How DEET Works

Fire up the citronella-scented tiki torches, and slather on the  DEET:  Everybody knows these simple precautions repel insects,  notably mosquitoes, whose bites not only itch and irritate, but also transmit diseases such as West Nile virus, malaria and dengue.

Scientists at Johns Hopkins have discovered what it is in the bugs'  molecular makeup that enables citronellal (the aromatic liquid used
in lotions, sprays and candles) and DEET, to deter insects from landing and feeding on you.

A Biting Female Mosquito with Her Abdomen Filled with a Blood Meal
A Biting Female Mosquito with
Her Abdomen Filled with a Blood Meal

Taste receptors on the insects' tongue and elsewhere are needed  to detect DEET.  Citronellal detection is enabled by pore-like proteins known as TRP (pronounced "trip") channels. When these molecular receptors are activated by exposure to DEET or citronellal,  they send chemical messages to the insect brain, resulting in "an  aversion response," the researchers report.

"DEET has low potency and is not as long-lasting as desired, so finding the molecules in insects that detect repellents opens the door to identifying more effective repellents for combating insect-borne disease," says Craig Montell, Ph.D., a professor of biological chemistry and member of Johns Hopkins' Center for Sensory Biology.

Scientists have long known that insects could smell DEET, Montell notes, but the new study showing taste molecules also are involved suggests that the repellant deters biting and feeding because it activates taste cells that are present on the insect's tongue, legs and wing margins.

"When a mosquito lands, it tastes your skin with its gustatory  receptors, before it bites. We think that one of  the reasons DEET is relatively effective is that it causes avoidance responses not only through the sense of smell but also through the  sense of taste. That's pretty important because even if a mosquito lands on you, there's a chance it won't bite."

The researchers identified two distinct types of cell surface channels that are required in olfactory neurons for avoiding citronellal vapor. The channels let calcium and other small, charged molecules into cells in response to citronellal. One type of channel,  called Or83b, was known to be required for avoiding DEET. The second  type is a TRP channel.

"We found that the mosquito-version of TRPA1 was directly activated by citronellal," says Montell who discovered TRP channels in 1989 in  the eyes of fruit flies and later in humans."This discovery now raises the possibility of using TRP channels to find better insect repellants."

There is a clear need for improved repellants, Montell says. DEET is not very potent or long-lasting except at very high concentrations, and it cannot be used in conjunction with certain types of fabrics. Additionally, some types of mosquitoes that transmit disease are not
repelled effectively by DEET. Citronellal, despite being pleasant-smelling (for humans, anyway), causes a rash when it comes into contact with skin.

John Hopkins Medicine

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