I know this is late but I'm adding this for future readers. Those are moth caterpillars. They can devour a large amount of petunias and FAST. They are a huge problem for petunias in some areas. They also can get into other plants. They devoured my knockout roses too.
You will need to treat the petunias with BT. It is relatively cheap and will only kill caterpillars. They eat the leaves that are covered with the BT and then they become paralyzed. You will need to treat several times. I do it once a month now as preventative-more often while trying to get rid of them. If I don't they come back. The smaller the caterpillar the easier it is to kill. The larger older ones are harder to kill. They run in cycles, they eat and then drop into the mulch and turn into moths. The moths come back and lay their eggs in the plants. This happens several cycles a season. Sorry I dont know the technical words. What I do know is that a few of them QUICKLY will become MANY. Also, they will be back every year until you get them under control. Good luck. These things have been a major pain in my butt.