How Plants Turn Chill Caterpillars Into Rampaging Cannibals

Among the world of insects, caterpillars seem like models of chill. Slowly chomping away at the periphery of some brilliant green leaf, they might as well be a feature of the plant itself.

But there is a caterpillar dark side. Of course, there's the glaring dark side of being among the more devastating agricultural pests on Earth, but there's also the fact that a great number of caterpillars are cannibals—swapping out tender foliage for the oozing innards of their peers. If you were to describe caterpillars in a word, it would be hungry.

A study out Monday in Nature Ecology and Evolution examines this phenomenon of caterpillar cannibalism from the perspective of plants. It's not just dumb luck that would-be herbivores start feasting on each other—plants make them do it by producing certain chemicals. Among these is methyl jasmonate, which is produced by some plants in response to external stresses, such as damaged leaves.

The defensive strategy being wielded by plants is intuitive enough, but is a bit more clever than simply repelling creatures that might like to feast on those plants. Take away one food source (leaves) and caterpillars are quick to turn to the next most available thing, which usually happens to be other caterpillars. Which means less leaf-eaters.

"It often starts with one caterpillar biting another one in the rear, which then oozes. And it goes downhill from there," study co-author John Orrock offered in a statement.

The new study describes experiments testing the above scenario. In a succession of controlled environments a fixed number of caterpillars (S. exigua, in particular) were presented with leaves featuring varying levels of methyl jasmonate. Given leaves with higher levels of the chemical, the researchers found that after a period of time there were less caterpillars left over and relatively unscathed leaves. As levels of the chemical declined, there were more caterpillars left over in the container and increasingly damaged leaves. Simple enough.

"Not only do these guys become predators, which is a victory for the plant, they are getting a lot of food by eating one another," Orrock said. "We struck upon a way that plants defend themselves that nobody had really appreciated before. From the plant's perspective, this is a pretty sweet outcome, turning herbivores on each other."