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Fine dining for the lonesome screwworm.

Fine Dining for the Loathsome Screwworm

Some 5.3 billion government-reared screwworm larvae went on a diet last year. The savings in their $100,000-a-week feed bill amounted to 25 percent.

Besides saving money, the new rearing system helps the Mexican-American Commission for Eradication of Screwworm produce healthier insects that have a better chance to compete for matings in the wild. The larvae are reared to mature pupae, sterilized by radiation, and dispersed as adults to stop reproduction of their own kind in nature.

By releasing sterile screwworms to mate with the wild population, ARS scientists succeeded in eliminating the screwworm from the United States as long ago as 1966 and more recently in Mexico. Now, thanks to cooperation from Mexican authorities, scientists are working to push their control over this dread parasite ever southward into neighboring Central American countries.

In nature, metallic-blue-green adult female screwworms - about twice the size of the common house fly - lay eggs next to flesh wounds on cattle, sheep, dogs, and other mammals, including humans. When the eggs hatch, the larvae enter the wounds and feed on living tissue.

But in a rearing factory, a diet of living flesh is out of the question. In an effort to simulate the parasites' ghastly natural preferences, scientists have developed other means of sustenance. For years, larvae were supported by acelate fibers as they fed on a liquid diet. Every 4 hours, workers had to vacuum off waste diet loaded with metabolic wastes and refeed with fresh diet.

This labor-intensive system has now been replaced by a gelled diet that incorporates the usual spray-dried bovine blood, spray-dried egg, and milk substitute.

"Larvae thrive on the gelled diet. It enables them to get together to feed, much as they would in nature," says David B. Taylor, an insect geneticist who is now at the ARS Bioscience Research Laboratory, Fargo, North Dakota, where he is looking for a way to genetically separate male from female larvae as a means of cutting costs.

Taylor and colleagues of USDA's Animal and Plant Health Inspection Service (APHIS) and Mexico's Ministry of Agriculture and Water Resources (SARH) conducted the scaled-up pilot tests, rearing 15 million screwworms per week at a factory in Chiapa de Corzo, Chiapas, Mexico.

In the spring of 1990, the entire facility converted to the new rearing method.

Improved efficiency has already offset the renovation cost. With less diet wasted by the new system, there is less disposal. And discarded diet may be processed one day soon into livestock feed supplements or soil conditioners for nearby farms.

Then there's the payoff of more and healthier sterile insects produced from egg to adult. "Within a month after complete conversion to the new system, we had more than 90 percent emergence of adult flies from pupae, as opposed to 85 percent under the former system," said H. Chris Hofmann, plant director.

The factory, which is the world's largest insectary, has recently been producing a quota of about 190 million sterile flies each week. With present floor space, "we could eventually expand operations to produce over 500 million flies per week," says Hofmann.

Factory employees sterilize screwworm flies for air release in Belize and Guatemala and in an area around the production site.

Until ARS entomologists (now retired) Edward F. Knipling and Raymond C. Bushland pioneered the sterile-release strategy in the 1950's, screwworms were the scourge of U.S. livestock.

Since the pest has been eradicated from the United States and Mexico, livestock producers and consumers have saved an estimated $10 billion in the United States and more than $2 billion in Mexico. By continually pushing the frontier of screwworm infestation south, USDA avoids the greater expense of trying to control the pest along the lengthy U.S.-Mexico border.

PHOTO : Screwworm larvae.
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Author:Hardin, Ben
Publication:Agricultural Research
Date:Jul 1, 1991
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