Aphids, also known as plant lice (and in Britain as greenflies), are small plant-eating insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions. About 4,400 species of 10 families are known, varying in length from 1 to 10 mm (0.04 to 0.4 in). Around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners. Natural enemies include predatory lady beetles (Coleoptera: Coccinellidae), hoverfly larvae (Diptera: Syrphidae), parasitic wasps, aphid midge larvae, crab spiders, lacewings (Neuroptera: Chrysopidae), and entomopathogenic fungi like Lecanicillium lecanii and the Entomophthorales.
Aphids are distributed worldwide, but are most common in temperate zones. Also, in contrast to many taxa, species diversity is much lower in the tropics than in the temperate zones. They can migrate great distances, mainly through passive dispersal by riding on winds. For example, the currant lettuce aphid (Nasonovia ribisnigri) is believed to have spread from New Zealand to Tasmania in this way. Aphids have also been spread by human transportation of infested plant materials. Aphids are the only animals known to synthesise carotenoids, an ability they gained by coopting the synthetic genes from fungi.