Scientists at the U.S. Department of Agriculture (USDA) have identified the component of the aromatic herb sweetgrass (Hierochloe odorata) that makes it an effective repellent of disease-carrying mosquitoes and other biting insects.

In a study that is being presented at the American Chemical Society's (ACS) 250th National Meeting & Exposition, experts from the agriculture department, as well as from the University of Mississippi and the University of Guelph, examined the effects of well-known tradition therapies that could help repel mosquitoes and other insects.

One of the remedies that the researchers looked into is a type of meadow grass known as sweetgrass, which is endemic in northern climates.

USDA researcher Dr. Charles Cantrell said that plant releases a sweet aroma that is effective at repelling mosquitoes.

To find out what active chemical in sweetgrass is responsible for repelling the disease-carrying insects, Cantrell and his colleagues subjected plant samples to steam distillation. This process involves having hot steam pass through the material of the sweetgrass followed by a reduction in the temperature. The resulting condensed liquid will then separate into water and oil, which contains the active chemicals that the researchers needed.

The research team then tested the aversion of mosquitoes to the sweetgrass oil by filling small vials a feeding solution that was colored in red to mimic human blood. They then covered the vials with thin membranes and coated those using different substances, such as sweetgrass oil and extracts from sweetgrass that were not obtained through steam distillation. They also coated some of the membranes using well-known commercial repellants ethanol solvent control and N,N-diethyl-m-toluamide (DEET).

Cantrell and his colleagues exposed the vials to mosquitoes and monitored the number of insects that would feed on the solution and those that would pass them by. The team also took note of which solution with specific coating type would the mosquitoes feed on.

The researchers discovered that the sweetgrass oil-coated vials had the fewest insect bites, matching even the potency of DEET in repelling mosquitoes.

When they purified the sweetgrass oil into 12 different fractions, the team found that three of these components have the ability to ward off mosquitoes. They then made use of mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy to isolate two active chemicals that are responsible for repelling the mosquitoes. These are coumarin and phytol.

Cantrell said that coumarin is already used as a base component for some anti-mosquito repellants available in the market, while phytol is believed to have insect-repelling capabilities in scientific literature.

Despite not having discovered a new insect deterrent in the study, the researchers said that they were able to provide a scientific explanation as to how a folk remedy for biting insects works.

Photo: Amit Burstein | Flickr 

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