Scientists say the widely-used insect repellent Deet is losing its effectiveness against mosquitos. Dr James Logan and researchers from the London School of Hygiene and Tropical Medicine published their findings in the journal Plos One. Deet, (N,N-Diethyl-m-toluamide), was first developed by the U.S. military following the effects of jungle warfare in WWII. While unclear for many years how the chemical worked, there has been more recent suspicion that mosquitos were becoming immune to it. Researchers studied Aedes aegypti, a mosquito species known for spreading dengue and yellow fevers. They found that on first exposure to a Deet-coated meal, the mosquitos were repelled. A few hours later they were offered a second chance, and the mosquitos bit despite the Deet, suggesting there is an immediate olfactory tolerance that develops. To test this, researches attached electrodes to the insects' antennae. "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," Dr. Logan explained. "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." An earlier study by the same researches found that genetic changes can take place to make the same species of mosquito immune to Deet, but this occurred in a lab setting and researcher were unsure whether this happens in the wild. This study shows that whether or not genetic mutations take place in the wild, temporary immunity occurs after exposure. Dr. Logan added that the study's findings shouldn't deter anyone from using the insect repellent in in areas at high risk for mosquito-borne disease, although scientists are hoping to develop new versions to be more effective.