The slugs and snails love mine so i scatter pellets around the area and this reduces it a lot.
I've also noticed those tiny bright green caterpillars too so i sprayed with an insecticide aswell.
Clearly you have to decide whether you want to be encouraging wildlife (by not using chemicals) or saving the plant (by using said chemicals) because you invariably end up losing 'one' to save the 'other' !

Or maybe you could use an Organic Garlic Barrier spray,if you didn't want to use Chemicals ? it feeds as well..Hope your Dahlia's survive ok..