If you want to protect yourself from mosquitoes and ticks—which seem to be everywhere this year—you might need to enlist a heavy-hitter. Insect experts reach for one thing: permethrin.
Here’s what to know about the synthetic insecticide, plus how and when to use it.
How permethrin works
Permethrin, which is derived from the chrysanthemum flower, has been used in some form since around 400 B.C. During the wars of the early 1800s, Napoleon dusted his soldiers with a permethrin-like powder to control fleas and body lice, which protected them from anemia and gave them a physical edge over their enemies.
Permethrin is a neurotoxin that targets an insect’s nervous system, triggering muscle spasms, paralysis, and eventually death. It functions as a contact insecticide, which means it kills via direct contact when a bug lands on a treated surface. That distinguishes it from DEET-based repellents, which work in a different way. DEET “blocks the sensors for an insect to find you, so you end up with a cloak of invisibility,” says Lee Haines, a medical entomologist at the University of Notre Dame who studies how insects spread diseases. “They’re attracted to your heat, but they can’t find you.”
To visualize how permethrin works, consider what would happen if you ate nothing but cotton candy for a week. “Can you imagine how hyperactive you would be?” Haines asks. The same thing happens to bugs: “If you see a mosquito that touches permethrin, it’s shaking uncontrollably, it’s flying uncontrollably, like it just had a lot of sugar. It’s buzzing around, and that’s because all of its nerves are firing at once.” That causes muscle spasms; once the insect’s muscles are exhausted, it becomes paralyzed and then dies.
If that sounds unpleasant, it is. “It’s not a nice way to die,” Haines says.
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Permethrin is odorless and colorless, and insects can’t detect its presence, she adds. That helps make it effective. In one study of Iranian military members, for example, the average number of mosquito bites in those wearing untreated uniforms was 108 per hour, compared to 10 per hour in soldiers wearing permethrin-treated uniforms.
Meanwhile, research has found that people who wore permethrin-treated socks and sneakers were 74 times less likely to get a tick bite than those wearing untreated footwear. Study participants who wore permethrin-treated shorts and T-shirts were about 5 and 2 times less likely, respectively, to get tick bites than those who were permethrin-free. The results highlight “the potential of permethrin-treated summer clothing for significantly reducing tick bites and tick-borne pathogen transmission,” the researchers wrote.
DIY vs. pre-treated clothes
How you apply permethrin matters. If you want to go the DIY route, opt for a product with a concentration of 0.5% permethrin. You can either buy a pre-diluted spray bottle, or dilute a concentrated jug yourself.
You can spray permethrin on your clothing, accessories, and gear—just not your skin. People often spray their pants, shirts, socks, shoes, hats, jacket, backpack, and tent, for example. (It doesn’t harm clothing.) You typically need to apply it 24 to 48 hours before going out; that way, it has time to dry and bond to the material you sprayed it on. Spray your clothing outside and while it’s off your body; hang it up on a clothesline, for example, and spray it evenly so that it becomes damp but not dripping wet.
Bobbi Pritt, a professor of laboratory medicine and pathology and the chair of the division of clinical microbiology at Mayo Clinic, always sprays permethrin on her gear outside on her screen porch, which is well-ventilated. Keep in mind that the liquid form can be toxic to cats, though it’s safe once it dries. At that point, “You don’t need to worry about having a cat in your lap,” says Pritt, who runs the blog Creepy Dreadful Wonderful Parasites.
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If you choose to spray permethrin on your clothing and gear, you’ll need to keep track of when you do it. Most products last up to six weeks or six washes—whichever comes first—and then need to be reapplied. That’s one reason why some people prefer longer-lasting pre-treated clothing, which is sold at outdoor-gear stores and can generally last up to 70 washes. It’s the only insect repellant used for factory treatment of clothing, according to the U.S. Environmental Protection Agency, and the EPA’s risk-assessment procedures “showed that permethrin factory-treated clothing is unlikely to pose any significant immediate or long-term hazard to people wearing the clothing,” according to their website.
Haines recommends opting for pre-treated garments rather than applying permethrin yourself. “It’s way better to get factory clothes,” she says. “It’s longer-lasting, and they’ve had to test it to make sure it works.” Plus, it’s more consistently applied and eliminates the possibility of human error from an imperfect spraying technique. It also removes the need for advanced preparation: People sometimes forget to apply permethrin to their clothing ahead of needing it, but with pre-treated clothes, you can grab and go.
What about natural bug sprays?
Permethrin is considered safe when used as directed. Research suggests that less than 2% of the insecticide is absorbed through the skin when it’s applied to clothing, and the EPA notes that there are no known risks of permethrin to women who are pregnant or nursing. It’s also safe for kids who are over 2 months old. “From all the studies and data that we have, it’s considered pretty safe as long as you use it according to the instructions on the container of the specific brand that you have,” Pritt says.
Still, some people prefer natural options. There are a couple of candidates: Oil of lemon eucalyptus can be applied to exposed skin as well as clothing and works well to repel mosquitoes and ticks, Haines says; make sure to get one with a 30% to 40% concentration, and dilute it with water before use. “It has a very strong smell, and you have to reapply it every two to three hours, but it works,” she says. Tea tree oil, which also needs to be diluted, is another popular natural insect repellent. It “reeks,” according to Haines, which keeps bugs away, but you have to reapply it as frequently as oil of lemon eucalyptus.
“What I try to tell people is that, yes, there are alternatives, but you have to use them responsibly if you want to protect yourself,” she says. “The natural answer is never as good as one that has been tweaked by a chemist to be higher efficacy.”
Making permethrin part of your routine
Pritt thinks of permethrin as “one piece of the puzzle—one tool in your toolbox of protecting yourself against ticks and mosquitoes.” When she goes out in the field to do research, she tucks her pants into her socks and wears tall boots. She sprays her entire outfit with permethrin the night before so it’s ready the next morning. She also sprays insect repellent on any exposed skin—often opting for oil of lemon eucalyptus, which she’s found works well for her.
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Haines takes a similar approach, but sprays a repellant with 20% picaridin on her exposed skin. “It’s really deadly on ticks,” she says. “It works on midges and mosquitos—it works on everything.” Plus, it typically lasts for eight to 12 hours. It doesn’t have a strong smell, she adds, and isn’t greasy. Pair it with your permethrin-treated clothing, and you’ll be much more protected from the blood-thirsty pests of summer.
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