Tobacco leaves emit warning chemicals that summon predators when mixed with caterpillar spit

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When hornworm caterpillars eat tobacco plants, they doom themselves with their own spit. As they chew away, a chemical in their saliva reacts with airborne substances that are released by the beleaguered plants. This chemical reaction sends out a distress signal that is heard and answered by the predatory big-eyed bug. The bug eats hornworm caterpillars. Drawn by the chemical SOS of plants under distress, it finds plenty to devour.

Many plants release airborne chemicals to defend themselves against very hungry caterpillars. These “plant volatiles” spread far and wide, summoning reinforcements to the plant’s defence. Some attract predators like the big-eyed bug; others call upon parasitic wasps that lay their eggs inside the caterpillars, which are soon devoured from the inside out.

Silke Allmann and Ian Baldwin from the Max Planck Institute studied a particular group of SOS chemicals called ‘green leaf volatiles’ (or GLVs), which are released almost immediately from wounded leaves. They come in two slightly different forms known as Z and E. When the duo injured tobacco leaves themselves, they released far more of the Z versions than the E ones. By contrast, leaves that were attacked by hornworms released both Z and E forms in equal amounts.

Allmann and Baldwin showed that something in the hornworm spit was causing this change. If they applied the saliva to a wound, without any caterpillar, the plant gave off the same balanced aroma of Z and E chemicals. For the moment, no one knows which of the many compounds in the caterpillar’s saliva prompts this change, but whatever the mystery chemical is, it directly converts the Z form into the E one. You can see the reaction taking place in a test tube, but only when the saliva of a hornworm is added. Extracts made from tobacco leaves won’t do the trick.

When Allmann and Baldwin tested the spit of other pests that eat tobacco leaves – the army beetworm and cotton leafworm – the leaves’ emissions were still Z-heavy.  This means that the release of Z and E volatiles in equal numbers sends out a very particular message: it says that the plant is being specifically attacked by hornworms, rather than any other type of caterpillar.

And the big-eyed bug is listening. Allmann and Baldwin tested the bug’s preferences by gluing hornworm eggs to tobacco plants and dabbing artificially prepared mixtures of Z and E volatiles. They found that the predator devoured more eggs when it caught a whiff of Z and E in equal measure, over a bouquet that smelled predominantly of Z. So by the very act of eating its favourite plant, the tobacco hornworm creates a chemical reaction that summons one of its predators.

This is a brilliant defence on the part of the plant but it seems baffling that the hornworm should produce a chemical in its spit that ultimately increases its risk of being eaten. It’s a mystery, but Allmann and Baldwin think that the caterpillars might benefit from the E volatiles, possibly because they protect it against infections. If that’s the case, the risk of summoning a marauding big-eyed bug might be worth the price of a meal and defence against disease.

Images by Tozuema and Eugene van der Pijil

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