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Plants cannot run or hide, so they need other strategies to avoid being eaten. Some curl up their leaves, others produce chemicals to make themselves taste bad if they sense animals drooling on them, chewing them up or laying eggs on them—all signals of an attack. New research now shows some flora can feel a plant-eating animal well before it launches an attack, letting a plant prepare a preemptive（先发制人的）defense that even works against other pest species.

When ecologist John Orrock of the University of Wisconsin-Madison sprayed snail slime—a liquid the animals release as they slide along—onto soil, nearby tomato plants appeared to notice. They increased their levels of an enzyme（酶）, which is known to prevent plant-eating animals. “None of the plants were ever actually attacked,” Orrock says. “We just gave them cues that suggested an attack was coming, and that was enough to cause big changes in their chemistry.”

Initially Orrock found this defense worked against snails; in the latest study, his team measured the slimy warning’s impact on another potential threat. The investigators found that hungry caterpillars（毛虫）, which usually eat tomato leaves greedily, had no appetite for them after the plants were exposed to snail slime and activated their chemical resistance. This nonspecific defense may be a strategy that benefits the plants by further improving their overall possibilities of survival, says Orrock, who reported the results with his colleagues in March in Oecologia.

The finding that a snail’s approach can cause a plant response that affects a different animal made Richard Karban curious, a plant communications expert, who was not involved in the study. “It is significant that the plants are responding before being damaged and that these cues are having such far-ranging effects, ” Karban says. The research was comprehensive, he adds, but he wonders how the tomato plants felt chemicals in snail slime that never actually touched them.

“That’s the million-dollar question,” Orrock says. He hopes future research will make out the mechanisms that enable plants to sense these relatively distant cues.