Gardenias are favorites in Augusta landscapes, either the bigger shrubs with the larger leaves (Gardenia jasminoides) or the dwarf ones with smaller leaves (Gardenia augusta 'Radicans') . I have both in my yard, and I have the large-leaved ones at my office.
It seems everyone enjoys the white, fragrant spring blooms. But there is a downside: whiteflies. Heavy infestations of whiteflies produce objectionable levels of honeydew, and the black, sooty mold that grows on it. Infestations can cause leaves to turn yellow and drop prematurely.
Whiteflies are only one-tenth to one-sixteenth inch long, and resemble gnats. When you shake a heavily infested plant, the air is instantly filled with a white cloud of these insects.
Whiteflies live on the underside of the leaves. If you go out and look underneath your gardenia leaves, you will see eggs that are waiting to hatch or the adults flying around. The tiny eggs are pale yellow to gray.
From eggs, whiteflies hatch into the nymphal stage and begin crawling around and sucking on the leaves. They will eventually develop in to the pupa stage. After that they turn into adults, emerging in April or May. They will be active two to three weeks, then lay their eggs and disappear until the next generation hatches out in August.
Insecticides can eliminate whitefly adults. Eggs, feeding nymphs and pupae actually defy insecticides; so, depending on what you use, one spraying will not give you adequate control.
Contact insecticides (which kill only on contact) include products such as 1 percent horticultural oil, insecticidal soap, and other insecticides containing the active ingredients pyrethrins, pyrethrums, resmethrins, cyfluthrin, and bifenthrin. Cyfluthrin, though, provides residual control for two weeks. Good coverage of your spray underneath the leaves is critical.
Systemic insecticides, in which the plant takes the insecticide into its system to protect it, are also options. They last longer than contact insecticides, so they may be better options.
Systemic insecticides to use are acephate, imidacloprid and disulfoton. You mix up a spray with acephate, which is commonly sold as Ortho Systemic Insect Spray. Disulfoton will come in granular form that is sprinkled on the ground. Common brand names for this are Di-Syston and Bayer Advanced Rose and Flower Care. The Bayer product also contains fertilizer. Imidacloprid will come in a liquid that you mix with water and pour on the ground. It is sold as Bayer Advanced Tree and Shrub Insect Control and is taken up into the plant through the root system.
Acephate and disulfoton should last for six weeks while imidacloprid should provide protection for up to 12 months. When you visit the garden center, look for these active ingredients on the product labels.
Whiteflies also feed on chinaberry trees, citrus and any plant in the privet family (includes ligustrum). In some cases, they will feed on viburnam.
The little flies are blown around by the wind and you may find them on other shrubs in your landscape. Don't be alarmed, though, because they are not causing damage to the other plants.
This means you don't have to spray everything in the landscape. That would be unnecessary, expensive and environmentally unsound.
Reach Sid Mullis, the director of the University of Georgia Extension Service office in Richmond County, at (706) 821-2349 or firstname.lastname@example.org.