The Very Hungry Caterpillars That Turned to Cannibalism

https://www.nytimes.com/2017/07/11/science/tomato-plants-caterpillars-cannibalism.html

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If you’re a hungry caterpillar and you’ve got a choice between eating a plant or another caterpillar, which do you chose?

You pick your fellow caterpillar, scientists have found — if the plant is noxious enough.

In a study published Monday in Nature Ecology and Evolution, scientists sprayed tomato plants with a substance that induces a defensive response — a suite of nasty chemicals — and found that caterpillars became cannibals instead of eating the plant.

“The plant rearranges the menu for the caterpillar and makes other caterpillars the optimal choice,” said John Orrock, an evolutionary ecologist at the University of Wisconsin — Madison who led the study.

His team’s findings support a growing body of research suggesting that plant defenses are far more sophisticated than we’ve thought. Plants can’t run or hide, but they possess powerful strategies capable of altering the minds of herbivores that try to eat them.

The fight begins when an insect bites the plant, which triggers an immunelike defense response. The plant produces chemicals that hungry herbivores find toxic, unappealing or difficult to digest. For example, caffeine and nicotine, both toxic in high doses, are byproducts of the defense responses of tobacco and coffee plants.

Dr. Orrock and his colleagues knew that a chemical called methyl jasmonate, which smells like limes or flowers, could induce this defense mechanism in tomato plants. They also knew that caterpillars, which eat the leaves of tomato plants, will turn on one another when the going gets tough.

The scientists wondered how a plant’s defense system would affect the caterpillars if they combined these behaviors.

They sprayed tomato plants with either a neutral substance or varying amounts of methyl jasmonate to create graded levels of defense in the tomato plants. “You crank up the methyl jasmonate, the plant makes more nasty stuff,” said Dr. Orrock.

Then they put each plant inside an arena with eight caterpillars and watched for eight days to see how the caterpillars would handle two choices: Eat the plant, or eat your fellow caterpillar.

The caterpillars munched the plants with no extra defenses down to bare sticks before turning on one another for nourishment. But faced with the well-defended plants sprayed with lots of methyl jasmonate, the caterpillars gave up on the tomato leaves early. And like desperate characters in a cartoon island mirage, fellow caterpillars became appealing steak dinners.

“You start the experiment with eight, and you end it with one or two,” said Dr. Orrock.

What makes this interaction more fascinating, scientists say, is the way some plants communicate defensive messages both within themselves and among other plants via chemicals that travel through the air. Once a plant is attacked, leaves on the opposite side of the plant, or on neighbors of the same species, detect these chemicals and plan early defenses.

And, although unusual, the signaling can work across species, too: Sagebrush, tomatoes and tobacco plants, for example, can share defense messages, said Richard Karban, an ecologist who studies the interactions between plants and herbivores at the University of California, Davisand was not involved in the study.

Dr. Orrock and his colleagues are now investigating what this airborne communication could mean for plants in conditions closer to those in the real world. Will a caterpillar move from one defended plant to its neighbor before cannibalizing? And if so, will the other plant already be prepared for an attack?

Dr. Karban said he thinks better understanding of these interactions could one day make us less dependent on pesticides, but he’s more excited about shedding light on the underappreciated complexity of plants.

“Plants don’t have noses, and yet clearly they can perceive and respond to smells. They don’t have eyes, and yet they are very sensitive to the light environment,” he said. “They’re just far more capable of sophisticated behaviors than we have traditionally given them credit for.”