New Mexico agriculture industry, agencies prepare as screwworm flies closer

Two cases of the New World screwworm, a costly pest that has devastated livestock producers and wildlife in the past, were found in northern Mexico in recent weeks, but the fly has yet to make it into New Mexico.

“It’s terrifying,” said Bronson Corn, New Mexico Cattle Growers Association president. “The chances of it ever crossing from the border on livestock is little to none, especially since the border between the United States and Mexico is shut down. But the wildlife aspect of it is quite concerning.”

New Mexico cattle organizations, agriculture agencies and veterinarians are preparing for the fly to potentially journey across the border by educating the public and planning for testing.

The New Mexico Agriculture Department is focused on educating people about the New World screwworm — a fly that resembles the housefly but has parasitic larvae — to create awareness but not alarm, said spokeswoman Jenny Green.

New Mexico State University extension offices throughout the state are equipped with sample kits that livestock producers can use to collect larvae of suspected screwworms. Those samples would be tested by the state entomologist, and any positive tests would be double checked by the U.S. Department of Agriculture.

Educational presentations on the screwworm will be held at extension offices over the winter months, said Tom Dean, Extension Southwest District director.

The United States government has spent millions to keep the screwworm south of the Darién Gap since it was eradicated from the country in the 1960s. The flies lay their eggs in the open wounds or mucus membranes of warm-blooded animals, particularly cattle. The insect can also affect wildlife, pets and, rarely, people. In its maggot stage, the screwworm eats through flesh, burrowing farther into a wound. Left untreated, it can kill a cow in a week.

If screwworm is suspected, the extension service recommends taking photos of the wounds and consulting a veterinarian. Larvae should be removed, the wound cleaned and the animal isolated.

In recent years the species has traveled north. Mexico recorded more than 6,000 cases as of September, Reuters reported.

Two recent Mexican cases were on cattle being transported through and inspected in the state of Nuevo León, which borders the U.S. That is a much different scenario than if the fly itself had migrated that far north, said Dr. Samantha Holeck, state veterinarian with the New Mexico Livestock Board.

“As far as we know, none of the native animals have been found with it in Nuevo León,” Holeck said. Surveillance of the area is ongoing and it will likely be several weeks before it’s clear if the fly has taken hold in that area, she said.

One case of the fly has been found in the U.S. this year on a person who was traveling from El Salvador into Maryland.

Since May, the United States has closed its southern border to cattle, bison and horse imports from Mexico in an effort to contain the fly. The government has also invested $21 million to retrofit a fruit fly sterilization facility in Mexico to produce sterile screwworms, and $8.5 million in a sterile fly dispersal facility in Texas.

The most successful strategy for managing and eradicating the flies has been producing sterile male flies and then distributing them in affected areas. Because screwworm flies only mate once, releasing sterile males causes population decline.

The USDA is planning to build a sterile fly production facility in southern Texas, with the capacity to produce 300 million sterile flies per week. It could take several years before that facility is fully operational, Holeck said. There is debate on that plan because of concerns about bringing the flies into an area that hasn’t been infested, according to Holeck.

“I think additional production of the flies will be critical to controlling it,” she said. “What we’ve got in production right now, my understanding is that it’s only about 20% of the capacity of flies that it took to eradicate it from the United States and Central America the first time around.”

The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service funded an estimated 90% of a Panama-United States Commission to eradicate the screwworm, building a facility in 2006 to release sterile flies. APHIS estimated in 2025 that investing in the sterile fly facility in Panama saved the U.S. cattle industry $2.3 billion annually.

A 1976 outbreak of the flies in Texas cost the state’s livestock producers and economy an estimated $1.8 billion in 2024 dollars, according to a report from the nonpartisan Congressional Research Service.

The species can also devastate wildlife. When Florida had an outbreak in 2016 near its Key Deer National Refuge, there were concerns about the survival of the endangered Key deer.

Holeck expected the screwworm to make it to New Mexico already, and believes good strategies have been put in place to slow it down. But the insect’s spread gets harder to control on a larger land mass.

“It just becomes very difficult to distribute enough of those sterile flies to really stay ahead of it,” Holeck said. “So that’s my biggest concern, is that no matter how good the strategies are, I’m afraid that it’s going to overwhelm all of that effort and still make its way here. So I think it’s important that we be prepared.”