Tenacious Whitefly Attacks Hibiscus

Question: What can I do to control the giant whitefly that has been attacking my hibiscus, and what other plants are the flies likely to attack?

T.R., Irvine

Answer: The giant whitefly (GWF) has been rapidly expanding its territory since it was first detected in Southern California in 1992. Monitoring and early detection are two of the best ways to control this pest.

In the early stages of an infestation, leaf removal can be an effective means of controlling the giant whitefly. GWF will select a few plants in your garden and remain on those plants until the population builds to extremely high levels before they will move to other plants in the yard. So removal of a relatively small number of leaves or selective pruning of heavily infested branches, when the giant whitefly first arrives in your yard, can make quite a dent in their populations.

Careful disposal of infested plant material is important to prevent spreading. Put clippings into a trash bag and throw out with the trash. Early detection means careful observation of both upper and lower leaf surfaces, looking for the whitefly’s telltale waxy, spiral rings and long, white, beard-like filaments.

You can also help to control the whitefly populations by spraying the underside of leaves with a strong surge of water.

UC researchers have concluded (in several side-by-side studies comparing pesticides with a strong surge of water) that water works as well as or better than chemical sprays.

It is recommended that you spray with water at least once a week or twice a week if populations are heavy. By spraying with water instead of chemicals, you don’t harm the beneficial insects that inhabit your garden and help to keep it in balance.

If you must spray, stay with short residual pesticides such as insecticidal soap or a horticultural oil.

Scientists from UC Riverside’s department of entomology have observed several insect predators feeding on GWF; however, the scientists say, control of this whitefly will require the use of parasites collected from Mexican GWF populations, where the insect is native.

So Far, Results Called Promising

John Kabashima, environmental horticulture advisor with the University of California Cooperative Extension in Orange County and one of the scientists who has been making field releases of GWF parasites in Orange County, stated that results are promising, but final conclusions on the parasites’ effectiveness will not be made until late next year. He also emphasized that home gardeners need to remember that biological controls will not completely eliminate the GWF, but rather will lower the population to levels that will not harm the plants or be easily observed.

Biological controls are not available for purchase by the general public. Hibiscus (darker colors preferred) is one of the giant whitefly’s favorite plants to inhabit, but the list of kinds of plants that they will attack continues to grow. They favor some avocados, begonia, boxwood, some citrus, eugenia, fruitless mulberry, ivy, nasturtiums and xylosma.

Fortunately, research is being conducted that looks like it may bring some relief to gardeners in the near future. Phone the hotline for a free pamphlet on the whitefly.

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Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners@yahoo.com. Calls and e-mail are picked up daily and are generally returned within two to three days.