I suspect earwigs are doing the nibbling as dahlias are a favorite of these night feeding pests. An effective cure is to place a small clay flowerpot upside down on a stake next to the blooms. Most often this is the stake that supports the dahlia. Next stuff damp straw or newspaper into the pot to attract the earwigs. Each morning you can carefully empty the pots onto to cement and stomp the earwigs or if you are really kind move them to your compost pile. You should also bait for slugs as they also love dahlias. Keep growing, Marianne Binetti