It's hard to say exactly what may be chewing on the petals. It does look like it could be from earwigs or a type of garden beetle. Slugs and snails can also cause damage to flower petals and leaves, but if you don't see any "slime" trails, it is likely one of the other culprits.  

You can venture out at night with a flashlight to see if you can catch what is eating them. There really isn't a spray or solution to prevent it beyond hand-picking off the insects or knocking them into a cup of soapy water. If you are hoping to use the blooms for cut flowers, you could net them with fine mesh insect netting to prevent further nibbling. Most double-blooming dahlias won't attract pollinators, but to prevent harming any beneficial insects that would help control pests like beetles and earwigs, we don't recommend using any pesticide on the flowers. 

Let us know if you have further questions. 

Emily