Riveting dramas play out daily on a nature preserve in southwestern Utah. A tobacco plant is quietly minding its own business when moth caterpillars attack, devouring its leaves. The plant sends out a chemical distress call to another insect species called "big-eyed bugs," which soon arrive on the scene to attack the caterpillars. Now, researchers have discovered that the alarm signal goes out at lightning speed—and in a strange twist, the caterpillars appear to help make the call.
Scientists have known for a long time that plants create complex, specific chemical signals that attract predators when they're under siege from herbivores. But those compounds take hours to a day for the plant to make. "It's day-old news," says study author Ian Baldwin, an ecologist at the Max Planck Institute for Chemical Ecology in Jena, Germany. After all, a predator wouldn't want to arrive only to find that the herbivores have already left. The new study, published tomorrow in Science, reveals a much faster distress call.
Silke Allmann, a graduate student in Baldwin's group, first got a hint of this signal when she was examining chemical analyses of green leafy volatiles (GLV), a group of chemicals that makes up the grassy smell of a fresh-cut lawn. Plants emit GLVs whenever they're damaged. But GLVs from tobacco plants come in two varieties, usually called the (Z)- and (E)-isomers. Allman discovered that when a plant is cut by, say, a lawnmower or a knife, it releases much more (Z)- than (E)-GLVs. But if a tobacco hornworm caterpillar (Manduca sexta) bites the leaf, (Z)- and (E)-compounds are released in equal amounts.
To see if this made a difference to predators, the researchers glued caterpillar eggs onto leaves and, next to them, dabbed a paste containing different mixtures of GLVs. The mixtures with more (E)-GLVs attracted more big-eyed bugs of the Geocori genus, which poked a hole in the eggs and ate the innards.
Allmann also looked for chemical reactions in the plant that would shift the ratio between different forms of the molecules, but she couldn't find any. So she wondered if enzymes from the caterpillar might cause the shift right after the plant releases its GLVs. Indeed, she found that caterpillar saliva converts the (Z)-version of one of the GLV molecules to the (E)-version. "That's where it got really weird," says Baldwin. "Why would a caterpillar do this to itself?" He speculates that the (E)-GLVs could help kill microbes in the caterpillars' gut.
The fact that the caterpillar's own spit helps sign its death warrant is a "weird and novel twist," says Jeffrey Conner, an ecological geneticist at Michigan State University's Kellogg Biological Station in Hickory Corners. Further experiments may turn up some reason why changing the mixture of plant compounds benefits the caterpillars, says Conner.