Yellow sticky paper from a plant nursery. I have no idea if the big box stores sell it. I get mine at the mom and pop nursery, and a 6" x 4" sheet lasts over a month..or until covered in bugs.
Also you can purchase whitefly parasitic wasps which work AWESOME in a greenhouse environment. I have not tested it in a field environment.
The yellow sticky traps work good also. But I end up also catching birds, frogs, mice and small dogs with the sticky traps.