DE is not effective in killing worms, and mites in my experience- it just doesn’t make sense, it can’t work- though it is still damaging, only it’s not damaging to your problem, it’s damaging to you birds. The DE is made of fossil remains, broken down over time by bodies of water, where the organisms lived. The DE is tiny, but when inhaled by chickens (who already have poor respiratory systems) it makes micro-cuts in their lungs- this is very similar to the effects of Insulation when you do not wear a mask (i can imagine that when you dust your bird with DE it would make them itchy from the tiny cuts, just as the micro cuts on your skin make you itchy when you are exposed to insulation) it causes you do cough due to the cuts made by the Tiny glass particles. Of course, if your birds breathe this in it can cause their respiratory systems to become very vulnerable. It truly is not worth the risk, despite the possible unrelated benefits when you do use the DE.
There are going to be people who disagree with what I’m saying, or tell me that I am wrong- but while it’s possible that parts of what I say is false, the undeniable truth is that it cannot possibly be good for your birds to breathe it in. So frankly, it is not at all worth the risk unless used as a complete last resort (although I would not use it at all)
sure there are some good stories about it working, and yes in some cases, while it really is almost never effective, people say it does work- There are going to be people that say it kills lice and mites, and maybe it did for them, but do you really want to dust your bird, in the thing that is so dangerous for them to breathe in?
(correct me if any of this is wrong, or if I need to add any information, I will happily edit this post- but please go by fact, not opinion or personal experience)