Most readers, I’m sure, are aware that mosquitoes are a transmission vector for a number of rather nasty diseases, including malaria, yellow fever, equine encephalitis, and dengue fever. The standard advice, in regions where mosquitoes are common, is to keep one’s skin covered, to the extent possible, and to use insect repellent liberally. One of the most common active ingredients in repellents is a chemical usually referred to as DEET (more formally as N,N-Diethyl-meta-toluamide or [IUPAC] N,N-Diethyl-3-methylbenzamide), an oily compound originally developed by the US military after the experience of jungle warfare in World War II. Various ideas have been suggested to explain why DEET works; today, the consensus seems to be that insects just don’t like the smell.
However, a report at the BBC News site suggests that DEET’s effectiveness can be reduced because mosquitoes can adapt to it. One type of adaptation is genetic. There are always some individual insects that are less susceptible to DEET than average, and heavy use of the repellent creates evolutionary selection pressure favoring that lack of sensitivity. (This is parallel to the evolutionary process leading to the development of antibiotic-resistant bacteria, or of herbicide-resistant weeds.) This is a process that takes some time to occur, though mosquito generations are of short duration.
Some recent research indicates that there is another, shorter-term form of resistance that occurs. Researchers at the London School of Hygiene and Tropical Medicine studied the effect of DEET on Aedes aegypti mosquitoes, which carry dengue and yellow fevers. The mosquitoes were initially given the opportunity to feed from a human arm which had been covered with DEET; the repellent did, in fact, repel them. However, when the same mosquitoes were presented with the same opportunity a few hours later, the repellent was significantly less effective.
To try to understand what was happening, the researchers measured electrical activity in the insects’ antennae (the location of the olfactory receptors). Somehow, the first exposure to DEET de-sensitized the mosquitoes, so that their olfactory response was diminished. According to Dr. James Logan,
We were able to record the response of the receptors on the antenna to DEET, and what we found was the mosquitoes were no longer as sensitive to the chemical, so they weren’t picking it up as well.
There is something about being exposed to the chemical that first time that changes their olfactory system – changes their sense of smell – and their ability to smell DEET, which makes it less effective.
The research paper [PDF available] has been published at the Public Library of Science, in the journal PLoS One.
More work will be needed to determine how long this short-term effect lasts, and whether it occurs in other species of mosquito. Using repellents containing DEET is still a lot better than using nothing, but understanding these effects may help us develop even more effective protection.