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.
Leaves are not the only part of the plant that can send signals when under attack that recruit beneficial organisms. When under attack by the western corn rootworm—a major pest—the roots of some varieties of corn have been shown to release a chemical that attracts a nematode that infects and kills rootworm larvae. During the process of breeding corn in the U.S., this ability to signal the beneficial nematode has apparently been lost. However, it is present in wild relatives and in European corn varieties and is, therefore, available for reintroduction into U.S. corn varieties.
Plants also have defense systems to help protect them from a broad range of viral, fungal, and bacterial attacks. Plants frequently contain substances that inhibit a disease from occurring whether the plant is exposed to the disease organism or not. In addition, antimicrobial substances are produced when genes within the plant are activated by various compounds or organisms—or a pest—in the zone immediately around the root (the rhizosphere) or by a signal from an infection site on a leaf. This phenomenon is called “induced resistance.” This type of resistance causes the plant to form various hormones and proteins that enhance the plant’s defense system. The resistance is called systemic because the entire plant becomes resistant to a disease, even far away from the site where the plant was stimulated.
PLANT DEFENSE MECHANISMS
Plants are not passive in the face of attack by insects, nematodes, or diseases caused by fungi and bacteria. Genes activated when plants are attacked or stimulated by organisms produce chemicals that
- slow insect feeding
- attract beneficial organisms
- produce structures that protect uninfected sites from nearby pathogens
- produce chemicals that provide a degree of resistance to pathogenic bacteria, fungi, and viruses
There are two major types of induced resistance: systemic acquired resistance (SAR) and induced systemic resistance (ISR) (figure 8.3). SAR is induced when plants are exposed to a disease organism or even some organisms that do not produce disease. Once the plant is exposed to the organism, it will produce the hormone salicylic acid and defense proteins that protect the plant from a wide range of pests. ISR is induced when plant roots are exposed to specific plant growth–promoting rhizobacteria (PGPR) in the soil. Once the plants are exposed to these beneficial bacteria, hormones (jasmonate and ethylene) are produced that protect the plants from various pests. Some organic amendments have been shown to induce resistance in plants. Therefore, farmers who have very biologically active soils high in organic matter may already be taking advantage of induced resistance. However, there currently are no reliable and cost-effective indicators to determine whether a soil amendment or soil is enhancing a plant’s defense mechanisms. More research needs to be conducted before induced resistance becomes a dependable form of pest management on farms. Although the mechanism works very differently from the way the human immune system works, the effects are similar—the system, once it’s stimulated, offers protection from attack by a variety of pathogens and insects.
When plants are healthy and thriving, they are better able to defend themselves from attack and may also be less attractive to pests. When under one or more stresses, such as drought, nutrient limitations, or soil compaction, plants may “unwittingly” send out signals to pests saying, in effect, “Come get me, I’m weak.” Vigorous plants are also better competitors with weeds, shading them out or just competing well for water and nutrients.
Many soil management practices discussed in this chapter and the other chapters in part 3 help to reduce the severity of crop pests. Healthy plants growing on soils with good biological diversity can mount a strong defense against many pests. For examples of the effects of soil management on plant pests, see the box on the right. The issue of plant health is so critical to ecological soil and plant management because it also influences, as we have just seen, the ability of plants to resist pests. Developing optimal soil health is, therefore, the basis for management of crop pests on farms—it should be a central goal that underpins crop integrated pest management (IPM) programs.