For most people, discriminating among foods is a luxuryfilet mignon is great, but rice and beans will do. For the tobacco hornworm, however, following one's dietary preferences is a matter of life or death. Indeed, research published in the current issue of the journal Nature shows that the larvae of this moth can become so chemically dependent on their favorite plants that they will starve to death rather than eat anything else.
To examine hornworm feeding preferences, Marta L. del Campo of the State University of New York at Binghamton and her colleagues raised the larvae on either of two diets. The first group was fed only the leaves of solanaceous plantspotato and other members of the nightshade family; the second group grew up eating non-solanaceous plants. Switching the insects from one regimen to the other yielded interesting results: whereas larvae reared on non-solanaceous diets happily feasted on the nightshade foliage, most of those brought up on nightshade leaves flatly refused the foreign food and died of starvation. "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," del Campo explains. "The receptors increase their responsiveness to this chemical, while maintaining low responses to other plant compounds."
Though it might seem maladaptive, the hornworm larvae's addiction to solanaceous leaves can pay off: larvae raised on nightshade plantsthe insect's natural hostsmature far more quickly than those reared on non-solanaceous foliage. As a result they spend less time in the vulnerable larval stage. At the same time, however, adult females occasionally lay their eggs on non-host plants. It thus pays for hornworm larvae to have the potential to become either specialists or generalists, depending on what food is first available to them.