I know it's way too late but in case anyone else comes here looking for answers; I don't know if this would work but I'm thinking maybe some dry ice in water? it should create a high CO2 level and a anoxic atmosphere for the mites and possibly kill them. Also, if anyone experiments with this idea, remeber not to leave the dry ice in there too long as temps are going to drop and you will kill the mites and freeze your plants too.