WASHINGTON — Even caterpillars shouldn’t take candy from strangers. Tasty little hairs growing on wild tobacco plants amount to “evil lollipops” that make caterpillars who eat them more likely to be eaten themselves, a researcher says.
Hawkmoth caterpillars (Manduca sexta) readily eat the hairs, reports Ian Baldwin of the Max Planck Institute for Chemical Ecology in Jena, Germany. But consuming them changes the odor of the caterpillars’ bodies and excretions, Baldwin said February 21 at the annual meeting of the American Association for the Advancement of Science. In experiments, he and his colleagues observed that the odor change makes the caterpillars more likely to become lollipops themselves for ants and other predators.
“It’s a lot like eating asparagus,” Baldwin said, although the distinctive scent of after-asparagus human urine doesn’t lure predators. Hairy projections like this wild tobacco’s, called trichomes, show up in a variety of forms, explains plant pathologist Barbara Illman of the U.S. Forest Service’s Forest Products Laboratory in Madison, Wis. Some trichomes secrete distinctive chemicals, putting the zest into cooking herbs and the sting into nettles. “We are only beginning to understand the diverse functions of trichomes,” she says.
Tobacco trichomes mark a newly recognized tactic in the battle between Nicotiana attenuata plants and the caterpillars that can chew them to shreds, Baldwin said. His team has analyzed the intricate chemical signals and ecological devices deployed in this war for more than a decade. Unlike cultivated tobacco species, this wild relative employs a full range of intact defenses undiminished by farmers’ interference. “It takes a lot to be a native plant,” he said.
One way tobacco defends against grazers is by doping its leaves with potentially deadly nicotine, but hawkmoth caterpillars have developed the metabolic machinery to survive far more concentrated doses of nicotine than people can. Wild tobacco species in turn can identify a hawkmoth caterpillar by substances in the insect’s spit and when munched by a hawkmoth will shift resources from nicotine production to various other tactics of chemical warfare.
Despite these battles, hawkmoths and the wild tobacco in Baldwin’s studies share an uneasy alliance. Adult hawkmoths are deft and effective pollinators, and the plants woo them with flowers that open at night and produce a scent rich in benzyl acetone, a hawkmoth favorite. After sipping nectar, female hawkmoths may lay eggs on the plants that lead to ravenous caterpillars. Under heavy attack, tobacco plants switch to blooming during the day and stop luring in hawkmoths, Baldwin and his colleagues reported in 2010.