The larvae of Manduca sexta , a moth nicknamed the tobacco hornworn, can become so chemically dependent on one of their favorite foods – the leaves of eggplant, potato or tomato plants – that they would rather starve to death than eat leaves from other plants.

"They can eat anything you put in front of them. But when they grow up eating only the leaves of [solanum , or nightshade family] plants like potato and tomato, the larvae reject everything else," says Marta del Campo, whose Cornell University doctoral research appears in the latest issue of the journalNature (May 10). She now works as a post-doctoral researcher at the State University of New York at Binghamton.

"When the hornworm larva feeds on plants from the nightshade family, its taste receptors become tuned to the plant chemical Indioside D , a steroidal glycoside compound that is made of a steroid unit and the sugars glucose, rhamnose and galactose. The receptors increase their responsiveness to this chemical, while maintaining low responses to other plant compounds," says del Campo.

"The addiction with the leaves is analogous to a chocolate addiction in humans," says J. Alan Renwick, Cornell professor of entomology and chemical ecology and senior research scientist at the Boyce Thompson Institute (BTI) for Plant Research, which is located on the Cornell campus. Renwick served as the chair of del Campo's graduate faculty committee. "The larvae are absolutely addicted to their host plants," he says.

If the larvae grow up eating nothing but nightshade plant leaves, that is all they will eat, the researchers found. When the leaves were taken away, most of the larvae starved to death within two or three days. Interestingly, if the larvae mature while eating any other food, the addiction does not set in. How can a larva distinguish between the nightshade family and other leaves? On the outside of the larval mouth there are four taste structures called sensilla styloconica , which the animal uses as something like a tongue to taste food. But whereas the human tongue is inside the mouth, the sensilla sit on the outside the larva's mouth. Within each of the sensilla there are four specialized taste receptors that chemically scan the surfaces of leaves to seek out their identity.

As soon as the researchers removed the taste receptors, larvae that previously had eaten only nightshade leaves began eating other plants. "This shows that the chemical information coming from these taste receptors completely controls the larvae's recognition of what is food and what is not," says del Campo. "In other words, the taste is not a brain-only phenomenon. It can also be a sensory phenomenon."

Joining del Campo and Renwick on the Nature paper, "Host Recognition by the Tobacco Hornworm is Mediated by a Host Plant Compound," are Frank C. Schroeder, a Cornell research associate in chemical biology; Ronald Booker, Cornell associate professor of neurobiology and behavior; Caroline Mueller, of BTI; and Carol Miles, SUNY Binghamton assistant professor of biology, who formerly was at Cornell.

Funding for the research was provided by the National Science Foundation and the Binational Science Foundation.