His team's findings, published in Nature Ecology & Evolution on Monday, support a growing body of research suggesting that plant defences are far more sophisticated than we have thought. Plants cannot 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 immune-like defence 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 defence responses of tobacco and coffee plants. Orrock and his colleagues knew that a chemical called methyl jasmonate, which smells like limes or flowers, could induce this defence 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 defence system would affect the caterpillars if they combined these behaviours. They sprayed tomato plants with either a neutral substance or varying amounts of methyl jasmonate to create graded levels of defence in the tomato plants. "You crank up the methyl jasmonate, the plant makes more nasty stuff," Orrock said. 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 defences 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," Orrock said. 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 neighbours of the same species, detect these chemicals and plan early defences. And, although unusual, the signalling can work across species, too: Sagebrush, tomatoes and tobacco plants, for example, can share defence messages, said Richard Karban, an ecologist who studies the interactions between plants and herbivores at the University of California, Davis and was not involved in the study. 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 neighbour before cannibalising? And if so, will the other plant already be prepared for an attack?

Karban said he thinks better understanding of these interactions could one day make us less dependent on pesticides, but he is more excited about shedding light on the under-appreciated 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 behaviours than we have traditionally given them credit for." The New York Times