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Tiny Wasps Help Win War on Whiteflies

Garden * Biological control of the pests, which feed on plants such as begonia and bird of paradise, is a slow process, but one that's working.

November 04, 2000|JULIE BAWDEN DAVIS | SPECIAL TO THE TIMES

Ask Orange County gardeners to name the most annoying and destructive pest in their yards today and most will point to the giant whitefly.

Since its discovery in San Diego County in 1992, the giant whitefly (Aleurodicus dugesii Cockerell), has rapidly spread throughout Southern California home gardens, public parks and arboretums.

Originally from Mexico and unchecked by any native predators, the whitefly, which resembles a tiny white moth, attacks a variety of plant species. These pests feed on the underside of leaves, creating white waxy spirals and long, hair-like filaments of wax that give a bearded appearance to affected leaves.

Their constant feeding weakens and sometimes kills plants.

They also produce a sticky solution, which accumulates on leaves and fosters the growth of black sooty mold fungus.

Although they've been found on a variety of plants, giant whiteflies have their favorites, including hibiscus, begonia, giant bird of paradise, orchid tree, banana, mulberry, xylosma, aralia and various vegetables.

Until the last year or so, controlling these pests has appeared to be impossible. But thanks to the work of University of California researchers and Master Gardeners, the giant whitefly's reign over the home garden is quickly coming to a close, said John Kabashima, environmental horticulture advisor for the University of California Cooperative Extension for Orange and Los Angeles counties.

Kabashima and his colleagues have been responsible for locating and introducing two parasitic wasps from Mexico that feed solely on the giant whitefly. Since 1997 these wasps have been released in the area and in the last year or so they have finally taken hold.

"We're seeing a steady decline in the whitefly populations thanks to the parasitic wasps," Kabashima said. "The wasps are spreading--we're now finding them in areas where we haven't yet made any releases."

Despite their small size, these stingerless microscopic wasps do a great deal of damage. They lay their eggs inside the whitefly larvae and when the wasp eggs hatch, their larvae feed on the giant whitefly larvae. The wasp then pupates and emerges, leaving behind a yellow or grayish-black shell of what would have been a whitefly.

"These wasps are very small and harmless to everyone but giant whiteflies," said Diane Fish of Tustin, a University of California Master Gardener, who has released the parasites throughout the county since March 1999. "The wasps' sole purpose in life is to destroy whiteflies."

Although it is effective, controlling whiteflies biologically with parasitic wasps is a slow process. It takes months for them to become established. But it is the only viable, long-term solution.

All other control methods are short term, Kabashima said. "Applications of pesticides and removal with water are only temporary, and pesticide use is actually detrimental," he said. "Introducing pesticides into the environment disrupts other natural systems, and actually kills off beneficials, including the wasps, which are more fragile. The whiteflies will come back, but the beneficial parasites may not, and you'll have a worse problem than you started with."

Biological control has been used for many years, dating back to the 1700s. The most recent success in California was with the Ash whitefly, which was controlled with another parasitic wasp.

In most cases, biological control methods don't completely eradicate a pest. Instead, they help regain balance to the environment. The wasps, for instance, won't destroy all of the whiteflies because they need a food source. Generally, they leave a small amount of whiteflies at the base of the plants, which is tolerable for most people.

For Pat Madsen, maintenance supervisor and arborist for the city of Tustin, the few whiteflies at the base of a handful of hibiscus at the city's Centennial Park is nothing compared to the state of the park a year ago.

"We have over 20 mulberry trees in the park, and in the summer of 1999 they were so infested by whiteflies that they looked like giant cotton candy trees," Madsen said. "At the end of last summer, they made a release of the parasitic wasps and this summer the trees were completely spotless. The only place you can find any whitefly--and they are minimal--is on our hibiscus."

Orange gardener Kittie Rau also saw a dramatic decrease in whiteflies on her property after the parasitic wasps were released. Because she had a severe infestation and her property is large, it was used as a release site.

"Before they introduced the wasps, my garden was completely infested," said the rare-fruit grower, who is also a Master Gardener. "The whiteflies nearly killed a number of my fruit trees and vines, including my cherimoya, blueberries and citrus. The ground often looked like it was covered with snow because of all the waxy filaments."

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