Technical Abstract: Whiteflies belong to the order Homoptera, and the family Aleyrodidae. They are tropical and subtropical in origin, and can be serious pests in field crops of the southern areas of the world and in glasshouses. Whiteflies have many pathogens, but nearly all known pathogens are fungi, which can infect their hosts directly through the exoskeleton. For example, twenty-five species of Aschersonia spp. are known to whiteflies in the genus Trialeuroides. Other fungi found in populations of Trialeuroides nymphs include Verticillium lecanii, Paecilomyces fumosoroseus, Paecilomyces fumosoroseus, Aphanocladium album, and Beauveria bassiana. Nymphs are most commonly infected, but adults have been found infected with Paecilomyces farinosis, Paecilomyces fumosoroseus, Verticillium fusisporum, and Erynia radicans. Aschersonia has rarely been isolated from Bemisia whitefly species, the fungal pathogens Verticillium and Paecilomyces are much more common. Beauveria bassiana has been shown to infect nymphs of Bemisia whiteflies if they are sprayed with conidial suspensions, but are rarely found outside of treated populations. When whitefly populations reach outbreak levels and climatic conditions are right, then fungal pathogens can cause natural epizootics that lead to a rapid decline in host populations. Some of these fungi have been developed in to biological pesticides for whitefly control, including Aschersonia, Verticillium lecanii, Paecilomyces fumosoroseus, and Beauveria bassiana.