Encarsia formosa is a small parasitic wasp that lays it eggs inside the nymph stage of greenhouse whitefly. One wasp forms inside the immature stage of the whitefly, killing the whitefly. Parasitized whitefly pupae turn black and adult Encarsia wasps emerge. Encarsia also kill young whitefly nymphs by feeding on them.
Encarsia formosa are purchased as parasitized pupae that are glued to a cardboard card. See the small round dark spot in the photo. The encarsia emerge as adults and search out whiteflies in the greenhouse to parasitize.
To learn more about biocontrol for whitefly see: Greenhouse Pests and their Natural Enemies and Biological Control: A Grower's Guide to Using Biological Control for Silverleaf Whitefly on Poinsettia