A whiff of catnip can make mosquitoes buzz off. Now researchers know why.
The active component of catnip (Nepeta cataria) repels insects. It does this by triggering a chemical receptor that can spur sensations such as pain or itch. Researchers reported this March 4 in Current Biology. The sensor is dubbed TRPA1. It is common in animals — from flatworms to people. And it’s what triggers a person to cough or an insect to flee when they encounter an irritant. Those irritants can range from cold or heat to wasabi or tear gas.
Catnip’s repellent effect on insects — and its effect of excitement and joy in felines — are well documented. Studies have shown that catnip may be as effective at deterring insects as the widely used synthetic repellent diethyl-m-toluamide. That chemical is better known as DEET. What hadn’t been known was how catnip repelled insects.
To find out, researchers exposed mosquitoes and fruit flies to catnip. Then they monitored the insects’ behavior. Fruit flies were less likely to lay eggs on the side of a petri dish that was treated with catnip or its active component. That chemical is called nepetalactone (Neh-PEE-tuh-LAK-toan). Mosquitoes also were less likely to take blood from a human hand coated with catnip.
Insects that had been genetically modified to lack TRPA1, however, had no aversion to the plant. Also, tests in lab-grown cells show catnip activates TRPA1. That behavior and lab-test data suggest that insect TRPA1 senses catnip as an irritant.
Learning how the plant deters insects could help researchers design even more potent repellents. They might be good for low-income countries hit hard by mosquito-borne diseases. “Oil extracted from the plant or the plant itself could be a great starting point,” says study coauthor Marco Gallio. He is a neuroscientist at Northwestern University in Evanston, Ill.
If a plant can make a chemical that activates TRPA1 in a variety of animals, none are going to eat it, says Paul Garrity. He’s a neuroscientist at Brandeis University in Waltham, Mass. He was not involved in the work. Catnip probably didn’t evolve in response to predation from ancient mosquitoes or fruit flies, he says. That’s because the plants aren’t on the insects’ main menu. Instead, these insects might be collateral damage in catnip’s fight with some other plant-nibbling insect.
The finding “does make you wonder what the target is in cats,” says Craig Montell. He’s a neuroscientist at the University of California, Santa Barbara. He also was not involved with the study. There’s also the question of whether the plant might send signals through different cells — such as those for pleasure — in the cat nervous system, Montell says.
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Luckily, the plant’s bug-off nature doesn’t affect people. That’s the sign of a good repellent, Gallio says. Human TRPA1 did not respond to catnip in lab-grown cells. Plus, he adds, “the great advantage is that you can grow [catnip] in your backyard.”
Though maybe don’t plant catnip in the garden, says study coauthor Marcus Stensmyr. He’s a neuroscientist at Lund University in Sweden. A pot might be better, he says, since catnip can spread like a weed.