A large tub of Diataomaceous Earth powder.
DE is organic, naturally mined, mineral product made up of fossilized remains of diatoms, a type of hard shelled algae. It contains no pesticides or chemicals and is safe and natural and harmless to animals and children (although use of dustmask is recommended in confined areas).
DE powder absorbs moisture thereby creating a dry environment which prevents moulds and fungi, and discourages the damp areas where mites and lice thrive.
We recommend sprinkling it all around the hen house weekly or whenever you change the bedding. During the summer we find it very effective to put copious amounts of DE powder all around our wooden hen houses, paying particular attention to nest boxes and perches.
A more cost effective way to buy - ideal for re-filling talc type bottles, add to chickens dustbath or sprinkle around the chicken house.