Petunias aren’t pretty when all chewed up. Diagnosis begins with detective work to identify the culprit. Examine buds very closely for damage. Pick a few and even check with magnifying glass for tiny caterpillars, which can turn the color of the flower they are eating. Also, examine the plants after dark (dusk) exploring the plants with your flashlight.

Small slugs can do this sort of damage and not leave a noticeable trail. In my garden there are black ones the size of a grain of rice.

Tobacco budworm damage commonly looks like this too. Here is a fact sheet from Colorado State Extension that includes some good photos. When I personally dealt with these, I noticed the problem early and that’s when I realized they were nearly invisible when very small, matching the flower in color. They were in a window box that was head-high, so I daily pinched flower buds to squish the caterpillars and hand-picked from leaves and flowers all I could find.

This WSU factsheet explains the lifecycle and difficulty with chemical management for budworms. The University of Nebraska page here has good photos and easy highlighted facts.