At the risk of becoming the Pied Piper of cats, you could use the garden herb catnip as a non-toxic insect repellent – and scientists have now nutted out why it wards off mozzies and other irritating bugs.
The active ingredient nepetalactone in the mint-like herb (Nepeta cataria) selectively activates the irritant receptor TRPA1 (transient receptor potential ankyrin 1) in certain insects, according to a study published in the journal Current Biology.
The protein is best known as the “wasabi receptor” (many people will be familiar with the eye-watering sensation caused by the Japanese horseradish), but although humans and other animals have it, catnip doesn’t affect us in the same way.
More intriguingly, tear gas – which contains mustard oil – activates the same irritant receptor in both mosquitoes and humans, says co-senior author Marco Gallio from Northwestern University, US.
“But it may not be a good insect repellent,” he adds, “as it makes people miserable too.” Even better, catnip doesn’t seem to deter bees (although aphids don’t mind it either).
Importantly, it’s particularly effective at repelling mosquitoes, which pose a major public health problem. Some studies show it to be as effective as chemical repellents such as DEET – if not more so – which many cannot afford or avoid due to concerns about toxicity.
And its use is not new. As lead author Nadia Melo, from Lund University in Sweden, and colleagues note, Pliny the Elder described several medicinal uses of it in his encyclopedia Naturalis Historia back around AD 77.
In the ninth century, they write, Bald’s leechbok “reports catnip as effective against everything from bedevilment (mix leaves with ale, chant 12 masses) to shoulder pain (pound leaves in ale, drink by fire)”.
In humans it was known to be soothing and calming, while for cats, rolling in it seems to evoke euphoria – apparently it gives them an opioid-like hit – and it’s thought the aim is to help them deter mosquitoes.
But it’s long been observed that mozzies and other insects are not nearly as keen on catnip – the team notes its historical use as a repellent against “pesky small creatures”, as referred to by Johannes Franck’s Speculum botanicum in the 1600s, and others.
Adding some modern molecular science to this, the collaborative experiment by Melo and colleagues at Marcus Stensmyr’s lab in Sweden was quite thorough.
First, they tested Pliny’s claim that catnip repels scorpions by allowing four Heterometrus cyaneus to choose a pot to hide in, one of which contained catnip. The scorpions all chose the pot with catnip, “displaying no apparent distress”. To be fair, the authors say the plant Pliny refers to as Nepeta may have been a different herb.
They continued with a vast array of experiments with different arthropods ranging from ticks, mites, aphids and planthoppers to bees, wasps, weevils, beetles, flies and mosquitoes, finding evidence to support the notion that nepetalactone is an irritant.
Then they used cultured cells expressing the TRPA1 genes – a molecular mechanism for “pain” and response to irritants discovered by Gallio’s lab – in flies, mozzies and humans to test if they are activated by catnip and nepetalactone.
Finally, the team tested catnip on mutant mosquitoes and flies without the TRPA1 receptor and found they lost their aversion to the herb. “TRPA1 mutant mosquitoes in particular do not avoid catnip any more at all,” says Gallio. “Cool.”
Now they’ve shown why catnip works and is so powerful, he says their study further supports its widespread use as a natural, safe repellent, accessible in poor countries afflicted by mosquito-borne diseases. “Great because it’s cheap and it grows like a weed.”
What can you do to avoid excessive feline affections while warding off the pesky bugs? Not a problem, says Gallio: “We like cats.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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