The present invention is related to a dispenser, which is filled with a gel comprising a C1 to C10 carboxylic acid and a thickener, comprising a polyester membrane. The invention further relates to said gel. It also relates to s method for preparing said dispenser, comprising a) mixing C1 to C10 carboxylic acid and the thickener, and b) filling the mixture of step a) into the dispenser. The invention further relates to a method for the control of mites in bee hives comprising putting said dispenser inside or near a bee hive. It also relates to a use of the dispenser or the gel for the control of mites or of nosema in agricultural, industrial or domestic environment.
