Almost every yard in the south has at least one gardenia. Planted by generations of gardeners for their exotically fragrant white blossoms, these tough evergreen shrubs are salt tolerant, drought tolerant and deer resistant. One thing they cannot resist is the tiny whitefly. These petite pests cause gardenia leaves to turn yellow and plants to appear dark or sooty. If this describes the gardenia in your yard now is the time to check for whitefly and determine if treatment is needed.
Adult whiteflies look like tiny white moths and are around one tenth of an inch in length. Like many pests, they are most often found on the backside of leaves. When infested plants are disturbed, whiteflies will flutter around for a few minutes and then resettle on the plant. Giving a bush a quick shake is an easy way to scout for whitefly at this time of year.
In reality, whiteflies are neither flies nor moths, but are most closely related to mealybugs, aphids and scale insects. They feed on plant sap with needle like mouthparts. Plants that are heavily infested with whitefly often have lots of yellow leaves and clouds of whitefly emerge when the plant is disturbed. In addition, like their scale, aphid, and mealybug relatives, whitefly secrete honeydew, a sticky sweet substance that attracts ants and wasps. Black sooty mold, a harmless fungus that grows on the honeydew, can cover whitefly infested plants, causing the leaves and stems to appear dark and sooty.
There are several types of whitefly in the south. Some feed on vegetables. Others are more common in greenhouses and on houseplants. The type of whitefly found on gardenia is known as the citrus whitefly. While citrus whitefly has been reported to feed on 38 different types of plants, they are almost always found only on gardenia or citrus trees in our area.
If your gardenia has lots of yellow leaves, or if the leaves and stems look dark and sooty you should check for whitefly. Recently emerged adult whitefly will be easily visible on the new growth. If you look on the back of older leaves you will likely find both adult and immature whitefly. In their immature stage whitefly resemble their scale relatives, looking like light orange, round, flat discs stuck to the back of the leaf.
Whiteflies produce several generations each season. If you have them on your gardenia bush now, they will likely persist all summer and into future seasons. On some bushes, whiteflies never seem to get out of hand. This is because their populations are kept in check by beneficial insects, including ladybugs, lacewings, and parasitic wasps. If you find whitefly on your gardenia but it is otherwise healthy with lots of clean, green leaves, you probably do not need to treat. In fact, applying pesticides can disrupt the balance between beneficial and pest insects, causing the pest insects to become the dominant species.
If your gardenia has whitefly, drops lots of yellow leaves and currently or in the past has been covered in black sooty mold, the plant would likely benefit from some form of intervention. There are many options for managing whitefly, including organic and synthetic pesticides. Some gardeners have reported taking their vacuum cleaner or shop vac into the yard and simply vacuuming off the insects. While effective, this method will need to be repeated once a month through the summer, or whenever adult whiteflies are present.
Insecticides that can be used by organic gardeners to control this pest include insecticidal soap and horticultural oil. One benefit of these products is they are less harmful to beneficial insects, as well as being safer for pets and people. To be effective horticultural oils and soaps need to be applied thoroughly to the backs of leaves all over the plant. They will need to be reapplied once a month through the summer.
Synthetic insect control products containing pyrethroid insecticides will also control whitefly, but are very damaging to beneficial insect populations. These include insect killers than contain the active ingredients permethrin, cyfluthrin, bifenthrin, and lamba cyhalothrin. These are sprayed on the leaves and must be reapplied over the summer.
Another synthetic insecticide option is imidaclopyrid, often marketed as Merit. This systemic chemical is applied to the roots, which absorb and move the chemical throughout the plant. The effects last for several months, so this product only needs to be applied once a season. Systemic products like Merit are less damaging to beneficial insects which do not feed on plant tissue, but recent research has shown they can harm pollinators which feed on plant nectar. Always read and follow all label directions when using any pesticide.
For assistance with pest identification and management, visit ces.ncsu.edu, where you can post your questions via the ‘Ask an Expert’ link, or contact your local Extension office. If you live in
Charlotte D. Glen is the horticulture agent with the Pender County Cooperative Extension of NC State University, College of Agriculture & Life Sciences. She can be reached via e-mail at Charlotte_Glen@ncsu.edu.