There are a number of organic insecticides available to control whitefly on plants growing in the garden. White fly breed very quickly and repeat applications will be necessary following the manufacturer's recommendations.
In the greenhouse instead of using insecticides, biological control can be very effective. This entails releasing the chalcid wasp encarsia into the greenhouse when whitefly are present. Encarsia are supplied as pupae on cards that are hung amongst the crop. This tiny wasp is harmless to humans and will keep the infestation of whitefly in balance. Encarsia can be introduced into the greenhouse when temperatures remain between 16-20° night and day.