This Caterpillar’s Stink Breath Keeps the Spiders Away

Wikimedia Commons
Wikimedia Commons / Wikimedia Commons

Bad breath usually won’t win you many friends, but at least it will keep your enemies at arm’s length, too. The larval caterpillar form of a moth called the tobacco hornworm, scientists have discovered, uses a blast of nasty air to fend off wolf spiders and other predators. The source of the stench is the hornworm’s preferred meal: tobacco leaves.

While human smokers look to nicotine as a stimulant, the tobacco plants it comes from use it as a defense against insects and other animals that would eat them. Nicotine is a poison, and an exceptionally fast-acting one, racing from the lungs to the brain in just seven seconds after inhalation. At a dose of 30 to 60 milligrams, it can cause convulsions, respiratory failure, and death (smokers only get about 1mg of it from each cigarette, FYI). 

Nasty stuff, and a good reason why most animals don’t munch on tobacco too much. The hornworm has a workaround for this, though, and can tolerate doses of nicotine hundreds of times higher than what would kill most other animals. Even the hornworm doesn’t hang on to the poison for long, though, and rapidly disposes of most of the nicotine in its waste. While getting rid of the nicotine, it can use a little bit of its food’s chemical weaponry as its own, and a team of German ecologists show in a new study how hornworms dispose of the chemical in an alternate way that helps defend them from predators. 

The researchers, from the Max Planck Institute for Chemical Ecology, found that a gene called CYP6B46 redirects some of the nicotine from the hornworm’s gut into their hemolymph, the arthropod analogue of blood, where it hinders and kills parasites. From the hemolymph, the nicotine can also be exhaled through breathing holes on the hornworm’s body called spiracles, creating a spider-repelling “defensive halitosis.”

Here’s a poor hornworm with its CYP6B46 gene silenced. The spider chows down on it without a second thought.

And here’s a spider versus a normal caterpillar and its noxious nicotine breath. After a face full of that, the spider tries to make a getaway up the side of the Dixie cup.