Plants are a lot less passive than their reputation makes them out to be. They foster helpful microbes, have internal systems of communication, and can even share information with their neighboring plants. When they're being eaten, their alarm signals call in predator species that consume whatever's eating them.

Now, a new paper suggests that predators aren't the only danger called in by those alarm signals. Indirectly, the signals induce a starvation-driven cannibalism among the erstwhile herbivores. The result is fewer insect pests and greater plant health.

Fine young cannibals

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.

Giving jasmonic acid to plants as the caterpillars got to work made little difference. Instead, the exposure had to start 24 hours before the plants started being eaten. Given enough time, however, there was a clear dose response: the more jasmonic acid a plant was exposed to, the more plant was left behind after caterpillars were placed on it. When a high-dose plant was compared to an untreated control, there was five times as much plant left by weight.

One of the reasons for that: the caterpillars were eating each other instead. The caterpillars on a treated plant ate twice as many of their peers as did caterpillars on a control plant. (The cannibalism on control plants likely came about because the caterpillars typically ate all its leaves, leaving them under food stress as well.) If you just gave caterpillars leaves from a treated plant, they tended to start eating each other sooner—and to eat more of each other—than if you fed them normal leaves.

This was true even if a single caterpillar was placed in with a bunch of dead ones; it ended up eating the corpses sooner.

Benefits of not being eaten

As far as the cannibals were concerned, this all worked out well. If you fed lone caterpillars leaves from treated plants, they had a relatively slow growth rate. Cannibalism boosted the growth rate of the survivors. It obviously worked out well from the plant's perspective, too. Not only were there fewer caterpillars eating it, but the ones on the plant were busy eating each other rather than the leaves.

What's not clear from this work is whether the jasmonic acid treatment reduced the nutritional value of the plant, made it unpalatable, or did actual harm to the insects that ate it. Any one of those scenarios seems consistent with the lower growth rate seen when lone caterpillars were fed treated leaves.

Before you go to look to buy some jasmonic acid to spray on your garden, a couple of things are worth noting. One is that it's not clear whether these defenses come at some cost to the plant. It could be that the energy put into a chemical defense (or the nutritional value taken out of the leaves, if that is what's going on) takes energy away from making delicious tomatoes.

The second issue is that the research involved enough caterpillars to completely denude a plant—the researchers put eight insects on a plant at a time when it only had three or four leaves. That's probably a bit unrealistic for typical conditions in the back yard. So understanding whether this would prove as effective in natural conditions would require additional work.

Nature Ecology & Evolution, 2017. DOI: 10.1038/s41559-017-0231-6 (About DOIs).