Image caption When the plant releases its chemical signal Geocoris insects (bottom, left) come to the rescue

Plants can summon insects to their aid to avoid being munched to death by caterpillars, scientists have found.

Leafy tobacco plants have evolved a "chemical SOS" that attracts predatory insects that eat the attackers.

In the journal Science, researchers revealed that the caterpillars' saliva activates this signal.

The modified signal causes Geocoris insects, which feed on the caterpillar larvae and eggs, to swoop in - rescuing the plant and gaining a meal.

The work was carried out by Silke Allmann and Ian Baldwin of the Max Planck Institute for Chemical Ecology in Germany.

They discovered that when the plants were attacked by tobacco hornworm caterpillars, Manduca sexta, the caterpillar saliva caused a chemical change in "green leaf volatiles" - pungent chemicals that the plants produce. The familiar smell of cut grass is generated by green leaf volatiles (GLVs).

The scientists studied the effect further by setting up a fake plant attack.

They glued caterpillar eggs onto two groups of tobacco plants, using cotton swabs to coat the eggs on one group of plants with the plant's own GLVs. The eggs on the other plants were treated with GLV mixed with caterpillar spit.

Image caption The insects respond to the plants' SOS and gain a freshly hatched meal

The plants "perfumed" with the plant chemical alone had only 8% of their eggs attacked, whereas plants perfumed with the plant and caterpillar-derived chemical mixture chemical lost almost a quarter of their eggs.

All of these missing eggs had been eaten by the Geocoris bugs, which were attracted by the chemicals.

The modified chemical seems to "betray the location of the feeding caterpillar", the researchers concluded in their paper.

"Why the larvae would produce such an apparently [disadvantageous chemical] in their saliva remains to be determined."