Each housing authority shall admit to occupancy in a farm labor center only single persons and families whose principal source of income is derived from agricultural work and employees of the authority whose residence on the center is necessary to its efficient operation; and such persons, families, and employees, if they qualify for admission under these standards, may be admitted to such occupancy without regard to whether or not they have low incomes. “Agricultural work” means work performed on a farm or in the handling, packing, processing, freezing, canning, or shipping of agricultural produce of the immediate area.