Caterpillar’s Bad Breath Scares Off Predators

The foul-breathed tobacco hornworm (Manduca sexta) may not get many dates, but the odor keeps predators at bay, a new study says.

The key to the hornworm’s halitosis is nicotine, a toxin it ingests while eating its favorite meal of tobacco leaves. New experiments show that the hornworm repurposes the nicotine, moving it from the gut to the hemolymph, the fluid that fills its circulatory system. The hornworm then excretes the excess nicotine in the hemolymph when it exhales. (Learn more about bugs.)

Hornworm larvae eat a tobacco plant in Utah’s Great Basin Desert. Photograph courtesy Ian T. Baldwin

Wolf spiders and other predators that eat the hornworm don’t have its defenses against nicotine and find the breath repulsive—and potentially poisonous, noted study leader Ian Baldwin, an ecologist at the Max Planck Institute for Chemical Ecology in Germany.

Although plenty of animals are known to have bad breath—”ever kissed a camel?” Baldwin quipped—this is the first discovery of halitosis advertising an animal’s potential toxicity, he said.

Dangerous Nicotine

Millions of smokers worldwide know nicotine—the main ingredient of cigarettes—but the chemical’s main role is as a chemical defense in tobacco plants.

Nicotine targets the neuromuscular junction—the place where nerve cells meet muscles—in animals. So when an animal eats a plant containing nicotine, its ability to breathe and move is significantly affected.

“This is why nicotine is such a great defense for plants: it poisons everything that uses muscles to move, and since plants don’t have nerves or muscles, it doesn’t poison the plant,” said Baldwin, whose study was published December 30 in the journal Proceedings of the National Academy of Sciences.

But the tobacco hornworm is a mysterious exception. As its name suggests, the caterpillar regularly dines on nicotine-containing plants, yet doesn’t seem to suffer any ill effects.

Baldwin and colleagues were investigating how the hornworms worked their chemical magic when they stumbled upon the importance of the hornworm’s bad breath.

Plant Defenses

When the researchers looked more closely at the hornworms, they found that the expression of a gut gene known as CYP6B46 increased after the worms ate tobacco leaves. This hinted that CYP6B46 was likely involved in the hornworm’s defenses against nicotine. (Also see “Woolly Bear Caterpillars Self-Medicate—A Bug First.”)

The next task for Baldwin and colleagues was to figure out exactly what this gene did. So they grew three sets of nicotine plants to look at the function of CYP6B46. One set was engineered to contain unusually low levels of nicotine. The second contained plants that had been grown to produce a small chemical that would thwart CYP6B46 from working. The third set of plants was a control group.

The researchers then let the tobacco hornworms feed on each set of plants and measured how many insects disappeared each night due to predation. The hornworms feeding on the nicotine-deficient tobacco were eaten more than the hornworms in the control plants and the hornworms in the plants that blocked CYP6B46.

This suggested the gene is key to the insects’ defense, but the scientists still needed to know how it works. More analysis revealed that CYP6B46 transfers ingested nicotine from the insect’s stomach to the hemolymph, where the insect breathes it out.

So, without a functional CYP6B46 gene, the hornworms exhaled less nicotine, making them more vulnerable to predators.

Now, if you’ll excuse me, I’m feeling the overwhelming need to brush my teeth.

Follow Carrie Arnold on Twitter and Google+.

Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com
  • Ima Ryma

    I am a tobacco hornworm.
    So guess what I do like to eat?
    Onto tobacco leaves I squirm.
    The plant does nicotine secrete
    To keep most animals away.
    But nicotine gives me a shot
    At not becoming someone’s prey,
    So I like nicotine a lot.
    When a spider or wasp tries to
    Snack on me, they turn tail and flee,
    Hit by all the bad breath I blew.
    They want nothing to do with me.

    The less likely predator death,
    The more I maximize bad breath.

  • Lai Wing Kong

    This similar worm damaged my plants, it can eat leaves drastically, reproduces eggs drastically; like alien. Beaware.
    This worm was found finally behind of leaves, otherwise my plants will be in trouble.

  • Dan Lee

    Ima Ryma// It seems poem you wrote here, funny to read.
    The story I read this section is so curious thing to know more about hornworms how they really live with tobacco. It seems something I’d like to know further about it~ nice information thx~^^ Happy new year~

  • yolis

    I have seen this caterpillar eating on a “Toloache” plant, that I have heard is toxic for human and pets, so this friend have a hard stomach!!!

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