It was never a matter of if the New World Screwworm would burrow its way back into American cattle, but when.

According to the modeling, it should have happened last summer. The first case was confirmed June 3 in south Texas.

With the closest case now in New Mexico, some 400 miles from the Arizona border, ranchers and dairy producers are preparing for a fight.

“Head in the sand isn’t going to work,” said Pima County Farm Bureau president Sarah King, who manages her family’s 55,000-acre cattle ranch near Tucson. 

“I don’t think anyone’s saying, ‘Oh, it’s already in the U.S., so we can just give up.’”

Screwworms are painful and costly

Screwworms pose no immediate threat to our food system.

They target living tissue, not fruit, vegetables or cut meat. Plus, officials say, any infected flesh would be detected during rigorous food safety inspections.

But a single female screwworm fly can lay up to 3,000 eggs in the few weeks that she is alive.

Once deposited, those eggs hatch within hours and grow into maggots that burrow deep, causing painful, foul-smelling injuries.

Livestock is a favorite target, but the parasitic insects can also infest wildlife, pets and, rarely, humans. Cases can be fatal if left untreated.

And while mass euthanasia is unlikely, even short-lived outbreaks could strain ranchers, who are already struggling to rebuild historically small herds.

A 1976 outbreak that affected 1 in 5 cows in Texas was estimated to have cost producers more than $770 million in today’s dollars.

How the USDA has prepared

Plans to contain this pest began taking shape nearly two years ago.

The United States Department of Agriculture (USDA) has spent hundreds of millions of dollars to accelerate production of sterile flies, which helped eradicate the pest from the U.S. and Mexico decades ago.

Female screwworms mate once during their lifetimes, and if they mate with sterile males, their eggs won’t be viable, crashing the population over time.

Officials also closed the southern border more than a year ago to cows, horses and bison, a move that has improved beef prices for Arizona ranchers but also imperiled feedlots that depend on young cattle from the Mexican state of Sonora.

There are no immediate plans to reopen those ports.

Meanwhile, millions of sterile flies were released within hours of the first screwworm confirmation, following a federal playbook that had been set in advance.

Teams set up a 20-kilometer “infested zone” to quarantine livestock and pets, as well as a wider surveillance zone to monitor flies and wildlife for signs of infection.

Most crucial now: Daily surveillance

Animals in these zones can still move to market or other producers if they pass an inspection and receive a permit.

But Canada has since blocked the import of livestock from infected areas, and Mexico has suspended cattle, horse and other animal imports from the wider U.S. 

Florida also enacted emergency measures blocking animals from infested zones and requiring inspection certificates of animals in an any state with a confirmed screwworm case.

Should an infestation occur in Arizona, the state Department of Agriculture intends to follow the federal playbook.

“But the biggest thing now is surveillance and vigilance,” said Ryan Wolker, the state veterinarian, who has worked close on its response. “The more eyes we can get on animals, the better.”

Because screwworms can work into even the tiniest cuts and scrapes, it’s important to watch daily for white eggs near umbilical areas, brands and castrations, as well as in ears, eyes and mouths. 

Enlarging or draining wounds and signs of discomfort are red flags.

Livestock producers can request free testing kits from their veterinarian, local cooperative extension office or the state’s Animal Services Division.

Arizona ranchers are prepping for change

Arizona dairies should have no trouble increasing inspections, said Jim Boyle, a Casa Grande dairy producer. Cows are already milked twice a day.

But daily inspections could prove more difficult on a sprawling cattle ranch.

“A trail camera or drone can only reveal so much,” King, the Tucson rancher, said. “A lot of this is going to require people, and that’s labor. It comes with a cost.”

And likely, long-term changes.

Arizona ranchers expect to spend more on veterinarian care. Some are considering which treatment medications to keep on hand.

Others are deliberating whether to delay branding and castration procedures until the summer fly season is over. Or whether to remove excess vegetation around remote water tanks to disrupt screwworm habitat.

“Better management practices will emerge,” rancher and Greenlee County Farm Bureau president Ty Kelly said. “We’ll see what they do in Texas and learn from that.”

Sterile flies are needed to win this fight

Meanwhile, ranchers say, sterile fly production must remain a priority.

This critical infrastructure dwindled once screwworms vanished decades ago, leaving only a fraction of the flies needed to eradicate them now.

Many are lobbying to rebuild a sterile fly distribution center in Douglas that was active until the 1980s.

And they are closely watching the research behind this fight, including a newly developed male-only sterile fly strain that could further speed eradication.

For now, sterile females are released alongside sterile males. If only sterile males are released, that effectively doubles the chances of wild female eggs never hatching.

“We’re moving in the right direction, but it all comes down to funding,” said Tim Petersen, the Yavapai County Farm Bureau vice president and a Camp Verde meat processor who also manages cattle ranches across the state.

“If there’s no funding in it for the long term, we won’t develop the tech we need.”


Joanna Allhands writes about water, land use and other issues important to the Arizona Farm Bureau. Reach her at joannaallhands@azfb.org.