How does insect repellent work? Insect repellents work by either preventing the insect from detecting the signals given off by people or by using chemicals that insects don’t like.
Insect repellents don’t have to repel all insects. They are generally designed to repel those insects that bite us. These are mostly mosquitoes, midges, ticks, and some types of flies. Other insects might be annoying, but they don’t bite us. In this way, they are different to insect sprays, which are designed to kill the insect. Insect sprays work by interrupting the fly’s nervous system so that their muscles lock up and they cannot breathe. Insect repellents only want to keep the bugs away. If you can keep these insects from biting, it reduces the amount of annoying, itchy bite marks, but it also reduces the incidence of dangerous diseases like malaria.
The first type of insect repellents work by masking the signals we give off that insects use to find us. Mosquitoes are very good at finding people. They can detect the concentration of carbon dioxide that we exhale. They can detect our body heat. They can smell the lactic acid in our perspiration and our body odor. Ticks use the same signals, and they can also detect vibrations as well. Ticks will climb to the tips of leaves and wait with their front legs extended. When they sense any of these signals, they will grab the passing host and climb on. Ticks can carry Lyme disease.
The most common chemical used in insect repellents is called DEET. DEET stands for diethyltoluamide. It is actually DET (Di+Ethyl+Toluamide), but it appears to have become DEET, and I can find no reason why. It’s possible that DET sounds too much like DDT, or possible “detonation”. DEET is a yellow oil. It is one of the most effective ingredients in insect repellents. The most interesting thing about DEET is that no one actually knows how it works. The theories are that it works because of two mechanisms. Mosquitoes and other insects have specialized receptors in their antennae and near their mouths. They can sense the carbon dioxide from over 30 m away and then home in on it. DEET works by interfering with those receptors and preventing the insect from detecting the carbon dioxide. If they can’t register it, they can’t find you. It may also block the other receptors that the insects have as well.
The second mechanism is that mosquitoes don’t like the way that DEET feels. Mosquitoes have cells on their feet that they can taste and feel with, somewhat like a tongue. Even if DEET interferes with mosquitoes’ ability to find people, there are so many mosquitoes that just through chance alon,e many of them will find you. If you have a layer of DEET on your skin, the insects land and then take off again. Experiments have shown that it is the way the DEET feels through their legs that is deterring them. It was long thought to be the taste, but other chemicals that taste the same way don’t deter the mosquitoes. If experimenters use a special glue to seal up the sense organs in the mosquitoes’ legs, they land and feed as normal. So, it is something to do with the way that DEET feels to these insects.
DEET was discovered by a scientist called Samuel Gertlier in 1944. He was working for the United States Department of Agriculture, but the work was for the United States Army. Mosquitoes and the malaria they carried were a huge problem for the American Army fighting in the jungles of Asia at the time. DEET proved incredibly effective. Originally, it was used in a 75% DEET solution, but research showed that mixing it with other chemicals made it last longer and stopped it from evaporating as quickly. Research has shown that DEET concentrations over 30% can actually be hazardous to our health, so modern insect repellents usually contain far less than this. They are still effective, but have to be reapplied.
Scientists have tried using genetics to breed mosquitoes that don’t have receptors for heat, carbon dioxide, and all of the other things, but they always seem to be able to find people. It seems that mosquitoes, like airplanes, have an awful lot of backup systems. It is actually only the female mosquito that bites, and even then, not many of the thousands of species of mosquitoes bite. They do it to get the proteins for their eggs. There has been a lot of work to eradicate the mosquitoes that carry malaria, but mosquitoes are also pollinators and an important part of the ecosystem, so it is a tricky problem. And this is what I learned today.
Sources
https://www.livescience.com/65334-how-does-deet-work-mosquitos.html
https://theconversation.com/how-do-mosquito-repellents-work-a-chemistry-expert-explains-244403
https://dermnetnz.org/topics/insect-repellent
https://en.wikipedia.org/wiki/Insect_repellent
https://off.com/en-us/education/insects-101/how-mosquitoes-find-you
https://en.wikipedia.org/wiki/DEET
https://www.scientificamerican.com/article/mosquitoes-see-red-literally-when-they-smell-human-breath
Photo by Vilvah Store: https://www.pexels.com/photo/display-of-mosquito-repellent-18066459/