Small creatures from the group of insects and worms which are pathogenic to humans, animals and plants can be controlled effectively by applying or introducing an aqueous percarboxylic acid solution containing one or more percarboxylic acids with 1 to 6 carbon atoms to surfaces and/or into water. A solution which contains peracetic acid and/or performic acid is preferably used. In the case of insects, the larvae of these are controlled in water using an application concentration of 1 to 5000 ppm of percarboxylic acid.