Whiteflies are not able to tolerate the winter climate outdoors. They will be found in indoor plants or greenhouse environments during the winter in zones 7 and colder...
If your plants are suffering from heavy infestations, use insecticidal soap. You can make your own by mixing 1 quart of water with 1 tablespoon of Castile soap. The insecticidal soap coats the eggs, larvae, and adults and suffocates them to death...