For decades, U.S. farmers, landscapers and builders tapped a seemingly endless supply of cheap labor: the waves of undocumented immigrants coming across the southern border. The workers arrived in time for harvests and construction booms. They did the low-wage manual labor that Americans were unwilling to do. By the 2000s, more than half of American farmworkers were undocumented, according to the Labor Department. But now — thanks to border enforcement, the surging cost of smugglers and changes in migration patterns — the number of people crossing into the United States illegally is nearing the lowest level in decades. There are more Mexicans leaving the United States than arriving there. For the White House, that might be a triumph. But for the agriculture industry, the impact is acute. Each year, its labor force dwindles. To fill those positions, employers have turned to temporary visa programs that recruit workers in Mexico and Central America.