Tobacco plants send out a chemical SOS call for “aerial reinforcements” from flying insects when the plants are attacked by leaf-eating caterpillars, according to a team of German scientists.

When the grubs begin chewing on their leaves, the tobacco plants send out a chemical “SOS alarm” that attracts predatory Geocoris bugs and directs them straight to the hornworm larvae, said the researchers.

The insects fly in and attack just as the grubs start to hatch from their eggs.

Other plants may have a similar capability, the researchers believe.

The “SOS chemical” is contained in compounds called green leaf volatiles (GLV) commonly emitted by plants when they suffer damage.

GLVs are responsible for the summery smell of freshly cut grass.

The researchers, writing in the journal Science, said they have discovered how the newly hatched hornworm caterpillars sealed their own fate when they started munching tobacco leaves.

Researchers said the saliva caused a subtle chemical change in one of the plant’s GLV compounds, switching it from a “Z” to an “E” form to produce a signal instantly recognised by Geocoris bugs.

“The plant cannot see its attacker, but plants can sense the digestive substances that attacking larvae have in their oral secretions when these substances come into contact with the leaves,” said researcher Silke Allmann, from the Max Planck Institute for Chemical Ecology in Jena, Germany.

This ability enabled the plant to summon help precisely when it was needed.

Geocoris is a voracious predator that devours both the eggs and young larvae of the tobacco hornworm, Manduca sexta. It feeds by stabbing its prey with a needle-like mouth appendage and sucking out the body juices.

In tests, the scientists glued hornworm eggs to growing tobacco plants and placed cotton swabs next to them impregnated with different combinations of the GLV chemical. Plants perfumed with the Z-form had only 8 per cent of their glued eggs attacked while those emitting the E-signal lost 24 per cent.

An enzyme in the caterpillars’ saliva was thought to trigger the Z-to-E conversion, which took less than an hour, said the scientists.

Geocoris smelled the chemical with its antennae and was able to pinpoint the exact location of the feeding larvae, attacking in under 24 hours.

Why the caterpillar evolved a saliva substance that endangered its life remains unclear. The scientists believe the same chemical that attracts Geocoris may also kill potentially harmful microbes eaten with the leaf tissue, so there is a trade-off.

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