Dropping millions of flies from planes seems like a nightmare; however, according to experts, it is the best bet against a flesh-eating threat that poses a threat to the livestock industry, poised to enter the southwestern border of the US.
An outbreak of New World screwworms— the larval form of a fly that is known to nest in wounds of warm-blooded animals and slowly eat them alive. Most Central American countries haven’t seen an outbreak in 20 years, but an infestation in 2023 has spread like wildfire. Cases have been reported in countries such as Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador.
The fly arrived in southern Mexico in November, leading to concern among US agricultural industry officials. This triggered the closure of several border-area cattle, horse, and bison trading ports. This isn’t the first time the US has had to battle these invasive pests. The nation eradicated the New World screwworm population in the 1960s and 1970s. The US did this by breeding sterile males and dropping them from planes to mate with wild, female flies. The strategy gradually reduced the bug population by preventing them from laying additional eggs. As the insects continue to spread north, experts hope that the approach may work again.
However, only one facility in Panama breeds sterilized New World screwworms for dispersal today, and hundreds of millions more sterile flies are needed to gain control over the outbreak. The US Department of Agriculture has announced plans to make a “fly factory” in a yet-to-be-decided town near the Texas-Mexico border. The battle against screwworms won’t be easy, quick, or inexpensive.
Parasitic larval forms of a metallic blue blow fly species are New World screwworms. Unlike all other blowfly species living in the Western Hemisphere, the New World screwworm feeds on the flesh of living animals, rather than dead ones. The flesh-eating maggots attack and feed on warm-blooded animals, such as horses and cows. In rare cases, they have been recorded infecting humans and domestic pets. After mating, the female fly finds a live host and lays 200 to 300 eggs in their wounds. Those eggs hatch in 12 to 24 hours and immediately start feeding on the flesh, burrowing deeper into the animal’s tissues and forming large wounds. After feeding on the tissue for several days, these larvae drop from the animal’s body and burrow deep into the ground. Later on, they emerge as fully grown adult flies.
There have been more than 35,000 New World screwworm infestations reported since the beginning of the outbreak, according to statistics listed on the Panama–United States Commission for the Eradication and Prevention of Screwworm Infestation in Livestock (COPEG) website. Cows make up about 83% of the affected animal species.
Treatment for infested cattle often involves antiseptic treatment, cleaning, and covering the wounds. If left untreated, screwworms can kill an animal in a matter of one to two weeks and spread to others. Daily monitoring and inspection of livestock is necessary to confirm that there are no infections. There are no vaccination methods or effective repellents against screwworms. Instead, ranchers should avoid branding, tagging, and other procedures that create potential entry points for the screwworms in livestock during warmer months. These tropical flies are less active during winter. While regional cattle is a major way by which these parasitic larvae spread and travel, animals like deer and rodents can also carry these infectious critters, making surveillance challenging.
Much like how a caterpillar goes into a cocoon before becoming a butterfly, the New World screwworm becomes a black, pill-sized pupa before growing up and emerging as an adult fly. The pupae are subjected to high-energy gamma rays in a sterile fly production facility, which breaks down the DNA of the males, damaging their sex chromosomes. When these sterile, impotent adult flies mate with females, they lay unfertilized eggs. Since the female flies mate only once in their short, 20-day lifespan, once these populations are exposed to sterile males, the populations die out in months or years, depending on the size of the outbreak.
While the details of dispersal are unclear, typically, adult flies are loaded into temperature-controlled containers and released from planes in sparsely populated, rural areas.
About 100 million sterilized flies are produced and dispersed aerially in affected regions each week in the COPEG facility. At this point, the dispersal attempts have been focused on the southern areas of Mexico and throughout Central America, where cases of infection have been reported.
The new dispersal facility is expected to be at the Moore Air Base in Hidalgo County, Texas, and is estimated to cost $8.5 million, per the release. Lawmakers estimate that the price tag for the production facility, also known as the “fly factory,” will cost around $300 million. USDA also has a $21 million plan to renovate an old fly factory in Mexico. Although the plans are expensive, it is a price worth paying to save the multi-billion-dollar livestock industry.
“When you offset the $300 million to the $10 billion of economic impact these flies would have, it’s an easy trade-off to understand,” Diebel, a rancher and the first vice president of the Texas & Southwestern Cattle Raisers Association, said. “Having (a domestic production facility) here is super important … to control the distribution of those sterile flies more efficiently.”
USDA also plans to reopen livestock trading ports in Arizona, Texas, and New Mexico that closed last year.
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