Thanks for writing. It sounds like whiteflies --- small, white insects that are about the size of gnats. Whiteflies can be extremely difficult to deal with because they're so mobile. You can try using an insecticidal soap or spray with neem oil --- but you'll need to reapply regularly (every week for a month or so).
Another option is to encourage beneficial insects that eat the whiteflies. Daisies, marigolds, calendula, sunflowers, zinnias, and other similar plants are good for attracting beneficial bugs.
---Justin, Senior Garden Editor, BHG.com