Abstract: A dispenser is disclosed. In one example, the dispenser relates to the distribution of formic acid vapour in beehives to combat mites. The dispenser has an evaporation surface which is delimited by a thin, gas-permeable, highly porous membrane made from a hydrophobic material and comprises an evaporation container and a reservoir, for the treatment dose of a liquid formic acid preparation. The evaporation container can be placed in a narrow air space of the hive, for example a comb space. The evaporation surface is exposed in a comb space of the brood nest or an air flow path. In one embodiment, the dispenser is equipped with a stackable packaging, and the treatment dose of the formic acid preparation is located exclusively in the hermetically sealed reservoir. The dispenser allows a very high evaporation rate per unit area and can be used without spatially extending the hive.