 Millions of Americans spray it on their bodies in the summertime to avoid getting mosquito bites, but the popular insect repellant chemical DEET appears to be going the way of antibiotics in losing its efficacy over time. A recent study published in the journal PLoS One reveals that mosquitoes are becoming increasingly less responsive to DEET, and are even now learning how to shut off their senses when exposed to it. For their research, Dr. James Logan and his colleagues from the London School of Hygiene & Tropical Medicine studied the effects of DEET on the Aedes aegypti mosquito type, which is known to bite during the day and transmit deadly diseases like yellow fever and dengue fever. Dr. Logan and his team exposed the mosquitoes to small amounts of DEET, and tracked their behavior throughout the course of several hours. What they observed is that, following three hours after exposure, many of the mosquitoes were no longer averse to DEET as they were during the first three hours. As it turns out, even minimal exposure to DEET resulted in the mosquitoes developing a type of immunity to the chemical, which was observed to no longer be effective beyond three hours after exposure. 'We think that the mosquitoes are habituating to the repellant, similar to a phenomenon seen with the human sense of smell also,' explains Dr. Logan about his findings. 'Our study shows that the effects of this exposure last up to three hours,' he adds, noting that mosquitoes previously exposed to DEET eventually adapt to it, and are no longer affected by it. It is believed that the sensory receptors of mosquitoes gradually shut down following exposure to DEET, which results in the creatures no longer being able to smell or sense it. As a result, these DEET-exposed mosquitoes are likely to return to human skin that has already been sprayed with DEET and try to tap it for blood, as if it had never been sprayed in the first place. The findings build upon earlier research by Dr. Logan, which hypothesized a genetic factor that resulted in some mosquitoes not responding to DEET. As reported by Nature back in 2010, some mosquito types have actually evolved to overcome the intended effects of DEET, thanks to a dominant gene that is sometimes passed down through mosquito families. 'That there might actually be a gene lurking in the background in mosquitoes that causes DEET resistance is the single most surprising result,' said Leslie Vosshall of Rockefeller University in New York City to Nature about the earlier study. 'This hasn't really been reported before.' 