Transgenic screwworms developed by
Agricultural Research Service (ARS) scientists could set the stage for
new, improved methods of eradicating the pest based on the sterile
insect technique (SIT).  

Pioneered by ARS entomologists nearly 55 years ago, the SIT is a
cornerstone of eradication programs implemented worldwide to control
not only the screwworm, Cochliomyia hominivorax, but also the
Mediterranean fruit fly, tsetse fly and other insect pests. By one
estimate, screwworm eradication efforts today save U.S. livestock
producers at least $900 million annually in potential losses.

The SIT involves sterilizing adult male flies with irradiation and
releasing them into the wild to mate with females. Their eggs' failure
to hatch diminishes the size of the next generation. Fewer flies, in
turn, mean fewer insecticide applications to protect livestock,
especially those with open wounds, where screwworm larvae feed.

But irradiating screwworms is costly. Irradiated male flies are also
less competitive than wild-type males. So, starting in 2004, the ARS
team--entomologists Margaret Allen and Steven Skoda and geneticist
Alfred Handler--began research aimed at developing genetically sterile,
male-only screwworms using transformation technology first tried on
Medflies, also targets of SIT-based eradication. Allen is at the ARS
Biological Control of Pests Research Unit in Stoneville, Miss.; Skoda
is a research leader with the ARS Livestock Insects Research Laboratory
at Kerrville, Texas; and Handler works at the ARS Insect Behavior and
Biocontrol Research Unit in Gainesville, Fla.

Using a genetic element called a "piggyBac transposon" as a vector, the
researchers introduced a green fluorescent protein (GFP) gene into the
genomes of eight screwworm strains. When viewed under ultraviolet
light, the transgenic screwworms emitted a fluorescent glow, helping
confirm GFP's activation. Caged mating experiments showed transgenic
male flies were as competitive as wild-type males, the team reports in
the journal Medical and Veterinary Entomology.

Once male-only screwworms are developed using the same transformation
method as that used for the GFP strain, the next phase would explore
inducing genetic sterility in the flies, which theoretically would
eliminate the need for irradiation. Their field release, however, would
hinge on an environmental impact assessment and regulatory approval.

ARS is the principal intramural scientific research agency of the USDA The research supports USDA's Animal and
Plant Health Inspection Service, which works with Mexico and Panama to
keep screwworms out of Central America.