In a deception that likely has evolved over thousands of years, a caterpillar that feeds on corn leaves induces the plant to turn off its defenses against insect predators, allowing the caterpillar to eat more and grow faster, according to chemical ecologists. The finding is one more revelation about the myriad of chemical signals that pass between plants and insects that scientists at Penn State and around the world have been discovering in recent years. In this case, the agent of deceit is the caterpillar's feces, or "frass." Plants are under constant threat of attack from herbivorous insects. Nearly 400,000 plant-eating insect species are known to live on 300,000 plant species. When these herbivores feed on plants, they not only cause mechanical damage but often deposit substances that can manipulate the plant's response to herbivory. These substances are analogous to the microbial-associated compounds that affect plant responses to pathogenic fungi or bacteria.