These tiny sucking insects often congregate in large colonies and are a particular problem in large greenhouses. They weaken plants by removing large quantities of nutrients and can also transmit disease.
Treat whitefly like aphids, by washing them from plants with a strong jet of water and by encouraging predators. Use soap spray or rotenone (as a last resort).
Image: Central Science Laboratory, Harpenden Archive, British Crown, Bugwood.org