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The stuff for swimming pool filters IS diatomaceous earth (DE) --it's the toxic version of DE and the bugs don't eat it and die.....
VERY IMPORTANT- the DE used in pool filters is DANGEROUS when inhaled. It's been heated (cooked at very high temps) and treated and that's what makes it dangerous.
Food Grade Diatomaceous Earth is not any more dangerous than inhaling the dust they dust bathe in anyways. It works mechanically like Kitsune described.
Wood Ash.....wood ashes is the way it was done in the olden days and works. Unlike the olden days...wood ash is not as available to people (only those with wood stoves or fireplaces that they use).
The act of dustbathing is the natural way that chickens help alleviate pest issues, but is not enough. Adding wood ash or DE to your chickens favorite dustbathing areas is a great way to have them dust themselves. Some will wait until they have a problem and then use fowl dust (which has chemicals in it), or spray made specifically for the problem. I use DE (FOOD GRADE) in their coops, nest boxes, and dust bathing areas and have had not problems...I like to prevent rather than treat. I also dump my wood stove ashes in the garden which they also dust bathe in and then "till" into the soil for me.
I did physically dust (with DE) a rescue chicken that had a huge infestation of feather mites (her feathers were actually falling apart in my hands). I dusted her two days in a row and I refreshed all the DE in the coop and dusting areas. Everyone is fine and no mites in sight!
That's what I do
sandra