If you notice your lovely petunias are looking a bit variegated, it's thrips. So tiny, their damage is what is noticeable. About the time you are thinking what a feast for the eyes, they arrive for a feast. If you can't bear to throw the plant away like me, it is most important to isolate it and let it remain so even after treatment. Avoiding infesting more, especially more valuable plants, is the objective. I used an insecticidal soap to slow the onslaught topically, then a systemic. A systemic is required because a topical cannot reach the innumerable hiding places. Feed and keep well watered. Though not as robust as originally, it looks pretty good. Mission accomplished. I prefer not to use chemicals, however, an apt treatment used sparingly and safely is sometimes necessary.