Lye in the form of burned hard wood ashes is what is needed. Lye is a naturally occurring component in ashes and acts as an irritant, it should kill external parasites like lice and hopefully force the others to move on.
I'll have to burn some wood then ha is it possible to just let them dust bathe in it?
Although where do you get hardwoods. Around here it's mostly pine, oak maple. Some birch. Most firewood is oak i think. Oak is hardwood right? Pine isn't?