What's the damage look like? A photo of partially eaten leaves would be useful - possibilities other than slugs or snails are leaf cutter bees, vine weevil, caterpillars, but the damage looks different with each one.

1.Leaves with numerous pale spots = Smut
2.Leaves with white dust like covering = Powdery Mildew
3.Leaves variously crumpled with irregular, pale yellowish patterns with insects visible more so with a magnifying glass = Red Spider
4.Leaves variously crumpled with irregular, pale yellowish patterns with no insects visible with a magnifying glass = Virus
5.Leaves wilt; stem interior with brown or black streaks = Wilt
6.Leaves with irregular holes and pieces eaten away with slime trails present on or near plants = Slugs and Snails
7.Leaves with irregular holes and pieces eaten away; no slime trails present on or near plants = Caterpillars
8.Leaves with small tattered holes, especially at tips of shoots; buds sometimes killed = Capsid Bugs

9.Leaves, shoots and flowers infested by colonies of black, green or yellow wingless and winged insects; Foliage sticky and sometimes sooty =Aphids

10.Stem with soft, irregular swelling = Crown Gall

11.Stem base rotted, cottony white mould growth present = Sclerotinia Disease

Thanks everyone, I think it may be vine weevil. Have not seen any cutter bees on the plant

Best order some Nemesys vine weevil nematodes for use as a soil drench then... and welcome to the site!

I have been discussing Dahlias on my blog. Slugs and snails are a problem but a few people have mentioned earwigs.

Earwigs and dahlias, natural bedfellows - but they tend to eat the flowers, not so much the leaves...

Can I ask why you have ruled out slugs/snails?

welcome to GOY too