Saturday, August 28, 2010

Insect Repellants

Johns Hopkins Medicine (Aug. 25, 2010)

Scientists Discover How Chemical Repellants Trip Up Insects

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.

Now, Johns Hopkins scientists 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 better understanding of these molecular-behavioral links already is aiding the team's search for more effective repellents.

In separate studies published on August 26, in Neuron and Current Biology, the Johns Hopkins 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 commonly used botanical repellent.

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.

"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 repellent 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."

In the second study, Montell and colleagues focused on the repellent citronellal. To measure repulsion to the vapors it emits, they applied the botanical compound to the inside bottom of one of the 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.

There is a clear need for improved repellents, 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.

References:

1.Avoiding DEET through Insect Gustatory Receptors.
Youngseok Lee, Sang Hoon Kim, Craig Montell.
Neuron, 26 August 2010; 67(4) pp. 555 - 561 DOI: 10.1016/j.neuron.2010.07.006
Link to Neuron abstract

2.Drosophila TRPA1 Channel Is Required to Avoid the Naturally Occurring Insect Repellent Citronellal.
Young Kwon, Sang Hoon Kim, David S. Ronderos, Youngseok Lee, Bradley Akitake, Owen M. Woodward, William B. Guggino, Dean P. Smith, Craig Montell.
Current Biology, in press, on line 26 August 2010 DOI: 10.1016/j.cub.2010.08.016

Link to Current Biology abstract

Link to Hopkins Medicine news release

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