Caterpillars!
There are some who will say it is earwigs but with the damage you describe I say caterpillars
If you look close you will see what came out the back end of the caterpillar this is the tell tale sign......droppings.
The culprits are generally moths, butterflies rarely touch dahlias!
Butterflies will land on single dahlias for the nectar but not to lay eggs!
I think of earwig damage as insect origami!
Earwigs eat the buds and as you may be aware within the bud every petal is already formed in minicule form. So one tiny bite from an earwig will damage dozens of petals.
The tell tale sign is; the outer petals (the first to open) are generally the only ones damaged (origami) unless the earwig has had a real feed!
