How do you overcome a strong enemy? Find an even stronger one of his. At least that's what wild tobacco plants do when attacked by insects, according to a new study published in today's Science.

Image: KESSLER/BALDMIN

Plants are constantly struggling to ward off a variety of predators, ranging from caterpillars to cows. To defend themselves, many have developed poisons and thorns, which tend to be very effective for a while. With time, however, predators tend to find ways around such defenses, or they simply become immune to them.

Taking a closer look at the wild tobacco plant (which grows in the Southwestern United States), a team of scientists from the Max-Planck-Institute in Jena, Germany have found that it resorts to a very different, less direct defense: when an herbivore predator such as the hawkmoth larva (see image) attacks a wild tobacco plant, the plant releases a volatile chemical compound (VOC) into the environment. These compounds signal to other predatory insects that there is a quick meal to be had. Indeed, the VOC leads them straight to their prey. Corn uses the same mechanism to attract parasitoids, including parasitic wasps that kill worms attacking corn plants by laying their eggs inside them.

Andre Kessler and Ian Baldwin, the researchers conducting the study, actually found that the VOCs go one step farther in protecting plants. When VOCs are emitted, the five-spotted hawkmoth is not only less likely to lay its eggs on tobacco plants, but predators are actually more likely to find and eat those eggs.

Although these mechanisms were detected earlier in a laboratory setting, the findings had remained controversial¿and some scientists doubted that they could be observed in a complex natural ecosystem. This is the first research to document VOC-based indirect defenses in the wild.