Plants can not only sense caterpillars that are munching on their leaves but they defend themselves by producing chemicals that will drive the bugs away. This is the startling conclusion of researchers at the University of Missouri who combines sound vibrations of munching caterpillars with chemical analysis of leaves. This new discovery adds to previous studies that suggested that plant growth can be influenced by sound and that plants respond to wind and touch.

 “Previous research has investigated how plants respond to acoustic energy, including music,” said Heidi Appel, senior research scientist at MU, according to materials released by the university. “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.”

 In the study, caterpillars were placed on Arabidopsis, a small flowering plant related to cabbage and mustard. Using a laser and a tiny piece of reflective material on the leaf of the plant, Appel and her collaborator Rex Cocroft, professor at MU, were able to measure the movement of the leaf in response to the chewing caterpillar.

Then they played back recordings of caterpillar feeding vibrations to one set of plants, but played back only silence to the other set of plants. When caterpillars later fed on both sets of plants, the researchers found that the plants previously exposed to feeding vibrations produced more mustard oils, a chemical that is unappealing to many caterpillars.

 “What is remarkable is that the plants exposed to different vibrations, including those made by a gentle wind or different insect sounds that share some acoustic features with caterpillar feeding vibrations did not increase their chemical defenses,” Cocroft said. “This indicates that the plants are able to distinguish feeding vibrations from other common sources of environmental vibration.”

 Appel and Cocroft say future research will focus on how vibrations are sensed by the plants, what features of the complex vibrational signal are important, and how the mechanical vibrations interact with other forms of plant information to generate protective responses to pests.

 “Plants have many ways to detect insect attack, but feeding vibrations are likely the fastest way for distant parts of the plant to perceive the attack and begin to increase their defenses,” Cocroft said.

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

 The study was published in the scientific journal Oecologia.

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