At this time of year, I suspect that the whitefly infested plants may be kale or other plants in the cabbage family. If so, they are most likely to be cabbage whiteflies, Aleyrodes proletella. See Cabbage Whitefly.
Cabbage whiteflies typically attack during the cool months, fall through winter. Even though the older leaves of cabbage, cauliflower, and broccoli may be heavily infested, you’ll still get good quality edible parts. But, if kale is the victim, these whiteflies colonize the choice edible portions – the young leaves.
It’s best prevent a possible attack by these whiteflies as soon as you seed or transplant the garden. At that time, install floating row cover which you can obtain from most garden centers. See How to Install a Floating Row Cover.
If your plants are already infested, you can rescue your crop with regularly scheduled search-and-destroy trips through the garden. When you see whiteflies, squish them. Periodically spritz the whiteflies with a harsh water spray. Or apply direct hits of insecticidal soap, diluted according to label directions. Repeat as needed. Some folks simply wash the leaves thoroughly just prior to using them.
If the above doesn’t apply to your situation, please tell me which plants are infested, and send several images of the plants and the pests.
We have no cabbage or kale. These are landscape plants and the whiteflies were prolific in warmer weather. I am afraid they are going to overwinter on the plants. Very difficult to squash them as they fly around and are hard to see.
Please tell me which landscape plants are affected.
Thought I'd touch base with you again about the whiteflies. Whiteflies come in various kinds. And each one of those specializes in attacking one or several different kinds of plants. Knowing which plant or plants is/are attacked is often the best way to determine an effective remedy.
If you were encountering clouds worth of whiteflies, they were probably on woody plants -- trees and shrubs. In that case, they may have been ash whiteflies.
If so, the likelihood of repeat problems next year and following is relatively small for several reasons. First, when the ash whiteflies arrived in Oregon several years ago, one of their natural enemies came with them. The truly good news is that the natural enemy has been very effective elsewhere.
Beyond that, sanitation -- raking up and discarding the leaves in the yard debris bin -- could help limit the whitefly population next season.
Pest Alert: Ash Whitefly has considerable information for you.