Tomatoes Can Turn Plant-Eaters Into Cannibals, Study Shows

Injured tomato plants can induce caterpillars to become cannibals. John Orrock

Plants are often seen as taking a passive role in their environments, just hanging out and soaking up the sunlight. But that impression is wrong—plants have many sophisticated ways of influencing their environment, and other plants and animals in it. And this includes leading herbivores down the garden path to cannibalism.

When tomatoes and other plants are munched on by caterpillars, they produce chemicals that act as an alarm signal to neighbors, leading them to produce nasty-tasting substances that ward off herbivores. Mimicking these conditions in the lab, researchers from the University of Wisconsin have shown that well-defended plants induce caterpillars to cannibalize each other.

"On well-defended plants, caterpillars become cannibals much sooner," says behavioral ecologist John Orrock, lead author of a study describing the findings published July 10 in the journal Nature Ecology & Evolution.

In the laboratory, Orrock and co-authors Brian Connolly and Anthony Kitchen sprayed tomato plants with a chemical called methyl jasmonate, which many plants produce in response to being chewed upon, and which induces chemical defenses. The researchers sprayed plants with a small or large amount of the chemical to mimic poorly- and well-defended plants. They then set caterpillars upon the plants, and observed what happened.

They found that well-defended plants had five times more biomass at the end of the experiment, compared to those which were poorly-defended, Orrock says. And in the former case, the caterpillars also ate each other.

Tomatoes benefit in two ways from caterpillar cannibalism: It leads to fewer herbivores, and those which remain are less hungry, he adds.

Other experiments showed that caterpillars would opt for cannibalism after eating a poorly-defended plant, but only after the leaf material was gone, Orrock says. Apparently caterpillars can make do quite well eating other caterpillars, although they (obviously) don't appear to prefer doing so, he adds.

The tomato plants "can change caterpillar behavior in a way that reduces caterpillars' density," which is fascinating, says Evan Preisser, a researcher at the University of Rhode Island who wasn't involved in writing the paper.

When people think of cannibalism, they often first consider carnivores, says Jennifer Thaler, a professor and entomologist at Cornell University. But cannibalism also is really important in herbivores. Thaler's work has shown, for example, that it isn't uncommon for half the eggs laid by herbivorous potato beetles to be cannibalized by other potato beetles. This paper shows another way that herbivore cannibalism may be ecologically important, and how plants may take advantage of it.

"Everywhere we look, plants are manipulating the organisms in their environment," Thaler says. "Nothing goes unaltered by plants."