Pushing back the silvery-green canopy of an olive tree, Don Stutsman examines clusters of ripening fruit and wonders whether he'll have enough hands to pick the blushing berries. He usually finishes his harvest by the end of October. But this year a number of unusual factors have collided to leave fruit hanging longer, jeopardizing his crop and highlighting the industry's dependence on illegal immigrants. A booming construction industry is offering better pay, and beefed-up patrolling along the Mexican border has made it harder for unauthorized workers to reach farms, offering a preview of what would happen if this source of...