I have to strongly advise against using ant powder. We tried it once in a bantam coop and them flapping about and inhaling the dust resulted in what appeared as a complete collapse of the immune system. We lost two of the three hens very quickly and despite removing all traces of the powder immediately, the third was ill for months.
As described earlier, steaming and trapping on the perches works extremely well. We had 5 coops with red mite at the beginning of Spring and all remain clear. A side effect of not using chemicals is the ants. They will sniff out red mite and enter the coops to eat them. They are the first sign you have red mite, long before traces are visible. The chemicals kill ants a lot faster than red mite unfortunately. We had three of the 5 coops with ants in, helping us out. They can't reach them all though because ants can't squeeze into the very small gaps mite can hide in, but the steamer can.