JOHNS HOPKINS (US)—Scientists now know how bug repellants like DEET and citronella-scented candles work to keep mosquitoes and other insects from putting the bite on you.

And knowing just what in bugs’ molecular makeup allows these products to protect you will aid the search for even more effective repellants, says the Johns Hopkins University scientist who led the research.

“DEET has low potency and is not as long-lasting as desired, so finding the molecules in insects that detect repellants opens the door to identifying more effective repellents for combating insect-borne disease,” says Craig Montell, a professor of biological chemistry.

Improved repellants could help prevent not only itch and irritation, but also transmission of insect-borne diseases such as West Nile virus, malaria, and dengue.

In separate studies published this week in Neuron and Current Biology, the researchers reveal how mosquitoes and other insects taste DEET—a man-made compound that’s been the most widely used insect repellent for more than 50 years—and smell citronellal, a common, aromatic botanical repellant used in lotions, sprays, and candles.

It turns out that three 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.

Tasty treat
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,” Montell explains. “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 Johns Hopkins study of repellants, conducted on fruit flies because they are easier to manipulate genetically than mosquitoes, began with a “food choice assay.”

The team filled feeding plates with high and low concentrations of color-coded sugar water (red and blue dyes added to the sugar), allowing the flies to feed at will. The researchers determined what the flies ate by the color of their stomachs: red, blue or purple (a combination of red and blue).

Normal flies preferred more sugary water to less sugary water in the absence of DEET. When various concentrations of DEET were mixed in with the more sugary water, the flies preferred the less sugary water, almost always avoiding the DEET-laced sugar water.

Flies that were genetically engineered to have abnormalities in three different taste receptors showed no aversion to the DEET-infused sugar water, indicating the receptors were necessary to detect DEET.

“We found that the insects were exquisitely sensitive to even tiny concentrations of DEET through the sense of taste,” Montell reports. “Levels of DEET as low as five-hundredths of a percent reduced feeding behavior.”

To add to the evidence that the three taste receptors are required for DEET detection, the team attached recording electrodes to tiny taste hairs on the fly tongue and measured the taste-induced spikes of electrical activity resulting from nerve cells responding to DEET.


Citronellal, despite being pleasant-smelling (for humans), causes a rash when it comes into contact with skin. (Credit: iStockphoto)

Stinky smell
In the second study, Montell and colleagues focused on citronellal. To measure repulsion to the vapors it emits, they applied the compound to the inside bottom of one of two connected test tubes, and introduced about 100 flies into the tubes. After a while, the team counted the flies in the two tubes. As expected, the flies avoided citronellal.

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 team tested flies with mutated versions of 11 different insect TRP channels. The repellent reaction to citronellal was reduced greatly in flies lacking TRPA1.

Montell discovered TRP channels in 1989 in the eyes of fruit flies and later in humans. His lab and others have tallied 28 TRP channels in mammals and 13 in flies, broadening understanding about how animals detect a broad range of sensory stimuli, including smells and tastes.

Better bug repellant
“This discovery now raises the possibility of using TRP channels to find better insect repellants,” Montell says.

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), causes a rash when it comes into contact with skin.

The DEET research appearing in Neuron was supported by the National Institute on Deafness and other Communication Disorders. The citronellal research appearing in Current Biology was supported by the National Institute on Deafness and other Communication Disorders, the National Institute of General Medical Sciences, and the Bill and Melinda Gates Foundation.

In addition to scientists at Johns Hopkins, researchers from the University of Texas Southwestern Medical Center contributed to the study published in Current Biology.

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