Citrus Whitefly Problems
I have seen several gardenias with citrus whitefly problems this summer. Whiteflies are not true flies, but are more closely related to scale insects, mealybugs and aphids. They are very small – about 1/10 to 1/16 inch long. They have a powdery white appearance and resemble tiny moths. The immature stage is scale-like and does not move. When infected plants are disturbed, the whiteflies flutter around briefly before settling again.
Both adults and immature forms of the citrus whitefly feed by sucking plant sap. The damage that they cause is similar to that caused by aphids. The infested plant may be stunted. Leaves turn yellow and die. Like aphids, whiteflies excrete honeydew, which makes leaves shiny and sticky and encourages the growth of sooty mold fungi.
The citrus whitefly was introduced from Asia. Until the advent of synthetic organic pesticides, this pest caused an estimated loss of 45 to 50 percent of the citrus crops in Florida and the Gulf states. Among many other host plants, gardenias seem to be highly susceptible. In fact, one of the infestations eradicated in California (at considerable expense) originated from a gardenia that had been smuggled into the state. The citrus whitefly has been reported on at least 38 genera of evergreen and deciduous plants. Potential hosts for citrus whitefly besides citrus plants and gardenias are chinaberry, privet, prickly ash, pomegranate, and Japanese persimmon.
Controlling whiteflies is not easy. Biological control experiments have been explored with three species of lady beetles known to feed on citrus whitefly crawlers and nymphs. However, they are seldom numerous enough to be an effect control. A tiny parasitoid wasp, Encarsia lahorensis, has shown promise and has been released in NC to help reduce citrus whiteflies. Lacewing larvae are also known to eat whiteflies.
Ideally, insecticides should be applied in late spring before the emergence of first-generation adult whiteflies. The spray should be directed to the undersides of the leaves. Horticultural oils are very effective when timed correctly and with several applications. They do less harm to the beneficials. Systemic insecticides containing imidacloprid (Merit and Bayer Advanced) or dinotefuran (Safari and GreenLight Tree and Shrub with Safari) are also highly effective when used properly.