Ecological Understanding of Insects in Organic Farming Systems: Plant Defenses against Insects

Introduction

Plant-feeding by arthropods is so common, and arthropods are so numerous, one may wonder why we are able to produce any crops at all. Plants have many characteristics that protect them from plant-feeding by insects (Pedigo and Rice, 2006; Eisner and Meinwald, 1995). These characteristics are usually called plant defenses. Plant defenses are generally classified as constitutive or induced. Constitutive defenses are always present in the plant, whereas induced defenses are synthesized or mobilized in response to plant injury. Many insects depend on plants for food, and have evolved mechanisms to overcome some plant defenses. Some plant-feeding insects have evolved ways to use plant defenses to their own benefit, by sequestering, or incorporating into their bodies, plant defensive chemicals and using them to protect themselves from predators (Metcalf and Lampman, 1989). An example is the cucumber beetle, which has overcome the chemical defenses of plants in the cucumber and squash family. When cucumber beetles feed on cucurbits, they incorporate bitter compounds into their body, and are less susceptible to attack from some natural enemies.

Constitutive Defenses Can be Mechanical or Chemical

Mechanical Defenses

Mechanical defenses include external structures that protect plant tissues from plant-feeders. Plant structural defenses on stems and leaves can deter, injure, or kill the plant-feeding organism. Leaves and stems can be covered with sharp spines or hairs called trichomes that are barbed or contain defensive chemicals. Defensive plant structures such as spines and thorns reduce feeding by large animals like cattle, goats, and deer by restricting their feeding rate. Some defensive compounds are produced inside the plant and secreted or deposited onto the plant’s surface, for example, resins, lignins, silica, and wax cover the surface layer of terrestrial plants and affect the texture of the plant tissue. Silica and lignins are indigestible to animals and grind down insect mouthparts. Some plants produce gum or sap flows in response to feeding injury, resulting in the insects becoming trapped.

Chemical Defenses

Chemical defenses are compounds produced by plants that are not directly involved in the normal plant growth, development or reproduction, and are often called secondary metabolites (Eisner and Meinwald, 1995). These chemicals often contribute to the distinctive flavors of plants. These secondary chemicals can render a plant species unsuitable as a food plant for some insect species. However, in some cases, some insect species have evolved mechanisms to overcome the defensive chemicals of a plant species and are able to utilize it as a food resource, thus becoming a potential pest. These metabolites include digestibility reducers and toxins.

• Digestibility reducers render plant cell walls indigestible to animals. The higher the concentration of digestibility reducers in the insect diet, the less nutrition the insect can gain from eating plant tissues. Digestibility reducers tend to be equally effective against insects that specialize on particular plants, and those that are generalists with a broader plant-host range. An example of a digestibility reducer is tannins in oaks.
• Toxins can interfere with an insect’s metabolism, often by blocking specific biochemical reactions. These chemicals are usually most effective against insects that specialize on other plant species and generalist insects that feed across plant types. Plant defensive chemicals include such compounds as alkaloids, cyanogenic glycosides, glucosinolates, terpenoids, and phenolics.
• Induced defenses include metabolic products and morphological and physiological changes in the plant that are induced by injury.

Plant defensive chemicals have long been used as insecticides. Some examples include: nicotine from tobacco; pyrethrin from flowers of Chrysanthemum species; azadirachtin from the neem tree, Azadirachta indica; limonene from Citrus species; and capsaicin from chili peppers. Often, plant defenses are lost during selective breeding of crop plants, rendering crop plant varieties susceptible to pests that their wild relatives can resist or tolerate.