Why US plans to drop millions of flies from planes

New World screwworms, the larval stage of Cochliomyia hominivorax, are not your average maggots. Unlike most blow flies that feed on decaying matter, this species eats the flesh of living warm-blooded animals. The parasite lays eggs in open wounds, and once hatched, the larvae burrow into the host’s tissue with sharp mouth hooks.

“After mating, the female fly finds a living host, lands on its wound, and will lay up to 200 to 300 eggs,” said Dr. Phillip Kaufman, head of the entomology department at Texas A&M University.

Though cattle are the most common victims, the parasite can also infect wildlife, pets, and, in rare cases, humans. If left untreated, infestations can kill animals within a week or two. Since early 2023, over 35,000 infestations have been reported across Central America, mostly in cows, as reported by CNN, citing data from the Panama–US Commission for the Eradication and Prevention of Screwworm (COPEG), and as reported by CNN.

A worker holds two small containers of New World Screwworm fly pupae at a facility that breeds sterile flies in Pacora, Panama. (COPEG via AP)

Fighting flies with flies

The approach to stopping the screwworm relies on mass sterilisation. In specialised facilities, screwworm pupae are exposed to high-energy gamma rays, rendering the emerging males sterile without harming their ability to mate.

“The females only mate once in their lifetime, which makes the technique highly effective,” Kaufman explained.

Once released — typically from planes flying over rural areas — the sterile males compete with wild ones, reducing the number of fertile matings. Over time, the population collapses.

Currently, only one facility exists for producing sterile screwworm flies — located in Panama. But with the outbreak advancing northward, US lawmakers and agricultural leaders say more production capacity is urgently needed.

New fly factory in Texas

In response, the USDA has announced plans to open a new facility at Moore Air Base in Hidalgo County, Texas. The production facility, still pending final location and design, is estimated to cost up to $300 million. Additionally, $21 million has been allocated to revamp an older facility in Mexico by late 2025.

While the screwworm is most active in warm months, ranchers are being advised to avoid branding or tagging animals during that time to reduce the risk of infestation.

There are currently no vaccines or reliable repellents.

(With inputs from CNN)