Question
What can I do?

Answer
Petunia-eating pests abound, but my guess would be these are Slugs.  What do you think?

Ther's a new Nematode on the market that waits around for a Slug to appear.  The unsuspecting Slug dies a slow death, taking with it all his infected friends.

But Nematodes are tricky to apply.  I would probably wait for more experience before recommending these.

In the meantime, put out one of those shallow dishes of beer and see if any Slugs decide to show up for lunch some night.  At least you can get them before they reach your Petunias.

Diatomaceous Earth is also effective.  But every time it rains, you have to re-apply.  Wet D.E. is worthless. Dry D.E. is ruthless.

There's also the copper ribbons you can deploy around your Flower beds with your Petunias.  They can't crawl over them because of a chemical reaction that triggers bursts of uncomfortable electric current through their slime, so they avoid it.  Search and destroy the ones slithering around inside the copper strip.

Let me know if you need any further information on these; they are available at most garden centers, and by mail order.  I vote for all of them.

L.I.G.