Last year I had shot-holes all over a miniature dahlia that was planted next to my fish pool. It was being eaten up by flea beetles. I was afraid to spray anything near my pool so I tried blending some garlic and making a spray. It did a pretty good job of keeping them at bay.
Bill

Earwigs could be doing it, too. Since you don't have too many dahlias you could use a trap like wadded up newspaper in a cup inverted on the stake. You go out in the AM & shake the cup into a bucket of water & get rid of them. If you have identification tags on the stakes the earwigs sometimes hide under them & you can a most satisifing SQUISH if you go round in the morning & just push the tag down onto the stake. You can also use an old piece of hose lying flat on the ground - they will crawl in there after their night-time meal & you can shake them out in the morning & squish them-- you get the idea- they like cool dark places to hole up in the daytime & come out at night to feed on your plants, so if you can catch them asleep you can get rid of a lot of them. I don't think you can EVER get rid of EVERY earwig, though!!

I don't know if you have slugs in Ontario, but they are another prime foe of the dahlia- mostly the leaves.

Earwigs are tough customers. I swear I've cut them up to 4 times with a paring knife or pruning shears and they still run. They're like cockroaches- could probably withstand a nuclear attack.

I agree with Poochella. It sounds like slugs to me. Almost every day, I have to pick them off and cast them back to the bush.

Do slugs eat flowers as well as leaves?

The only sure way of finding out what's eating them would be to go out after dark with a torch and catch the "eaters" in the act. If it's slugs/snails, I can recommend using slug pellets that contain iron phosphate instead of metaldehyde - it doesn't seem to harm any wildlife other than the slugs/snails themselves (although research could prove this not to be the case - as with most things that sound too good to be true!) If it's earwigs, then do as Plantlady2 advises. You can use plant-pots as well as cups.