Try feeding mint herbs of any variety. Also dust baths with wood ash, rosemary, mint, DE, lavender, marigold mixed in it. You can hang or place lavender in coop to freshen and repel insects or herbs mentioned above in nest boxes to prevent them in it. Someone else may have more advice as far as chemical treatment but I usually do herbal or natural unless absolutely necessary.
Try putting some of those herbs in nest boxes, especially the one she likes. Just toss em in with whatever bedding you put in the box. You can also put a bit of de in there too. Will she let ya handle her much? You could try getting a handful of the dust from bath and petting it in to her a bit if she tolerates being handled.