TERESA VEGA'S first son was two when a flood carried rubbish, dead animals and disease through the canals of Oaxaca, her desperately poor home state in southern Mexico. The boy started vomiting, got diarrhoea and ran a fever. There was a doctor a few hours' walk away, but Ms Vega and her husband, Marco Lopez, had no money to pay him. They could do nothing, she says. They watched their son die.

Ms Vega now says this event is the reason for everything she and her husband have done since. When they had another son, Erminio, they decided that they had to make money in case he also fell ill. But Oaxaca offered them no jobs, save for a bit of maize-harvesting every July. Teresa's younger brother Felix had already left for America to find work in California's fruit and vegetable fields. In 2005, seeing no alternative, Ms Vega and her husband set out to follow.

Little Erminio would not have survived the journey, so Ms Vega and her husband had to leave him behind, in the care of Mr Lopez's father. Erminio was one at the time. That was the last time Ms Vega saw him. Now 26, though she looks a decade older, she knew she was running another risk, because she was seven months pregnant again. But she and her husband made their way north nonetheless. Then came the crossings.

The crossings—invariably plural, because most attempts fail, leading to deportations and renewed attempts—are a seminal event in virtually all the stories of the undocumented farmworkers who labour in America's fields. The border is their threshold and their first glimpse of El Norte, the promised land in the north.

But for la migra, as they call America's immigration and border officials, it's “like catching deer,” says Felix. He and his wife and cousins, six in total, were deported three times before succeeding at the fourth attempt, and the humiliations at the hand of la migra still sting.

Everyone's quarry

Once they walked all night through the desert of Arizona, slashing themselves on fences of barbed wire and running out of water, before border-patrol agents ambushed them. The agents tied them up, shouted at them, threw them into a van and then into a freezing jail, where they slept on a bare floor for several nights until enough migrants had been rounded up to fill a bus that took them back to the Mexican side.

On another crossing Mexican bandits waylaid them. They pointed guns, stole their food and stripped them naked. Because the Vegas speak an indigenous language called Mixtec and understand little Spanish (and no English), Mr Vega's wife and the other women did not understand the bandits and feared they would be raped. They were not, but then had to cross the frigid night desert without clothes, food or water, until la migra caught them again.

Gonzalo Vega, yet another cousin, made the trip with his wife, five months pregnant, and his two younger brothers, who were seven and ten at the time. He carried all their water and food, but the children struggled. After a day and two nights of walking they were desperate for sleep, but Gonzalo didn't let them rest in the freezing cold lest they not wake up again. He could not light a fire, because la migra would have seen it.

They threw themselves into ditches whenever the border patrol's SUVs approached. Once Mr Vega's wife fell hard onto her bulging belly. The worst moment came when la migra caught them again, beat Gonzalo and threatened to take his brothers away from him. When the family was allowed to remain together, even the cold jail floor felt good, he recalls. Gonzalo's group succeeded on the fifth try.

If and when the border is crossed, the paved but hostile vastness of America is the next challenge. Usually a family member already on the other side will pick the migrants up in a car. Many then make their way to the farm towns of California.

Often they take the same roads on which the “Okies” travelled en masse in the 1930s as they fled the depressed dust bowl of Oklahoma, Texas and Arkansas to seek a living in California. These Okies are for ever etched into America's psyche as the Joad family in “The Grapes of Wrath”. Comparing the Mexicans who toil California's fields to the Okies in John Steinbeck's classic novel is a staple of the Latino left. That does not make it any less accurate. Joads then and Vegas now are pushed by the same need, pulled by the same promise. Now as then, there is no clearing house for jobs in the fields, so the migrants follow tips and rumours. Often, like the Joads, they end up in the right places at the wrong times. Felix Vega and three of his group, including his wife, were dropped off in Oxnard, famous for its strawberries. But they arrived out of season, so they slept on the streets, then in a doghouse, then in somebody's car. For two months they did not bathe and barely ate. Finally, they found jobs picking strawberries and made their first money in America.

And thus they joined the vast undocumented workforce that undergirds America's food supply. The government estimates that more than 80% of America's crop workers are Hispanic (mostly Mexican), and more than half are illegal aliens. But Rob Williams, the director of the Migrant Farmworker Justice Project (which represents farmworkers in court), considers those numbers grossly misleading because they rely on self-reporting. He estimates that more than 90% of farmworkers are sin papeles (without papers), just as the Vegas are.

The devil's work

The price of strawberries

Farm work has, for most crops, become no easier since Steinbeck's day. Strawberries, the crop the Vegas started out with, are nicknamed la fruta del diablo (the devil's fruit) because pickers have to bend over all day. “Hot weather is bad,” says Felix Vega, but “cold is worse” because it makes the back pain unbearable. Even worse is sleet or rain, which turns the field into a lake of mud. The worst is picking while having the flu.

Every crop exacts its own particular discomfort, as this correspondent discovered on an August day picking grapes in the very part of the San Joaquin Valley where Steinbeck's Joad family looked for work. Working with two Mexican brothers and a young Mexican couple, he cut the grapes, collected them in tubs and periodically dumped them into a wagon pulled by a tractor.

The lanes between vines are exactly as wide as the tractor, so the little group had to duck into and underneath the vines all day long. They crawled alongside the tractor, trying to avoid having their feet run over. Within hours this correspondent's shins were bleeding as the wagon's metal protrusions slammed into them, which seemed unavoidable. With an encouraging smile, a co-worker pulled up a trouser leg to reveal his own scarred shin.

Because the pickers were squatting or kneeling under the vines and twisting to reach up for the grapes (the low-hanging fruit proving the trickiest), their necks and shoulders were soon in agony. Standing up to relieve their backs thrust their heads into the vines, which are covered in pesticides. There are many cases of birth defects and cancer in the families of farmworkers. But as the heat climbed above 100°F (about 40°C), the vines, soaked in toxins or not, became allies. The air underneath them is stagnant, as in a sauna, but their foliage is the only available shade.

Just as the heat threatened to overwhelm this correspondent, the woman in the group broke into a slow Mexican song, which somehow helped. But heatstroke is common in the fields. In 2008 Maria Isavel Vasquez Jiminez, a 17-year-old Mexican girl who was pregnant, collapsed while picking grapes and died two days later.

Hungry amid food

As Tom Joad in Steinbeck's novel discovered, many farmworkers, even as they spend their waking hours picking food for others, can barely afford to eat. Between harvests they have no work. When they do work, their wages are meagre. The workers picking grapes with this correspondent got $8 an hour. That is vastly superior to the $9 a day—not hour—which the tractor driver says he used to get at home in Mexico. But costs in the United States are higher too.

Teresa Vega makes about $65 a day during the strawberry season, as does her husband. But they now have two daughters living with them, Luisa, four, and Maritza, two. So Ms Vega must, perversely, hire a babysitter while she is working. That costs $50 a day.

Most of what remains pays their rent for a trailer in Watsonville, just outside Steinbeck's home town of Salinas. The trailer is dilapidated, but Ms Vega tends to it lovingly. By the door hangs a picture of the Virgin of Guadalupe, Mexico's patron saint. There is even a small television set.

But the trailer has no air conditioning or heating. On this day, after a downpour, it smells musty. Teresa explains, in Mixtec through her brother's translation into Spanish, that in the winter Luisa and Maritza are always ill. On the counter that serves as the kitchen there is no fresh food, only a jar of protein powder.

After their expenses, very little is left over for her husband's blind grandparents in Mexico, for Teresa's diabetic father and above all for their son Erminio, who was the original reason they came. Western Union, a service that remits cash, takes another painful cut whenever they send money home.

Aside from poverty, the other consequence of being sin papeles is having to live “in the shadows”. This is the difference between today's Mexicans and yesterday's Okies, between the Joads and the Vegas (although Tom Joad was also on the run from the law). The Okies were poor, disdained and hungry. But they were American and white, often Scottish-Irish. They could not be deported.

“The hardest part is not being free, not being able to go out,” says Felix Vega. “It's like being in a jail.” Any contact with official or bureaucratic America might lead to deportation and thus separation from his wife and sons—Victor, seven, and Jesus, four— who were born in America and are thus citizens.

This anxiety extends to every aspect of work and life. In the fields, undocumented workers hardly ever protest when contractors or growers abuse them. Merely getting to the fields and back is risky. Undocumented farmworkers have to drive long distances, but they don't have driving licences. Any brush with the police is dangerous. Felix Vega stays below the speed limit and comes to a complete halt at stop signs.

His cousin Gonzalo has been pulled over three times—because of “the colour of my skin”, he thinks. Like many indigenous Mexicans from Oaxaca, the Vegas are short, squat and dark. Last time the cop claimed that Gonzalo's tyre had touched the centre line as he was driving. Local police are not supposed to enforce immigration law, which is a federal matter, but they can impound the cars of drivers without licences, so they took Gonzalo's. He had to pay a $1,580 fine, then to buy a new car for $1,500. The expense set his finances back by years.

In Steinbeck's novel, “the migrant people, scuttling for work, scrabbling to live, looked always for pleasure, dug for pleasure, manufactured pleasure,” often of a boisterous sort. For undocumented migrants, however, those pleasures are not available, for they might attract attention.

On those Sundays when he is not working, Felix Vega goes to church, then walks with his sons to a public park. Beyond that, he stays off the streets. He has never been to a cinema. Nor to a hospital: when family members get sick, they use folk remedies. His sister Teresa, who lives quite a distance away, hardly ever lets her girls play outside. Luisa and Maritza spend almost all of their time in the trailer, on the mattress that completely fills the far end of it and serves as a family bed and playpen.

“The hardest part is not being free, not being able to go out”

Gonzalo Vega and his wife and daughters—Diana, two, and Esbeide, ten months—live in a single room with one mattress and one chair. He used to let Diana (with whom his wife was pregnant during their crossing) play outside. But then the American neighbours, who seem generally hostile, complained about noise and threatened to call the cops. “It's always the same: they have papers and we don't,” he sighs. So now Diana stays inside and is told to keep quiet.

Gonzalo's younger brothers—the two he brought over the border—live in another town. They spend almost all their time studying, Gonzalo says, because he has told them that the best students might get papers and become legal. He knows that might not be true, he says, but it keeps them out of trouble.

Yet a life without pleasures is not a life without joys. For the Vegas, the children are the joys. Felix's older son, Victor, is trilingual in Mixtec, Spanish and English and has the naughty cheek of a boy who is legal. He goes to a nearby state school. Felix, beaming with pride, worries that its classes are too crowded and its teachers bad, sounding like any middle-class American parent.

“I don't hate Americans,” says Felix. “Some are racist, but there are racists in Mexico, too.” Here in America, he says, those Latinos who have papers sometimes discriminate against them more than the gavachos (non-Hispanic whites) do.

But all the Vegas feel hated much of the time. Some people hurl racial slurs at them, give them dirty looks or call them “wetbacks”, a term of abuse recalling someone who has just swum the Rio Grande. Felix Vega says that the mood has become noticeably more hostile this year, perhaps because a controversial state law in Arizona has legitimised such animosity. That law, parts of which have been suspended by a federal judge, would make illegal immigration a state crime and oblige local police to enforce it.

Its fans correctly call the Vegas and their ilk “illegals”. This is often taken to mean “criminal”, yet being in the United States illegally is actually a civil offence; it is the illegal crossing that is a criminal offence. The migrants and their sympathisers therefore prefer “without papers” or “undocumented”. “They think we're criminals, but we came here to do good and we're all children of God,” says Felix Vega, touching the cross around his neck.

The stolen jobs no one wants

At a time of high unemployment, many Americans are convinced that these aliens take American jobs. As a test, this summer the United Farm Workers (UFW), the main agricultural union, launched a campaign called “Take Our Jobs”, inviting willing Americans to work in the fields. In the following three months 3m people visited takeourjobs.com, but 40% of the responses were hate mail, says Maria Machuca, UFW's spokesman. This included e-mails such as one reading: “We're becoming more aggressive in our methods. Soon it may come to hands on, taping bitches to light posts.”

Only 8,600 people expressed an interest in working in the fields, says Ms Machuca. But they made demands that seem bizarre to farmworkers, such as high pay, health and pension benefits, relocation allowances and other things associated with normal American jobs. In late September only seven American applicants in the “Take our jobs” campaign were actually picking crops.

That was the point, says Arturo Rodriguez, the UFW's president. America's farm jobs, which are excluded from almost all federal and state labour regulations, are not normal jobs. Americans refuse to do them. The argument about stolen jobs is “just a façade” for a coarser scapegoating, says Mr Rodriguez, and “we demonstrate the hypocrisy.”

Teresa, Felix and Gonzalo Vega only nod sadly when asked about the rancour, the Arizona law, the politics. They feel they had no choice in coming illegally. Would they do it again? “No, not if I had known what lay ahead,” says Felix. But after a silence, he corrects himself. Yes, he would, because even though he doesn't think he'll ever get papers, he has two sons who are American and could be lawyers or writers one day, living openly.

Teresa Vega is the most reticent. She admits that her “plan didn't work”. She hears that Erminio, at home in Oaxaca, is not doing well. He is often ill. “He needs love” and doesn't get enough, she says. But then she, too, reverses herself. She always thinks of her first son, the one who died because she had no money to save him. Yes, she would come again.

People like the Vegas will always keep coming, no matter the fences that go up on the border and the helicopters that circle above. For they are like the Joads. As Steinbeck wrote: “How can you frighten a man whose hunger is not only in his own cramped stomach but in the wretched bellies of his children? You can't scare him—he has known a fear beyond every other.”