Caterpillars die rather than switch.

Researchers have found a compound in potatoes and tomatoes that turns tobacco hornworm caterpillars into addicts.

In the lab, at least a third of hornworms become so addicted to the compound, indioside D, that they starve rather than switch to food without it, reports Marta del Campo, now of Binghamton University in New York. In the May 10 NATURE, she and her colleagues at Cornell University say the substance hooks caterpillars by changing the sensitivity of chemoreceptors near the worms' mouths.

The hornworms, Manduca sexta, eat whatever plant they hatch on. That's often a member of the nightshade family, such as potatoes, tomatoes, and eggplants.

The researchers reared some hornworms on nightshade foliage and others on wheat or alfalfa. Then, the scientists switched the diets. Those insects used to wheat or alfalfa readily ate nightshade meals. Many nightshade feeders, however, died of starvation because they rejected substitute foods. Working with plant extracts, the researchers eventually traced such fatal brand loyalty to indioside D.

The caterpillars carry the equivalent of taste buds oil whiskers just outside their mouths. Monitoring nerve activity, the researchers discovered that one set of receptors becomes more sensitive to indioside D after exposure to the compound. "This is a very simple learning system," del Campo says. When the researchers removed the sensory hairs of caterpillars, the former addicts would eat pretty much any foliage.

Indioside D's addictive powers most often aren't bad for the caterpillars with access to nightshades. The chemical, which seems to be unique to the plants, revs up feeding, so addicted hornworms grow rapidly.