Caterpillar
© Diana Meister/iStockphoto



That's the sound of a caterpillar chewing on a leaf. But the real surprise is what happens next: the plant reacts to the noise by churning out chemicals that repel predators.


The discovery was made in 2014, by researchers Heidi Appel and Rex Cocroft from the University of Missouri.

It's been known for a while that sounds can affect the way plants germinate, and the expression of some of their genes, says Appel. "But just why plants were sensitive to airborne sound was a mystery".

Self-preservation is as good an evolutionary strategy as you get, so the pair set out to test whether plants were able to respond to the miniscule vibrations caused by having their leaves chewed.

With the help of a laser and some reflective tape, Cocroft -- an expert in bioacoustics -- recorded the vibrations made by a caterpillar chewing on leaves of a mustard plant, and then played the soundtrack to other plants of the same type. Control plants were played two hours of silence, in a kind of botanical John Cage tribute.

Appel then measured the levels of bad-tasting chemicals that the plants produce to deter predators, like glucosinolates, which gives mustard its sharp flavour. Sure enough, the plants that 'heard' the chewing sound had higher levels of those compounds when they were later grazed upon. So the feeding vibrations had primed the plants for later attack.

Appel says priming defence systems like this are very common: "[It] provides a way for plants to avoid the cost of producing chemical defenses until they're actually needed".

And to make sure the plants don't just react to any old vibrations, the team did the same test with harmless wind noise and insect song, and the plants didn't react at all.

"Our research provided for the first time an ecological reason for their vibration detection," says Appel.

Just how the plants detect the vibrations, and whether it's a widespread trick of the plant trade is the subject of their current work.

Eavesdropping plants. Who knew!