Aphid, or Plant Louse, a small insect that destroys crops and ornamental plants. Most aphids are green, red, or brown; some are gray, yellow, black, or pink. Nearly all aphids are less than 0.2 inch (6 mm) long. Their bodies are soft, and their legs are slender and weak. Some adults have two pairs of weak wings.

The aphidThe aphid , or plant louse, is a tiny, soft-bodied insect that feeds on plant juices.

Many aphids are named for the plants they destroy; for example, cotton aphid and apple aphid. Aphids damage plants by sucking the juices from buds, leaves, and shoots or from the roots. They exude a sugary liquid called honeydew, which sticks to the plant and causes the growth of various harmful fungi and molds. Some kinds of aphids cause the formation of galls (small swellings) on plants. Aphids can also spread plant diseases.

Ants play an important role in the destruction caused by aphids. Certain species of ants protect aphids from enemies, shelter them from bad weather, tend their eggs, and carry them from one host plant to another to feed. Such aphids are often called ant cows, because the ants eat the honeydew they produce, and even "milk" the aphids by stroking them with their antennae to increase the flow of honeydew.

Most aphids have a complicated life history that alternates between different types of generations: winged and wingless; parthenogenetic (asexual) and sexual (including bisexual); and viviparous (giving birth to live young) and egg-laying.

Natural enemies of aphids include various other insects and some birds. Some aphids cover their bodies with waxy secretions from their abdomens; these secretions harden, becoming difficult for predators to penetrate. Farmers and gardeners control aphids with insecticides.

Aphids are in the order Hemiptera and the aphid family, Aphidae.