Egg FIGURE 7–3 Life cycle of an insect with incomplete metamorphosis. Adult Naiad Naiad Egg FIGURE 7–4 Life cycle of an insect with complete metamorphosis. Adult Larva Pupa Classification Based on Feeding Habit Insect pests can also be classified based on how they inflict damage on the plant through their eating habits. On this basis, there are two categories of insects: chewing insects and sucking and piercing insects. Chewing Insects Chewing insects, as their name implies, chew plant parts (e.g., petals, leaves, stems, fruits, and flowers) during feeding. They have chewing mouthparts. Their damage is not limited to the aboveground parts of plants but also includes roots. This group includes larvae such as caterpillars and grubs, as well as adults such as grasshoppers, beetles, and boring insects. The tissue of the plant is destroyed in the process of feeding, the damage caused being more serious as the insect matures. Damage from chewing insects is easy to identify (Figure 7–6). Symptoms include the following: Defoliating. Insects such as leaf beetles, caterpillars, cutworms, and grasshoppers devastate the foliage of plants by chewing portions of leaves and, in severe cases, stripping the plant completely of leaves. Defoliation causes a reduction in photosynthetic area, thereby reducing plant vigor and productivity. 7.4 Insects 225
FIGURE 7–5 (Source: USDA) Caterpillar of cotton bollworm. FIGURE 7–6 Typical damage caused by a chewing insect pest such as a cabbage worm. (Source: Photo provided courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University) (a) (b) FIGURE 7–7 Sucking insects: (a) aphids and (b) mealybugs. (Source: Photos provided courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University) 226 Chapter 7 Biological Enemies of Horticultural Plants Boring. The chewing mouthparts of borers are used to bore channels into succulent tissue—stems, fruits, tubers, and seeds. Examples of pests that bore into plant parts are corn borers and white grubs. Leaf mining. Unlike borers, which bore directly into the tissue, leaf miners only tunnel between epidermal layers. Root feeding. Root feeders damage plants from below in the soil by chewing roots and other underground structures. Examples are white grubs and wireworms. Sucking and Piercing Insects Sucking and piercing insects puncture the plant part on which they feed in order to suck out plant fluids. Examples of such insects include aphids, scales, mealybugs, thrips, and leaf hoppers. Insects in this group are often small in size (Figure 7–7). Even in their adult stage, they can be microscopic, making them difficult to readily detect and control. Their effect is often recognized as curling up or puckering of leaves (leaf distortion) or bleaching of leaves. Sucking and piercing insects also damage fruits. They are often found on the undersides of leaves. Certain sucking insects may inject toxins into the plant in the feeding process. Another characteristic of damage caused by
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