The recent appearance of the sweet potato whitefly in Sutter County has cast a shadow on the future of melons and other crops grown in Northern California. The voracious strain B of the sweet potato whitefly has an appetite for up to 600 different types of plants. "Melons are one of their favorite foods, and that has melon growers in the Sacramento Valley gravely concerned," says Carolyn Pickel, a University of California farm advisor for Sutter and Yuba counties. In addition to melons, the pest might also zero in on processing tomatoes, also a significant crop in the Sacramento Valley. An invasion of the sweet potato whitefly in the Imperial Valley during 1990 virtually wiped out the melon crop, forcing farm workers into the unemployment line and costing the local economy an estimated $130 million. The pest turned up late last summer in a Sutter County watermelon field and has since been identified in eight other locations in the vicinity. Scientists don't know how well the sweet potato whitefly will survive the Northern California winter or whether it will be able to multiply to the devastating levels it did in Southern California.