I believe you have what is called a bud worm. they leave a sticky film where they have been chomping. Are you finding holes in the flower part of the plant? the worm will turn the color of the petunia it is eating and blend in and is difficult to see. They also leave behind tiny specks of worm poop, that resemble the seeds from the seed pods on the plant. I believe a moth is responsible for leaving the larva there to chomp the plants.If you ask me how to get rid of them? I wish I knew so I could get rid of them.