had no ethanol and contained just 33 percent DEET. It also included polymers that thickened the spray to help slow its evaporation—and it remains the go-to repellent for today’s military.
U.S. authorities approved DEET-based products, with various concentrations of the chemical, for the general public in 1957. Currently about 120 EPA-endorsed DEET mixtures, made by about 30 companies, are sold and considered safe for people of all ages, according to EPA evaluations (though users should avoid getting these products in their eyes or mouth). DEET-based repellants are a popular choice for people who spend extended periods in outdoor places with mosquitoes and other biting insects.
PMD is a strong DEET alternative, Fonseca says, especially for people who are seeking a naturally occurring chemical or put off by DEET’s strong odor and greasy texture. PMD is found in eucalyptus plants and, combined with water and a small amount of ethanol, is sometimes sold under the name “oil of lemon eucalyptus.” Icaridin is another synthetic option, Fonseca adds, noting that the latter is often sold in lower concentrations than DEET and PMD and therefore must be applied more often.
How do DEET and other chemicals repel mosquitoes?
How exactly DEET works is still a subject of debate, says Aaron Gross, a toxicologist at Virginia Tech’s Molecular Physiology & Toxicology Laboratory. Some studies suggest that DEET targets specific receptors in mosquitoes’ antennae and blocks their ability to smell chemicals in exhaled breath and on skin that attract them to humans. Other research proposes that the chemical toys with mosquitoes’ brain by manipulating their neuronal receptors. “Everyone seems to have a different opinion,” Gross says. “We really don’t have a clear, concise way of knowing how DEET exerts its repellent.... All we know is that it effectively disrupts the mosquitoes’ host-seeking behavior.”
“It is the oldest tried-and-true gold standard among all of the repellents. Back when I was a graduate student doing field work in a mosquito-filled bog, DEET was my best friend.”
PMD and icaridin are no less mysterious. Many experts believe that, like DEET, these products somehow hinder mosquitoes’ senses and thus their ability to hunt down blood meals. But the exact mechanisms are still unknown—and Gross notes that testing such insect repellents’ efficacy isn’t particularly easy or enjoyable: most trials require a human subject to stick a repellent-treated forearm into a mosquito-filled cage. “I work with a friend who once needed to test a repellent,” Fonseca says. “He coated the front and back of my hand with the spray and I put my hand inside the cage of mosquitoes. It was too late before I realized that we hadn’t sprayed the stuff between my fingers, and when I took out my hand, I saw that I’d been bitten on the inside of every single one of them.” This, she adds, clarifies the importance of ensuring every bit of exposed skin is coated if one is relying on a bug spray. How often a repellent should be reapplied depends on the concentration of the product’s active ingredient; sprays with a higher percentage of that chemical work longer, but they may be smellier and stickier. “It’s always best to just follow the label because all of the formulas can be a little different,” Fonseca says.
What about other mosquito-repelling products that aren’t applied on skin?
Clothing treated with the insecticide permethrin is very safe and effective, says Eric Day, an entomologist and manager of Virginia Tech’s Insect Identification Laboratory. Rather than actively repelling mosquitoes, permethrin—a synthetic version of a chemical that occurs naturally in chrysanthemum flowers—paralyzes and kills insects by overstimulating their nervous system. As a repellent, it’s too harsh to be applied directly to skin, but permethrin doesn’t cause irritation when small amounts are infused in clothes. It’s still necessary, though, for people to apply bug spray to any bit of skin that is not covered by the treated clothing.
There are also some candles that are made with citronella oil, which is found in some species of lemongrass, that can help ward off mosquitoes and other pests when burned. But insects tend to avoid smoke in general, Fonseca says, so it’s difficult to say if citronella is what really does the trick.
Day says that most other products such as bracelets, bug zappers and high-frequency sonic devices haven’t been scientifically validated and typically aren’t worth buying. “Whenever I look in a bug zapper there are very few mosquitoes and usually just other bugs from the environment,” he says. “A lot of these products market themselves as ‘natural’ alternatives to things like DEET, but they don’t actually do much.” Some marketers also claim that certain plants with specific chemicals and strong fragrances can deter mosquitoes, but there is scant scientific basis for this.
How can people prevent mosquitoes around their homes?
Female mosquitoes lay their eggs in stagnant water, and Day says cleaning out gutters and emptying rain buckets or planter pots is a good way to keep the insects at bay—especially in areas where mosquito-carried diseases and pathogens such as malaria, West Nile virus and Zika virus are prevalent. If mosquito larvae are spotted in still water, some research suggests that adding a few drops of boric acid to the water can poison the larvae and prevent them from hatching.
As climate change progresses and global temperatures rise, it’s likely that some warm and wet environments will see growing mosquito populations and more disease outbreaks. Having a strong insect repellent on hand is becoming increasingly important, Fonseca says, and she encourages people living in mosquito-infested places to start considering it a part of their regular skin-care habits. “I think that one of the biggest problems is that we really haven’t been that good at diligently using repellents,” she says. “We’re starting to get better at this with sun protection by putting on SPF products every morning with our face lotions and creams. But I think it’s time that many of us, especially those really sensitive to mosquito bites, start incorporating repellents into our daily routines.”
