Frozen Fly Eggs Thawed, Reared To Adulthood

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.

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