October 15, 2020

A decadeslong sterile fly breeding and irradiation program eradicated screwworms from North and Central America and maintains a barrier against them in Panama.

Genetic modifications may improve the efficiency of such control methods, allowing further eradication campaigns in the Caribbean and South America. In a presentation Aug. 21 during the AVMA Virtual Convention 2020, Max Scott, PhD, a professor of entomology at North Carolina State University, described genetic modifications used to create all-male screwworm fly populations, rather than the current crops of sterile flies that more often mate among one another than with the wild flies targeted.

Screwworms are devastating parasites for livestock, and they infest a wide range of warm-blooded animals. Screwworm flies lay eggs on wounds or orifices, and their larvae bore into living tissue.

By raising and irradiating screwworm pupae, the U.S. government produces about 2 billion sterile screwworm flies each year to maintain a barrier against the flies’ entry to Central America. Starting in 1957, the U.S. and partner governments used that technology for a series of eradication campaigns that eliminated the flies from North and Central America.

Female screwworm flies mate only once, so the campaign worked by introducing sterile male flies to compete with wild ones. But Dr. Scott said the breeding program lacks a method to separate mass numbers of flies, so the batches contain sterile females and sterile males that mate with one another more than with the wild flies targeted in the eradication campaigns.

Releasing only male flies would make the campaigns upward of 10 times as efficient, he said.

Using sequence-binding domains from a bacteria and a virus, Dr. Scott’s research team created a switch upstream from a protein essential for female development in screwworm flies. That switch activates in the presence of tetracycline or its derivatives, such as doxycycline, allowing female flies to live only as long as their diet includes the drug, which is fed to batches for breeding and withheld from batches for distribution.

The modifications are in testing with partners from the U.S. Department of Agriculture, and further research could make all female flies develop into males.

Screwworm flies briefly reinfested the U.S. in 2016 and early 2017 during an outbreak among wildlife in the Florida Keys. USDA officials said the eradication effort involved the release of 154 million sterile flies, along with 17,000 animal inspections and 700 hours of surveillance.