Red mite
are the one thing that would eventually make me stop keeping hens, however all is not lost. This is what I have done when I've had a bad infestation in the past.
Clean the bedding out as carefully as you can trying your best not to spill it on the ground and burn or dispose of it away from your premises. DO NOT put it on the compost. The mites will just troop out of there and back onto a passing hen.
If you have roofing felt on the hen house remove it and dispose of it away from your premisses. Red mite just love felt to live under felt and unless you can get under it to spray you will just be fighting a losing battle.
Do not replace with felt but use Onduline or similar. (It looks like black or sometimes green) corrugated sheeting.
I use Poultry Shield but another member here (HF) thinks it is just not so bubbly washing up liquid. The point of Poultry Shield (or washing up liquid) is to permeate the waxy coating these red mite have. Make up the solution in a garden pressure sprayer, most garden centres will sell them, they can be a bit pricy but will be well worth the investment. Drench you house, perches and the ground surrounding the house and if your birds are in a small run do the run for good measure. Take special care with all joints in the woodwork. Have your sprayer ready and waiting if and when you remove any felt, you will be repelled and repulsed by what you will find there, heaving
Remove all perches and anything else that's removable. I paint mine in a product that is not good for you but used to be used on fences. I am not allowed to recommend this step but am simply making you aware of what I do....
I keep a spare set of perches and paint this product on and soak the ends in a bucket of it for a few days and then leave to dry/soak in until the next time I need to treat the next lot of perches.
Make a slurry of diatomaceous earth of water and a really good glug of washing up liquid or poultry shield. You are looking for the consistency of gloss paint of pouring custard. Paint all the interior of your hen house including perches and nest boxes.
Put you house back together and instead of using your usual bedding put shredded newspaper in the house. Remove this in two days and burn or dispose of it off your premisses. Repeat all the spraying and cleaning out every three to four days for not less than three weeks as you want to catch the newly hatching eggs.
Keep your hen house winter and summer as dry possible.
Failing all this there are chemical resolutions which I'm sure others will tell you about. It's a big task. I did this on one large hen house and three small ones last summer and (hands held together in prayer) I've not had any red mite this year.
Best of luck.
Orchardlady
Also every time you introduce new hens give them a squirt of Frontline for cats and dogs, one squirt on the back of the neck, one under one wing and a final squirt up the bum!
Keep all new birds quarantined from your main flock and don't introduce them until you are absolutely sure they and their quarantine quarters are red mite free.