I took a little detour last week away from summer science into the dark lands of antibiotic resistance. But having talked about mosquito attraction a couple of weeks ago, it only makes sense to talk about the opposite: mosquito repellence.
It’s quite likely that if you’ve sprayed bug spray on yourself or your kids before going outside in summer, that bug spray was full of DEET. According to the National Pesticide Information Center, some 30 percent of the US population uses at least one DEET-containing product every year.
Formally known as N,N-dietheyl-meta-toluamide, DEET’s listed as the active ingredient in about 570 products registered with the Environmental Protection Agency (EPA; which is tasked with regulating insect repellents), at concentrations ranging from 4% to full-on 100%. That list of products includes sprays, lotions, wrist bands, wipes, etc.
As with many technologies, DEET’s first home was with the military. The Army wanted a better bug repellent after World War II, and after testing thousands of compounds started putting DEET into troops’ hands in 1946. The EPA approved DEET for public sale in 1957, and today it’s considered effective against a long list of biting bugs, including black flies, chiggers, deer flies, fleas, gnats, horse flies, mosquitoes, no-see-ums, sand flies, and ticks.
Effective mosquito repellence is about more than a pleasant summer BBQ or forest hike. In developing countries, DEET is often used to treat bed nets, which help repel night-biting mosquitoes that transmit malaria, as well as to fend off mosquitoes carrying other little nasties like yellow fever and dengue. Here in the Northeast, it helps protect against hungry skeeters harboring West Nile virus and Eastern Equine Encephalitis virus, as well as ticks carrying Lyme disease.
But here’s the funny thing about DEET — despite the fact that it’s used so much by so many people in so many places for nearly 70 years, we have no idea of how it actually works. There are theories, of course: that it keeps mosquitoes from detecting lactic acid or some other attractant, that mosquitoes don’t like its smell, that they find it irritating. But its exact mechanism of action has yet to be sussed out.
While we don’t know how it works, we do know that, just as microbes get resistant to antibiotics, mosquitoes are getting resistant to DEET — a serious problem for people in countries where DEET-treated bed nets are a mainstay in malaria control programs. Just as it’s not clear how DEET works, it’s not clear how mosquitoes become immune to it. But again, there are theories, such as structural changes in the mosquito’s “nose” or the idea that a mosquito may only get turned off by DEET once and ignore it afterward.
DEET isn’t the only kid on the mosquito repellent block, though. There are a few other chemicals you may find in the bug spray isle at your drug store, in particular:
- Picaridin. Also called icaridin, this is an artificial version of the chemical piperidine, which gives black pepper its kick. (No, that’s not to say you should start marinating yourself in black pepper paste to keep mosquitoes away.) First synthesized in the 1980s, it got EPA approval in 2005. Here’s a list of picaridin-containing repellents.
- IR3535. Technically a biopesticide, it’s structurally similar to the amino acid alanine, and has been available in the US since 1999. Here’s a list of products containing IR3535.
- Lemon eucalyptus oil. Interestingly enough, this is the only naturally-occurring mosquito repellent that’s EPA-approved for both safety and efficacy. Most natural repellents are tested for safety only, and don’t necessarily score so well on the effectiveness front. Here’s where you’ll find lemon eucalyptus oil.
(By the way, if you’re trying to find the right repellent for you, depending what bugs you want to repel, what ingredient you want to use, and what time of day you’ll be out and about, check out these repellent finders from NPIC and EPA.)
There’s also research going on for new repellent technologies. One that’s been getting a lot of news the last couple of weeks is called the Kite Patch. It’s a paper patch worn on the skin or clothing that releases a chemical that overstimulates a mosquito’s CO2 sensors, essentially driving it crazy and driving it away. Olfactor Laboratories, the company that’s developing the patch, is raising money on Indiegogo to conduct field tests in Uganda.
Just as with microbes, we’re in something of an arms race with mosquitoes. We have repellents, they have probing prosbosci and the ability to smell us out at long distances. As climate changes and mosquito ranges shift, it’s a race that will literally heat up.