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After Just a Few Hours, Bugs Aren't Bugged by Bug Spray

Update Date: Feb 21, 2013 12:14 PM EST

Anyone who has been lucky enough to spend time in the tropical areas probably knows how important insect repellent is. However, it can often feel fruitless to spread the spray on your skin, because it seems like mosquitoes pierce through the barrier anyway. Now, science has confirmed what those anecdotal experiences have shown: mosquitoes learn to simply ignore insect repellent. After just hours, mosquitoes become resistant to DEET spray and bite anyway.

DEET, which is short for N,N-Diethyl-m-toluamide, was first developed by the American military. Used in most insect repellents, it is not quite clear how the chemical works, but it appears that most mosquitoes simply just do not like the smell. According to the BBC, some research has indicated that genetic changes can make some mosquitoes immune to the smell of DEET, though it is not clear whether that trait has appeared in the wild.

According to the Smithsonian, research was not conducted among mosquitoes that have that immunity. The study was conducted with some remarkably calm volunteers. Aedes aegypti mosquitoes were placed in a mesh metal cage. Human volunteers placed their arms about an inch over the cages. One arm was sprayed with 20-percent DEET spray; for comparison, a can of OFF! Spray contains about 25 percent DEET. Meanwhile, the other arm - the control - was sprayed with nothing. Researchers found that 70 to 80 percent of mosquitoes attempted to reach the plain arm, while 10 to 20 percent of mosquitoes attempted to reach the DEET-sprayed arm.

Then the experiment was recreated a few hours later. Researchers found that the percentage of mosquitoes who attempted to reach the human-scented arm remained the same. However, about half of the mosquitoes exposed to the DEET spray hours earlier attempted to reach the DEET-sprayed arm.

The scientists suspected that perhaps chemicals in the DEET spray were interacting with those in human skin, so they performed the same experiment with a heating device, to which mosquitoes are naturally attracted. They behaved the same way as they did with human skin. In addition, a third experiment, in which mosquitoes were exposed to DEET spray after exposure to normal human skin, found that only 10 percent of mosquitoes attempted to bite the DEET-sprayed skin.

Researchers believe that the answer lies in their antenna. Electrodes on the mosquitoes' antenna found that mosquitoes were less sensitive to the spray. This capability is concerning, since insect repellent is not used to prevent bug bites, but also serious mosquito-borne illnesses, like malaria and dengue fever.

However, they say that people should not stop using DEET spray for two reasons. The same percentage of mosquitoes did not attempt to reach the DEET-sprayed arm, so it would appear that the spray still holds some protection. In addition, scientists have not yet developed a tool more effective than DEET spray.

The study was published in the journal PLoS One.

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