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.
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.
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.