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Whitefly causes bleak times for growers.

Whitefly Causes Bleak Times for Growers

It attacks lettuce, tomatoes, cantaloupe, watermelon, cucumber, sugar beets, and squash. It also goes after cotton, beans, and soybeans and has a real penchant for ornamentals, especially poinsettias.

A truly diverse and varacious appetite distinguishes the sweetpotato whitefly (Bemisia tabaci), which lives on the underside of leaves. It not only reduces yields by sucking life-giving nutrients from host plants, but it also spreads deadly plant viruses. Adding insult to injury, it leaves behind a sugary substance that makes an ideal growth medium for sooty mold on host plants.

ARS entomologist John W. Neal says when you walk through a greenhouse of plants that are badly infested, the disturbed insects look like a cloud of dandruff. Alone, a mature whitefly looks somewhat like a piece of ash from a burning log, while immature whiteflies are scale-like and semi-transparent.

Since 1986, this tiny pest has been causing serious economic problems for U.S. vegetable, cotton, and ornamental growers. First recorded on tabacco in Greece in 1889, the sweetpotato whitefly is found on crops throughout the world.

"No satisfactory chemical control exists for it on any crop," says Richard S. Soper, ARS program leader for biocontrol. "One of the most disturbing things in its ability to become resistant to almost any chemical used against it."

How significant are problems caused by the sweetpotato whitefly in the United States?

"Last season, it cost Florida tomato growers between $20 and $40 million, wiping out 25 percent of the crop," according to Wayne Hawkins. Hawkins executive vice president of the Florida Tomato Exchange, says this estimate doesn't include handling or transportation costs.

"The whitefly spells devastation for Florida tomato growers," Hawkins says. "It causes tomatoes to ripen unevenly, resulting in white longitudinal streaks on fruit and an increase in internal white tissue. Before ripening, however, there is no visible sign of whitefly damage on the tomato or the plant. Since we harvest and ship mature green tomatoes, the insect damage is not detectable until arrival at some distant market."

Tomatoes are not the only targets; the sweetpotato whitefly has homed in on squash with a new disorder, called squash silverleaf, which reduces fruit quality and yield.

"Although it's of unknown origin, every case of this disorder we studied was associated with the sweetpotato whitefly," says Raymond K. Yokomi. Yokomi is an ARS entomologist at the U.S. Horticultural Laboratory in Orlando, Florida.

Desmond R. Jimenez, an ARS chemist at the Orlando lab, is working with Yokomi to determine how the whitefly causes squash silverleaf and irregular ripening of tomatoes. "We're looking for a phytotoxic factor (poisonous to the plant) in the insect that could be responsible for these disorders," Jimenez says.

In California, the sweetpotato whitefly spreads the lettuce infectious yellow virus. [See Agricultural Research, September 1988, pp. 10-12.]

Hawkins says a new whitefly-transmitted gemenivirus outbreak last year caused inestimable damage to Florida tomatoes. This problem is readily apparent and the disease spreads rapidly through a field - plants are stunted and they turn from green to either a mottled yellow or purple. The geminivirus has definitely been linked to the sweetpotato whitefly. "The Florida tomato industry established a Sweetpotato Whitefly Task Force that went to Congress last year for funds to increase ARS research efforts to control the spread of this pest," Hawkins explains.

In addition to Hawkins' Florida Tomato Exchange, the Task Force includes the American Association of Nurserymen, Florida Fruit and Vegetable Association, National Cotton Council of America, Society of American Florists, United Fresh Fruit and Vegetable Association, and Western Growers Association.

Not Just the Veggies Suffer

It's not only vegetable crops that appeal to the sweetpotato whitefly. Evidently, cotton is just as appetizing. "There seems to be something in the cotton plant that attracts the whitefly," ARS entomologist Hollis Flint says. "It walks around on the underside of plant leaves, finds a good spot to draw out the phloem sap, and becomes sedentary, sucking away." The insect has developed a digestive system that excretes the sugars it doesn't use from the sap, says Flint, who is located at the ARS Western Cotton Research Laboratory in Phoenix, Arizona.

"This sticky stuff then coats the cotton lint in open bolls," Flint says. In addition to serving as a growth medium for molds, this sugary substance creates a big problem when the cotton is picked and goes to gin. He says an even bigger problem occurs when the sticky cotton is processed for cloth. It clogs up the machinery.

John Neal and Jo-Ann Bentz, entomologists with the ARS Florist and Nursery Crops Laboratory in Beltsville, Maryland, are also working on possible controls for the pest.

"We now have a natural product that kills, sweetpotato whitefly nymphs," Neal says. "Similar to Safer's soaps now used in greenhouses, this biosoap seems to dissolve the waxy cuticle of the pest. We've been testing it for years in the laboratory."

Neal's biosoap is made of extracts from a species of greenhouse-grown Nicotiana, a relative of the tobacco plant. Having its own wetting agent, the extract can simply be mixed with water and sprayed on plants. Neal got his extracts from chemist George Buta of the ARS Plant Hormone Laboratory in Beltsville.

"We're evaluating 66 species of Nicotiana and so far have found 8 very effective ones," Neal says. He credits ARS retired agronomist George Pittarelli for originally noting resistance of Nicotiana species to whiteflies.

Since the whitefly is a worldwide pest, ARS is studying natural enemies in other countries, hoping to import likely candidates for evaluation in the United States. The ARS European Parasite Laboratory. Behoust, France, will begin a project this spring to identify important natural enemies of the sweetpotato whitefly. Foreign exploration could include Pakistan, southern Europe, the Caribbean, Central and South America, the Near East, and North Africa.

PHOTO : The underside of a squash leaf is dotted with eggs from these sweetpotato whiteflies. (K-3922-11)

PHOTO : Entomologists Kim Hoelmer (left) and Desmond Jimenez examine an acrylamide gel containing proteins extracted from squash plants were colonized with sweetpotato whiteflies. (K-3927-17)

PHOTO : Entomologist Raymond K. Yokomi collects sweetpotato whiteflies in a squash field for studies of squash silverleaf and tomato irregular ripening. (K-3926-5)

PHOTO : The beetle Delphastus pusillus Casey is a promising predator for sweetpotato whitefly control in greenhouses. It is not considered a picky eater, though it prefers sweetpotato whitefly eggs, to the fully developed adult. (K-3922-17)
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Title Annotation:U.S. Agricultural Research Service looking for ways to prevent damage to many different crops caused by the sweetpotato whitefly
Author:Stanley, Doris
Publication:Agricultural Research
Date:Jan 1, 1991
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