Aphids have also been spread by human transportation of infested plant materials.
Aphids are in the superfamily Aphidoidea in the Sternorrhyncha division of the order Hemiptera.
Late 20th-century reclassification within the Hemiptera reduced the old taxon "Homoptera" to two suborders: Sternorrhyncha (e.g., aphids, whiteflies, scales, psyllids, etc.) and Auchenorrhyncha (e.g., cicadas, leafhoppers, treehoppers, planthoppers, etc.) with the suborder Heteroptera containing a large group of insects known as the true bugs.
The most recent authoritative classifications place all extant taxa into a single large family Aphididae.
Despite their names, taxonomically, the woolly conifer aphids like the pine aphid, the spruce aphid, and the balsam woolly aphid are not true aphids, but adelgids, and lack the cornicles of true aphids.
Aphids, adelgids, and phylloxerids are very closely related, and are all within the suborder Sternorrhyncha, the plant-sucking bugs.
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies, or whiteflies (not to be confused with "jumping plant lice" or true whiteflies), are small sap-sucking insects, and members of the superfamily Aphidoidea.
Around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners.
They vary in length from 1 to 10 millimetres (0.04 to 0.39 in).
Natural enemies include predatory ladybugs, hoverfly larvae, parasitic wasps, aphid midge larvae, crab spiders, lacewings, and entomopathogenic fungi such as Lecanicillium lecanii and the Entomophthorales.
Aphids are distributed worldwide, but are most common in temperate zones.
In contrast to many taxa, aphid 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.