Researchers have identified compounds in sweetgrass - an aromatic herb native to Europe, Asia and North America - that keep mosquitoes at bay and could add to the arsenal of insect repellents.
Native North Americans have long adorned themselves and their homes with fragrant sweetgrass (Hierochloe odorata), a native plant used in traditional medicine, to repel biting insects, and mosquitoes in particular.
Now, researchers have identified the compounds in sweetgrass that keep mosquitoes at bay.
Charles Cantrell with his team at the US Department of Agriculture suspected that the active insect-repelling chemicals must waft off sweetgrass at ambient temperatures and, like essential oils from lavender and other plants, could be extracted using a process known as steam distillation.
In collaboration with researchers at the University of Guelph and the University of Mississippi, Cantrell's team performed steam distillation on sweetgrass samples and evaluated its oil for the ability to deter mosquitoes from biting.
To test the mosquitoes' aversion to the oil, the researchers filled small vials with a red-coloured feeding solution that mimicked human blood and covered the vials with a thin membrane.
Then, they coated the membranes with different substances: the sweetgrass oil, alternative sweetgrass extracts obtained without steam distillation, the gold-standard insect repellent N,N-diethyl-m-toluamide (DEET) or the ethanol solvent control.
Then, the mosquitoes got the chance to either bite the membranes to get to the blood or pass them by. The researchers observed what the insects did, counting how many mosquitoes went for a bite of each type of "blood" vat.
Of the sweetgrass extracts, the steam-distilled oil got the fewest mosquito bites, matching the repellent potency of DEET.
Researchers then purified the oil into 12 fractions and again checked their ability to ward off the bugs. They found three fractions that repelled mosquitoes as well as the oil.
Using nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry, the researchers identified two chemicals in these active fractions that seemed to be responsible for putting off mosquitoes: phytol and coumarin.
Coumarin is an ingredient in some commercial anti-mosquito products, while phytol is reported to have repelling activity in the scientific literature, Cantrell said.
The research will be presented at the 250th National Meeting & Exposition of the American Chemical Society (ACS) in Boston.
