Graphical representation of the interaction between the tobacco hornworm Manduca sexta and of the predatory wolf spider Camptocosa parallela: The exhaled via the tracheal toxic nicotine deters the spider

Photo: Danny Kessler; graphics: Pavan Kumar and Sagar Pandit, MPI chem Ecol.

Ecologists have discovered that a caterpillar feeding on tobacco leaves exhales a small fraction of the ingested nicotine to protect itself from being eaten by spiders.

The tobacco hornworm lava's previously unknown defence mechanism was identified by ecologists from the Max Planck Institute for Chemical Ecology.

Wolf spiders would usually reject the nicotine-fed larvae before biting into them and injecting a mixture of digestive enzymes and poisons.

To make sure that the spiders weren't put off because there was nicotine on the larvae's skin, the researchers washed the larvae. The spiders still weren't interested in eating the larvae.

The team put this defence mechanism to the test by manipulating the genes of the tobacco plant and of the hornworm larvae.

Wolf spider attacks a Camptocosa parallela tobacco hornworm in the laboratory

Photo: Pavan Kumar, MPI chem. Ecol.

On one hand, the team planted tobacco plants that were deficient in nicotine to see what effect that would have on caterpillar populations. They also took some hornworm larvae and silenced an enzyme (cytochrome P450 enzyme) that they use to catalyse the nicotine when they eat tobacco leaves. The team then observed what happened to those caterpillars feeding on nicotine deficient plants and compared the results to the caterpillars that lacked the active catalyser for the toxin.

The team found caterpillars that ate the nicotine-deficient plants were much more likely to be eaten by the nocturnal predator -- the wolf spider Camptocosa parallela, which normally avoids eating the larvae. Spiders tend to assess their prey by tapping it with chemosensory endowed legs and palps.

The spiders also preyed on those larvae deficient in the P450 cytochrome. This suggested that the enzyme plays an important role in the defence mechanism.

Further analysis showed that the enzyme is involved in transporting the ingested nicotine into the caterpillar's bloodstream (haemolymph), which lets the nicotine be exhaled from its spiracles (holes in their exoskeleton through which the creature breathes). Caterpillars exhale nicotine and this works as an anti-spider mechanism, although the "defensive halitosis" doesn't protect the caterpillar from other predators.

The paper is published in PNAS.


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