A. The main suspect here is likely to be earwigs. They have a liking for dahlias and chrysanthemums and cause damage to other plants. They attack at night.
Fill some small flowerpots with straw and place them inverted on bamboo sticks. Check in the morning. The earwigs hide in them from the light. Scooped out grapefruit halves will similarly entice slugs in during the day.