Fire up the citronella-scented tiki torches, and slather on the DEET: 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 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.
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."
"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, but the research 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.
The researchers found that the mosquito-version of TRPA1 was directly activated by citronellal.This discovery now raises the possibility of using TRP channels to find better insect repellants."
And there is a clear need for improved repellants because 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.
And citronellal, despite being pleasant-smelling (for humans, anyway), causes a rash when it comes into contact with skin.