Whiteflies - Silverleaf whitefly nymphs on Poinsettia (leaf underside)

The primary whitefly species in greenhouse are the greenhouse whitefly (Trialeurodes vaporariorum) and silverleaf or sweet potato whitefly (Bemisia tabaci). The lifecycle of both species consist of eggs, nymphs (seen here), pupae and adults. Whiteflies are found on the undersides of leaves of poinsettias, but also feed on many annuals, perennials, herbs and vegetable bedding plants in greenhouses. Both nymphs and adults have piercing-sucking mouthparts, which they use to feed on plant fluids. Nymphs may secrete honeydew that serves as a growing medium for black sooty mold fungi. Severe infestations may defoliate plants.

See the fact sheet from University of Connecticut "Managing Whiteflies in the Greenhouse" .