Study confirms efficacy of sweetgrass plant used by native American tribes to repel mosquitoes

Taking a cue from native American tribes that had for centuries used sweetgrass plant, a species common throughout large swaths of the US and Canada, to repel insects, researchers in the US conducted studies to evaluate how effective it was vis-à-vis the most common synthetic repellent, DEET (N,N-diethyl-m-toluamide).

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 DEET or the ethanol solvent control.

Then, the bugs 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.

"Then you take the mosquitoes and squish them on some paper," says Cantrell. "If they have the blood mimic in them, you see it right there on the paper."

Of the sweetgrass extracts, the steam-distilled oil got the fewest mosquito bites, matching the repellent potency of DEET.

The next step was to figure out the exact chemicals that give the sweetgrass oil its anti-mosquito power.

The researchers 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, he adds, while phytol is reported to have repelling activity in the scientific literature.

So although Cantrell didn't find brand-new insect deterrents in this experiment, he is happy to have demonstrated that "we were able to find constituents that are known to act as insect repellents in a folk remedy, and now we understand that there's a real scientific basis to this folklore."

On summer evenings, we try our best to avoid mosquito bites by dousing our skin with bug repellents and lighting citronella candles. (See: Mosquitoes use smell to see their hosts)