If the layer of mold is really thick, the leaves will not receive adequate sunlight, which can weaken the plant. Whitefly control starts with the application of a dormant oil spray in late winter or very early spring. Oils are available as dormant oil, horticultural oil, or superior oil. One common brand is Volck Oil. Oils should not be applied if the temperature will drop below freezing for at least 24 hours after applications.
Oils kill by sealing off the air supply to the immature whiteflies.
All surfaces of the leaves must be covered with the spray solution. In the spring, the gardenias will need to be sprayed with some type of insecticide. Acephate will need to be applied every 10 to 14 days, while dimethoate should be applied about once a month. As an alternative, an insecticidal soap can be used. Insecticidal soaps only kill whiteflies that the spray contacts directly.