Tobacco leaves emit warning chemicals that summon predators when mixed with caterpillar spit
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.
Reference: Science http://dx.doi.org/10.1126/science.1191634
Images by Tozuema and Eugene van der Pijil
More on plant defences:
Tobacco plants foil very hungry caterpillars by switching pollinators to hummingbirds
The 13,000-year old tree that survives by cloning itself
Review: Royal Institution Christmas Lecture – the 300 million year war
Are red autumn leaves a warning sign to insects?
The fiery taste of chillies is a defence against a fungus
Go Further
Animals
- This ‘saber-toothed’ salmon wasn’t quite what we thoughtThis ‘saber-toothed’ salmon wasn’t quite what we thought
- Why this rhino-zebra friendship makes perfect senseWhy this rhino-zebra friendship makes perfect sense
- When did bioluminescence evolve? It’s older than we thought.When did bioluminescence evolve? It’s older than we thought.
- Soy, skim … spider. Are any of these technically milk?Soy, skim … spider. Are any of these technically milk?
- This pristine piece of the Amazon shows nature’s resilienceThis pristine piece of the Amazon shows nature’s resilience
Environment
- This pristine piece of the Amazon shows nature’s resilienceThis pristine piece of the Amazon shows nature’s resilience
- Listen to 30 years of climate change transformed into haunting musicListen to 30 years of climate change transformed into haunting music
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
History & Culture
- Séances at the White House? Why these first ladies turned to the occultSéances at the White House? Why these first ladies turned to the occult
- Gambling is everywhere now. When is that a problem?Gambling is everywhere now. When is that a problem?
- Beauty is pain—at least it was in 17th-century SpainBeauty is pain—at least it was in 17th-century Spain
- The real spies who inspired ‘The Ministry of Ungentlemanly Warfare’The real spies who inspired ‘The Ministry of Ungentlemanly Warfare’
- Heard of Zoroastrianism? The religion still has fervent followersHeard of Zoroastrianism? The religion still has fervent followers
Science
- Here's how astronomers found one of the rarest phenomenons in spaceHere's how astronomers found one of the rarest phenomenons in space
- Not an extrovert or introvert? There’s a word for that.Not an extrovert or introvert? There’s a word for that.
- NASA has a plan to clean up space junk—but is going green enough?NASA has a plan to clean up space junk—but is going green enough?
- Soy, skim … spider. Are any of these technically milk?Soy, skim … spider. Are any of these technically milk?
- Can aspirin help protect against colorectal cancers?Can aspirin help protect against colorectal cancers?
Travel
- What it's like to hike the Camino del Mayab in MexicoWhat it's like to hike the Camino del Mayab in Mexico
- Follow in the footsteps of Robin Hood in Sherwood ForestFollow in the footsteps of Robin Hood in Sherwood Forest
- This chef is taking Indian cuisine in a bold new directionThis chef is taking Indian cuisine in a bold new direction
- On the path of Latin America's greatest wildlife migrationOn the path of Latin America's greatest wildlife migration
- Everything you need to know about Everglades National ParkEverything you need to know about Everglades National Park