Will aphids damage petunia plants

Yes, aphids will damage petunia plants.

Specifically, aphids alter the appearance and well-being of any plant that they infest. They cut entry points in stems to drain the plant of its hormone-, mineral- and sugar-rich watery saps. They introduce diseases and viruses through the sheaths which they leave to keep entry points open and through their piercing mouthparts known as stylets. They leave a plant disfigured and unsightly as well as susceptible to ant invasions and black sooty mold through the honeydew which they excrete.