Thanks for writing. Whiteflies are notorious greenhouse pests, but they can also thrive in the garden and raise havoc.
You might look to beneficial insects. For example, ladybugs and several types of tiny wasps attack whiteflies. Or you can spray horticultural oil or insecticidal soap. Alternate the sprays by applying the oil one week and the soap the following week for best results.
---Justin, Senior Garden Editor, BHG.com