Spitting in Singapore may get you thrown in jail, but in nature there are far more lethal results. When a caterpillar drools on a corn leaf, the offended vegetable releases chemical vapors that attract parasitic wasps, which lay eggs in the caterpillar. The larval wasps eventually consume the unsuspecting insect. Now scientists have discovered that the caterpillar is its own worst enemy: A chemical in its spit triggers the corn's call for help. The finding, reported in today's issue of Science,* could lead to better ways to arm crops against pests.
Over the last several years, a group led by biochemist James Tumlinson of the U.S. Department of Agriculture's Agricultural Research Service center in Gainesville, Florida, has been peeling away the complex defense mechanisms of crop plants to devise ecologically benign methods for insect pest control. For its latest work, Tumlinson's team knew that corn plants release a potent wasp attractant--a blend of terpenoids and indole--when under attack from the beet armyworm caterpillar. But they didn't know the chemical trigger. Mere mechanical damage to corn leaves doesn't elicit vapors; the plants produced the organic compounds only in response to the spit. Eventually, the scientists found the irritant, which they dubbed volicitin.
It's unclear why the caterpillars continue to produce a chemical that leads to their own demise. "That's a big mystery," says Tumlinson. "It must have some important role." It could help in digestion, he speculates, or be a key precursor for other metabolically important chemicals, such as prostaglandins. "At this stage many scenarios appear possible," writes Edward Farmer of the Institut de Biologie et Physiologie Vegetales in Lausanne, Switzerland, in an accompanying commentary.
Deciphering the elaborate defense strategy could someday better arm plants against insect pests. Spraying volicitin on all crops isn't the answer, says Tumlinson, because "causing all plants to produce more volatiles would just confuse the wasps." But making the pests easier for the wasps to find, for example, could be very beneficial. "We're not looking at this as a silver bullet," says Tumlinson, "but as one component of an ecologically based pest-management system."
During herbivory, wounding activates the systemic expression of defense genes through the octadecanoid signal pathway. In parallel, inset saliva containing chemical elicitors, such as volicitin, triggers the plant to release a bouquet of volatile compounds that attract parasitic or predatory insects to the herbivore. Volatile release stimulated by volicitin may also depend on the octadecanoid pathway, raising the possibility of cross talk between this molecule and the wound-induced expression of defense genes.