EL CENTRO, CALIF. — NORMALLY by now, the 10,000-acre fall cantaloupe harvest would be nearly complete here in the Imperial Valley's irrigated desert.
But this year, just a lone 90-acre field of cantaloupe was being picked last week, the yield perhaps only half the usual 450 cartons per acre.
For the past two months - and especially during last week's harvest - plumes of dust signaled the daily parade of curious state and local agriculture officials, journalists, and local farmers pulling up to tramp out and examine how the crop was doing.
Just a touch of a leaf releases a puff of a thousand sweet-potato whiteflies, which leave behind a damaging sticky excretion.
The tiny insects - a new, voracious strain of a pest common in gardens and farms - have brought a giant challenge to the southern California desert farming region.
"This is one of the worst threats we've ever had," says Nick Toscano, an entomologist at the University of California, Riverside, and chairman of the California Whitefly Task Force. "It's the only insect in my experience that compares with the locust plagues in Africa in its ability to devastate everything."
Because this area produces 90 percent of the fall and winter vegetables grown in the United States, the little pest has the potential to affect the grocery bill of any American who enjoys fresh produce throughout the winter.
Almost 100 percent of the 1991 fall melon crop here was destroyed by the new B-strain whitefly. Late summer clouds of the insect were so thick that local residents recall breathing them in when they went outside. Crop losses were estimated at $121 million. Consequently, farmers planted virtually no melons this fall.
Concerns about the pest are spreading north to the larger, more productive San Joaquin Valley, where the new strain was discovered in fields as far north as Fresno in September, says James Duffus, a research leader with the US Agriculture Research Service in Salinas, Calif.
He explains that the B-strain whitefly can feed on a range of up to 500 plants, with cantaloupe and cotton being their prime targets here. The B-strain was detected first in 1986 in Florida on poinsettia plants and later in Texas and Arizona.
But here, apparently aided by the high summer heat in the desert, the new strain reproduced so rapidly that its devastating effects were magnified.
"What's unique [about the infestation] is that it happened overnight. Usually it takes a longer period, and we have time for preparations to grow into it," says Henry J. Voss, director of the California Department of Food and Agriculture.
After last year's explosion of the pest, the whitefly didn't reproduce as rapidly during the cooler winter and spring. Farmers were able to produce their usual winter and spring vegetable and alfalfa crops before whitefly numbers were noticed increasing again in the summer.
Agriculture officials hope this apparent sensitivity to cooler temperatures will keep the whitefly from multiplying rapidly in the San Joaquin Valley, which is cooler than the desert.
But looking at the gradual spread of the whitefly species worldwide, that theory could prove wrong, scientists say.
The whitefly is believed to have come to the Americas from the Middle East. It has largely been confined to tropical and subtropical areas between the 30th parallels, says Mr. Toscano.
While scientists are uncertain how the new B-strain started, he says, it represents an expansion in the reach of the whitefly species to areas further north and cooler, like California, Arizona, and Texas.
The insect lays waste to plants in a number of ways: by extracting nutrients from the plant; by excreting a sticky honeydew that gums up cotton fiber destined for textile mills and creates a mold that can kill the plant; and by transmitting plant diseases.
Unlike other pests that may slow development or kill a few plants, the whitefly has "the potential to destroy a total crop," says Toscano.
Insecticide is not considered an effective way to control the pest because the chemical falls on the top of leaves, while the bug lives on the underside of leaves, says David Ritter, coordinator of the Imperial County Agriculture Commission whitefly program. Because the insect thrives on so many plants, entire regions would have to be sprayed, he explains.
While state and federal agriculture officials are experimenting with the release of beetles that feed on the whitefly, the main strategy to control the bug at present is to reduce the amount of food available to it.
MANY farmers are not planting a crop at all. Some wait for cooler weather to plant winter vegetable crops. Other growers with the area's most common crop - alfalfa - are drying out their fields, so they aren't attractive to the pest. Agriculture officials also encourage farmers to destroy fields immediately after harvest and destroy weeds and other plants in fallow fields.
As a result, whitefly populations this fall appear to be down .
"We've got it somewhat under control, but it's controlled at the expense of production," Toscano explains. He adds that by "living with the insect" in this way, local farm economies could be reduced by as much as 25 to 35 percent.
Meanwhile, as the pest has altered harvest schedules and volumes, markets have been disrupted, explains Roy Strahm, a third-generation valley farmer.
Melon prices this year have been higher than usual because there was no Imperial Valley crop, he says, but prices for other items were unexpectedly low.
After the valley's 1991 fall melon crop was wiped out by the whitefly, farmers delayed planting their winter vegetable crop. That crop came to market at the same time as crops of northern California farmers, causing a vegetable glut and low prices to consumers. The result, says Mr. Strahm, was devastating losses to farmers here who had just suffered a total melon-crop loss.