Freezing weather kills them off in the environment, so the chickens don't pick up more. It can prevent reinfestations from the outdoors, so that is good. It's up to you to work on the ones living on the chickens or in the coop.
Whether you use DE or a pesticide, you need to treat your chickens and your coop, including your nests. For DE, you need to apply it to the chickens heavily, as it's physical action is what kills the mites. Use a dust mask, when dusting. With a pesticide, it's using a biochemical action to kill the mites and should be applied using the directions. Using more pesticide that the directions call for makes it more toxic to the chickens.
Doing everything you can to encourage dust bathing is really important when battling external parasites. It's their natural defense. Sometimes, chickens not raised by other chickens are slow to learn about dust bathing. In areas that get a lot of rain and have heavy clay soil, it can be harder for the chickens to find the fine, loose soil they need to dust bathe. Dry clay can be scratched up, but wet clay is just a mess. They can't dust bathe in mud. Sometimes, you need to provide a covered area that stays dry. That can also be a good place to add things like DE or wood ash.
Dust bathing helps them smother a couple of mites, before they turn into millions.
