The United States has a plan to combat screwworm. It involves a lot of flies

Flesh-eating parasite The fly that poses a major threat to livestock has returned to the United States after 60 years. This week, the USDA confirmed the presence of New World screwworm in a calf in South Texas.

It was eradicated in the United States in 1966, then as far south as Panama by 2006, and recently resurfaced in Mexico, making it likely that the screwworm will eventually re-enter the country, with models showing it could arrive as soon as the summer of 2025. It took a little longer, but the screwworm has arrived. In order to avoid an outbreak, officials are using a tried-and-true technique: releasing lots and lots of adult screwworm flies.

Screwworm infection occurs when female flies lay eggs in open wounds or other parts of the body of warm-blooded animals. When the eggs hatch, the worms emerge and feed on living tissue before turning into flies. As adults, screwworm flies do not bite or feed on meat. Scientists in the 1930s and 1940s believed that if they could prevent female flies from reproducing, they could break the cycle. At that time, New World worms were killing hundreds of thousands of livestock annually, mostly in the American South and Southwest.

In the 1950s, researchers at the USDA made a breakthrough when they applied radiation to male screwworms and rendered them sterile. When released into an infested area, sterile males mate with wild female insects and produce non-viable eggs. No offspring are produced, and the population is stunted. This technique is known as the sterile insect technique, and was first used successfully on the island of Curacao, off the coast of Venezuela. It took only seven weeks to eradicate this pest, and these efforts saved the island’s goat herds, which were a vital source of food.

This technique takes advantage of the fact that female New World screwworm flies mate only once in their lives. “The sterile insect technique is perhaps the most eloquent example of a completely successful biological control mechanism,” says Sally DiNuta, an assistant professor of veterinary medicine at the University of Florida. “The life cycle stops. No offspring are produced. It has been very successful.”

For many years, the dense stretch of rainforest between Panama and Colombia known as the Darien Gap served as a biological barrier where sterile flies were released to prevent the spread of screwworm northward. But insects began to break through the barrier in 2022.

To prevent an outbreak in South Texas, the USDA has closed off an approximately 12-mile zone around the infected calf and is conducting targeted releases of sterile screwworm flies from trucks. This is in addition to the 4 million sterile flies dropped by air every week in the region. In anticipation of the screwworm’s northward movement, in February, the agency shifted its efforts to disperse 100 million sterile flies per week to focus on the area along the U.S.-Mexico border.

“Although this development represents a serious threat to our livestock and wildlife, it does not surprise us,” USDA Secretary Brooke Rollins said during a House Agriculture Committee meeting Thursday.

She said about 400 million flies were needed weekly to overcome the screwworm. Currently, the United States can only produce about 100 million flies per week at a facility in Panama.

A sterile insect facility in Mexico closed in 2012, but the USDA is investing $21 million to help renovate and convert an existing fruit fly facility in Mitapa, Mexico, to produce an additional 60 to 100 million sterile flies per week. The facility is expected to be operational this summer, according to the USDA.