McAllen, Texas (CNN) For decades, tons of fresh produce have passed through the McAllen Produce Terminal Market -- a critical gateway connecting Mexican fruits and vegetables with US kitchens and restaurants.

But that rapid flow of commerce is slowing down, as U.S. Customs and Border Protection personnel get diverted to handle an influx of migrants. That means truckloads of produce are languishing for hours or days near the border.

Now, with President Donald Trump's threat to close the border , business owners who are already suffering delays worry they'll go out of business.

"In three days, I lost between $800 and $1,000," said Oscar Corral of Perlag Imports.

Oscar Corral said two shipments of produce he was expecting didn't show up on time, costing him $800-$1,000.

Corral was expecting three truckloads of cilantro and lettuce Saturday, but only one shipment arrived. The rest came two days late.

"If the delays escalate to a closure, I will likely go out of business."

Federico Vera agrees. He said he saved for 20 years to achieve his dream of becoming a small business owner. He now runs Fedevera Produce, importing onions, tomatoes and avocados.

Vera said he's suffered delays getting his inventory and worries about the future.

"In a week or two, I'm going to start losing money," he said.

What's behind the big slowdown

The massive delays stem from Customs and Border Protection redirecting 750 officers from US ports of entry to "help Border Patrol agents care for migrants," the agency said Wednesday.

"CBP officers will be helping with processing, transportation and hospital watch," it said.

"There will be impacts to traffic at the border. There will be a slowdown in the processing of trade. There will be wait time increases in our pedestrian and passenger vehicle lanes ... but this is required to help us manage this operational crisis."

CBP's Laredo sector, which includes the McAllen area, isn't saying how many officers there have been reassigned. But it said fewer truck lanes are open due to the reassignments.

Truckers and produce face uncertain future

Javier Ruiz Rojas has brought truckloads of broccoli, tomatoes and lettuce to the US for 19 years.

It normally takes him 10 hours to drive from Guanajuanto, in central Mexico, to the US border near McAllen. But once he got to the border this week, it took him another eight hours just to cross it.

"It's just too long of a wait," Ruiz Rojas said.

Business owners at the McAllen Produce Terminal Market fear going out of business if there's a border closure.

Jorge Zamarripa's wait at the border has tripled -- from two hours to over six hours. He doesn't get paid for the extra hours.

But it's not just truckers who are suffering. Zamarripa said American consumers who want fresh produce could be affected, too.

"The produce gets old very quickly -- like tomatoes, cucumbers and green peppers. Those are products that need to be taken out very soon," said Zamarripa, who picked up his load from the Mexican city of Reynosa.

If the delays get much worse, such produce "won't make it to Illinois, Missouri or even Canada."

Zamarripa said he wants Trump to fix the mass slowdown, "and fix it quickly."

"If he fixes it, produce will arrive fresh to American kitchens," Zamarripa said.

What a border closure could mean

Closing the US-Mexico border "would inflict severe economic harm on American families, workers, farmers and manufacturers across the United States, the US Chamber of Commerce said.

"US trade with Mexico exceeds $1.7 billion daily, and nearly half a million people legally cross the southern border every day as workers, students, shoppers, and tourists," said Neil Bradley, executive vice president of the Chamber of Commerce.

"Even threatening to close the border to legitimate commerce and travel creates a degree of economic uncertainty that risks compromising the very gains in growth and productivity that policies of the Trump Administration have helped achieve."

Corral, who's already lost about $1,000 in three days, said he has a message for Trump:

"The US can't supply (all) its own produce," he said. It relies on Mexico.