The chemical I want to talk about today is a widely used and very useful chemical called N,N-diethyl-3-methylbenzamide. Most people know it by its acronym, DEET.
I was recently wondering just how DEET does its thing of making mosquitoes not bite people. (Why yes, the recent gap in my posts means I was on vacation. There may have been mosquitoes involved.) Obviously, I started by searching Google Scholar.
To my interest, it seems to be one of those (many) chemicals where science doesn't know exactly how it works, at least not well enough (yet) to synthesize a better fit based on that knowledge. There was some talk of specific chemical receptor targets in a few of the papers, but I didn't see anything in my searches about success in making a new repellent. From what I read, however, there is a pretty good understanding of the mechanism in general:
It seems mosquitoes think that humans smell tasty, and DEET smells bad.
Personally, I also think DEET spray smells bad, but it goes below my detection limit within a few seconds of putting it on, so I can deal with that. My spray can of DEET is only a 25% solution, and nothing on it says what the other 75% is. However, everything needs an MSDS, so I found the MSDS for the can of DEET sitting on the table in front of me. (Same Pest Control Product Act registration number and everything.) The other 75% is largely ethanol and water, so I'm guessing the smell is actually due to the DEET. Do not spray in enclosed spaces or with the nozzle pointing upwind; if any overspray drifts into your mouth it tastes awful. (I learned this a long time ago. Yuck.)
I also found out that DEET was discovered in a shotgun-approach search for insect repellents initiated by the US army in 1942. They basically screened every chemical they could find or synthesize for repellency, then on the repellency hits, they tested for skin irritation, toxic effects, and so on, to find something they could use as an insect repellent. DEET was the winner in the mid-1940s and it's still the one that new insect repellents are compared against when doing efficacy tests.
There are other insect repellents available of course. Most are nowhere near as effective as DEET.
Continuing in the vein of biting insects, if one forgets to put on a layer of DEET because there happened to be no mosquitoes visible, forgetting that there are other biting bugs around (*ahem* why do you ask if I did this?) there's always a product called "afterbite".
Which also should not be used in an enclosed space.
Because contrary to my usual modus operandi, I used it without reading the ingredients or the warnings, and immediately had to lie down and wait for the ammonia vapour to dissipate. Embarrassing, because I have worked with ammonia and I know very well what it does to a person, even at 3%.
According to even some very old references I've found, the itch of many insect bites is caused by an acidic venom, and it's the alkali nature of ammonia that relieves the itch, through simple neutralization—which means that a paste of baking soda and water should also work if you don't have ammonia at hand. That old article also refers to a "wet cake of soap" but I'm not sure if modern bar soap is as alkali as soap used to be. My bites having long faded before I got home and read that, I couldn't test it.