This is a name given to various pests which can be found on euphorbias. The adults, which are about 1.5 mm and look like tiny flour-covered moths, and the yellow-green larvae, 0.3-0.7 mm long, prefer the under-side of leaves, the adults usually occupying the upper leaves and the larvae more mature plant parts. They suck plant sap and excrete honey dew which can lead to black mould. In serious infestations the plant can turn yellow and dry back.
Infected plants should be isolated where possible and must be treated. Yellow sticky strips can be used to catch the adults, but it will also be necessary to spray at least 4 times at 5-10 day intervals with pyrethrum, soft soap or insecticide (including systemic). A leaf-shine spray also helps against the larvae. As the eggs are unaffected by these methods, repeat sprays will be necessary.
White fly can also be dealt with biologically with the parasitic wasp, Encarsia formosa.