Tobacco hornworm caterpillars are insects that feast on a type of wild tobacco plant. That meal provides more than nutrients. It also makes the insects' breath so stinky that some predators flee.

A researcher team found the bad-breath defense by comparing insects raised on normal tobacco plants with others that had dined on tobacco plants genetically modified to produce no nicotine.

When a hornworm caterpillar ate normal tobacco leaves， the insects breath - which comes out of tiny structures called spiracles - turned nasty， the scientists found. That halitosis was strong enough to drive away wolf spiders. These predators normally would feast on hornworms.

At an outdoor site in Utah， the scientists placed the genetically modified plants alongside others that make nicotine. Caterpillars that fed on the ordinary plants were more likely to stay alive at night. But those dining on nicotine-free leaves disappeared in larger numbers. Wolf spiders were eating them up， the researchers discovered.

The scientists wanted to know how the caterpillars turned nicotine into a defense for themselves. They identified a gene that helped caterpillars move nicotine out of their gut so it could be puffed out in their breath. In the lab， the biologists placed wolf spiders and hornworms together in a dish. The spiders attacked. But they quickly fled if the caterpillars had been eating ordinary tobacco leaves. Not so for caterpillars that had been eating tobacco genetically modified to block the insects use of nicotine as a self-defense. Those caterpillars became the spiders lunch.