Researchers identify insect repelling compounds in sweetgrass
As well as being a nuisance, mosquito bites are also linked with the spread of more serious diseases in some parts of the world, such as malaria, and the research team from the US Department of Agriculture, in collaboration with the University of Guelph and the University of Mississippi, turned their attention to the compounds in sweetgrass that keep the bugs at bay.
Sweetgrass (Hierochloe odorata) is a native plant used in traditional medicine, native to northern climates, and the two chemicals identified in it that seem to be responsible for putting off mosquitoes are phytol and coumarin.
Coumarin is an ingredient in some commercial anti-mosquito products, while phytol is reported to have repelling activity in scientific literature.
“We found that in our search for new insect repellents, folk remedies have provided good leads,” says Charles Cantrell, PhD, of the USDA.
“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.”
Testing the folklore
Cantrell and his team 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.
So, Cantrell’s team at the USDA in collaboration with the other research teams, 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-colored 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 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.
They noted that of the sweetgrass extracts, the steam-distilled oil got the fewest mosquito bites, matching the repellent potency of DEET.
From this they were then able to figure out the exact chemicals that give the sweetgrass oil its anti-mosquito influence.
