A pinch of catnip can make mosquitoes buzz and now researchers know why.
The active components of catnip (Nepeta cataria) repel insects by triggering a chemical receptor that stimulates sensations such as pain or itching, the researchers report on March 4 in Current Biology. The sensor, nicknamed TRPA1, is common in animals — from flatworms to humans — and responds to environmental irritants such as cold, heat, wasabi, and tear gas. When irritants come in contact with TRPA1, the reaction can cause people to cough or flee an insect.
The repellent effect of Catnip on insects – and its euphoric effect on felines – has been documented for millennia. Studies have shown that catnip can be as effective as the widely used synthetic diethyl-m-toluamide repellent or DEET (SN: 9/5/01). But it was unknown how the plant repelled insects.
Thus, the researchers exposed mosquitoes and fruit flies to the catnip and monitored the behavior of the insects. Fruit flies were less likely to lay eggs on the side of a petri dish that was treated with catnip or its active component, nepetalactone. Mosquitoes were also less likely to take blood from a human hand clad in catnip. However, insects that were genetically modified to lack TRPA1 had no aversion to the plant. That behavior, along with laboratory-grown cell experiments showing that catnip activates TRPA1, suggests that the insect TRPA1 perceives catnip as an irritant.
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Puzzling about how the plant deters insects could help researchers design potent repellents that may be easier to obtain in developing countries affected by mosquito-borne diseases. “Oil extracted from the plant or from the plant itself could be a great starting point,” says study co-author Marco Gallio, a neuroscientist at Northwestern University in Evanston, Ill.
If a plant can manufacture a chemical that activates TRPA1 in a variety of animals, none will eat it, says Paul Garrity, a neuroscientist at Brandeis University in Waltham, Massachusetts, who did not participate in the work. He Catnip probably did not evolve in response to predation by ancient mosquitoes or fruit flies, he says, since the plants are not on the main menu of insects. On the contrary, these insects can be collateral damage in the catnip fight with some other insect that bites plants.
The discovery "makes you wonder what the target is in cats," says Craig Montell, a neuroscientist at the University of California, Santa Barbara, who also did not participate in the study. The question is not just whether catnip targets TRPA1 in cats, but also whether the plant can send signals through different cells (such as pleasure cells) in the feline nervous system, Montell says.
Fortunately, the nature of plant bugs does not affect people: a sign of a good repellent, says Gallio. Human TRPA1 did not respond to catnip in laboratory-cultured cells. In addition, he says, "the big advantage is that you can grow (catnip) in the garden."
Although they may not plant catnip in the garden, says study co-author Marcus Stensmyr, a neuroscientist at Lund University in Sweden. A pot can be better, he says, as the catna can spread like a weed and take over a garden.