Scanning electron micrograph of a glasshouse whitefly, Trialeurodes vaporariorum, on a tomato leaf. The picture shows an adult fly (pale brown) together with seven eggs (vertical blue cones, 0.1mm tall) and a domed larva (left of centre, yellow oval). At the top edge of the picture is an empty larval case, after emergence of an adult fly. Larvae develop in four stages, or instars. The first is mobile, but later instars lack legs and are sessile (as here). The adult is covered with a fine wax powder, shed particles of which are visible on the leaf, most clearly in the foreground. This covering contributes to the difficulty of whitefly control by means of insecticides, as does their ability to develop genetic resistance. T. vaporariorum is often no more than a nuisance in cool glasshouses, but some whitefly species are serious pests as vectors for plant pathogens such as begomoviruses.