My petunia flowers are being eaten - the whole flowers are eaten mainly in the night. They are predominantly the purple ones. Whatever it is doesn't eat the plant at all.
It's difficult to know what exactly is attacking your petunias without a photo and without careful personal examination. But here are some tips on petunia pests on GardeningKnowHow: https://www.gardeningknowhow.com/ornamental/flowers/petunia/problems-with-petunias.htm