Looks like it could be feather mites which live in the quills/shafts of the feathers and cannot be seen by the naked eye. This may be why she is picking at herself, trying to alleviate the itching.
Most of the time you will need to treat the whole flock and clean/treat the coop/bedding as well. From the info links I have provided it seems the standard treatment of choice for feather mites is Ivermectin.
You will also want to make sure they do not have worms as well. You would want to address those FIRST before treating with Ivermectin.
Vetericyn spray/ointment or plain neosporin can be applied to the wounds.
You might want to consider upping the protein in their feed, either by feeding an all flock 18-20% protein food, a "feather fixer" formula, giving tuna, salmon, scrambled/hard boiled eggs or meats to the flock.
While DE and Ash are somewhat effective in prevention/control of mites/lice, once the infestation gets to a certain point, treatment with "chemicals" is generally the best course of action.
The below links give you info on feather mites along with pictures and treatment(s).
FEATHER MITE INFORMATION:
WORMING AND TREATING LICE/MITES AT THE SAME TIME:
BATHING A CHICKEN: