California farms are heavily dependent on immigrants to fill their physically demanding and often low-paid jobs, and it is no secret that many of those immigrants are in the country illegally.

Employers contend that they have no way of knowing when a worker submits a fake Social Security number on their I-9 forms, the federal document on which employees attest they are citizens or otherwise legal. Supporters of tougher immigration enforcement say that employers are just winking at the issue.

A common way for the government to root out illegal labor is to audit companies’ I-9 records. By federal law, ICE sends companies a letter before beginning an audit, and by the new state law, companies must pass that news on to their employees.

When Bee Sweet Citrus, a large grower and shipper of oranges, lemons and grapefruit in Fowler, in Central California, plastered on the wall an advisory about ICE’s inspection plans, the company lost some packers, sorters and administrators, one employee said.

“I resigned out of fear,” said the worker, Ana, who asked that her surname be withheld because she is undocumented. She said she hung on until Feb. 16, close to the deadline for the company to submit paperwork to ICE, so she could make another month’s rent. (The company said it was complying with state and federal laws regarding the ICE audit, but declined to comment further.)

Faith in the Valley, an advocacy group based in Fresno, said it had received a spate of calls from workers rattled after seeing the notices about audits. Manuel Cunha, the president of Nisei Farmers League, a growers’ organization, said, “I have never seen workers quit like this before.”

“There is fear and hysteria,” he said.

ICE has also stepped up activity in urban areas, conducting sweeps and delivering audit notices to hundreds of businesses in San Francisco, Oakland and Los Angeles.