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Close-up of screwworm larva: Link to photo information
Tusklike mandibles protruding from the screwworm larva's mouth rasp the flesh of living warm-blooded animals. A wound may contain hundreds of such larvae. Click the image for more information about it.

Entomologist Roger Leopold places screwworm embryos in a liquid-nitrogen storage unit: Link to photo information
Entomologist Roger Leopold places screwworm embryos in a liquid-nitrogen storage unit to be transported to the germplasm unit in Fort Collins, Colorado. Click the image for more information about it.

Frozen Fly Eggs Thawed, Reared to Adulthood

By Jan Suszkiw
February 4, 2005

Adult screwworm flies reared from frozen eggs now ensure the continuation of laboratory colonies of these parasites critical to research and sterile-insect releases, Agricultural Research Service (ARS) scientists report.

Considered the backbone of screwworm eradication, sterile-insect releases (SIR) helped eliminate the parasites from the United States and Central America. Prior to eradication, completed in 1966, screwworms cost the U.S. cattle industry hundreds of millions of dollars in losses annually. Screwworm maggots cause harm, and sometimes death, by eating the flesh in open wounds of livestock, people and pets.

In the SIR approach, male and female screwworms are mass-reared and sterilized so that when released, their matings with wild flies produce no offspring. Freezing screwworm embryos eliminates the need to continuously rear colonies of the flies, especially if there's no immediate need for them, according to Roger Leopold, an entomologist with the ARS Red River Valley Agricultural Research Center in Fargo, N.D. Storing frozen, or cryopreserved, eggs costs pennies per day, compared with expenses of thousands of dollars per month to maintain the caged adult flies.

Cryopreservation involves several steps, but culminates with the eggs' immersion in liquid nitrogen at minus 196 degrees Fahrenheit. Scientists thaw the eggs and revive them using fetal bovine serum. Eight to 12 hours later, larvae emerge and are reared to adulthood on an artificial diet.

In August, Leopold and entomologist Dennis Berkebile, with the ARS Midwestern Livestock Insects Research Unit in Lincoln, Neb., cryopreserved and transferred 25,000 fly embryos--representing 10 key screwworm strains--from Lincoln to a backup repository at the ARS National Center for Genetic Resources Conservation in Fort Collins, Colo.

ARS interest in cryopreservation stemmed from concern that, without a backup system, entire screwworm colonies could be lost to calamities such as fire or disease outbreaks, derailing years of research aimed at preventing the parasite's reintroduction.

Read more about the research in the February 2005 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.