Researchers Silke Allmann and Ian Baldwin of the Max Planck Institute for Chemical Ecology in Germany have discovered that tobacco plants possess unusual defense mechanisms. The plants have evolved a "chemical SOS signal" that attracts predatory insects when hornworm caterpillars eat their leaves.

Allmann found that tobacco plants can sense the digestive substances that attacking larvae have in their oral secretions. After studying this mechanism, the scientists found that an increased amount of (E)-2-hexenal is emitted when the leaves are attacked.

The scientists believe that caterpillars need this substance for protection from harmful organisms. The substance is a strong antibiotic agent that kills bacteria on the wounded leaves. Therefore, the detrimental aspects of the substance are balanced out by protection from bacterial infections, the researchers believe.