DE hurts their lungs and eyes. Permethrin is recommended for chickens and i have used it for chickens but haven’t tried to use it on ducks (yet) let’s tag
@Wyorp Rock
@Eggcessive @aart @azygous
too
they know alot about parasite treatment
there is also a spray i have called manna pro...
have you checked her oil gland
majestic waterfowl sanctuary has a great article about wetfeather and oil gland problems. Scroll down to Oil Gland
http://www.majesticwaterfowl.org/mmissue71.htm
to me she looks older than six months also possible could be parasites although ducks don’t get them...