Figure 7-1 External features of an insect
Based on the type of mouth parts and their feeding habits, insect pests are classified into two main groups: 
Biting and chewing insects: The mouth parts are adapted for biting and chewing solid food from the plants or its products, e.g., grasshoppers, termites, cockroaches, larvae of moths and butterflies.
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Piercing and sucking insects: The mouth parts are modified to pierce the plant tissues and suck the sap from cells of leaves, buds and fruits.
Crop plants are attacked by insects at all stages of their growth. Insects also attack all parts of the plantroots, stems, leaves, fruits and even the seeds. Very often, it is the larvae that cause damage to crops, but in some cases, both the young and the adult (e.g., grain weevil) attack crops. The insects cause damage either in their attempt to feed or lay eggs on host plants. 
They feed on leaves and reduce the leaf area. As a result, photosynthesis in plant is reduced. This arrests plant growth and, as a consequence, the final yield of the plants is reduced. Some examples of these insects are grasshoppers, leaf beetles, caterpillars or butterflies and moths.
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