Most insect repellents have been found to actually lose its effectiveness against mosquitoes.
At first, mosquitoes are repelled by their initial encounter with the substance, but they then get used to the smell and later ignore it, according to scientists from the London School of Hygiene and Tropical Medicine.
After placing electrodes on the mosquito’s antennae, the researchers were able to measure what was going on. Ultimately, the mosquitoes were growing less sensitive to the chemical.
There is something about being exposed to the chemical the first time that changes their olfactory system – changes their sense of smell – and their ability to smell modern repellents, which makes it less effective.
Source:
http://www.foxnews.com/health/2013/02/21/mosquitoes-learn-to-ignore-insect-repellent-deet-after-first-smell/#ixzz2Lh3kJPxk
http://www.bbc.co.uk/news/science-environment-21519998