Natural odours may keep mosquitoes at bay

Published in The Hindu on August 28, 2009

Culex

An environmentally safe mosquito repellent to which Culex mosquitoes may not be able to develop resistance may become realty. Culex mosquitoes cause dengue in humans.

Mosquitoes are attracted to carbon dioxide emitted in human breath. This is well known. Now, scientists from the University of California, Riverside, have found a way to make Culex mosquitoes lose the ability to sense carbon dioxide.

The germ of an idea to blunt their ability to sense carbon dioxide came from studying fruit flies. Fruit flies emit carbon dioxide when they are subjected to stress like vigorous shaking or electric shock.

Fruit flies have specialised neurons in their antennae that are sensitive to carbon dioxide. This gas emitted by fruit flies alert other flies and they stay away from the area of danger. This is their way of communicating danger to other fruit flies.

However, fruit flies do not keep away from ripened fruits despite plants emitting carbon dioxide. Dr. Anandasankar Ray, an entomologist became curious to understand this.

After screening 46 odours, Dr. Ray identified two odours — 1-hexanol and 2,3-butanedione — that strongly inhibit carbon dioxide sensitivity of the neurons. These two odours are emitted only by fruits that are ripening, thus allowing fruit flies to find their source of food.

“The presence of these odorants significantly and specifically reduces carbon dioxide-mediated avoidance behaviour, as well as avoidance mediated by Drosophila [fruit flies], stress odour,” notes the paper published in the journal Nature.

The researchers found that both the odours inhibit carbon dioxide response in a dose-dependent manner, irrespective of whether their application was initiated before or after the presentation of the gas.

Dr. Ray also found that with increasing concentrations of one of the odour (2,3-butanedione), the carbon dioxide neuron is silenced “well beyond the period of application.” In other words, the fruit flies’ insensitivity to carbon dioxide prolonged for a long time even after the odour was no longer present.

But in the case of Culex mosquitoes, the 2,3-butanedione was not effective. But two other odours, 1-hexanol and 1-butanal were very effective to make the mosquitoes insensitive to carbon dioxide.

“These odours are the first reported inhibitors of carbon dioxide-sensitive neurons in mosquitoes and may provide a valuable resource for the identification of economical, environmentally safe, volatile compounds that may reduce mosquito-human contact by blocking responsiveness to carbon dioxide,” notes the paper.

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