Marigold, there are different grades of Diatom powder, even available for humans. Also, my hens do peck at it in the dust bath when I put it in. I didn't think it was a dehydrating powder and it isn't advertised as such. The way I understood it worked was by breaking up the exoskeleton of the mites/lice and killing them by cutting them with the very fine particles. I haven't read anywhere that it dehydrates them. Perhaps you could let me know where you have seen this.
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I've just observed this dehydrating effect from using it in dustbaths, as I said especially on droppings, and in redmite powder which I suppose is just medicated Diatom. Yes, it is a silica compound, and is abrasive to the exoskeleton of parasites, as you say, but it also dries them up as well as grinding them away (ouch!) So these effects would make me wary of trying to get hens to eat large enough doses of it to try to treat or prevent worms, though as you say, they do sometimes eat it out of the dustbath etc. I just can't see any advantage in using it for worms when there are other ways which have been proven to be more reliable and safe.
I do agree with you about the multiplicity of products on the market nowadays, without which you'd think no chicken could thrive and survive! I think you're implying that many of the 'problems' we are encouraged to buy 'remedies' for could be prevented or ameliorated by better management in the first place, using sound management practices which might have been better understood in the past by more experienced keepers who had grown up with poultry.
