Hungry caterpillars may sound an alarm for plants

Can plants “hear” hungry caterpillars? Vibrations caused by caterpillars munching through leaves can lead to plants hiking up their chemical defences against being eaten, a recent study in the US suggests.

For the experiments, the researchers simulated the mechanical vibrations caused by Pieris rapae caterpillars chewing on thale cress (Arabidopsis thaliana) leaves. They found that if leaves were pre-treated with vibrations that mimic caterpillars feeding, they produced higher levels of defensive chemicals when real caterpillars attacked compared with untreated leaves.

The plants also seemed to discriminate between the vibrations caused by chewing and those caused by the wind or by insect song. "How such selectivity is achieved is an open question in plant sensing," write the researchers in Oecologia.

“Previous research has investigated how plants respond to acoustic energy, including music,” says researcher Heidi Appel from the University of Missouri. “However, our work is the first example of how plants respond to an ecologically relevant vibration. We found that feeding vibrations signal changes in the plant cells’ metabolism, creating more defensive chemicals that can repel attacks from caterpillars.”

The findings prompt questions about how mechanical vibrations could elicit local or even distant reactions in plants, and there may be some application in warding off pests too.

“Caterpillars react to this chemical defence by crawling away, so using vibrations to enhance plant defences could be useful to agriculture,” says Appel. “This research also opens the window of plant behaviour a little wider, showing that plants have many of the same responses to outside influences that animals do, even though the responses look different.”