If the henhouse is made of thicker wood it gives them more cover from any anti-mite treatment. We have a farm pond and the only way I've ever found to kill them was to weight the house with a breezeblock, tie a rope on the house and submerge the whole thing for a couple of days. Needs drying out for a couple of weeks but works every time. First dig your pond ..... Have, in the past, cleaned, power-washed, heat-gunned and dusted a house then left it empty outside for a year - and still found red mite in it!