Answer: Unfortunately I am not able to guess what specifically is bothering your dahlias. Snails or slugs can sometimes damage dahlia foliage. These are nocturnal but will leave behind a telltale silvery trail. They can be baited with products containing iron phosphate. Read and follow the label directions.
Grasshoppers can also attack dahlias but unfortunately are very difficult to control, especially later in the season such as now. A biological control called Nosema can sometimes be useful if applied early in the season, but it is slow to work. (Keep this in mind for next year.) Carbaryl (the ingredient in Sevin) will also kill grasshoppers but is a contact insecticide without residual action and thus of limited help with the fast moving grasshoppers.
Sometimes animals such as deer, rabbits or woodchucks will nibble on dahlia plants. These can sometimes be deterred using one of the commercially formulated repellents labeled for the appropriate animal, or by fencing them out.
I hope this helps you trouble shoot.
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