http://www.digitaljournal.com/article/344133

Some mosquitoes ignore DEET repellent

Posted Feb 23, 2013 by Tim Sandle
New observations of mosquitoes suggest that the widely used insect repellent DEET appears to be losing its effectiveness.
Mosquito  carrier of dengue fever
Mosquito, carrier of dengue fever
According to a new study, some mosquitoes, just a few hours after their first exposure to the insect repellent DEET, are able to ignore it and are no longer repelled by it. The observed mosquitoes seem to be able to ignore the chemical's noxious smell. This comes from a new study published in the journal PLOS One.
For the study, scientists looked at the Aedes aegypti mosquitoes. This species spreads the parasitic diseases dengue and malaria.
DEET (N,N-Diethyl-m-toluamide) is a slightly yellow oil. It is the most common active ingredient in insect repellents. It is intended to be applied to the skin or to clothing, and provides protection against tick bites, mosquito bites, chiggers, and other insects that can transmit disease. The chemical was developed by the United States military to protect troops undertaking jungle warfare during World War II.
To understand the diminished effects of the chemical, as the BBC has reported, researchers based at the London School of Hygiene and Tropical Medicine attached electrodes to the antennas of female mosquitoes as the insects sought to feed on a human arm covered in DEET. Three hours after the chemical repelled the mosquitoes, some returning insects were less sensitive to the unpleasant smell.
According to the Daily Telegraph, upon examination, the resistance was linked to a lower response in the neurons in the mosquitoes olfactory receptor (which provides the 'sense of smell').