It turns out that cannibalism is widespread among the insects that otherwise spend their time munching on plants. “It often starts with one caterpillar biting another one in the rear, which then oozes,” said University of Wisconsin–Madison’s John Orrock in a press release describing his work. “And it goes downhill from there. At the end of the day, somebody gets eaten.”
It’s considered a stress response to a lack of food. What surprised Orrock was that this behavior sometimes took place on plants. You know, the things these caterpillars are supposed to be eating. If the food’s right there, why would these insects be turning on each other?
It all comes back to the chemicals a plant releases to say “watch out, I’m being eaten.” This is typically some chemical relative of jasmonic acid, a regulator of plant stress responses. While jasmonic acid can be used to coordinate a plant’s own response to stress, it also gets out into the environment and alerts other plants that something stressful is going on.