I have both in my yard and I have the large-leaved ones at my office.
Both are popular shrubs because of their white, fragrant, spring blooms. But the down side is whiteflies. If we don't control them, the plants become unsightly because heavy infestations of whiteflies produce honeydew and the black sooty mold that grows on it. Infestations can cause leaves to turn yellow and drop prematurely. If you control the whiteflies, then you won't have the sooty mold.
Whiteflies are tiny insects, only about 1/10 to 1/16 inch long, and resemble small gnats. When you shake a heavily infested plant, the air is filled instantly with a white cloud of the insects.
Whiteflies live on the underside of the leaves. If you go out and look underneath your leaves right now, you will see eggs waiting to hatch or the adults flying around. The eggs are about 1/100 inch long, pale yellow to gray.
From the eggs, they hatch into the nymphal stage. Then they will begin crawling around and sucking on the leaves. They will develop into the pupa stage. After that, they turn into adults, emerging during April or May. I first saw them on mine about a week and a half ago and sprayed for them last weekend.
Whiteflies will be active two to three weeks, lay their eggs, then disappear until the next generation hatches out in late July or early August. So we get to deal with their damage twice a year, with the worse damage being done by the August generation.
Whiteflies like to feed on certain plants. The most common (other than gardenias) are chinaberry trees, citrus and any plant in the privet family (includes ligustrum). In some cases they will feed on viburnam.
There are tons of old privet growing along fence rows and old house sites that produce a lot of whiteflies, but they seem to have little real damage.
Insecticides eliminate whitefly adults and nymphs still in the crawling phase. Eggs, feeding nymphs and pupae actually defy insecticides; depending on what you use, one spraying will not give you adequate control.
Labeled 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.
Other options for control (probably more desirable because of not having to apply as often) are systemic insecticides. The plant takes the insecticide into its system to protect it. Systemic insecticides to use are acephate, imidacloprid, and disulfoton. With acephate, you will buy the concentrated liquid and mix with water in a sprayer. 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 out on the ground, or a granular that you do the same. It is sold as Bayer Advanced Tree and Shrub Insect Control. Both insecticides will then be taken up to the plant through the root system.
Acephate and disulfoton should last for six weeks, and imidacloprid should provide protection for up to 12 months.
Common company names you will find are Ortho, Bayer, Ferti-Lome, Green Light, Safer, Spectracide and Bonide. When you visit the garden center, just look for the active ingredients or ask someone to assist you.
You will oftentimes find whiteflies on other shrubs in your landscape because they blow in the wind and land on them, but they are not causing that plant any damage. So spraying everything in the landscape is unnecessary, expensive and environmentally unsound.
REACH SID MULLIS, THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY, AT (706) 821-2349 OR SMULLIS@UGA.EDU.