These abnormal growths are known as galls and result from the way the insect feeds on the plant. Insects destroy fruit and seeds. Some insects beat us to the harvest, feeding directly on the fruit or seeds we are trying to harvest. A worm in an apple probably is the larva of the codling moth. The ear of sweet corn often is already inhabited by corn earworms.
Insects transmit plant diseases.
Often, the insect itself causes little damage when it pauses to feed on a plant. However, many insects can pick up viruses, bacteria and other disease organisms as they feed then later transmit them to healthy plants, resulting in severe damage and even death of the plant. For example, aphids transmit viruses that cause disease in wheat and barley, bark beetles move fungi which cause Dutch elm disease that kills American elm, and the aster leafhopper can inject the bacteria causing aster yellows, a common problem for lettuce, carrots and many flowers. Insects transmit diseases to humans and animals . Insects and other arthropods also can transmit diseases to animals.