When you cut grass, it smells fresh. For plants, this smell means "I am hurt." When caterpillars eat corn leaves, the corn sends out a special smell. This smell calls small wasps that lay their eggs inside caterpillars.

The wasps' babies then eat the caterpillars from the inside. So the plant calls a bodyguard! Nearby plants can also notice the smell and get ready to protect themselves.

The fresh smell of cut grass is actually a sign that plants are damaged. In 1990, American researchers found that corn seedlings chewed by caterpillars released large amounts of special smelling chemicals. Damaging the leaves artificially was not enough, but adding liquid from a caterpillar's mouth caused the release.

Female parasitic wasps, which lay their eggs inside caterpillars, learned to use this smell to find their targets. In 2001, a study of wild tobacco showed that such smells helped predators eat more pest eggs. Nearby plants respond too. In 2014, a Japanese team found that tomato plants absorbed a smell from damaged neighbors and changed it into a substance that works against caterpillars.

In 1990, Turlings and colleagues reported in Science that corn seedlings under attack by caterpillars gave off large amounts of terpene volatiles. Damaging the leaves mechanically produced little of the scent. Putting fluid from the caterpillar's mouth on the wounds did the trick. Female parasitic wasps, which lay their eggs inside caterpillars, learned to follow this odor to their hosts, so the plant was recruiting the enemy of its enemy.

Biologists call this indirect defense. The green smell of cut grass belongs to the same family of injury signals. Field evidence came in 2001, when Kessler and Baldwin released certain volatiles from wild tobacco. Predators ate more pest eggs, and one compound, linalool, deterred moths from laying eggs; the pair estimated that volatiles could cut pest numbers by more than 90 percent.

Neighbors pick up the message too. In 2014, Sugimoto and colleagues showed that tomatoes absorbed a green-leaf volatile from infested plants nearby and turned it into a compound that works against caterpillars. Nobody yet knows how far such signals carry in open fields, or whether the sender gains anything when its neighbors arm themselves. In 2023, researchers filmed calcium signals racing through leaves after exposure to the smell.