Abstract: The invention provides a method of dipping a number of animals of a species with a pesticide using a formulation of the pesticide which is subject to stripping of the pesticide during dipping of successive animals or groups of animals, the method including:establishing a predetermined level of a dipping liquid containing a first concentration of the pesticide (C.sub.IC) in a dipping vessel the concentration providing safe and effective pesticidal treatment;dipping the animals in the dipping liquid; andmaintaining the predetermined level of the dipping liquid in the dipping vessel by addition thereto of a replenishment composition containing a second concentration of the pesticidethe second concentration of the pesticide (C.sub.CR) being greater than the first concentration of the pesticide, to thereby maintain a safe and pesticidally effective concentration of the pesticide.