Aphids, the vampires of the plant kingdom, are members of the order Hemiptera (the true bugs) in the class Insecta. As are other hemipterans, aphids are equipped with highly modified mouthparts that permit the injection of salivary juices and the siphoning off, in this case, of phloemic plant juices. Analogous to the anti-clotting substances of mosquitoes, leeches, and many endoparasites, aphid saliva contains a compound that prevents the normal healing of a plant wound until the aphid has feasted. Two tubes or cornicles, resembling the dual exhausts of an automobile, excrete a sugary waste product known as honeydew.

Known also as plant lice or ant cows, aphids create significant economic hardships for farmers, horticulturists, and home gardeners by causing leaf curling and other deformations. These insects are also responsible for sooty molds, wilting, damage to fruits and vegetables, and in some cases, they act as vectors for plant diseases. Although aphids rarely kill their plant hosts, the mouthparts are sometimes contaminated during feeding on diseased plants. When the aphid moves to the next plant host, it acts as a carrier, infecting other plants with its tainted mouthparts.

Most aphids are specific to a type of vascular plant, but some are generalists. Almost every plant species has a corresponding aphid species to which it is susceptible. Although there are commercial insecticides for killing aphids, environmentally friendly alternatives utilizing biological controls, such as living ladybugs, green lacewings, and parasitic microwasps, have created a new commercial industry. These environmentally friendly agents are ideal for integrated pest management that is particularly appealing to organic produce farmers and their customers.