Hi, 

The chewing on the leaves in the first 2 photos does look like either earwigs, slugs or snails. Whatever you are seeing more of in this damp cool weather since we have had for a lot this spring. All of those prefer moisture. You can try to set traps for them but putting a wood board, log, piece of bark or rolled up newspaper nearby. You would have to check it every morning to empty it into soapy water to make a dent in the population. 

The last photo, we are not sure what the issue on the leaf is except it looks like the leaves may have been scorched. It could be that they are too wet from all the rain and the roots are rotting, especially with the entire plant wilting. Is the soil well draining enough? Did you overwinter these in the ground or did you plant fresh this spring? Are there multiple plants that are wilting?

You can send back a photo of the entire plant/planting area and that may help us. 

Emily

There is a reference page from Penn State Extension that could be helpful. We asked a staff member who grows a lot of dahlias and she thinks it could be verticillium wilt or a crown rot or wilt from being too wet potentially.