WEATHER CAUSES RETURN OF WHITEFLY.

Byline: Joshua Siskin

The prolonged spell of warm weather we have experienced this winter has been good for the giant whitefly. As long as temperatures stay mild, this insect pest will continue to be fruitful and multiply.

Like other tropical and sub-tropical insects, it may never truly ``overwinter'' - a term which, in the entomologist's lexicon, connotes hibernation in egg, larval or adult state until spring arrives.

The giant whitefly arrived in Southern California from Mexico in the early 1990s. After infesting San Diego gardens, it arrived in our area, first wreaking havoc in Hollywood and West Los Angeles before find its way into the Valley.

I have been watching the giant whitefly for nearly a decade and have become familiar with its mode of operation. Although it has favorite target or host plants - hibiscus, xylosma, Richmondensis begonia, giant bird of paradise, false aralia, citrus - it can show up almost anywhere. I have seen it in geranium plantings, on king palms growing in clay pots and among gloriously flowering cannas.

Yet in assessing whether your garden is susceptible to giant whitefly attack, sun exposure and other cultural conditions are no less important than the presence of particular plant species.

A few days ago, Andria Strand described her giant whitefly predicament to me in an e-mail. ``I have a yellow-flowered hibiscus in my yard,'' she wrote. ``Recently it started to get what looks like white hair on the backside of some of the leaves. I've never seen this before. Also, the overall plant looks like it is yellowing and maybe dying. Is there something I can spray on it or do I have to tear it out so it doesn't affect my other plants?''

First of all, you are lucky that your hibiscus is yellow and not the more common red variety. When giant whitefly infests a red hibiscus, it does so rapidly and completely. Infestation of yellow hibiscus takes place more slowly. If you catch it early on, you can keep it under control simply by removing the tainted leaves. Giant whiteflies are only slightly mobile. When an egg hatches, the emerging nymph often grows to adulthood and dies attached to the same leaf on which it was born.

The yellowing foliage you describe is typically encountered on hibiscus during the winter and has nothing to do with whiteflies. The plant is in a state of semidormancy and some leaves turn yellow before dropping off. With the arrival of spring, new green foliage will replace foliage that has fallen during the winter.

The immobility of the giant whitefly is illustrated by the condition of plants at a Hollywood apartment building whose landscape I have monitored for several years. The hibiscus and xylosma in front of this building are regularly loaded up with giant whiteflies. However, in a first-floor atrium, just a few steps up from the ground floor, other hibiscuses are growing without a trace of whitefly. The reason for this is that whiteflies are not true flies and, at best, flutter a bit while going from one leaf to another. Their capacity for vertical flight is strictly limited.

The giant whitefly gets its name from its size, which is about 3/16 of an inch and nearly three times larger than that of other whiteflies. Whiteflies are sucking insects with kinship to aphids, mealybugs and scales.

If your hibiscus is a foundation plant - which means it is planted right up against a house or building - it will be more difficult to keep the giant whitefly under control. Only half of your plant will be exposed to the sun and air. The other side will be eternally shaded and bereft of any breeze passing over or circulating through its leaves. Where sun and air are blocked out, the giant whitefly feels most at home.

There is no chemical remedy for the giant whitefly. Rinsing the flies off of foliage with a strong stream of water - syringing - is as good as any pesticide application.

TIP OF THE WEEK: J. Thurman, in an e-mail from the Antelope Valley, says he want to treat his peach tree for pests and has been told to make a ``sticker application,'' a procedure with which he is unfamiliar. In such an application, you mix a surfactant, which has a detergentlike effect, with your pesticide of choice. A surfactant makes water stickier so that the chemical solution you apply will stick to the peach foliage, buds or bark and not bead up and drip off. Any well-stocked nursery should carry surfactant, also called ``sticker'' or ``wetter sticker.'' Surfactant can easily burn plant tissue, so make sure you use it sparingly, in accordance with instructions.