yep, red spider mites. Spray with water, have them stand above a tray of water on stones, and keep the humidity high. Also spray with an insecticide and in about a week, take off half the affected leaves. Then a week later, take off the rest. New leaves will grow from the axils.and these will be fresh.
You probably should feed the plants as well, since they must be growing in pots, and dahlias are heavy feeders.
Even better, put them outside, preferably in a big pot, or in the ground, and water deeply at least once a week.
All the best
Peter
