Hi Toni,

It looks like earwig damage to me. Not much likes to eat dahlias, but earwigs love them.

Control is difficult for me. My garden is a perfect environment for them (lots of decaying materials and places for them to hide). To discourage them, you need a tidy garden, less or no mulch, etc. That's not going to work for me, but you might consider that next year. 

You can take down their numbers by going out at night and shaking them off the plant into a bucket of soapy water. They will run or drop off the plants when they see you coming, so you have to be quick. You can also place traps in your garden--some people use rolled up damp newspaper--so that they congregate in the trap during the day and then you dump them in the soapy water.  There are chemical controls you can use, including organic options, but any of these options will kill other invertebrates as well. You can do an internet search for earwig control and you will find products available in stores. 

Earwigs are a serious pest. They will eat a wide variety of plants. I struggle with them in my own garden, so you have my sympathy. 

Lindsey K. Kerr, MS, MHP (she, her, hers)
Consumer Horticulture Educator
Michigan State University Extension