Synonyms and other taxonomic changes
for recent summary of classification and higher taxa relationships see(1)
~1100 spp. in ~250 genera of 28 families in our area; estimated almost 8,000 spp. in 1,137 genera of 45 families worldwide(2)
keys to quarantine spp. in(3)
Parasitic on plants, they drink plant fluids through their piercing, tube-like mouthparts. Like many parasites, they tend to specialize in specific groups of plants: knowing what plants they feed on can help greatly in identification.
While there's some variation, they start as free-moving crawlers, with the females becoming less mobile as they mature. In most groups, the females attach to a single spot and lose legs, antennae, etc., so that they begin to look more like some kind of growth than an insect. Both mobile and non-mobile types develop thick protective layers of wax or other inert substances, often in elaborate shapes- so it's hard to see the actual insect underneath.
The adult males are rarely seen (or at least rarely noticed), but look somewhat like winged aphids:
The female lays her eggs where she is (often in a large sac hidden under her own protective covering), and the crawlers hatch to move on to new feeding sites.
Most gain the protection of ants by secreting a sugary substance called honeydew.
Normally only a minor pest (mostly because they spread diseases), but non-native species that lack natural enemies can build up to devastating numbers. Some of the earliest and most dramatically successful uses of biological control have been against this group.
Whiteflies undergo a similar legless, attached stage as nymphs, but then both males and females develop into winged adults.
Howell, J. O. and M. L. Williams. (1976). An Annotated Key to the Families of Scale Insects (Homoptera: Coccoidea) of America, North of Mexico, Based on Characteristics of the Adult Female. Annals of the Entomological Society of America, Vol. 69, No. 2: 181-189. (Oxford Academic