Vanessa, an illegal immigrant, has harvested fruit in Kerman, Huron and Madera for four years. Until this summer, she had never seen a white face in the fields.

Then one day, four teenagers showed up at a cherry orchard. They didn't speak Spanish, and they didn't seem to know what they were doing.

"Everybody was surprised to see them there," Vanessa said.

It didn't go so well for the newcomers. Within an hour, all four had quit.

At the heart of the debate over illegal immigration is a question that burns as hot as the afternoon sun hovering over the Central Valley: Are illegal immigrants doing the work that no one else wants, or are they stealing jobs from Americans and dragging down their wages?

To some extent, both are true.

As Vanessa's story shows, some jobs might go unfilled — even in tough times — without illegal immigrants.

But there are drawbacks. Illegal immigrants push down wages for legal workers in food-processing, factory and service jobs, economists say. Because illegal immigrants will work for almost any wage, employers have little reason to pay other workers more. Sometimes jobs that low-skilled Americans would be willing to do, such as washing dishes and cleaning bathrooms, are instead taken by illegal immigrants.

Illegal immigrants help the nation's private-sector economy by providing cheap labor — something that is especially critical for the Central Valley. But their competition with low-skilled American workers and their strain on local government budgets cancel out that boost for that nation's overall economy, some economists say.

In the end, illegal immigration in the Valley means businesses are big winners while many blue-collar workers lose out.

Farmworker groups have tried to prove that we need illegal immigrants. In 2006, as Congress was considering immigration reform, immigrant workers around the country either stayed home or joined protests for the May 1 "Day Without Immigrants" economic boycott.

In the central San Joaquin Valley, the United Farm Workers of America estimated that tens of thousands of field workers didn't show up for work. Restaurants, landscape contractors, food manufacturers and growers all struggled to make it through the day with skeleton crews.

With immigration reform again on Congress' agenda, the UFW has tried to bring attention to the role of illegal immigrants, which it says account for half a million farmworkers. It launched a campaign in June called TakeOurJobs.org that invited anyone to sign up online for a farming job.

The campaign was mostly meant to grab headlines — and it did. UFW President Arturo S. Rodriguez was invited to appear on Comedy Central's "The Colbert Report" to talk about the campaign.

"Americans do not work in the field because it's very difficult work, requires a lot of expertise and the conditions are horrid," he told host Stephen Colbert.

More than 10,000 people registered on the website; only 11 people actually went out to work in the fields — including Colbert.

Economists say illegal immigrants play a key role in the nation's economy: Their willingness to work for low wages helps keep American businesses competitive and lowers the cost of goods and services.

But their economic benefit is very small. One researcher estimated that it represents only the slightest fraction of the country's gross domestic product — 3 cents for every $100 the economy generates. That means that most businesses are only marginally more efficient thanks to illegal immigrants, although some businesses that rely heavily on them — including those in agriculture — benefit greatly.