Insects monitor toxin ramp-up. (Spying on Plant Defenses).

A common caterpillar can sense when a plant is gearing up to manufacture insecticidal toxins, according to new findings on plant-insect warfare. What's more, that early warning system kick starts the insect's manufacture of detoxification enzymes that will be needed when the plant's attack finally comes.

Other researchers had detected plants' use of insect compounds as triggers for defensive chemistry. But a new study of Helicoverpa zea caterpillars represents the first example of a turnabout, says one of its authors, May R. Berenbaum of the University of Illinois at Urbana-Champaign. When the caterpillars attack a celery plant, the compounds that the plant immediately produces--jasmonate and salicylate--turn on caterpillar antitoxin genes, Berenbaum and her colleagues report in the Oct. 17 Nature.

The beauty of this system for the caterpillar is that many plants making wildly diverse toxins start their manufacturing process with jasmonate or salicylate, explains Berenbaum. Keying on such widespread chemicals may explain how the caterpillar infests more than 100 plant species from diverse families. Midwesterners call the insects corn earworms, but farmers elsewhere grumble about cotton bollworms and tomato fruitworms.

Berenbaum's collaborator Xianchun Li of Nanjing Agricultural University in China says that the researchers dosed some laboratory caterpillar feed with one or the other of the two chemicals. Caterpillars given either diet showed activity of four genes within the so-called P450 group, but regular feed didn't activate the genes. Feeding the insects with celery plants that had been damaged recently also kicked on the P450 genes.

Earlier studies had found that corn-earworm P450 genes make substances that detoxify one of celery's natural insecticides.

To see whether the gene activity actually protects the caterpillars, researchers exposed some of the insects to jasmonate and salicylate. Those insects that got the chemical warning grew faster and were more likely to survive on celery leaves than did caterpillars with no priming. The forewarned caterpillars also did better than unprepared ones on feed dosed with a purified celery toxin.

The results suggest that scientists may need to rethink proposals to spray jasmonate on crops to trigger plants' natural defenses, says Berenbaum.

Jennifer Thaler of the University of Toronto, another investigator of plant-insect warfare, says she wouldn't have predicted the new results, even though they make splendid sense. "People think of these organisms as being passive, and they're not," she says.