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Under a blazing sun bearing down on the Florida peninsula, Silvia, a Guatemalan immigrant, labored in the staked-down tomato fields that stretched into near-infinity, patrolled from above by ragged gangs of black vultures.

For female migrant workers, the rows could be dangerous places, with threats ranging from sexual harassment and groping to outright assault. As farm laborers, they have few options if they get fired, says Silvia, speaking through a translator.

“Bosses who would have more power to force you to go out with them and pay you more to do it ... that happened to me,” says Silvia.

Sexual harassment and assault has for decades been part of a massive, market-driven, exploitation of largely Mexican, Haitian, and Guatemalan migrant workers. A decade ago, US attorney Doug Molloy called Immokalee and its vast corporate tomato fields the “ground zero” of human slavery in the United States. According to reports and lawsuits, abuses against both legal and illegal migrants ranged from beatings and death threats to wage theft, from illegal trafficking to unhealthy exposure to farm chemicals, as well as outright slavery and rape.

Such conditions, added to the threat of deportation, underscore how the migrant laborer in the US today remains at once the most critical as well as vulnerable player in a US agricultural sector valued at $226 billion, according to the World Bank.

Farm worker exploitation, nationally, remains routine – despite strengthened congressional protections this year. Yet for most of Immokalee’s field hands, blatant wage abuses and physical cruelty have largely – and almost suddenly – vanished from the tomato rows.

A decade-long farm worker-led effort to push corporations such as Walmart and Yum! Foods to demand farmers submit to “clean labor” audits yielded, for the first time, the introduction of shade tents, mandated water and bathroom breaks, and a “penny per pound” bonus that has distributed $22 million so far directly back to tomato pickers over four years.

That can amount to $80 a week for many workers – basically, an extra day’s pay per week. Ninety percent of corporate farms that dot the Florida peninsula now submit to the audits, which are performed in the field under the supervision of a retired state judge.

“In the past three years, [the tomato fields in Immokalee] have gone from being the worst to the best,” according to Susan Marquis, dean of the Pardee RAND Graduate School in Santa Monica, Calif.

Rich Clabaugh/Staff

Ethics in the produce aisle

But if Immokalee shows how quickly and dramatically supply chain pressure can curb labor abuses, historic conditions here are only a sliver of an exploitation problem that fills the American shopping cart, says former Immokalee farm worker Nely Rodriguez, who, along with Silvia, now works with the Coalition of Immokalee Workers. The goal now, says Ms. Rodriguez, is to “expand the program to Georgia, Maryland, Virginia, Alabama – to every sector where farm workers are struggling.”

In North Carolina, for example, 39 percent of the state’s 150,000-strong farm workers report being illegally trafficked or otherwise abused, according to the June 2014 issue of the International Journal of Rural Criminology.

As a Haitian migrant picker named Peterson says, one only needs “eyes in the field” to see the difficult conditions. Peterson, who is using a pseudonym, holds out a paycheck for a recent day’s work in Florida, showing he made $69.77, after taxes, for a 13-hour workday in the cucumber fields. That’s even though he technically makes $8.50 an hour, the state minimum wage.

To that end, the CIW and other farm worker groups, including The Farmworker Alliance of Central Florida, are going beyond picketing big corporations to instead leverage US consumers directly to use the Immokalee project to force reform more deeply into the darkest corners of the American cornucopia.

A certified “Fair Food” label for produce that is grown and packed ethically is part of a nascent “clean labor” movement that spans from the pepper fields of South Florida to small organic farms in California’s valleys. Immokalee tomatoes certified as “clean” from labor abuses are available at Whole Foods stores throughout much of the country. This fall, Giant and Stop & Shop in New England began carrying the label. Similar labels such as “Food Justice Certified,” which expands farm worker protections to organic products, are also beginning to crop up from New York to California.

Such sourcing clues can be a powerful initiative, experts say, given that the labels tap directly into a US consumer base that has begun to turn once routine shopping decisions into moral guideposts.

“The label allows personal judgment: What are you willing to tolerate and what are you willing to ignore?” says Mr. Molloy. “And that’s a difficult question to avoid when the product is right there literally in folks’ shopping cart.”

Indeed, the popularity of “fair trade” coffees and chocolate suggests that a certain portion of US consumers do, to at least some extent, attach a value to purchases that benefit farm workers, whether in the US or overseas.

“A tomato is a tomato, so I’d pay a little more” to make sure workers are treated fairly, says Angela Carter, a young Atlanta woman exiting a Kroger supermarket in the city’s Edgewood neighborhood.

While the label strategy may not be a guaranteed success, over the past decade, the Immokalee workers so far have managed nothing less than “revolutionary changes in the structure of the agriculture sector,” says Rutgers University labor economist Janice Fine.

And labor experts see a lot of promise in using labels as a tool to spread the reforms seen in Immokalee to other agricultural centers around the US.

“Labeling is a way of asserting the ability of consumers to act according to moral values that they want to implement in their lives,” says Alan Rubel, a philosophy professor at the University of Wisconsin in Madison, who specializes in information ethics and policy. “The label tells consumers the way in which their food would fit with their moral views.”

But challenges remain in making sure that promise is realized.

Some supermarket chains, including Kroger and Publix, have balked, claiming they can already assure shoppers that their vegetables are grown without exploitation. (Farm worker groups disagree.) Moreover, some market experts like Mr. Rubel worry that a proliferation of value-guided labeling will dilute their impact. And the idea of certifying products for ethical labor practices is a different proposition than, say, organic or non-GMO labels that serve primarily as family health guides.

For Yesica Ramirez, a former field hand who now works for the Farmworker Alliance in Apopka, Fla., the labels present another paradox, given higher prices that come with certified “clean labor” products.

“People who work the fields are consumers, too, and they would be impacted more if food is more expensive,” says Ms. Ramirez. “We are changing, we can do it, but it’s still a challenge.”

Luis M. Alvarez/AP/File Farmworkers pick tomatoes at Taylor & Fulton Tomatoes in 2006, in Immokalee, Fla. Over the past 10 years, the tomato fields of Immokalee have gone from being called “ground zero” for human slavery in the US to among the best in the country, experts say.

‘Pig pens filled with humans’

Exploitation of farm labor has a long, sobering history in the United States, running all the way back to slavery. In the 20th century, the US imported foreign labor from Mexico under the brutal “bracero,” or “strong arm,” program, that began after World War II and ran until the 1960s. And despite endless documentation – as well as farm worker rebellions like the Delano Grape Strike in California in the late 1960s – corruption and abuse remain entrenched.

As early as 1940, the journalist Ernie Pyle bemoaned conditions in Immokalee, where he described “pig pens filled with humans.” Given decades of such reports, the US Congress strengthened protections with laws such as the Migrant and Seasonal Agricultural Worker Protection Act of 1983 and the more recent Justice for Victims of Trafficking Act of 2015.

But according to US prosecutors, farm labor exploitation, even more so than sex trafficking, is a persistent and largely invisible crime, hidden amid grungy labor camps and behind ominous “no trespassing” signs at field gates. Exploited workers often don’t know their rights, and even if they do, are often afraid to assert them, adds Ramirez, the former Apopka farm worker.

Only about 80,000 of an estimated 1.3 million migrant workers have farm, or H2, visas. And even visa holders are often loath to complain about abuses, given that their working papers tie them directly to a single employer, often a farm labor contractor, who can send them home at will.

Recent farm worker abuse cases include a 2014 federal lawsuit that alleged that an Oakley, Idaho-based Christmas tree grower forced five Hispanic visa holders into horrific working conditions in the Sierra Nevada Mountains in California. Threatened with death of they didn’t keep working, the men claimed they were fed rotting food, forced to drink unfiltered river water, and endure rashes from the chemicals they were told to handle.

In July, a federal court in the Northern District of Ohio unsealed a 15-count superseding indictment “charging three defendants with luring Guatemalan minors and adults into the United States on false pretenses, then using threats of physical harm to compel their labor at egg farms in Ohio,” according to a Justice Department press release.

Other cases have involved tomato pickers locked in box trucks under armed guard and first-graders laboring in Michigan blueberry groves. At the same time, lawsuits documenting wage abuses also have increased, although the numbers are still small.

The number of pending cases involving farm labor abuse in front of the Equal Employment Opportunity Commission rose from 1 in 2010 to 10 in 2014.

Profound market pressures, many of these cases show, can make even former farm workers complicit in labor abuse.

The road from farm to table is a long one, with many middlemen. Downward pressure by corporate food forces – and ultimately demanded by US consumers looking for lower prices – has squeezed margins for every layer, especially the foreign migrants in the fields.

The farm-to-table “supply chain is there to protect the party above. In agriculture, it’s really hard to get a labor force, because it’s very hard work,” says Professor Fine. “Prosecutions have showed that the traffickers were often the farm labor contractors, who controlled the housing, the food, the pay, and supervised the worker in the field.”

Melanie Stetson Freeman/Staff Crew chief Francisco Dominguez stands by the bus he uses to transport his team of farm workers out to the tomato fields, on Sept. 24, 2015, in Immokalee, Fla.

‘Mostly, we take care of each other better’

Francisco Dominguez is a farm labor contractor, albeit a new and different breed.

Today, his colorful school bus – where workers’ rights papers are affixed next to the door – is followed by a pickup truck pulling Porta-Potties – a distinct shift from past practices, when women especially would not drink water during scorching days so as not to have to take a bathroom break, and be docked pay – or lose their job.

The transformation of shadowy labor contractors to farm-hired crew chiefs has turned out to be the most fundamental shift in the fields, labor economists say. But it is also representative of the broader “Fair Food” strategy: Push for specific concessions demanded by workers who witness actual abuses, unshackled from more academic pleas for social justice.

In Mr. Dominguez’s case, he was one of the original Florida farm workers to agitate for changes in the 1990s, particularly in the safety of orange pickers. “They had the wrong ladders, so many people would fall off and hurt themselves, even get killed,” he said. Those early efforts had at least one measurable impact: Workers were given ladders that were safer.

Another example shows how difficult it can be to get even small reforms on the farm. Only recently did farm workers win a concession they’d been demanding for years: That they be allowed to fill tomato baskets only to the brim, not piled as high as they would go. Since workers are paid in part by how many baskets they pick, extra-full baskets represented a 10 percent extra margin for farmers – money that now goes to the workers.

Another crew chief, a burly-chested man named Jaime Zamora, stands in a salt-stained shirt behind a truck in front of a popular Immokalee taqueria. Free-running chickens peck at the gravel at his boot-laced feet. He runs his crew of melon pickers around south Florida and all the way to up to Cordele, Ga., where he says he puts them up in a motel “so they can watch TV and relax” after a day in the field.

“A lot of things have changed,” he says. “Mostly, we try to take care of each other better.”

Melanie Stetson Freeman/Staff Nely Rodriguez works at the food cooperative at The Coalition of Immokalee Workers headquarters, a nonprofit that fights for worker rights, on Sept. 24, 2015 in Immokalee, Fla. The group is putting fair food labels onto produce from farms that have signed onto providing good labor practices for its agricultural workers.

US and slavery 'should never be in the same sentence'

But there are complexities to US farm labor policy that still give Immokalee a fortress-like air. Reporters are often barred from the the farms by threats of felony convictions, meaning it’s difficult to independently confirm assurances that field conditions have improved. Moreover, the partnerships between farm worker groups such as the Coalition of Immokalee Workers and multinational cucumber and tomato growers remains a tenuous one, given the market and cultural realities at play.

Historically, farm worker exploitation has been overshadowed by larger immigration politics in the US. For Americans to understand labor trafficking and rectify abuses remains a tall order, given the vast cultural chasm that spans between the average American (including law enforcement) and farm worker communities.

Mr. Molloy, the former US attorney for South Florida, made prosecuting slavery cases his legacy.

One case that stands out is a young woman who was being held as a slave on a farm – unpaid and unable to leave. Her unusual religious convictions turned out to be a key to solving the case. The girl explained that she felt trapped because her boss had clipped a lock of her hair and kept it in a vial around his neck. When the man was booked, Molloy asked to see the possession envelope. Sure enough, there was the girl’s hair in a vial – a piece of evidence that led to his conviction.

In Molloy’s mind, the “fair food” label faces a challenge similar to the one he had persuading American juries that they should care about what happens to exploited and abused farm workers like the girl with the vial of hair.

“There’s an educational aspect to it, but, in this political time period that we’re in, when we’re hopefully trying to honestly sort out our feelings and policies on illegal immigration, it’s important to remember that this is different than it’s being portrayed,” says Molloy. “These [farm workers] are not scavengers coming in to steal jobs.”

Success is possible, he says, largely by appealing to a sense among Americans “that the words 'United States' and 'slavery' should never be in the same sentence.”

[An earlier version of this story implied that Silvia had been assaulted, which CIW denies.]

Oct. 26: Modern slavery: Labor trafficking is everywhere and nowhere

Nov. 2: Trafficking: One woman's journey from Staten Island slavery to her own boss

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Nov. 9: From England's pews, a quiet abolitionist finds his voice on slavery

Coming Nov. 23: How technology is freeing victims of trafficking