Mycki McKay can remember when carloads of seasonal workers from Central and South America rolled down the drive at Helen’s Flower Farm & Greenhouses in Riverhead during the busy spring and fall looking for work. "We used to get them in caravans," she said. But this year is different. "I don’t think we had one," said the retail manager of the sprawling East-End nursery and farmstand the McKay family has run for decades. The large seasonal influx of migrant workers who cultivate plants at greenhouses, prune grapevines and harvest corn and pumpkins across Long Island has been driven into the shadows, wary of the federal crackdown by Immigration and Customs Enforcement agents. Large groups of workers who used to mass for work in public places are now rare, and the seasonal influx of workers has diminished, making recruitment of workers more competitive, farmers and their advocates say. Deportation fears have driven some migrants to self-deport, while farmers worry that other worker...