In the 20th century, centuries of research into biochemistry finally started to pay big dividends for humanity, with major inventions ranging from antibiotics to DEET, an organic compound which proved extraordinarily useful in repelling mosquitoes. Unfortunately, it is becoming increasingly clear that the 21st century will be nature's backlash against the biochemical revolution. Hikers and other outdoor lovers will doubtless be most disappointed to hear that DEET-resistant mosquitoes are now joining that backlash.
DEET, or N,N-diethyl-meta-toluamide, is an oil first developed by the U.S. military during the early Cold War period as an insect repellent to aid its soldiers in jungle warfare. It was thought to block the smell receptors of insects like mosquitoes and ticks, which are normally highly attuned to chemicals emitted in the sweat and the breath of animals like human beings. Subsequent research has cast some doubt on that theory, but regardless of the cause (one theory is that it simply produces an odour that mosquitoes find extremely offensive), it worked. DEET-containing sprays and lotions, when slathered over the skin, could deter insect bites for hours at a time.
Unfortunately, this deprived mosquitoes of the food they relied on for survival - which set up the same sort of survival-of-the-fittest adaptations as our chemical warfare against bacteria and bedbugs has, in other contexts. Sooner or later, evolutionary theory dictated that some mosquitoes would probably develop a mutation that made them able to tolerate DEET. If that happened, they would be able to reproduce faster and more successfully than their DEET-vulnerable cousins, because the vast supply of human blood wandering around would have suddenly become accessible again.
And, unfortunately, that is now what seems to be happening. In May 2010, there was widespread media coverage of a study published in Nature by British researchers who had found that some mosquitoes had developed a genetic mutation which rendered them immune to DEET. James Logan and his teem at Rothamsted Research, according to coverage in Scientific American, were able to isolate mosquitoes with the DEET immunity and confirm through breeding experiments that it was being provided by a single dominant gene. The discovery actually emerged by chance from other research into the actual mechanism by which DEET works (at which point the scientists realized it didn't appear to work at all on some mosquitoes, and decided to find out why). Which specific gene in mosquito DNA was responsible for the immunity to DEET is still unknown - but what is known is that such a mutation would provide a key advantage to mosquitoes in areas with sizeable human populations.
DEET is simply a convenience for most people in the West - albeit a huge convenience, as anyone who has ever wandered into a mosquito-rich swamp and forgotten their can of bug spray can testify. It's an even more serious matter, though, in regions where preventing mosquito bites is actually key to limiting the spread of diseases like malaria. If we enter an era in which DEET is no longer effective, we will have to come up with some other chemical of comparable effectiveness.