The following biological control agents include both types, but should be considered with reference to their practical use in horticulture. For example, the use of the parasitic wasp Encarsia formosa is only eff ective if introduced

early at a density of one wasp scale per plant, when the whitefl y pest problem is no greater than one whitefl y per twenty plants. They also requires a minimum temperature of 20°C, otherwise they do not fl y around from plant to plant and walking gives less eff ective control.