The term pests is here employed in the widest possible sense to include insects, mites, nematodes and other animals which feed on or otherwise damage any parts of a plant, and weeds which compete with other plants for air, light and nutrients. Disease-causing organisms of plants -- bacteria, fungi, viruses and virus-like organisms (phytoplasmas) -- are classified as pathogens. Thus the plant pest is any living, non-pathogenic agent, which injures or limits the growth of a plant, and is of importance to man when crop yields are seriously damaged, either qualitatively or quantitatively.
Of the animal pests, rodents may on occasion be so abundant as to seriously damage unharvested crops. Other animal pests which may be of local importance if their breeding is allowed to progress unchecked include certain birds, mammals and reptiles. Biting-insects, mainly at the caterpillar stage, may feed on the foliage, the shoots, the flowers and the fruits, causing extensive damage and reducing yields. The sucking insects such as the aphids and the plant bugs live on the sap of plants. This may cause direct injury, as in the feeding of a large population of mites which causes the foliage to turn brown and become ineffective, but sucking insects are of even greater importance as the vectors of disease. Nematodes are particularly destructive when they damage the roots of a plant, hindering normal development. Weeds cause a constantly heavy annual toll in lost crop production. They compete directly with crops for air, light, space and nutrients and thereby reduce absolute yields. By being present in the harvested crop they reduce its value and usefulness, perhaps even rendering it useless.