Plant Defenses, Management Practices, and Pests
Before discussing the key ecological principles and approaches to soil management, let’s first see how amazing plants really are. They use a variety of systems to defend themselves from attack by insects and diseases. Sometimes they can just outgrow a small pest problem by putting out new root or shoot growth. Many plants also produce chemicals that slow down insect feeding. While not killing the insect, it at least limits the damage. Beneficial organisms that attack and kill insect pests need a variety of sources of nutrition, usually obtained from flowering plants in and around the field. However, when fed upon—for example, by caterpillars— many plants produce a sticky sweet substance from the wounds, called “extra-floral nectar,” which provides some attraction and food for beneficial organisms. Plants under attack by insects also produce airborne (volatile) chemicals that signal beneficial insects that the specific host it desires is on the plant. The beneficial insect, frequently a small wasp, then hones in on the chemical signal, finds the caterpillar, and lays its eggs inside it (figure 8.2). As the eggs develop, they kill the caterpillar. As one indication of how sophisticated this system is, the wasp that lays its eggs in the tomato hornworm caterpillar injects a virus along with the eggs that deactivates the caterpillar’s immune system. Without the virus, the eggs would not be able to develop and the caterpillar would not die. There is also evidence that plants near those with feeding damage sense the chemicals released by the wounded leaves and start making chemicals to defend themselves even before they are attacked.